This problem has stumped me now for nearly 8 months. I am about to give up, but I'm willing to make one valliant last effort. There is a car show next weekend that benefits child cancer patients...lets at least do it for the kids .

The car: 1991 Formula AFR Stealthram 355

The Problem: Hits an unwanted "wall" at 2600. No matter what is attempted (WOT), it will not exceed 2600 rpm. It pops and sputters on anything above 35% throttle. During this time the tach goes haywire also. The car idles fine and the tach reads normal at idle.

I put the MSD distributor on the Mercruiser distrib dyno and it read excellent to 10K rpm. The testing of the distributor included the ignition module, cap, and pickup coil. I have also tried two stock distirbutors. The pickup coil and ignition module are new.

All other aftermarket equipment (Coil, retard control, ignition box, etc) has been eliminated as the cause by replacement with new or bypassing. I have replaced the igniton wires and the plugs.

The ecm has been eliminated as well as the prom by replacement with known working components from a friends car.

I have removed the entire engine harness, unloomed it, and visually inspected all connections. I also tested continuity for every wire.

What circuit(s) or component(s) when failing could cause this? Any similar experiences? Any engine wiring diagrams to post and analyze together?
Thank youuuuu!

 

Heh, there is a component that I found that causes symptoms like you describe, acts like an ignition problem but isn't. I had the same thing happening on my 91 Formula and it turned out to be the coolant temp sensor. As the engine got warmer the problem got worse and worse. Give it a try, for $7 it is a very cheap fix to a frustrating problem.

 

That would be SWEEEET. What exactly did your car do when you were experiencing the problem?

I think I can just do a resistance test on the thing first. What is room temp resistance for the CTS anyone? And while we're on it, maybe a bad MAT can have the same effect?

I will also try to make up an ALDL cable tomorrow so I can scan the computer for strange readings like MAT and CTS outputs.

 

Here we go. Will check this first thing tomorrow and update.

 

Resistance of CTS checked good for room temp at sensor and at ECM pins. Also checked for short to ground on these wires. MAT checked out good as well. Keep 'em coming... Thanx

ALDL cable finished. Will hook it up tomorrow -- too late tonight (noise)

 

Based on what you say either something is preventing revving or possibly you may be having some sort of RFI interference. Try to go back and figure what the circumstances were when it started. The tach going crazy is a clue. Are all the grounds tight and to a good chassis ground? Try adding temporarily another engine to chassis ground. Are the plug wires being routed too close to the electronics, and are they supression type. Solid core non suppression type can cause havoc with electronics. Try having someone rev it and simply try moving around wires, if it changes it may steer you in the right direction. A scope is a good tool for this sort of problem. Try a capacitor from the 12 volt side of the coil to ground. Y can temporarily wrap aluminum foil around the wiring to see if it changes. Let us know how Y make out. The only other thing I have seen stonewall the engine like you say was a plugged cat on a friends Mustang, course it was glowing red also. LOL

 

Thanks for the help!!!!!!!!

Based on what you say either something is preventing revving or possibly you may be having some sort of RFI interference. Try to go back and figure what the circumstances were when it started.
I was driving the car in town without any configuration changes at the time it happened. It's been about a year since the original occurance, so memory of the place and exact circumstances have escaped me.


The tach going crazy is a clue. Are all the grounds tight and to a good chassis ground?
I think the first thing in order is to find all of the ecm, tach, and ignition related grounds and temporarily jump them to a known good ground. Can someone help me with recommended grounds (pins) to re-check at the ecm, distributor, etc?

Try adding temporarily another engine to chassis ground.
I have a ground strap the size of my arm run from the bell housing to an aarm bolt.

Are the plug wires being routed too close to the electronics, and are they supression type. Solid core non suppression type can cause havoc with electronics.
Not sure what type they are, but the wires are MSD spiral wire. I bought these brand new after the car started doing this in an attempt to diagnose the car.


Try having someone rev it and simply try moving around wires, if it changes it may steer you in the right direction.
The harness has been completely removed, disassembled, and re-installed, so there's been a lot of movement in the harness. The jiggling/shaking method has been exhausted.

A scope is a good tool for this sort of problem.
I may be able to get a hold of a scope, but what shall I scope? The ignition wires with an inductive pickup, the drs, or ?


Try a capacitor from the 12 volt side of the coil to ground.
COOL- What's this do? What size of Cap?

U can temporarily wrap aluminum foil around the wiring to see if it changes.
Okay, I'll try that! <willing to try anything and everything>

Let us know how U make out.
WILL DO!

The only other thing I have seen stonewall the engine like you say was a plugged cat on a friends Mustang, course it was glowing red also. LOL
No cat, 4" exhaust. Unless a cat (a real one) made a nest in there, I think it's free and clear.

 

'730 ECM '746 Harness
------------------------------------- --------------------------------------
BA1 N/C N/C
BA2 N/C N/C
BA3 N/C N/C
BA4 (MAP +5v ref.) C14
BA5 (TPS +5v ref.) C14 (shared at sensor)
BA6 (IGN +12v) A6
BA7 N/C N/C
BA8 (ALDL Serial Diag) A8
BA9 N/C N/C
BA10 N/C N/C
BA11 (Fuel Pump relay) A1
BA12 (Sys. GND) A12

BB1 (BATT +12v) B1
BB2 N/C N/C
BB3 N/C N/C
BB4 N/C N/C
BB5 (TPS, CTS, MAT GND) A11
BB6 (MAP GND) D2
BB7 N/C N/C
BB8 N/C N/C
BB9 (VSS Low - NOT USED with Buffered VSS on pin BC6!)
BB10 (VSS Hi - NOT USED with Buffered VSS on pin BC6!)
BB11 (VSS output 4000ppm - NOT USED with Buffered VSS on pin BC6!)
BB12 N/C N/C

BC1 (VSS output 2000ppm - NOT USED with Buffered VSS on pin BC6!)
BC2 N/C N/C
BC3 N/C N/C
BC4 N/C N/C
BC5 N/C N/C
BC6 (VSS {Buffered}) A10
BC7 (EST Bypass) D5
BC8 (EST Ctrl) D4
BC9 (A/C Signal) B8
BC10 N/C N/C
BC11 (Injectors 1,3,5,7) D14
BC12 (Injectors 2,4,6,8) D16
BC13 N/C N/C
BC14 N/C N/C
BC15 N/C N/C
BC16 (Battery) ??

BD1 (Sys GND) D1
BD2 N/C
BD3 N/C
BD4 N/C
BD5 N/C
BD6 (Sys GND) D1
BD7 (Sys GND) D1
BD8 (Distributor Ref.) B5
BD9 (EST Ref Low) B3
BD10 N/C N/C
BD11 N/C N/C
BD12 (A/C Pressure Sw.) A8
BD13 N/C N/C
BD14 (4th gear Sw.) C7
BD15 N/C N/C
BD16 (Park/Neutral Sw. Signal) B10

GE1 N/C N/C
GE2 N/C N/C
GE3 (IAC "A" Hi) C5
GE4 (IAC "A" Low) C6
GE5 (IAC "B" Hi) C4
GE6 (IAC "B" Low) C3
GE7 (SES Lamp) A5
GE8 (Fan Ctrl) ?? ('746 doesn't control fan?)
GE9 (EGR Ctrl) ?? ('746 doesn't control EGR?)
GE10 N/C N/C
GE11 N/C N/C
GE12 (ALDL Diag Term) A9
GE13 (Fuel Pump Signal) B2
GE14 (O2 Signal) D7
GE15 (O2 GND) D6
GE16 (CTS Signal) C10

GF1 (Shift Lamp - M/T) A7 (Manual Trans only)
GF2 (AIR Valve - Port) C2
GF3 N/C N/C
GF4 (AIR Valve - Converter) C1
GF5 N/C N/C
GF6 (TCC ctrl) A7 (Automatic Trans only)
GF7 (Evap. Canister Purge) A3
GF8 N/C N/C
GF9 (Knock Sensor Signal) B7 (splice E to C of esc module)
GF10 (VATS/Pass-Key) B6
GF11 N/C N/C
GF12 N/C N/C
GF13 (TPS Signal) C13
GF14 N/C N/C
GF15 (MAP Signal) C11
GF16 (MAT Signal) C12


From what I see here, I need to double check ground at
BD1
BD6
BD7
for sure


I am unsure, but I might need to check
BA12
BB5
BB6
GE15
These are sensor ground references I belive-- do I need to verify these are grounded to chassis?

What does the EST bypass circuit look like?
What Pins on the Ignition Module should I check for good ground?

 

When I was having troubles the engne would act just fine while still cool, but as the engine warmed up it would stumble and cough if you gave it more than a little gas and the tach would bounce around as the engine was trying to accelerate.

 

The engine does it at all times, even seconds after a cold startup.

 

UPDATE

Finished ALDL cable and ran car
Misdiagnosed the thing, waste $50 on a new MAP sensor and still no go.
Trying again, I finally caught something...
The computer thinks the car is at 3000 when the car is really idling. Okay, there's the problem. After fiddling with it more, I got it to throw a code. ESC failure which is code 43. On 730 cars, the ESC is built into the ECM. Can anyone provide me any advise? Diagnostic flow chart perhaps? Many thanks.

 

Another wierd thing. I set the TPS voltage to 0.54V at the ECM PIN. When scanning; however, the TPS voltage is 0.00 at idle according to moates software!

The TPS does linearly increase with throttle all the way to 5V on the software readout and at the ECM pin. The only difference is the closed throttle TPS reading via digital voltage meter (0.54) versus the software (0.00).

 

I feel like I'm talking to myself...

I pulled the wire off of the knock sensor and tried to start. Still acted the same.

Here a procedure I pulled out of another tread. I'm going to try this procedure and report my findings. Stay tuned!


disconnect the knock sensor
put the ingition in the on position
grab a multimeter and test from the knock sensor wire to a ground it should read between 4-6 volts. if its more or less, read the lower 2

if the voltage is between 4-6 vols
probe the knock sensor terminal and the block you should get a reading of 3300 and 4500 ohms
if its between the two check thde harness and sensor connetor if thats ok remove ecm and be sure the mem-cal is properly seated into the ecm, if the memcal is sat in perfectly... replace the memcal.
If the ohms are not between the two, the knock sensor is bad


IF NOT :the voltage is over 6, the wire from the knock sensor could be to closed to secondary igiition wires or the wires is shorted to voltage or its a fulaty ECM.

IF NOT: voltage is less then 4, the wire is open or shorted to a ground or its a faulty ECM

 

Results of this test...

I do have good voltage to the knock sensor wire. It is 4.98V, cool.

I was unable to get a resistance reading between the block and the knock sensor pin. The digital voltage meter simply remained off line.

My conclusion...
These cars always run fine without the ks plugged in, so I dont think this is the cause of the code and ignition problem. I will probably replace it...but not until I look at some more issues.

Memcal?

Any help on diagnosing this code 43? Thanks!

 

Okay, I tried another ECM and memcal and prom...same problems. Both ECMs threw code 43. It looks like it may be eliminated to just wiring or interference now.

I think the next step is for me to completely bypass ALL code 43 related wiring. This will take the help of someone smarter than me and with more technical reference documents available to them. Can someone with a GM manual post 91 underhood wiring diagrams so we can figure out what needs to be bypassed exactly? Thanks!

Can bad O2 readings cause this problem? Such as a grounded or shorted O2 signal wire? Just turning some gears...

 

Hello JMatlock88,

I do have this picture that you can take a look at, Don't know if it will be helpful or not. If it is helpful to you, I can e-mail a clearer picture to you.

 

ya please email it! I'll host the clearer one! Thanks! What ECM / Vehicle is in the pic?

 

It's out of a 91 - 92 firebird.

 

Here is a pinout of the ignition module. I am going to check continuity to the ecm (and check for ground or short) for pins A, C, and D. From my understanding, if these wires have issues, it may be the cause of the false RPM readings in the scan tool.

 

Man, if that isnt the best kept secret! Okay, here's the diagram...

Basicly, I'm coming up with the same thing. I've already checked the knock sensor wire-- it's working great. So the only other ESC related wires are A,B,C, and D on the ignition module. Anyone agree or disagree here? I will also aligator clip the body of the distributor to ground just in case.

If none of these systems are faulty, I think I'm as far as I can go with this one.

 

Have you tried disconnecting your battery for 30 seconds to clear the stored codes in the ECM?

 

Yes, I flip my rear battery cutoff switch to clear the codes. Also, switching the ECMs/Eproms back and forth did the same thing.

 

May seem lame, but I like basics. How about a timing light, that might help. Check base timing with EST wire disconnected, then hook it up and look for advance. Linear advance may rule out some items for you. With meter hooked up to KS and engine running tap the area of the block with a wrench-NOT the sensor itself, you should see momentary retarding of the timing as the sensor responds to the vibration. The sensor is a piezoelectric device similiar to an old microphone design, you will probably read infinity until it senses some vibration in the 6-8 hz region.

 

The scanning software may be reporting percent of TPS, if so then 0% at idle is correct.

To double check this connect scanner, key on, engine off, depress gas pedal and see what the scan tool reports.

RBob.

 

The 3,000 RPM while idling is something to look at, I'll get back to that.

I don't think the ESC code 43 is the problem, just a symptom of the problem. For now use a 3.9K ohm resistor and wire it in place of the knock sensor. (run it from the knock sensor wire to ground on the engine someplace). This will eliminate any problems created by the knock sensor picking up noise.

As for the 3,000 RPM ECM report, along with the other symptoms, there is something confusing the ECM. The ECM is only grounded to the engine block. This is accomplished via the ECM wiring harness. In the stock location the battery has a heavy ground wire to the block. Plus a smaller ground wire to the fender, then the ground strap(s) from the back of the head(s) to the firewall.

With the battery in the rear (I read about a cutoff switch, so the battery is in the back?). Is the battery - run to the engine block or to the frame? If to the frame then a heavy ground needs to be placed between teh engine block and the frame/firewall.

The heavy ground to the a-arm bolt doesn't count. The a-arm is bolted to the K-member which is then bolted to the frame. If the battery is in the back then a heavy ground is required from the engine to the frame or firewall (the frame and firewall are welded together).

If the battery is in the stock location then check that there is at least one ground strap from the back of a head to the firewall. And that the bolts are tight.

Another possibility is that the alternator is bad. An open diode will cause ripple on the vehicle power system. This can/will cause the ECM to be a little confused. Can also affect the ignition module. Can also check the scan tool to see what the ECM is reporting for both pump & vehicle voltage.

The ECM calculates the engine RPM by measuring the amount of time between distributor reference pulses (DRP). The ECM has internal counters to do this. So either the distributor isn't sending a proper stream of DRPs, or the ECM can't properly measure the time between them.

I know you tried another distributor, was it a stock GM unit? If you have an MSD box or something also try the engine & scanning without it. See if it changes the reported RPM.

Open the EST connector (also known as the BYPASS, tan w/black stripe) and check the reported RPM. See if it changed and/or if it is now correct. Note that opening the EST will cause a code 42, ignore it.

RBob.

 

I am looking at TPS voltage readout on the scan tool. WOT is 5V, idle is at 0.00. If it were %, it would range from 0-100 or 0-1. There is a % TPS readout also, but it always remains zero. Not sure why.

Will do all of this and report! Thanks!

I agree! Will do this and report.

I don't really have stock ground locations. As it sits, my 0 ga wire is grounded to a large rear bumper (metal crash zone) bolt (battery in rear, yes). The original left head grounds are on the firewall. The right head grounds are still on the right head. There is no strap going from the head to the firewall. I do not suspect this as the problem because I have attached a jumper cable to the motor and then clamped it all over the engine bay. The starter cranks perfectly. The problem started plenty of time after all of these ground modifications were done.


Now I could definetly suspect this! I had the alternator tested good, but then again who really knows. How about trying to start the car without the alternator? I will report what the scan tool sees.

Yes, it was two stock GM distributors. I will scan and report what RPM is reported without the Crane box and then also with the EST disconnected.

Thanks for your help folks!!!! <Out turning wrenches> :lala:

 

For the TPS, the scan tool was reading right on today... 0-100%, 0.54V at idle and around 4.25 at WOT.

All wires going from distrib ignition module to ECM checked out good.

Placed grounds all over the place, including one from engine to firewall and then linked all of my other grounds to them too. No difference. Also aligator clipped the distributor housing. No difference.

I ran the car w/ the alternator big plug disconnected. No difference.

The car actually runs differently (much worse) without the Crane box hooked up. It won't rev over 1500 rpm-- feels like a timing issue.

The only thing left to do is grab a resistor for the knock and also do knock test w/ the hammer. I'm not getting anywhere today

 

Forgot to mention...the scan tool is reading normal rpm today instead of reading higher-than-actual rpm as before. May have been a screw up in my cable that's layed out on a breadboard.

I will log some at idle and post it soon.

 

Here is the data log for 30 seconds or so of idle with the Crane box attached. Otherwise the thing wont idle.

Save target as...

http://members.***.net/jmatlock89/Gm040602.csv

 

Replaced the coil with another OEM coil. No go.

Replaced msd distrib with an OEM distrib. Need another hand before I can get it timed. Awaiting results.

 

Oh and a note about the timing-- the moment you crack the throttle, the timing jumps all over the place. Why?

 

I'm not an electronics tech or anything, but I'd be willing to bet that it is an open somewhere.

 

Whenever I see wierd stuff whether it's a TV, two way radio system or the nieghbors VCR I drop back and punt-normally to the power supply. How about measuring the 12 volts DC to the ign box, then rev it. See what happens. If you have a decent meter then try using the AC volts scale. If you measure a large percentage of AC volts it may be screwing up the electronics. Thats where the scope is handy but a decent DVM should work. All this has to be tied together somehow. If there is a lot of crap on the 12 volt line go to Radio Shack and get a 1000mf 50 electrolytic capacitor. One end will have a band or a stripe, that is the neg end-ground it to a good ground-the other end or positive attach to the 12 volt hot wire. See what happens. It's just a WAG but I have seen lots of wierd stuff with high power 2 way transmitters when there is a electric issue in a car. Dan

 

There is probably built in circuitry which filters power supply ripple, but it has opened up or burnt up or whatever you want to say.

I think putting that cap in there is an excellent idea. If it works then leave it, but something tells me something isn't right.

I'd be looking over the grounding points very carefully, and do basic voltage checks where it counts.

Hopefully the ECM worked wehn you gave it back to your friend, sometimes those will get ruined by an electrical fault in the car, thus swapping one in would prove nothing.

 

What's your fuel pressure when the problem occurs? What's the fuel volume that your pump is delivering? When th eproblem occurs, does it smell rich, or lean, or normal? Take a resistance reading on your injectors.

Just a few quick ideas to look at.....I'll think about it some more....

 

I just talked to him earlier tonight, and he said that pressure was good even when it while it's dying... he has a digital gauge setup on his A pillar pods, but I'm sure he'll chime in sometime to give you the numbers he has on it.... the resistance on the injectors is probably a good idea.

 

FP is unchanged and good. Injectors = GOOD IDEA. I will test the resistance across each and report. They have less than a few thousand miles on them, but it's worth a look for sure!

As an update,
Swapped distributors with another stock one. I am not sure if this distrib is a good working unit, but I thought I'd try it. Now the symptoms have changed... I can't get it to idle to time it. (I am not a newbie -- the plug wires are on right and the thing is not 180 out.) I suspect a bad distributor. I'll swap in another stock unit and hopefully get back to where I was before.

 

Just a shot for you. But a long time ago I had problums like you and It turnd out to be my stock tach. Disconected it a bingo......

 

Thanks for the idea! Unfortunately, the original tach was removed (literally) and the problem still exists. I replaced the tach with an in-dash autometer, still the same problem. The only thing I havent done is cut the OEM tach wire. I am trying that today and will report asap.