Ok, first off let me say that I swapped in a KG3 alt about 1-2 weeks before this started. I feel like it's related but I could be wrong. Aside from the alt swap, nothing else was done that could cause this.

When starting the car, it will die right away unless I give it throttle and slowly let off. The idle will fall to 200-300 then slowly rise to normal. When I put it in gear it will drop again before rising to normal. When coasting to a stop it does this too, but if I am going slow, say below 20, and let off the gas it will sometimes just die. After driving on the highway for a few miles it seems to clear up mostly. Its still not perfect, but it won't drop low enough to die when coming to a stop. With the AC on it actually seems to help due to the pcm raising the idle for it. I've drivin at least 100 miles like this and still haven't gotten a check engine light. When starting the car the fuel pump primes and seems normal.

Now I think it could be related to the alt, but the alt itself isn't the issue. It generates plenty of amps even at idle, around 13-13.5. I'm wondering if my stock belt could be to small and putting to much load on the engine at idle. It went on ok when I swapped the alt but it was noticeably tighter. It doesn't squeal or anything so I don't know if this could be it though.

So far I've cleaned the MAF, TB and checked for any blockages in the intake, and changed the IAC and spark plugs/wires. Didn't help. I've checked for vacuum leaks and haven't found any. Not that I can see from under the hood at least, and I don't hear any either.

There is no surging while driving. While under throttle or coasting at speed it runs fine, so I don't think its the TPS, or could it? Everything I've found about bad TPS seems to be related to surging or high idle.

I don't think its the VSS, that doesn't seem to be related to low idle issues, and my speedo doesn't act odd at all.

I have a buddy with HP Tuners and I intend to borrow his scanner when I get the chance. For now though I'm at a loss. What does this seem like to you? Any help would be very much appreciated.

 

Just a basic question, have you checked all the grounds? Our vettes give us crazy issues if/when a ground gets loose.

 

You may remember a few weeks back when I posted about the car not starting, just after the alt swap. That was actually a ground issue. It was the grounding strap that went to the block and was held in place by the small alt bracket in the rear of the alt. I guess from my angle, on my back under the car I never saw it when I took that bracket off. It's bolted to the block now and that was the first thing I checked. It's still snug in place. I haven't hunted all the grounds down to check just yet, I'll look into it. It's just odd that after I got all the issues sorted out with the alt, it ran fine for maybe two weeks, nothing was touched, and now this. It's just random.

 

The randomness of it is what made me thing grounding issue. Next thing on the list I'd look at is the tune, and making sure all the tables are still setup. I had a similar problem with my C5Z when I first bought, when the A/C was on it would run like crap, at times it would die, slowing to a stop for a light would cause it to die and symptoms were similar to yours, except yours is just the opposite, when the A/C is on its running fine. Regardless, turned out on mine the A/C tables hadn't been setup during the recent tune I'd had done, so the car was having worlds of issues.

 

I didn't think this a likely cause, but my radiator busted on me. It started seeping out of the seems under pressure, and over time will now leak even when cold. I drove it that way for those few weeks and just carried water with me. It never ran hot but would leak enough after a drive that I would have to add some the next morning. Is there anything under or near the bottom of the radiator that may have been damage by water constantly dripping on it? I've ordered a new radiator already so I'll be pulling this bad one out this week.

 

I'd double check all your vacuum lines. It's either a vacuum leak causing your IAC to go wild or....... is your A/C not working either?

Also, check your intake tube to make sure no air is entering it after the MAF. That'll cause some weird stuff sometimes.

 

This morning I went out and checked all vacuum hoses and electrical connections that are accessible from the top, everything was fine. I was in my work cloths so I couldn't crawl under it just then. I unhooked the battery for a few mins while I was checking stuff, then hooked it back up. Damn thing started right up without a single issue. Idled fine for a few mins then I shut it off and went into the house thinking it might be all good now. Get in this evening and its now like before. Died right away, had to restart and give it throttle to keep it running. No sounds, no lights, just as before.

So I ask you. What could be the issue that unhooking the battery(clearing the pcm of any recent codes) will fix, but will then continue on the 2nd start and on, that has NOT set off the check engine light?

 

Just because there is no light doesn't mean there is no code.

 

Hooked up HP Tuners and scanned it this morning. No DCT's. So whatever it is, it's not getting any codes for it.

 

Does your AC work? I had a weird issue that would cause a stumble/stall because my AC wouldn't turn on.

 

The AC works fine. If anything it may help a little because of the slight rise in idle for having it on.

 

Idle air control valve maybe?

 

Played around with the scanner program, as I'm new to HP Tuners. I finally was able to pull a code, though it was showing as an old code. The car started up fine and had normal idle too.

P0135: O2 heater circuit bank 1 sensor 1.

So I guess my O2 is bad. Would that cause the weird idle problem? When I had the table/chart up with the car running, I was getting NO reading from my WB/AFR. Is that normal?

 

That can cause issues. And your O2 should have a reading. They are not used in open loop, but you should still get a reading from them.