So I just got this sweet 94 Corvette, and am LOVING it. My first one. When I first got it, it had some misfire at higher RPM's so I installed a new MSD cap, rotor, and coil, as well as Taylor 8.2 wires and TR55's gapped at 050. After I figured out that I'd re-installed the Opti on wrong,(IT CAN BE DONE, I DID IT!) It now purrs like a kitten, and the high rpm miss is gone. So now 4 weeks have gone by, and I'm fixing little bugs like the AC buttons not working, so I clean the contacts, etc. all the typical corvette issues.

Anyways, on my way to work Monday morning, after 20 minutes of 70 mph driving, I'm coming off the interstate, down to 30mph and when I went to accelerate, it feels like there's no power. I hear it popping and backfiring, and it sounds like I'm only running on half the engine. I pump the throttle several times, and I get it over 3K and she takes off like it's supposed to. But once I let the RPM's fall again, it's stumbling and popping again. This goes on for about 30 seconds to a minute or so, and I'm feathering the throttle, then flooring it, whatever, trying to get moving with traffic again, and it suddenly clears up. I'm back doing 70 again, but feathering the throttle I can feel it popping and missing, but I'm at speed and it keeps that speed barely, but I have no real power if I'm just feeding into the throttle, until I'm almost WOT, or unless I floor it, then it runs like a scalded ape, just like it should, until I left off the throttle again.

So I limp to work 2-3 minutes later, and it's idling fine. I check under the hood for obvious things like hoses off, or wires off, but see nothing. I kill it and go into my office, come out 8 hours later, and drive home with no issues. I took my motorcycle to work the next day, and that evening, when I get home, I take the Vette for a spin. This time, after about 5-10 mins, it starts popping and backfiring again, and I swear I'm only running on four cylinders it feels like. So I get home ASAP, test the MSD coil, it's at 9500 ohms, and it idles just fine. I just can't drive it. I kill it and let it cool off about 15 minutes, cranks fine, take off and it drives fine again, for about 3 minutes, and it's spitting and pooping all over itself again. I limp home, and when I get out of the car, it's idling perfect again. Hit the throttle, it takes the gas like a champ. Can my coil be doing this? It and the ICM is bolted right to the cylinder head, and it's sandwiched between the heater hose and the head, so It's in a pretty hot spot. The car is fine when it's not up to full operating temp yet. Any thoughts? OH yeah, NO CODES, not one.

Here's some more info on some testing I did this evening. I hooked up a DVM to my TPS, couldn't get a stable reading with just the key on, so I kept it hooked up and cranked the car. After cranking, I now have a steady .87 vdc at idle. I moved the throttle, and I got smooth response from the voltage and the engine. I let the car warm up to 180, and left the DVM hooked up, and set it up on my windshield where I could see it with the hood shut. I went for a little ride, and after a few minutes of driving at 55, she started acting up. my TPS was constant at about .97 before and after it acted up. As I changed throttle position, the voltage changed correspondingly, nice and smooth, no dips, jumps or other burbles. So I ruled out the TPS then and there. I got back home, it was running like crap, but when I slowed to pull into my garage, it cleared up. So I stopped, held the brakes, and put a little load on the engine, almost enough to spin the tires, and sure enough, it started missing like before. As long as I held it with a slight steady load, it never popped or backfired, it just ran like crap, and sounded like it was missing on at least 2, possibly 3 cylinders. I wouldn't say it's rich either, seems lean if anything, no fuel or flooding smell. I let off the throttle, my TPS signal went back to idle, and the car smoothed out and ran perfect. Put it in park, and held the throttle to about 3k on the tach, and it purred like a kitten. Now what do you think about that? Only messes up under load. Somebody has got to know something about this, where I can look next, because everything is checking out so far. Checked the coil again, showing 10,300 ohms, don't know if that's getting excessive or not. I also moved the coil/ICM assembly up out of that little hole it's in, just moved the bracket out from behind the PS reservoir, and rolled it up higher by the valve cover. At least there it's getting some air and isn't completely pressed against the head, but still hot none the less. Any thoughts?

 

Check the fuel pressure at key on, started and after shutoff. My fuel pump began to fail (burned up on me due to a clogged filter) and would run fine cold and warm (in parked/neutral) as long as I wasn't under load. As soon as I got it out on the road and it warmed up, bam, no power, limping, bogging. Ran fine cold, just when it went into closed loop it would fall on its face.

Could also be injectors, that once warmed up, don't spray (atomize) the fuel the way it should.

 

What should I expect to see for pressure at idle/ load conditions? I think it's about 38# idling, haven't check it hot or running, but will do that tomorrow.

 

same issue with my 93.

replaced so far..Injectors (FIC) Coil (MSD) ICM (GM) spark plugs and wires (AC Delco) Cleaned TB , replaced Fuel Pump (OEM Delphi)

I have a feeling it the dreaded optispark, what else could it be.?


LT1 Corvettes = Moneypits

 

Does no one do a search?
The LT1 cutting out at operating temp seems to be a recurring theme.
Doesn't matter if its a 92 or a 96.

You need to dig in there check all the wiring harnesses.
If you search for ICM testing or just ICM it should bring up lots to read.

I had the same issue and the 92 seemed to be eating ICM's like they were candy.
Once I fixed the wiring harness between the opti and ECM I've not had a problem.
Yours could be the same or a different connector. Opti should be the last thing to replace.

 

Actually, if you look at my post, I mention that this began on Monday. Why do you think I waited to post the question on Wednesday? I'll tell you why. Because I spent 2-3 hours on Monday and another 4-5 hours on Tuesday SEARCHING forums for testing procedures, possible culprits, etc. I finally posted because I had run out of tests to do and options. Besides, when searching, you have to search one topic ten different ways because what I call a miss, you may call a spit, or backfire, or pop. So I've done my searching, and I felt it was time to spell out my problems in case someone could relate using my terminology. If you only knew how many little problems I've solved already using SEARCH on these forums, then you wouldn't assume that I haven't used SEARCH before. Now, I'm off my soap box. Thanks for the suggestions though, I'll search those terms and see what pops up and post back on my results, I hate it when people don't post the results or the fix. Thanks again,

 

I quite agree, to many posts do not have a ending or a thank you. After so many people sometimes try to reason out the answer, the end result is not given. However some do and are appreciative which are the ones you remember.

As for you problem........
It looks like you're about to do the fuel pressure checks as TONY_A has suggested which is a good place to start. If you drive it with the gauge connected and it falls below 35, that would indicate a definite problem. Should be 35 to 43 under almost all driving and starting conditions. If that’s the case (<35), I would change the filter to start.

I would also give a look at the FPR. Pull off the vacuum hose and see if any fuel comes out. If it is leaking back the FPR could be bad.

Another possibility or suggestion would be a heat sensitive (intermittent) ICM. Just may be acting up at certain times.

I would not quite run to the opti till you eliminated any fuel or ICM /(coil) problem.

 

I developed this very problem today. I put in a new optispark about 100 miles ago (Chandler Motorsports unit). I'm having similar symptoms when my Delphi opti let go a few weeks ago (screws fell out). Both started with a single "hick-up". I believe that was when the first screw fell out. When the second screw broke, it was all over- popping, barely running etc. This time however, it will idle fine, but mid throttle has terrible backfiring. It is sitting in a parking lot right now, probably will be on a flatbed tonight. After 10 years, I think it is about time to say bye-bye.

 

It's my O2 Sensors, Let me give you the whole story.

Last Saturday I noticed I had a blown fuse for the O2 sensors, so I put in a new one and saw smoke coming from the passenger side of the engine compartment. It was the Heated O2 sensor wire, and it completely melted in two before I could pull the fuse. I did pull it, but the next day I put the fuse back in, and it all seemed ok, so I left it in. Sunday was the first day I noticed it, but didn't give it much throught, but Monday it was worse. So Friday morning, I was on my way to work in the car, and I thought about that fuse so I pulled it. Immediately the car started running better, but not perfect. I stopped at an ATM and killed the car while I conducted my business and talked to my wife on the phone. I started the car again after about 2 minutes, and it ran perfect. I drove around a bit, and I wanted to be sure, so I put the fuse back in, and sure enough, before I could get up to 45 mph, it started acting up again. I stopped, removed the fuse, restarted the car after a couple minutes, and for the last 2 days, and 300 miles, I haven't had a single issue. Who'd have thought that the o2 sensor on the passenger side could cause this kind of problem? I'll replace the O2 sensors in a week or so and give more feedback if the problem comes back. Thanks for the help guys.

 

Mine was the Optispark.
want to see a bad Optispark check out my thread.
http://forums.corvetteforum.com/c4-t...-question.html

BTW..128K on it

 

Congrats! Troubleshooting these things can drive a man crazy...

 

Sure can !!! when these LT1 misfire it can be anything.
Better name for this motor would have been MF1 (Miss Fire 1)

 

How can you just say bye bye. There's so much talk about your vettes issue in these forums you should be able to figure out what to do.

 

where is the fuse for the o2 at?

 

It's in the fuse box in the passenger compartment, fuse #20. It feeds all 3, O2 sensors heating elements.

 

92 vette has a miss. I have put a new optispark plugs and wire and the problem remains. What should be my next move? The car just started acting up one day on the way home and has not got worse. Should I get a new coil?

 

See if you can isolate a bad cylinder. Pull plug wires one at a time or pull injector plug off. Look for a no change in idle speed.

Did you check for sparking in a dark area. Rev the engine.
Check connectors and pins for corrosion. ICM, coil, opti, etc.

 

I did that and found that if i pulled the wire off the number 1 injector it has no effect on the motor. I did have to replace that injector last year. However, I used a cheaper one and I think that it might be the problem, so today I ordered a new OEM injector from Ecklers; It was not cheap, but I believe that it might fix the problem. If this is not the issue what do you think will be the problem. I figure if the injector does not work I will run a comp test on the cylinder.