Chevy Corvette Forum banner

1 - 8 of 8 Posts

·
Registered
Joined
·
7 Posts
Discussion Starter #1 (Edited)
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. But I did have an issue with no voltage on the outside pink w/black stripe wire going to the ICM for a few minutes, but when I wiggled the wires, voltage came back. But when the car was running, and I wiggled this wire, it simply dies outright, no stumbling or popping, just like as if someone turned the key off. It wouldn't start without voltage on that wire either. I know I gotta fix this short, but that doesn't seem like it's causing the misfire/popping, unless it's low voltage, just enough to keep running, but not enough to power the whole spark needs of the engine? I don't know. Anyone ever experience no voltage on those wires to the ICM?

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?

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. (Oh Yeah, I fixed the short on the Pink w/black stripe wire to the ICM, so we can rule that out) Anyways, after cranking, I now have .87 vdc at idle, and steady. 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. The only reason it will backfire is because of the throttle position, 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?
 

·
Registered
Joined
·
9,756 Posts
You need to pull the codes before you go any further as they will point you in the right direction. And you need to check the ignition timing with the EST wire disconnected. Then verify you have enough fuel pressure at idle and on the highway while your car is in motion. You can do that by attaching a fuel pressure gauge to your windshield with duct or strapping tape. The popping and backfiring indicates an ignition problem so make sure your spark plug wires are separated and are in good condition with about 25,000 ohms each. Also I suggest cleaning your computer plug-ins with contact cleaner and then re-connect them several times to ensure a good connection. It has been proven 95% of all electrical problems are the result of a bad connection so it's imperative your connections are good.
 

·
Registered
Joined
·
9,756 Posts
Sorry, should've mentioned there are no codes, I will test the fuel pressure this evening when I get home. Thanks.

Remember the basics: If you have adequate fuel pressure and a good hot spark at the right time your engine will run great. Backfiring indicates either your ignition timing is way too far advanced or you have two wires crossed.
 

·
Registered
Joined
·
7 Posts
Discussion Starter #7
It could be a faulty o2 sensor. Especially if it runs fine in open loop.
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? Good call. 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.
 
1 - 8 of 8 Posts
Top