On a 92, could a bad ECM cause the engine to idle rough and run rich without throwing codes? Watching with Datamaster, the Short Term and Long Term counts keep dropping down to 108 and I get a popping through the exhaust at idle. It has new O2 sensors, new plugs and wires, and all the sensors (MAP, IAT, Coolant Temp) seem to be reading correctly. I guess my last thoughts are opti, ECM, Ignition module, or coil. You can also hear it missing or popping through the exhaust if you rev it to around 3,000 from idle and hold it there. Any thoughts?

 

I would take the opti out of this list... here is why:

If the optical portion is failing, then you will get either codes 16 or 36. That's a given. If the optical portion is working correctly, it won't throw those codes... and it will throw them rather quickly, from my observations. As soon as the timing feedback isn't recieved or sent correctly, the codes start flying.

If it was a high voltage problem, it would manifest in a different manner... what happens inside the high voltage side is thus: The contact points get carboned up. The ECU tries to compensate (adjusts timing) for the bad contacts, resulting in a wondering idle. At about 4500 RPM on up, it will result in a stumble. Now, if these symptoms have been around for a while, the next phase could be what you are experiencing. However, most of the time, folks replace their opti before it gets worse...

Check to be sure the coil wire is not grounding out. That will result in very poor drivability.

Does the engine run fine when cold, then all hell breaks loose? Or is the engine up to closed loop op temp? Or warmer, but not quite up there yet?

Here is my thinking:

If it's warmer, but not up to closed loop, and it starts to run like crap, I would blame the coil/ignitor assembly. Also - check the harness to the coil/ignitor assembly. There has been a history of those connectors failing, leading to all sorts of drivability problems, but no codes!!!

If the engine is up to op temp when the problems start (closed loop), I would wonder about the ECU... but I would check the coil/ignitor first, and the harness.

If it's bad when cold, the harness sounds like a good starting point.

Make sure the O2 sensors are TIGHT! a little leak there could cause all sorts of problems. I assume this has been going on for a while, tho, right?

Any mods we need to know about? Headers? recent exhaust work? Mileage? Maintenance history of these components?

 

I don't know what engine you have but if it is the LT1 check all of the connectors between the ECM and the Opti including the one on ethe passenger side of the manifolf, the ones on the coil and ICM and those to the opti. If any of these are damaged or have intermittant contact you can get any number of the symptoms you describe.

 

Thanks Guys, I will check the connectors tonight. It has done this since I bought the car about a year ago, but has gotten worse recently. If I start it in the garage and just let it run, It will start to pop slightly just before it hits closed loop, and then will miss more once it is in closed loop.

 

check the connections... do a search for user "jet-jock" in archives. He had this exact problem last year and replaced the coil/ignitor harness (Ecklers part) and everything was fine.

 

It is an LT1... only the LT1 family of engines used the optispark... the L98 used the conventional HEI ignition system.

 

Sorry I missed the word opti in the first post and there is no info on his engine on his information sheet. Thanks

 

If your BLM's are at 108, that means that the O2's are living mainly above 450mv (rich). At 108 your ECM is not in control and can't pull enough fuel out to lean out the A/F ratio. I have had this exact same problem with my 92 to a lesser degree since I installed my headers several yrs ago. Only my BLM's are split (118-140) ECM is in control. After chasing this problem I feel that the 'popping' your hearing is unburt fuel in the exhaust stream being ignited by the cats. I have replaced all the sensors, opti, plugs, coil, ignitor, etc but can't get the problem sorted out. So I just live with it. The car runs like a scaled dog to the rev limiter and never misses a beat. The only thing left to check is the harness / connectors as suggested above or replace the ECM. Let me know (or post) if you find the answer to the problem.

 

Checking to see if this one was figured out.

 

Not yet. Checked over all the connectors to the coil and ignition module and drove it this morning, same problem. However, when it first starts it runs perfectly smooth, no popping out the exhaust or anything. It's only as it starts to warm up that it starts to pop out the exhaust and feel like it's misfiring a little.

 

When my '92 ECM went, it only happened after the engine was warmed up and heated the ECM under the hood.

Tom Piper

 

What were the symptoms? Mine is not throwing any codes.

 

.....and I have the red headed stepchild motor but your symptoms remind me of the beginning of the failure of the coils on my primary injectors.

The symptoms started out as a random "miss" and then went to an obvious "miss". The hotter the lump got the worse the "miss"....eventually. My exhaust smelt lean @ the right tips & the right bank plugs looked like no fuel was getting in there...clean as the driven snow. A scan of the system in closed loop showed the left side as rich....oh this is my first EEC car so I don't know much about this stuff.

Anyway did you do an ohm test on the injector coils? Did you check the fuel pressure reg for gas in the vacuum line? Did you do a fuel pressure test to see if the injectors are leaking? I know I'm all over the place with the suggestions....sorry but it seems to me from your description that the problem is linked to engine temp.


Tom

 

ksphar, How do you know it is running rich? Have you got anything solid to indicate it is rich other than the BLMs on Data Master? If there is an issue somewhere these numbers may be wrong, your ECM may think its rich and it may not be.

Are you 100% certain the plug leads are all on their correct plugs?
Are all the plugs firing properly? None of them damaged/cracked?

Have you done a compression test? What about a coolant CO test?

Does it blow any kind of smoke at all?? (hot or cold)

When you say its smooth at first start, but then develops the issue, would it have reached "closed loop" by the time the issue shows?

All these things are alot cheaper to check than replacing the ECM, and any one or more of them could cause this!!!

ittlfly, Do you have an after market throttle body on your engine by any chance??

Bogus, got the CD today, you are the man. Thanks mate

 

it's the heat thing I am hoping to test...

Do you have any of those cans of compressed air? If not, get some - you know, computer dusters!

Ok, take the ECU off the mount and open the bottom where the memcal goes. start the car and let it start to fail... once it freaks, spray some duster in there, but with the can inverted. This will cause a cold spray to pour out. If the engine starts to run better, you have your culpret.

If no change occurs, make your way to the coil and ignitor, do the same thing... if there is no difference... well... the ECM might not have gotten enough of a dose... but this is a good way of checking.

 

[QUOTE=Casethecorvetteman]ksphar, How do you know it is running rich? Have you got anything solid to indicate it is rich other than the BLMs on Data Master? If there is an issue somewhere these numbers may be wrong, your ECM may think its rich and it may not be.

Are you 100% certain the plug leads are all on their correct plugs?
Are all the plugs firing properly? None of them damaged/cracked?

Have you done a compression test? What about a coolant CO test?

Does it blow any kind of smoke at all?? (hot or cold)

When you say its smooth at first start, but then develops the issue, would it have reached "closed loop" by the time the issue shows?

All these things are alot cheaper to check than replacing the ECM, and any one or more of them could cause this!!!


ittlfly, Do you have an after market throttle body on your engine by any chance??

I know where your're going with this but no, it's stock.

 

No worries mate, i thought youd probably know what i was heading towards there.

 

I was working with the car today and was using datamaster, and when i shut the car off, i tried to restart it and every time it would run for 2 or 3 seconds then die. i unhooked the datamaster cable and after about 2 or 3 more attempts it fired and stayed running. I tried this again later and one time it just cranked for about ten seconds without firing. The car was firing the ignition, had good fuel pressure, but would not stay running. What could be causing this? Also, it still runs rich at idle and pops through the exhaust. However, if i shut it off, and restart, it runs better in open loop until it hits closed loop again, then runs rich.

 

if the datamaster is causing the engine to quit, it's time for a new ECM.

 

Yeah that puts a new spin on things.