Hello folks. Brand new to this forum, but I've been on corvetteforum.com for a long while and, frankly, I'm not getting much help over there for my weird, intermittent but very nasty problem.

I'll make a long story as short as possible: I went over a speedbump a month or so past, and halfway across the motor died. Tried to restart, but all I had was a cranking motor, either no spark or no fuel. I let her sit for 10 minutes and she fired up finally, and everything was good. Went into a car wash later that day, when the undercarriage cleaner hit the car, motor died again. Same deal, 10 minutes, easy restart. In both cases, when the motor died, the coolant temp pegged at 0, but I've checked the coolant temp sensor and associated wiring and haven't found any problems.

Ever since then, the car will occasionally go a little crazy; coolant temp will bounce from LOW to actual temp, or just bounce like crazy in general; oil temp would do similar things. On a long drive, the car suddenly started accelerating on her own; I put the clutch in and the motor revved to 2500 RPM and stayed there for about 8 minutes before she finally came back to normal idle. Thankfully I was on the freeway and was able to shift around this new high idle to keep up with traffic without speeding too much.

Now, once the car is above about 180 degrees, all these things will happen sort of randomly; the coolant gauge will go haywire, oil temp generally will read LOW, throttle will do its own thing, and motor will die. After the motor dies, it may or may not immediately restart. The problem is getting worse in that, on mild acceleration, the car will buck violently 2-3 times, continue normally, then buck again. Scary. The bucking feels like someone is shutting off the motor for an instant and then restarting.

So, I recently got the car in the air and I found this:



As the pic explains, the wire going to the oil temp sensor and the driver's side engine ground were all melted together. I thought, hey, I found the problem! I separated the melted wires, used liquid electrical tape to seal them back up, and figured things would be fine.

They aren't. The car still does everything it did before the fix. I checked the grounds behind the headlights, the connectors are clean as a whistle and the female pins aren't bent out of shape; the driver's side motor ground is fine also.

Here's the kicker: checking the codes from the DIC, I get no codes. Nothing. Nada. Every time I check the codes and the first one comes up "PCM: No Codes" I want to yell at the car.

Oh, I also tested the ignition switch, it seems to be working great, and I pulled apart the door accordions and disconnected all the door hardware. That caused a couple codes, but only the ones I'd expect, and didn't solve the problem.

Here's a short video of what's going on. In this case I'm actually using the throttle, it isn't the car doing it itself; I was seeing if I could get anything to change by modulating the throttle some. Even still, the coolant temp is doing its weird thing. Watch it around :50 and you'll see it peg low, and if you listen close you'll hear a bump at :54 that is the motor stuttering.

http://www.youtube.com/watch?v=BdbeX_q8DMo

Please, folks. I need some help here. Someone on CF said it could be a PCM failure caused by the ground short on the oil temp sensor. If that's so, how can I test for it? I don't want to just throw parts at the problem.

If only the car would give me a code....

Thanks for the help guys.

 

Welcome to LS1Tech.

PCM's are pretty sensitive units, any kind of shorting can screw it up. If you did have some wires touching, you could definitely have some PCM issues.

Maybe you can swap out the PCM with a used unit from somewhere and test it. Make sure there aren't any more wires that are melted together along the harness though, if you can.

Any idea why those wires melted in the first place?

 

Hello , how's it going besides You're vehicle kind of acting out on it's own . First off a scan tool that can record or even read live data might be helpful . I used to own a 80' Z-28 that when the car was about 8 years old would die out driving down the road ,,,,,,might start right up ,,,,,,,might take 5 min . I searched and talked to mechanics and finally I dropped the gas tank and cleaned the fuel pick up sock and it never did it again .Now some of what You said sounded similar and I could see the computer over-compensating for a lack of proper volume and/or pressure . I would run a fuel volume and a pressure test . As far as the rest is concerned it could need some updates flashed in to the computer possibly . Hope this helps and good luck .

 

You said you just pulled the wires apart and taped them up? You need to go back over them and cut the section of bad wire out and replace it, if it melted it could have corroded. Although I dont think its your fuel being that your temp sensor is messing up along with your oil sensor check it anyway. Like stated earlier go over those wires the most you can to make sure their not shorted anywhere else. May want to replace the sensors also being they could have been shorted out and messed up.

 

No idea how those wired melted, but I did follow them up through the harness as far as I could and didn't find any problems. That area is a little bit close to my headers, but not close enough that I'd expect any melting. The wires were definitely touching, though, unfortunately.

I didn't know swapping out a used PCM would work; I thought they had to be programmed with the VIN and such in order to operate? If I were to do this, I would need a PCM with a Corvette tune already in it, not one for a 4.3 or something, correct?

I really feel like this is more electrical than fuel; when the car is bucking, its as if it has great fuel pressure and is pulling, then suddenly loses spark, then gains it again. However, that being said, I don't want to throw out ANYONE's troubleshooting advice, so I will see about finding a gauge and some fittings to check my fuel pressure at the rail.

I took a good look at the wires when I pulled them apart; they had melted together into a great little sealed block, and the wire looked brand new where it had melted through the insulation. I used liquid electrical tape, which is a product that is a kind of thick rubbery goo that dries into the consistency of wire insulation, much better for a repair of insulation than actual tape.

I will think long and hard about replacing the sensors..both coolant and oil temp sensors work great, when they work, and I don't want to spend another ~$60 if I don't have to...but it might come to that.

 

Alright, so I wanted to buy a used PCM to try it, but dealers and the local shops are telling me it'll cost upwards of $115 to program it with my VIN. They say they have to do a total PCM flash. Is this true? I thought so long as I had a Corvette PCM, it would just be a VIN change.

I guess I could get a reman at O'Reillys, they program your VIN and ship it to you I believe. (EDIT: Not true, it comes with no OS and no VIN - ugh).

What's my best option here?