jshepit

Will it damage the engine to operate in Field Service Mode? Pins A and B jumpered?? Risks???

rough

dont think it would since you have to turn to key to acc and have those jumped to pull codes,

jv9999

iTrader: (1)

It won't hurt anything.

jshepit

Thanks for the responses guys. Curious though, do you know why the engine would run smooth with Pins A and B jumpered and crapping when accelerating with them removed? I get the sense that jumpering A and B and putting the ECM into Service Mode may be eliminating some of the sensors the ECM needs to process for smooth operation. I'm just not sure which sensors might be taken out of equation. Any ideas?

Guest

why would it run smooth with them jumped together??

rough

maybe one of the electrical guru's will chime in and be able to help

87SSTPI

iTrader: (7)

When the pins are jumped it causes the engine to run in 'I believe' closed loop mode, so yes it would have an effect on it.
I'm no expert by any means, and I'm sure someone on here should easily be able to explain this a little more.

alvanwie

Some ecm's advance the timing 8-10 degrees when in aldl mode. Be careful could cause damage.
You may want to check your timing.

IROCThe5.7L

I was always told not to start the car when pulling codes. Not because of engine damage, but a slight chance of harming the ECM.

yeah low

And what I was told is that jumpering the A-B pins on the ALDL also disables the electronic advance in the distributor... Try taking out the Ignition control module and have it FULLY TESTED.

By fully I mean carry it somethere they can tes LOW RPM, HIGH RPM, EST/SPOUT, ETC... I kinda think that you are dealing with this issue.

Also, look under the hood at the 4 wire harness (EST DISTRIBUTORS) to see if the advance disable plug has come unplugged, or gotten corroded. This plug should be NORMALLY plugged in, right guys?

/YL

jshepit

Thanks everyone. This is great stuff, please keep it coming. I took the car into a mechanic today to run a scan on the car under load (this is when the engine misses). Sounds like there is mixed thoughts on what exactly happens when the ECM in jumpered (Service Mode). It clearly changes the operating parameters of the engine (hence it runs smooth in Service Mode), but it also still seems to indicate a faulty sensor.

I already have a new Ignition Module, that I'll put in, and with some luck maybe this is it. I'll also check the timing while in Service Mode (something I haven't done yet). More to come....

thanks eveyone for the input.

rough

Keep us informed on what the mechanics tell ya

jshepit

They just called and suspect the ECM as bad. They said that the scanner readings were bouncing inconsistantly on some readings and when the had the scanner plug-in (key on/no start) the ECM showed no data. It is my understanding that the ECM should still provide sensor data even though the engine isn't running. Showing no data is a bad sign. Does anyone know a quick way or two to verify whether the ECM is ok? I would think that taking sensor readings during driving that these readings should be fairly linear based on driving conditions, but certaining not jump around inconsistently.

Does anyone have thoughts on this analysis? Where could I get a good ECM replacement for a stock 305 TPI?

rough

I got the same quote as you did, mine does not bog as bad as yours sounds. But it seems to drive fine and i get no codes.

jv9999

iTrader: (1)

According to the shop manual, running it with the A+B jumpered puts it in "field service mode". If it's in open loop it the CEL will flash 2.5 times a sec and 1 times a sec for closed loop. No codes will be stored in FS mode. That's all it really says on the subject.
----------
Hmmm, did it flash code 12 with A+B jumpered? Ask them what they will charge you if they guess wrong...

jshepit

It does flash code 12 when jumpered. I see it all the time. I mean it's possible the ECM is bad (and I don't have a scanner, or may the EPROM), but hard to say at this point. I spoke with the technician (mechanic) and he seemed pretty knowledgable (I'm pretty knowledge with the time I've spent in research), but without my own scanner I have no way to know. If the scanner is picking up inconsistent readings ( and in some cases no readings at all, like; RPM, OS sensor and engine temp), maybe the ECM is fried, but will operate ok in Service Mode.

I'm getting a little frustrated at this point, but determined to solve the problem. I still feel that major engine have been eliminated and I'm dealing with bad sensors (all that didnt register with ECM during scan test are brand new) or the ECM is flaky. Probably not a stretch from what I've heard about these OBD1 systems.

Can anyone recommend a good solid place to get an ECM replacement? and should I replace the EPROM too (like get a set, ECM And EPROM together)?

Sounds like a new avenue to investigate.

In the mean time I plan to jumper A and B and test the timing to make sure it's not too far out of wack and if ok, drive the heck out of it until I solve it.

This about how I feel right now. Help and suggestions are welcome.

thanks to everyone thats offered input.

jeff

jv9999

iTrader: (1)

That seems like an odd way for the ECM to fail. Sure, it's possible, but I would think it more likely that the guy does not work on many 80's ODBI cars anymore and maybe his scanner was set for the wrong ECM.

The PROM is specifically designed for your engine/trans/gear ratio so if you replace it, it needs to be the same. I'd try someone else's scanner before I bought an ECM.

tinymay

Just had to replace my ECM on my '88 350. Not sure your ECM is same but just a little checking would tell. Preferred an AC-Delco unit but hard to find and I really wanted a "go to" guy if it did not work right. Chose to go local instead of the web. Here is the one I picked...

http://shop.advanceautoparts.com/Pro...pe=248&PTSet=A

By the way, you will reuse your existing chip.

AaronIROCZ

iTrader: (5)

Check for bad grounds, they can make everything way out of whack. I had one once that read negative rpms.

jshepit

thanks everyone. I have done a pretty thorough investigation on the grounds, but I can surely appreciate the affect of bad grounds. I'll check again. Not sure exactly the best way to check ALL grounds other than a good visual and ohm it out. Suggestions are welcome.

thanks for the tip on an ECM. I think it would probably be wise to get a second opinion on the ECM Scan I had done. The ECM seems to react (when jumpered A&B) as expected so hard to believe it died, but who knows. I'll have it doubled checked.

One thing I've picked up that I'm not sure of is if the ECM does fail in goes into somewhat of a protection mode and will ground the fan connection so it runs when the key is turned on. My fans runs all the time. Even when starting on a cold day. Any thoughts I this theory?

rough

have you traced your fan wires to see if they run into a relay ect or are they just hacked up so when you turn the car on they stay on ? to answer your fan question.

jshepit

Not yet. I've been pretty focused on trying to get it to run smooth during acceleration with the jumper (Pins A and B) removed. The car is pretty much stock and orginial. there is no clear evidence of any hacking at the wiring, but like I said I really haven't pursued this issue yet. It seems to warm up fine. I just heard that one of the safety mechanisms build into the system is if the ECM were to fail it would force the fans on to protect against overheating (since the ECM may not be able to flip the switch). I have no idea how true it is.

My biggest issue is trying to figure out why it seems to run smooth with A & B jumpered, but misses during acceleration with it removed. So far I'm stumped.

Vader

In case you haven't found it already:

GM OBD I System Operation

This generation of engine control systems is a refinement of the first engine control computers and systems. First generation engine controls have some control over spark timing, a few output devices like the EFE system and EGR, and a minor role in fuel mixture adjustment. The OBD I system is designed to control and sense a wider range of functions than the earlier systems, including complete control of fuel delivery, and has proven to be much more reliable and effective.

The system has two basic modes of operation when the engine is running - Open Loop Mode and Closed Loop Mode. In a nutshell, Open Loop mode means the ECM is controlling the engine with essentially no feedback from the engine sensors - the control loop is said to be "open". Closed Loop mode means that the ECM sends control signals to the engine, scans the sensors for feedback to determine further corrections, then adjusts accordingly. The process is continuous, and the control loop is said to be "closed" since the sensors effectively communicate back to the ECM regarding the effectiveness of any adjustments made.

Open Loop Mode

When the starter is engaged, if the coolant temperature indicates less than 100̊F and the intake air temperature is within 20̊F of the coolant temperature, the cold start injector provides a spray of fuel or the cold enrichment subroutine allows longer fuel injector pulses on the service injectors. The cold start or cold enrichment subroutine is of eight seconds duration at a maximum. On a cold-start injector system the extra fuel is distributed to each cylinder via a air distribution system built into the bottom of the intake manifold. If the engine temperature is greater than 100̊F, the cold start injector/subroutine is disabled by the cold start switch or ECM. Upon startup the ECM utilizes information in the PROM to establish the initial pulse rate for the injectors and the engine starts. At this time the engine is operating in Open Loop mode and will continue to do so until the engine warms up sufficiently to enable the oxygen sensor.

Closed Loop Mode

After a warm up period the ECM scans the sensors. If all sensors are operating and within their proper ranges, the ECM will enter Closed Loop operation. This means that the sensors are dynamically controlling the engine fuel and spark. The ECM receives information on intake air flow, coolant temperature, intake air temperature, exhaust gas oxygen content, barometric pressure, manifold pressure or vacuum, air conditioner status, power steering pump load, EGR operation, evaporative canister valve operation, engine RPM, vehicle speed, transmission gear selection, fuel pump status, combustion detonation, and throttle position. This information is used to calculate the proper ignition timing advance and pulse width for the injectors, and fires the injectors for the calculated period. This procedure is repeated continuously in very rapid sequence to maintain the optimum fuel air ratio. The electronic spark control components provide maximum advance, if engine knock is detected the spark is automatically retarded. This too, is a continuous process. It should be noted that the following components are matched for optimum performance - distributor EST module, ESC module, knock sensor and ECM PROM. These components are not interchangeable between 5.0L - 5.7L engines. In the event the information received is higher or lower than the normal range, a code will set in the ECM, and the Check Engine or Service Engine Soon lamp will light.


Block Learn / Integrator

The factory ECM/PCM has a learning capability which allows it to make corrections for minor variations in the fuel system to improve performance and driveability. There are two learning features. The Integrator and Block Learn (INT and BLM) and Block Learn Memory (BLM) cell. The INT and BLM feature is normal with a value of around 128 (the possible range is 0-255). If this value is higher than 128, it indicates that the ECM is adding fuel to the base fuel calculations because the system is running lean. A value lower than 128 indicates that the ECM is taking out fuel because the system is running rich. The integrator is a short term corrective action while the BLM is along term correction. The BLM value will change if the integrator has seen a condition which lasts for a longer period of time. There are from two to sixteen different cells which the ECM modifies, depending on RPM, airflow or manifold air pressure and other conditions such as AC on or off, etc. The ECM learns how much adjustment is required in each cell, retains it in memory, and applies these adjustments when the engine operates in that cell or RPM/Load Range. These features of the ECM allows the system to adjust itself constantly to your engine and assure peak performance for stock and other than stock engines.

NOTE: When the vehicle power is disconnected for repair or to clear diagnostic codes, the learning process has to begin all over again. To reinitialize the ECM, drive the vehicle at operating temperature with moderate acceleration and idle conditions for at least twenty minutes.

Performance PROMs typically change the parameters for fuel flow, fuel cut-off and spark advance-timing and will allow increased fuel flow and modify the spark advance curves during rapid acceleration. They can be programmed to tolerate differences in idle quality and manifold vacuum due to altered cam profiles and fuel systems.

Field Service Mode

On the OBDI ECMs, you can jump 'A' and 'B' terminals on the ALDL while the engine is running.
WARNING! This must be done after the engine is running. This is called "Field Service Mode" and will not harm the ECM.

If the ECM is in Open-Loop mode, the SES light will flash rapidly, about 2½ times per second. If it's in Closed-Loop mode, it will flash about once per second. When in Closed Loop mode, flashing less than once per second indicates the ECM is enriching the mixture above the 128 count base line. Flashing more than once per second indicates the ECM is leaning the mixture below the 128 base line.

Backup Fuel Mode

A 3.9K resistor installed between ALDL terminals 'A' and 'B' forces the ECM into Backup Fuel mode. This mode forces the ECM to use predetermined fuel calculations in the ECM PROM instead of the learned inputs in active RAM. This is more commonly referred to as the "Limp Home" mode of operation. Backup fuel is usually around 12:1 or richer from the factory.

ALDL Mode

A 10.0K ohm resistor installed between ALDL terminals 'A' and 'B' forces the ECM into Diagnostic Data Request/ALDL Mode. This sends a request for serial data string to be sent from the ECM via the ‘E’ terminal at 160 baud. If handshaking occurs within X seconds or after the first data string is send, the ECM switches to 8192 baud and will remain in the data request/serial data string mode until the ignition is turned off.

Miscellaneous

An easy way to tell whether the PROM has been installed correctly in the PCM, and contains valid code set, is to watch the SERVICE ENGINE SOON light as you key on. Make sure the ignition is off for at least twenty seconds. Turn the key to the run position without starting the engine and watch the SES light. It should illuminate, then go out, then illuminate again and stay illuminated. This is called a SES light bounce. If you do not get a bounce then something is wrong and damage could result. If you try to start/run the engine when a proper SES light bounce is not present, then the computer will default to backup fuel and spark mode because it cannot execute a valid computer program. This can happen if the PROM is in backwards, or not seated, or contains a faulty program, or if the PCM has been damaged electrically.

When in Field Service Mode, the integrator is disabled, but the ECM will still operate using BLM data, unlike the Backup Fuel Mode.

jshepit

Great data, Thanks! I havent seen it compliled all together quite like this , but the basic content of your listing is understood and known. Based on this data I'm making an assumption that I have a bad sensor (or is not being registered by the ECM) based on the fact that it runs good with A&B jumperd, (which takes the sensors out of play). Am I making a good assumption here? Could it possibilty run good with A&B jumpered even if I have a bad ignition module, pick-up coil or electronic spark module? I keep getting a lot of feedback that these can cause missing/jerking symptoms I get with the ECM in normal operating mode (jumper removed).

I appreciate your thoughts. This thing is driving me crazy.

Vader

Your '88 should have the '165 ECM. There should be a EST bypass in the system. It would be very easy to disconnect that to determine if the unstable operation is related to timing.

The O² and CTS inputs should also be checked.

If you have a printout or data file from the scan, read the sensor live data.

tinymay

Excellent info, Vader. Thanks!!

jshepit

how do you initiate a EST bypass?

tinymay

Look about 6-8 inches from the ECM on the "tan" wire for a oxygen sensor type connector and unplug it. The ECM will not control the timing when you crank up now. Using a timing light on plug #1, check for 6* advance and note any misfires or erratic timing mark movements. If it is rock steady on 6*, base timing is good.

jshepit

Vader and Tinymay. thanks for continuing to respond to this tread. It's getting me closer to isolating my problem and I appreciate the help. One thing I did notice (and should have checked earlier) is that the SES light just stays in the on position (no on/off then on bounce) with the key on but no start. According to your thread Vader this is a sign the ECM or memory is possibility damaged. After a month of chasing my problem it seems like I may be narrowing in on the ECM (which will be my very last replacement), but could fix the problem. I plan to try the EST bypass that you Vader and Tinymay discussed and see what happens.

It's just amazing though how well it runs in Service Mode. Cant wait to see it running with everything operating correctly. Without the jumper its real erratic (missing and spudering). you can push through it (1-4 gears), but when cruising in 4 or 5th it just spuders a lot. Put the jumper in and BINGO, runs great under all conditions.

Any other thoughts on how to evaluate the ECM or PROM would be appreciated.

tinymay

Well, gotta tell you... This last episode of "no start" on my 350 was a mind blower because my SES light stayed on all the time. In fact that sucker had been on steady for about a year running just fine so I never bothered with it. When it died I was hosed. No way to tell what sensor was broke. Once my distributor, coil, grounds, sensors, plenum, fuel rail and injectors were all tested and passed, my only choice was the ECM. Replaced it, cranked and SES now works. If I had only known up front. So my bet is if your SES is always on, the ECM is partially fried. Mine was the injector switch but it could be any circuit causing bit blockage. Keep us posted. I'm curious.

jshepit

What ECM did you put in Tinymay? I'd like to find a good one other than just a stock aftermarket replacement, but I could go this way depending on whats recommended. The car is stock except for the 3:73 rear end. From what I can tell I should be able to get an ECM that is tweaked for this (rear end) upgrade versus stock.

Any thoughts?

tinymay

The one above in post #18 is exactly the one I put in but I went to a local auto parts store instead of buying on the web just in case it did not work. Only drawback was I had to pay tax. My preference was to get an AC-Delco for about $40 more but could not find one in stock. I think the electronics are the same but the difference is how each re-manufacturer cleans and seals the board's components. That's the only difference I found so far. Besides, any changes made to running parameters are in the chip. You will re-use your current chip and place it in a new ECM, if you get one.

HEY!! Found a link for the ACDelco:
http://frugalmechanic.com/auto-part/...odule-assembly
Best price I've seen for the 88999194. Hope it's not a mistake. And remember you have to pay a refundable core charge. That was another reason I went to a local store. I didn't want to pay shipping twice. At least you have two choices for ECM replacement if you have to go there. Keep tweakin!!

thekevin

Well I am having the same issues you are, been fighting a miss under acceleration for the past couple of weeks, replaced, IAC, Intake temp sensor, plugs, plug wires, cap, rotor. The problem seems to come and go until i jump A and B on the ALDL then it drives great, it may not pull as hard as it did higher up in the rpm range, but if its not adjusting the timing that might explain that. The weather is about to get nasty here so I probably wont be changing a whole lot til the weather gets better, I am open to any suggestions on where to start.

Oh, and my car is a 85 Z28 TPI 305/700r4, all stock except exhaust and is my daily driver, about 250 miles a week.

RED_DRAGON_85

WOW!!!
good info
subscribed...





on a side note, my 85 has similar problems where the throttle response is terrible until the engine warms up fully.
like if i am trying to navigate a parking lot at light throttle when the engine is cold, i give it gas like normal but nothing happens. i give it more, nothing nothing nothing, then all the sudden im going 10 mph...
weird.

ill have to try the jumper thing

tinymay

Been there!! and dont have any good hair left...

Only suggestion would be to remove the connector off one sensor at a time and go for a drive. Each one should make something change. When you remove the right one, it will drive like it does now. My stab in the dark.

rgarcia63

There's an overtemp fan switch on the passenger side head that isn't controlled by the ECM it will turn the fan on whether the ECM works, or not.

thekevin

I found a couple more interesting things about mine over the weekend, it actually ran worse when I unhooked the EST wire, it was missing all of the time. I am still experiencing the same issue although not as bad driving it with my O2 sensor and EGR valve disconnected. Im patiently waiting for my usb aldl cable to get here so I can hook it up to my laptop and hopefully tell what is going on.

Azrael91966669

it dosnt hurt the ecm infact to set min idle you have to jumper the a&b wait 30 seconds for the iac to extend then unplug the iac connector and est wire
then start the car

neagan

iTrader: (7)

RE: This part of Vader's post:

An easy way to tell whether the PROM has been installed correctly in the PCM, and contains valid code set, is to watch the SERVICE ENGINE SOON light as you key on. Make sure the ignition is off for at least twenty seconds. Turn the key to the run position without starting the engine and watch the SES light. It should illuminate, then go out, then illuminate again and stay illuminated. This is called a SES light bounce. If you do not get a bounce then something is wrong and damage could result. If you try to start/run the engine when a proper SES light bounce is not present, then the computer will default to backup fuel and spark mode because it cannot execute a valid computer program. This can happen if the PROM is in backwards, or not seated, or contains a faulty program, or if the PCM has been damaged electrically.

I went right out to my IROC and performed this quick test, but am unsure of what should be straight forward results.
When I turned the ignition to the 'On' position there was such a quick Engine Check Light *blink* that I missed it the first two times. Is that what I'm looking for? Usually when I'm checking for a code the light blinks quite purposely, maybe taking 0.5 seconds. This one is like your not sure the engine light had even enough time to fully come on; it's like a rattle snake strike in less than the first second after clicking over the ignition to 'ON'.

Plus, I'm not getting my ALDL port to show a connection with an ACTRON ScanTool.

Vader; if you're reading this; did you used to have a lightning fast Toyota 4-Runner???
Thanks guys for initiating this thread in the first place. Nitro-Nicky

355tpipickup

iTrader: (5)

im goin to keep an eye on this thead as i am havin problems also. my motor wont even start and when i turn the key on the fuel pump primes then my fans kick on and stay on...they never used to do that. plus my check engine lite turns on and never goes off. i try to jump the A & B to read coads the same way i have done befor but this time it makes no difference at all with anythin....everythin stays the same. Im startin to think after reading this i may have a bad ECM "730" im so tired of problems and tryin to figure them out on this project....

neagan

iTrader: (7)

Hey Idaho Boy/355tpipickup-

How's the winter chill treating you! My family is all over by Twin; just off the canyon where Evil Kenevil 'tried' to jump the snake, way back when. Meridian is beautiful; lucky you. I've got some great friends that moved out there on Carlton Ave about 5 years ago.

I feel your frustration on your computer issues. A new one is going to be about $300 bucks. I'm close to throwing up my hands and replacing mine just to be sure. I hope you figure yours out. I've been chasing sensors and open/closed loop issues ever since I bought the dang thing. Now that I went and paid for an actual computer test machine, it won't even recognize through the ALDL serial port. It's been a long-long line of domino's; each one more frustrating then the last. I sure hope you've got a warm dry place to work on it! Best of Luck, Nitro-Nicky

rgarcia63

355 I replied to your last post on your no start issue. Your statement above does actually point to a bad ECM.

jshepit

Wow!! I started this tread and listened to everyone very closely and here is where I am. I've had the car into two mechanics and had them run a scanner test on the system, both said the ECM is bad and one said the MAF was fried. Ok, so I purchased a MAF and ECM and walla, no change! could the faulty readings the mechanics get when they use the scanner be the EPROM and not the ECM? sometimes I think the term "ECM" is over used and you really need to seperate ECM from EPROM because they don't come as a set when you purchase. I'm now looking for how to get a replacement EPROM, but not sure where to go.

It's been a little frustrating tracking this problem. Oh by the way, my SES light doesn't bounce when the key is turned on. If it does it's darn fast and I can't see it. It seems that this leads me to a bad ECM or maybe EPROM.

Does anyone have anything thoughts whether the PROM could cause all these symptoms or I'm I chasing my tail again.

thanks a ton guys! It's nice to dialog the problem and I appreciate your suggestions..

jeff

jshepit

I'm completely lost now. I have replaced everything I can think of and this 305 TPI and it still misses. If you push real hard it kinda of gets through it, but if your just maintaining a constant 45 it spits and spudders. All new sensors, ECM, EPROM (GM OEM), Fuel injectors checked, fuel pressure checked (although I was told to check under higher RPM which I havent done yet and fuel pump was replaced), complete tune up. I've had it into my mechanic twice now and it's led him to say faulty, MAF, ECM and EPROM. All replaced and nothing has changed.

But, if I go back to what started this string jumpering ALDL pins A & B it runs smooth. We don't run it like this, but it eliminates the miss. At least we know the electronic module works good beacuse it's my understanding thats pretty much all it runs on with A & B jumpered. It's starting to really get me down.

Any last minute thoughts on what to do to isolate this miss or eliminate it?? I'm just about ready to drop it off at the mechanic and just say fix it. it's been the $$$ signs that scare me hear, but I guess since everything has been replace it's probably mostly labor to track down some fault wire somewhere that I can't seem to find.

Thanks,

neagan

iTrader: (7)

I can't remember; are you running an MSD and/or any 'special' type of spark plugs???
Also, if you do not see your Engine Lite bounce on, it means that the bulb or connection has an issue. You really need to eliminate that particular problem right away. Chase your grounds down and rebuff the connections for the lite and at the computer; verify power at the light socket. Sorry you had to wait so long for an answer. AND: I am NOT a proffesional mechanic- Nitro-Nicky

jshepit

Thanks for responding. No MSD or special spark plugs. This baby is pretty much stock, from an engine perspective. I'll review the lite bounce again. It is my understanding that it's pretty quick so maybe I'm just missing it. Trying it a night is probably my best bet. Assuming it is bouncing, any other thoughts? Almost everything I can think of has been inspected or replaced. This one definately has me baffled.

Thanks,

jshepit

An update to an old problem. We have done everything we can think of and this 88 305 TPI still sputters/misses when trying to maintain a steady speed, but sputter/miss is eliminated with the jumper (A & B) installed. We just finished rebuiding the top end (new injectors, many new sensors, new fuel pressure regulator, new intake, etc...) and WOW, the same exact miss when trying to maintain steady speed.

The only thing I can think of that hasn't been replaced is the Knockout sensor, the EGR solenoid and the speedometer is real whacky. What I mean by whacky is as you gain speed the speedometer shows your going much faster than you are. The faster you go, the more its off.

If anyone has any ideas at all on why this 305 TPI won't run smooth at steady speed I'm willing to pay for your thoughts.

It starts everytime without fails and seems to run so smooth until you try and maintain any steady speed (30mph up to even 75). I've had the computer diagnosed 3 times now and it still hasen't uncovered the fault. I get no SES light at all at anytime.

Any thoughts on a good mechanic in northern colorado would also be helpful. It may be time to let someone that knows more than me work on this problem.

devgro2

a vendor on ebay screenname ragged has good stuff on his site , all gm tpi ,tbi injectors sensors and ecms ,have boughtthings from him before you can get a ecm for $50 he backs up all his items so hes pretty reputable ,good feedback too, check him out

Kodfish

iTrader: (1)

What about maybe the VSS signal being bad to the ecm? You say it only happens when at a constant speed so maybe you have a faulty VSS that is feeding the ECM bad info. To troubleshoot this you really need a scanner; look at your VSS pulses and see if they are consistant when it starts to miss. If not then that may be your fault.

jshepit

This was not an easy problem to solve. It did take a handheld scanner while driving, but what solved the problem was removing the metal plate under the electronic control module in the distributor housing. It turns out it was creating an intermittent ground and therefore causing the interminttent miss. Wierd problem!