Slow89Iroc-Z

I have been trying to diagnosis a rough running engine for about 3 years now!!! Basically the car shakes like it’s missing. Well I have done the usual stuff like a full tune-up and checked to make sure there is no arching from the wires in the dark. I replaced the dizzy with a new GMPP unit. I have also checked for vacuum leaks buy plugging all vacuum lines...no change but replaced all vacuum lines to be sure. I also checked around the intake manifold for leaks with a propane torch and carb cleaner but nothing was found. I checked to make sure the electrical grounds were clean and tight. I also shot the header primary tubes with a digital pyrometer and all tubes were around 800 degrees while at idle. Everything looks fine. I have also tried a new intake gasket 3 times along with a new manifold. I had the heads redone with new seals and a valve job along with a port and polish. I ran a compression test that came back all within 5%-10% of each other. I swapped cams, replaced the timing chain, roller rockers, pushrods and ran the valves a number of times but still no luck. I have gone through about 500 chips just for the idle....running it rich, lean...and timing from 10 all the way to 35 at idle and no luck.

Well now I am thinking that the junkyard ECM I am running is not good??? I tried doing a cylinder balance test, unplugged the IAC valve and EST and started pulling injector plugs. Well I started on the passenger side and pulled one off....it made a very little difference but a difference...continued pulling and replacing injectors on the passenger side one at a time with the same results... Moved to the driver side and did the same but this time when I pulled them off and let it run the car started to die after about 10 seconds. I did this to all of them on the driver side and all responded the same way. This was very different than what the passenger side did.... So I went to recheck the pass side and pulled off 2 plugs at once and the car responded slightly worse than when one was pulled but it still was running....pulled 3 off and the car was still running.... Tried this on the driver side and the car died when I pulled 2 off.

So what does this mean??? Is there a problem with the ECM, injectors, fuel rails, injector harness???? I am out of ideas here!!!

Damon

Certainly another junkyard ECM for $20 would be worth the investment, even if just for peace of mind. You don't need new- even if the junkyard computer has a problem it's not likely to be the SAME problem as your current one.

TPI intake?

How does it run above idle? While driving around town?

800* on all 8 tubes doesn't fit with a whole bank of injectors not firing.

justjake

not all that familiar with our ecm's but i think they are bank fire (4 cyls at a time). if this is right, sounds like the passenger bank isn't firing completely (hence the slight change). the drivers bank may be enough to keep it running. check with a noid light (plugs between injector and harness) and check fuel and injectors though not as likely...

edit: anybody know the voltage to look for? maybe compare the two banks? ecm is certainly suspect, check harness too... maybe intermittent with pyro reading right at isle? low voltage or crappy pulse?

Slow89Iroc-Z

I am running a Holley Stealth Ram...it is the brand new casting from them...not the one with the sealing problems... I am sure of this. Above idle and under load it still runs ruff...if you give is gas in park the engine will shake and you can feel it missing. I am not 100% sure on the temps on the primary tubes....the cooling fans kept comming on and off and were cooling the header tubes...I do remember seeing lower temps on the pass side tho...not sure if it was from the fan. Ill run it again but AI remember that the pass was around the 750* and the driver side was all over 800*. But then again the fan is on the pass side that was running.

I dont thing a noid light will help since I can feel all the injectors firing.....I am not sure if they are firing correctly???????????

justjake

got a scope? time to check signal. got a friend with the same ecm?

Damon

True, TPI style engines are batch-fired bank to bank. Possible ECM problem but I THINK the 2 banks of injectors are individually grounded. Lose a ground, lose a whole bank. Something to keep in mind.

I'm not in officially over my head with how these systems actually operate so double check with someone who really knows the electrical side of this EFI system.

Slow89Iroc-Z

I dont have a scope...I am trying to track down a known good ecm to swap in but its proving difficult.

I have checked the grounds on the back of the heads to the firewall and they are tight and clean....also the ground from the battery to the block is clean and tight..... Just to be on the safe side I also used a set of jumper cables and attached it to the battery gnd and to each head to see if it helped....

RBob

iTrader: (1)

If you are still using the original Multec injectors, or even replaced them with another set of Multec's, I would put a decent set of Bosch injectors in. The roughness sounds just like a set of Multec's.

Is the O2 sensor on the drivers side manifold? If so, then that can cause the difference between sides when pulling injector plugs. Pull one on the driver side, O2 goes up, ECM adds fuel, and things get worse. Pull one on the passenger side and no difference in the O2 to the sensor.

If you try pulling injector plugs again disconnect the O2 sensor. That will prevent it from going into closed loop.

There are two injector fuses, one per 4 injectors. However, the ECM has one driver to ground them all at once (batch fire).

RBob.

sofakingdom

iTrader: (1)

Actually, the "ground" is supplied by the ECM driver. The driver "grounds" one side of the injector coils in order to fire them. 12V is constantly supplied to the other side of the injector coils.

However, there are 2 12V feeds to the injectors; via 2 fuses in the fusebox. One fuse for each side of the motor. INJ 1 and INJ 2 or something like that. One of those could be blown. You can check for 12V at the injectors that don't have any effect on how the engine runs; if there's no 12V, start looking at fuses.

"Feel" isnt worth a crap. When one side fires, it will drop the fuel pressure, which will transmit to the other side as a pulse in the fuel rail. Forget that, and do some real troubleshooting and measurement.

I sure wouldn't just jump on replacing the ECM; mindlessly swapping out big expensive parts without troubleshooting, just because you don't understand them and get frustrated, usually doesn't get very satisfactory results.

Slow89Iroc-Z

Well the injectors I am running are 30lb SVO red tops set at 30lb/hr in the prom. Good point about the ecm seeing the change on the drivers side but not the passenger side. Ill force open loop in the chip and try again.

sofakingdon Ill check the inj fuses in the box.....
Quote: "Forget that, and do some real troubleshooting and measurement" What do you suggest I do next? I have ohmed out the injectors and they are about the same. I am going to check the ecm grounds on the back of the pass head again but other than that I am stumped???

Merlin

Monitor the supply voltage to each injector bank while the engine is running. Unless you have a wiring or supply problem, the voltages should be identical. You can also measure circuit resistance from the injector harness to the ECM. Even though there is only one switching transistor (Q1) to operate the injectors (ground them) there are two parallel paths to the ECM - One from each injector bank (D15, D16).

I understand that you installed a new distributor, looked for arcing around the wires, and performed "a full tune up." Are all the plugs the same heat range, with the same gaps?

Slow89Iroc-Z

all plugs are autolite 3924 with a gap of .045 just installed last week...double checked gap before installing........no change with new plugs.

sofakingdom

iTrader: (1)

Check for 12V on one side of the injectors?

Check for ground pulses from the ECM on the other side ("noid light")?

Pull the fuel rails with the injectors, put fuel pressure on them, see if they're actually firing? Fuel comes out?

Apply 12 V directly to all the injectors with clip leads or something and observe the spray pattern?

etc. .....

Merlin

Or you can bring it to me. I'll have it repaired, fully evaluated, and back to you by August at the latest.

Bring gas money...

Slow89Iroc-Z

Merlin u

Slow89Iroc-Z

Well I tried a new ecm and it runs the same.....

Checked for 12V on one side of the injectors......its there had to unplug the harness from all injectors to verify this but it was there.

Check for ground pulses from the ECM on the other side...I unpluged the coil and all injectors and hooked up an analog meter to the harness while cranking and the needle jumped on each one.

Pull the fuel rails with the injectors, put fuel pressure on them, see if they're actually firing....Cant do this one....the Holley rails don't have clips to to keep the injectors from comming out when under pressure....

Apply 12 V directly to all the injectors with clip leads or something and observe the spray pattern....Cant same as above???

Can it be the way the fuel lines are routed??? See this post... https://www.thirdgen.org/forums/alternative-port-efi-intakes/354201-does-matter-how-fuel.html

KWIK84

iTrader: (5)

Unplug all injectors, install fuel press gauge, power up system to bring up pressure, then fire each inj individually by supplying power and ground to each injector, and getting pressure back up before firing next injector if all injectors are working properly your pressure drop from firing the injector should be close t the same. Just a note cause i'm sure someone will say something about it if i don't, when you fire the injectors have some way to make sure that you are keeping all of them open for the same amount of time or your testing will be no good. Try Radio Shack for a momentary switch that you can put inline on your ground side jumper wire.

Slow89Iroc-Z

Ok did some more checking yesterday. I hooked up the fuel feed and return lines line Holley recomends but it didnt make any difference. See this post: https://www.thirdgen.org/forums/showthread.php?t=354201

Now since I had the fuel rails off I flipped them over with the injectors in them and used tie wire to hold them in the rails while I pressurized the rails. I then had someone crank the motor over while I watched them spray...all of them sprayed the same and with a good patteren and none were leaking.

I also ran the valves again with no change.

I am starting to believe that what I am seeing is not a miss...Something is causing the motor to idle ruff. At higher rpms and during a brake torque the motor seems smoother. I am lost.......

I am running a base timing of 8 degrees with the computer advanceing it to 25 degrees at idle. Idle MAP is in the 50-60kPa range and desired idle speed is 850rpm. Any sugestions?

Vader

Get back to the basics. Perform a cylinder leakage test to see if all the valves are closing fully and sealing and if all the rings are sealing well. Verify correct plug gaps. Run the firing order again just to be certain you didn't pull the "5-7 swap" or something similar that has been done so many times.

Did you degree the cam when it was installed?

Slow89Iroc-Z

I dont have a compressor or guage to perform the leakdown test. I did do a compression test tho. I just did a tuneup last weekend while bringing it out of storage and all plugs were checked and regapped when installed...none were dropped. Firing order is correct...went over it a few times already.... Cam was not degreed in but it is the second one I have put in this block with both cams I had the same problems....