94 Corvette LT1 Troubles

[Fastmax32168]

So I have been having an ongoing problem with my 94 Vet, and it just keeps getting worse. It started by acting like it was running out of fuel and dying on me a few times and not restarting until it cooled off. It seemed to be a fuel pressure issue, but I have gone all through and even upgraded the system and have determined it is not in the fuel system. I have good fuel pressure. I replaced the ignition module for good measure. The fuel trims tell me its running lean but I cant find a vacuum leak anywhere, and all the sensors seem to be reading correctly. Since I was messing with the tune, I went back to a tune when it was running great, but that hasn't corrected it. It certainly seems to have all the symptoms a failing opti, but before replacing it, here is what is confounding me. The long term fuel trim is adding as much fuel as it possibly can, and the PCM still says its running lean. If I flash in a new tune, it clears the fuel trims, and when I start the car will barely keep running it is so lean, and immediately begins to add more fuel to the long term trims until it gets to the max trim of +25, at which point it will keep running but poorly. It misses and sputters and goes back and forth between hitting on all cylinders and not. I do not understand how a failing opti would cause the fuel system to go lean, I would think it would be sending raw fuel through and the 02 would read rich. Am I wrong about that? Any other ideas why the car would suddenly begin to run so lean on both banks not related to inadequate fuel pressure? I have had this car almost 15 years and it has never given me grief like this before, I guess eventually I had to hit the LT1 lotto. Thanks

[NomakeWan]

Okay, so let's nail down a few things. For one, if loading a fresh known-good tune causes the car to die from being insanely lean in open-loop operation, then we can at least confirm that the lean condition is real. In addition, since adding tons of fuel allows it to run (albeit very poorly), we can be sure that the control loops are working as they should. If the car is still actually running lean at this point (it doesn't smell rich, does it?), then that points to a mechanical failure.

Lean means not enough fuel is reaching the cylinders. This could be caused by a few things.

1. Incorrect fuel pressure. You did say that you have "good" fuel pressure, but is it correct fuel pressure? The PCM is expecting a very specific pressure from the fuel system, and if it isn't getting it, it won't be able to regulate the injector pulse widths correctly. If you want to use a different pressure from what the PCM expects, you have to make sure that you change it in the tune.

2. Fuel rail blockage. If fuel isn't actually making it to the injectors, there you go.

3. Fuel injector failures. This happening on both banks would point to a cascade failure. It's unlikely that multiple injectors have failed but it is possible. You say you "upgraded the system" but didn't indicate what all that entailed, so I will point out that it will be critical to check and make sure all 8 injectors are actually firing, and if not, whether it's a failure of the injector(s) themselves, or a failure of the wiring to them. They do live in a very harsh environment and those wires aren't super thick. If the injectors fail to fire correctly, then there's your lean condition.

4. Physical blockage of intake runners post-injector. If something got stuck pre-injector and nothing else was wrong, you'd have a rich condition. If something's stuck post-injector then you could potentially have a lean condition. I don't find this one likely since it's on both banks and there just isn't that much distance between the injectors and the combustion chamber, but it's a distant possibility.

I haven't experienced your issue on either of my Corvettes, but an engine is just a big air pump. If you know what's going in and what's coming out, then there are only a few places that stand out as culprits.

[sherlock9c1]

Is the motor stock, or modified? Are the intake and exhaust stock? Still running MAF and not speed density? No leaks on the exhaust between the heads and O2 sensors? Have you verified fuel pressure while driving down the road? Are the EVAP and PCV systems intact and working? AIR system still intact? Or is it not off? What do the spark plugs look like?

[Fastmax32168]

The fuel system is a brand new racetronix pump with the wiring upgrade harness, new filter, new regulator. I have 42 PSI. Its a good thought on the fuel rail blockage, the filter I removed certainly seemed awful clogged up when I tried to blow through it. I will pull the rail tomorrow and check it out.

[Fastmax32168]

The motor is heavily modified, but it has been this way a long time and the problem developed out of the blue. Still running maf. No leaks. Fuel pressure verified. AIR system long gone. It went from perfect to crap in one drive.

[steveo]

tell us exactly what you have checked since that 'one drive' and how.

for example

- dropped individual cylinders with eehack to see if it was engine-wide or isolated to a cylinder (observations?)
- checked injector pink wire voltage was 12v on both banks with a multi meter (read xx volts)
- checked fuel pressure with a mechanical fuel pressure gauge at the rail while revving the engine and vacuum line capped off (read xx psi)
- checked optispark with an oscilloscope, verified both high and low res pulses
- checked spark plug wires at night by misting them with water and looking for heavy arcing (no arcing? slight?)

and go start to finish, dont miss anything, because the thing you are missing is probably what the problem is

[JimCT_9C1]

I do not understand how a failing opti would cause the fuel system to go lean, I would think it would be sending raw fuel through and the 02 would read rich. Am I wrong about that?

Lots of good advice in the above posts.
After the running checks including arcing wires etc, as suggested pull and read your spark plugs. This will confirm how lean or rich your cylinders may actually be.

To your question, O2s know nothing about fuel, only O2. I had a similar problem and it was ignition related (bad plug wire). Essentially no spark at a cylinder, so it was pumping unburned air/fuel (high O2) into the exhaust. O2s saw that as lean and added lots of fuel and was gas fouling the other cylinders. The plugs told the tale.

So yes, ignition problems can cause apparent lean readings from O2s, but the plugs may say otherwise.

Hope this helps,

Jim

[Fastmax32168]

I am going to work on it today, checking out some of the advice given, but here is the story of how I got here. Several months ago I took the car for a spirited sunday run and everything was fine. On the way home it starts running to where it will take less and less throttle like its not getting enough fuel. Finally it shuts off completely and I coast into a parking lot. I can hear the pump running but it seems to have very little pressure at the rail and its a no start. I phone a friend and we dolly it home. In the driveway a few hours later it starts right up and drives perfect around the neighborhood. The fuel pressure seems light so I go ahead and replace the filter and the original pressure regulator, cheap and easy. Fuel filter seems pretty clogged. Fuel pressure goes up a few pounds and I think I fixed it. Next spirited sunday drive the exact same scenario happens. Runs perfect for a while then takes less and less throttle finally shuts off and gets another dolly ride home. This time its a no start until the next day when it fires up but doesnt run quite right this time. I replace the ignition module thinking maybe it is heating up and failing, again cheap and easy and I had one in my treasure trove. I drive it around the neighborhood with a fuel pressure gauge while datalogging and the pressure is well below 40 and its adding fuel trims like crazy. So it seems like its again, not getting enough fuel. Been wanting to upgrade the fuel pump so I install a racetronix pump and and wire kit. Now we have in spec fuel pressure but it is still running like crap, actually its worse. It is still adding fuel plus missing and sputtering, which it wasnt doing before. About a month in time has elapsed since the last ride and the pump replacement and it seems to be degrading with time so that may have nothing to do with the new pump. If it wasnt for the fuel trim scenario I would think the opti has failed, but what is really throwing me is why when I flashed a known good tune in yesterday, (thinking I possibly changed something that caused an issue since I was tuning when this all started) the car went so lean I had to pump the fuel like crazy just to keep it running until it added in enough fuel trim. Then its back to staying running but awful crummy, I certainly dont want to drive it anywhere. So I havent done a lot of these diagnosis yet because the missing and sputtering is something that was not happening until this last session, it seemed like I was chasing a fuel pressure problem up until now. So I either have a new problem, or the continued development of the original one and I was on the wrong path. At least its bad enough now that I can chase it. I hate intermittent problems, it gets really frustrating sometimes. Theres my sad story, will report back later today with my findings.

[In-Tech]

Hello,
A couple things I have seen before.
MAP sensor non complete failure. "Spring" return mechanism breaks inside causing very low kpa readings into leaner maps. Quick easy test, unplug map sensor, if it runs better, replace. Or, use scanner and look at kpa numbers if you are familiar. Can be intermittently better and/or worse.
MAF sensor non complete failure. The wire gets coked with ozone, dust, oil, etc and will cause it to run lean as it has less output. Quick easy test, unplug maf sensor, if it runs better, replace. Or, use scanner and look at hz numbers if you are familiar.
One note on MAF replacement. A shit ton of junk out there. The delphi replacement isn't even the same maf and includes a wiring adaptor and truly needs the maf tables re-calibrated. The AC-Delco is a reman and usually good but not always. The autozone, oreillys, pep boys, napa remans are generally garbage.
-Carl

[In-Tech]

Of course we have all seen efi distributor problems, opti or not, with incorrect and intermittent rpm signals but the above is an easier check.

[Fastmax32168]

I compared a datalog from when it was running well to its current state and the readings from all the sensors appear to be in the same range in both logs at similar load and RPM settings. I did try unplugging the MAF to no avail, but not the MAP, although I will give it a go when I get it back together. I managed to pull a few plugs this morning, not an easy task on a longtube equipped C4, and they look pretty good, nice golden tan color. I also pulled the injector rail, did a semi scientific test, Ohm ed the injectors, verified they click on, and sprayed brake cleaner through them while activated looking for an obvious issue to no avail. Plenty of nice clean fuel pours out the empty rail when I turn on the ignition as well.

[Fastmax32168]

I tested for 12V at the injectors, I reassembled the injector rail and it seemed to run pretty decent until it warmed up, then it progressively got worse, although never as bad as it was yesterday. I unplugged injectors one by one and there are two that I had difficulty discerning if that cylinder is actually pulling its load. Not that there was no change, but it did not seem as pronounced as the others. I managed to get a timing light on all the plug wires, and all of them have a nice steady signal even when it is sitting there missing. I guess my plan now is to let it cool off move a few injectors around, and see if that moves the cylinders that dont seem to be pulling a full load.

[sherlock9c1]

What are the fuel trims across RPM and MAP? Are they uniformly lean or are they more lean at idle, or at some other condition?

What MAP are you seeing at idle? What's the idle RPM? Do you remember what MAP you used to see at idle before this started happening?

Is your PCV system still hooked up? Have you checked all of your vacuum lines for cracks? Try spraying WD40 around the vacuum lines while it's running?

[Fastmax32168]

What are the fuel trims across RPM and MAP? Are they uniformly lean or are they more lean at idle, or at some other condition?

They seem uniform across RPM settings, as much as I can discern while its sitting still.

What MAP are you seeing at idle? What's the idle RPM? Do you remember what MAP you used to see at idle before this started happening?

The MAP readings are virtually the same on old datalogs as they are now. Actually most all the readings are the same and all the sensors seem to be functioning with no codes or CE light.

Is your PCV system still hooked up? Have you checked all of your vacuum lines for cracks? Try spraying WD40 around the vacuum lines while it's running?

I have checked it for vacuum leaks and cant find any. Honestly I have difficulty equating a possible vacuum leak with a total shutdown and failure to restart which is where this originated, then progressed into running but missing. However that can be said for much of what I am checking now, including bad plug wires and a failing injector. what could cause both of these scenarios?
I guess I am operating under two possible theories, one being that because the car sits for long periods of time, it gummed up the fuel filter and injectors, and the injectors are still partially gummed up, causing uneven cylinder distribution and the PCM to add fuel thinking its lean. The second being that the opti is crapping out and causing lots of weirdness.

[sherlock9c1]

What injectors are you running?

[Fastmax32168]

30lb ford style red tops

[steveo]

the opti isn't just for spark, it's for engine position reference, which means injector firing is also based on that sketchy sensor. a badly messed up opti sensor will show up in a datalog as wild rpm flucutations.

[sherlock9c1]

Try turning the key on with the engine off, and wiggle the optispark 4-pin harness on the passenger side. If you hear the injectors clicking, this is indicative of a bad harness. They do fail, and you can buy replacements cheap on Ebay. There's a reason most optispark vendors now include that harness.

[beasleyrb]

I just spent 6 months finding a problem very similar to yours. I don't know if your car has the MAF Burn Off Relay and the MAF Burn OFF control Relay but my 88 Corvette does. I had a bad MAF Burn OFF Control Relay. Bad contacts on the relay resulted in low voltage. I didn't find this. A troubleshooter with Oscilloscopes, tools and over 25 years of experience did. It took him two days to find it. I checked very wire in the loom, every sensor to ground, every voltage at the ECM and replaced about 90% of the parts on the engine that have to do with fuel supply and things that control the injectors, etc. You name it, I checked it per the service manual or replaced it out of frustration. I finally gave up and had this young fellow come trouble shoot it. He has all the equipment and the experience. No one wants to be a part changer but it could be the most obscure thing you can imagine. I doubt I ever would have found this because I thought I had Good MAF reading on my scan tool. Again where the experience comes in. I feel your pain my friend. I hope you find the problem and report back so it may help others.

[NomakeWan]

The LT1 does not use a MAF relay of any kind, unfortunately.

[Fastmax32168]

I finally got some time to play with this again today, I actually do have a few codes here now. One in the CCM that is H41 Loss of ECM serial data. and the other in the ABS module H72 Serial data link fault. I know the one in the ABS has been there intermittently for a while, sometimes my traction control light comes on, other times it works fine so I am not sure if that issue is related to my current problem. As to the CCM H41 code, Am I correct that the CCM has nothing to do with fuel and spark after it tells the ECM its OK to start? That leaves the ECM in charge of running things correct? Could that leave me with the ECM itself being bad? I have flashed it about 100 times over the years, and I can still read and flash it now. Its an expensive WAG part. Not sure how I should proceed at this point. Help appreciated. Thanks.

[NomakeWan]

Both of those codes are historical codes set by connecting to the PCM and flashing. When you shut up the bus and interact with the PCM directly it causes a loss of serial data to other modules. This is expected. Those codes can be reset from the dash and will not come back until the next time you connect to the car and shut up the bus.

But yes, the CCM has nothing to do with controlling the engine. All it can do is disable the fuel pump relay (though not permanently; the fuel pump is also tied into the oil pressure switch) and not send the VATS activation signal to the PCM (which will kill the injectors if the PCM is set to operate with VATS).

[spfautsch]

Are cranking times fairly long or does it pop right off when you start it?

Sounds a bit like a failing hi-res signal - particularly when you mention it started manifesting symptoms when hot and they disappeared after cool-down. How old is your opti?

[beasleyrb]

This isn't going to help much but I feel your pain. I had a problem evade me for 5 months. It turned out to be a relay in a circuit I never considered could be a problem. Caused the ecm to get a bad signal. Not no signal but off enough (low voltage) to screw up what the ecm thought it should be doing. I guess I'm saying consider EVERYTHING in the fuel and ignition system. Something changed and your engine performance went south. It's probably a sensor or 5 volt signal back to the ecm. Like I said, this wasn't going to help much.