Crossposting from General Tech on advice from fellow member.

Posting to this forum as this is an engine you guys know much better than the Fiero guys would - it's a speed-density 350 TPI running a '92 8262 ECM, essentially a 7730 w/$8D map. I built it 20 years ago and it's been sitting for about the last 10 years, and I've been trying to get it running properly again. it ran well as recently as three years ago and this happened suddenly while taking it out for a drive. It's not reporting any codes at the ECM, just flashing 12's. I know that generally points to an ignition problem. It starts up immediately, but has a rough idle and is sluggish to rev. It's drivable but is cantankerous at best, with backfires. I've recently put new cats in it and it just has bullet mufflers that shouldn't clog.

This is a TRUE DUAL EXHAUST with headers, dual cats and dual mufflers with ONE ACDelco O2 sensor on the front (odd) bank only.

First step - Replaced all injectors
Second step - drained fuel tank and refreshed with 5 gallons of premium
Third step - replaced distributor cap
Fourth step - new plug wires
Fifth step - look for vacuum leaks with propane torch. Replaced a few aged hoses around the cruise control, but no apparent leaks.

Since starting this thread, the following critical observation has been made:

Even (rear) bank (2,4,6,8) exhaust is blowing cold, fuel-saturated air, and warms up slowly.

Additional diagnostic steps performed in this thread:

6. Disconnected entire rear bank of injectors. Runs the same.
7. Checked plugs in rear bank. Good, but wet with unburned fuel.
8. Replaced ICM. No change.
9. Tested spark with spark tester at distributor. Tests good.
10. Ran compression check on rear bank. OK at 135-145 across all four.
11. Connected MT2500 scanner to ECM. Once going into closed loop, Integrator goes to 129 and BLM goes to 136, but the ECM otherwise thinks everything is fine on the front bank. No O2 sensor in rear bank.
12. Checked injector fuses, all OK.
13. Injector wiring tested. All red wires on injector harness tested good +12V to ground, and no shorts to ground on any injector harness dark green wires.
14, Verified plug wires match firing order.
15. Cleaned and re-gapped spark plugs to .035.
16. Ran compression check on front bank. OK, 155-175 across all four.
17. Removed and cleaned ECM, Moates adapter, CALPAK and flash EEPROM connections.

I have a replacement rotor for the distributor, but the original wasn't coming off and I didn't want to make it worse trying to force it, so I cleaned it up with some emery cloth and stuck with it for now.

Plugs are NGK R's and are properly gapped at .035.

None of these fixes has made any difference and the issue persists. I recorded a short video so you guys can see & hear what it's doing.


Suggestions for next steps welcome.

 

Video from scanner. It hadn't gone into closed loop because it hadn't fully warmed up yet.

 

the o2 seems to be switching, so you'd think it would as long as it's up to temp. the tune might be an open loop tune?
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all I have is questions, not ideas really. Just going to throw out ideas you've probably already checked.

watching the o2 voltage id it will graph, most scanners will. might be able to tell if it's really lean or really rich when you try to drive it an it's being cantankerous. might help to know.

it's a little odd to me that the o2 seems to be switching even thou it's in open loop and integer is locked at 128 which is no fuel added or taken away.

maybe pull up your tps voltage and map value and see if they are reading normally. I've heard of dead spots in the tps not throwing codes.

confirm the timing at the crank matched the timing on the scanner?

have you done a compression check, maybe an intake valve isn't sealing well, that makes them run badly as it messes the map signal up.


is coolent temp reading correctly in the scanner? it's basics needs coolent/intake temp to right, tps and map for the fueling side. timing of course has to be right. the dizzy isn't off a tooth? plug wire reversed? 5/7 I believe will still run half way decent.

yeah, I got nothing. Hopefully others know better.

oh just in case, the propane leak test doesn't work like it does with a carb. only why I've found is to graph the o2 sensor voltage on the scanner and spray around watching the 02 or fuel trim change. the computer, at least when running right will reach to fast to hear a change.

do you remember what cam is in it? might help sort out what was done to the tune to make it run.
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Thanks for your thoughts on it. It has an LT4 HOT cam. I'll check on your suggestions. It had said the O2 was ready, but I think I may not have hit the minimum time limit to go into closed loop on it by the time I started shooting the video. I saw elsewhere that it's about two minutes. It sounded like something changed at about 30 seconds and I expected that to indicate closed loop, but it was probably just too soon. I'm also going to try using starting fluid instead of propane to check for leaks.

 

Went at it again today and have some new observations. It eventually went into closed loop after getting up to temp. Integrator then went to 129. While the left side exhaust is hot and loud, The right side exhaust is blowing cold and quiet with slight bluish smoke when revved. It looks like the right side (rear bank) is basically not firing at all. I'm trying to figure out what would cause that to happen.

 

Pulled all four rear bank plugs. All four wet and gassy but otherwise properly gapped and fine. I can't figure out why this would happen, but it seems to point to the ICM. But would only one side of it fail?

 

Code 12 is an indication there is no reference pulse from your distributor with your engine off and key on, which is normal. If the check engine light is off with the engine running, and no codes are being stored, it's fine. No ignition problem prevalent as per your ECM. You said it happened out of nowhere after already having it tuned well? Sudden backfiring, rough idle, and rough driving. This is a Lean indication, especially with a BLM of 136. Have you checked your actual fuel pressure?

- Rob

 

Yeah, no codes are actually being stored. There is a fuel pressure gauge on the fuel rail. It reads about 38psi when the engine is idling and goes up to around 44psi when revving. Removing the vacuum line takes it to about 48 psi.

 

I just seen that you wrote one side of your true dual exhaust is blowing cold and is quiet, while the other side is hot and firing. You mentioned you bought new cats and mufflers, but did you check for blockage anywhere else on that side of the exhaust system? You said the car was parked for 10 years, was it parked outside? Any small animals/rodents running about?

- Rob

 

Just so we're on the same page crazyd, with true duals and dual cats, if one cat were clogged then the gases would back up and that injector bank would saturate your plugs, the cylinders won't fire, and you will misfire. These are your symptoms, even after replacing the majority of your ignition system as well as your injectors. Now, you already stated you replaced the catalytic converters, are you absolutely positive that there is not other obstruction in that side after sitting for 10 years?

- Rob

 

I can't say for certain on the exhaust, Rob. It's all welded together with no easy way to inspect it. I have bungs on both sides but only one side is in use for O2, the front (odd) bank, and the other is for a future wideband. The engine ran perfectly three years ago when taking it for a drive and then this happened suddenly. I don't know for sure if critters aren't a factor, but a plugged exhaust seems like it might explain a lot of what's going on here.

 

Quick check would be to disconnect exhaust and run open header for a short test. Depending on o2 location it won't go into closed loop, but it would tell you pretty quickly if idle clears up on start up.

 

Exhaust plumbing on the rear bank is complicated. I'm going to get a backpressure tester and see what, if anything, it reveals first.

 

The plug is seized up in the port to test backpressure and won't budge, so that idea's out.

 

Heat it. Heat the bejeezus out of it. Heat it with a torch and it'll come right on out of there. Heat works. HEAT it up and spin it out.