Crazy C5 Corvette is driving me and several pro tuners crazy. The tuner I use has tuned hundreds of cars over the last 25-30 years, and has seen some pretty weird things happen in that time. But he has always figured out the issues. Figures my car would be the "not this time" example. This guy has developed friendships with several other pro tuners, some in different states. I'd say we're talking about over 100 years and maybe 700+ cars of combined tuning experience. They are all stumped, and they've never before seen any car do what mine is doing. What is it doing? Well, it sounds simple enough, and this happens ONLY ON THE FIRST COLD START OF THE DAY. Over 67°-68°F, it starts like a showroom new car. Anything under 66°-67°, something somewhere in the electronics slams the throttle blade completely closed, which stalls the engine immediately. If you try to restart it by giving it a little pedal, it fires and streaks to 2,500-3,000 rpm, not cool on your first start of the day, before much oil has pressurized the bearings! Not good!!! Has anyone had this issue, or know anyone who has? This has some of the better tuners in the country baffled. I try to be logical, and ask myself how the computer can read ambient temps? My 2000 C5 has the later 85mm LS6 MAF, so the IAT is read by the MAF, NOT the separate IAT sensor in the airbridge that the 1997-2000 models have. So the MAF is one way the PCM knows ambient temps. Another is the coolant temp sensor. I will say that if it is, say 40° ambient, when I put a small (750w-1,500w) electric space heater on the floor for 15 minutes to 1/2 hour, about 2 feet away from the front bumper, it fires right up. After completely warming the engine, you could shut it off for 5 hours, 8 hours, whatever, and it'll fire right up again. First start of the day is the ONLY time it gives us Hell, then its OK for the rest of the day. Sure wish I had a good old fashioned idle adjustment screw on the TB!!!!! Please help, guys. Thank you-Grinder11.....

 

Anyone??? I know its an unusual problem when tuners from 4 different states have never heard of it! I will say I've learned something new today. A friend of mine here on Tech called me and told me that he has seen this happen only 1 time in all the years he's been doing LS stuff. Said it was a battery within the PCM. Until his call, I never knew there was a small battery inside the PCM!! Apparently a 2032, or similar sized little battery. He told me to turn the key on in the car (DO NOT START!) and leave it on for 2-3 minutes. Then turn it to the off position. Then immediately try to start the car. It fired for maybe a half second, as usual. Then the throttle shut and it stalled, as it usually does when ambient is under 66°-67°. So I learned the PCM has its own internal battery. I am leaning towards something corrupt in the MAF tables when ambient is under that 66°-67° mark. I put my little 10"×5"×5" electric space heater on the floor about 1 foot away from the front bumper. Let it run for 1 hour, turned the key, and car started as good as any new car, as it usually does after the pre-heat. Very frustrating.....

 

Without the tune file at minimum, and better yet a log of this issue happening, nobody can do any more than speculate.

 

He is going to see if he can get the files.
We can coach him through the upload process.

He is even older than me and lots of boomers just never needed computer skills.

 

Based on the totality of circumstances it doesn't sound like a tuning issue to me, unless maybe it's related to engine coolant temp (IAT should read at ambient after 5-8 hours of sitting, when you say that it will still fire up OK, but ECT may still be above that ~67°F threshold depending on where it's parked).

The throttle snapping shut sounds like a failure of the DBW throttle electronics, possibly triggered by the unit dropping to a certain temp. Could be wiring or internal fault. I can't think of anything in the tune that would tell the throttle to completely shut off idle airflow based on an ambient temp reading.

I wasn't aware of any button-sized battery inside the PCM. First I've heard of that. Are you sure this isn't being confused for a capacitor?

One of the things missing from your initial explanation of the issue is a history of when it started. Was something changed with the tune or setup that triggered the condition? Did it come out of nowhere? Has it gotten worse over time or just been exactly the same since it appeared?

My gut tells me that if every tuner is completely unable to find the source of this, and you're dealing with the best of the best tuners, then the problem is likely not in the tune itself. Has anything that's been done by any tuner had any impact on the issue at all? Even a little? If not, I think the problem is a hardware or wiring issue.

 

Yes, I heard about this battery from a very knowledgeable guy here. Yes, the issue came up after a cold start tune change. I hate to say that because many may see that as a smoking gun. I think it's a coincidence, because the change happened in early summer. This means it was a good 3 months, maybe 4, before temps were cold enough for it to show up. The same tuner did the original tune. He is VERY good. He also re-flashed the PCM with a tune made in 2019, 2 or 3 years BEFORE this tune change and cold start issue. It made no difference. Im also leaning towards it being not tune related. Something may have gone south between the time the change was made and the issue showing up 3-4 months later. The LS6 MAF is 20 years old, and I cleaned it well before the tune. I'm also thinking this is either a DBWTB or MAF related issue. Tuner said the coolant temp sensor looked like it was working correctly on his HPTuners program. He said there was no red flag change at that 66° ambient temps, neither over or under it.......

 

Why not take a blow dryer or heat gun and try and heat up individual sensors to see if you can trace down the problem to a specific one.

 

Along with a can of freeze spray. Commonly used for troubleshooting temperature based electronics issues.

BTW, no battery in a P01. I just took one apart. 4896 to be exact.

 

That's what I figured. If there were batteries in these PCMs to fail, at this age it would be a common condition with pretty well documented symptoms. So we can scratch that off the list.

I like the ideas mentioned above regarding using heat/cold on specific sensors and components to see how/if the condition changes. The more data that the OP provides, the more I feel that the issue is not related to the tune but rather a hard electronics/sensor or wiring related problem.

 

I “know” Grinder from the Corvette forum as I also have a C5.
He hasn’t mentioned what throttle body he has. If it’s the original, the TPS would be suspect. It can easily be replaced. If it’s an LS2/3 then not so easy.

The TAC and PCM on a C5 live in an unfriendly environment behind the front wheel. Certainly could be issues there.

 

I had a weird idle issue with my Ford van(2500 rpm jump sometimes when put into park) and no codes in the computer. Changed the TPS and that solved it. Easy enough to try.

 

Turns out he has an ls2 TB

 

So it was a simple hardware module mismatch?

 

Unknown. We haven’t seen the tune. The LS2 TB plays nicely with the C5 OS as I’m sure you know.

I was referring to the ability to simply change the throttle position sensor.

 

I don't mess with DBW enough to keep track of which works with what. I just know some don't play well together, if even 1 piece of the puzzle is wrong.

 

Ok guys. Mike finally got me the tune file. I looked it over and found the following in the idle section. See .jpg file.
This is compared to a stock file.
And, I compared it to my current C5 file which is also a A4 2000 Coupe.

In this area, the only difference between my C5 and a stock file is the ETC area scaler which was changed for the larger TB. it's a number that is commonly used.
And of course, BRAF which makes sense as it's a modified car.
All the rest of my idle adaptives are stock.
His have been messed with.

This could be it? Dunno. But these idle adaptives certainly control the throttle blade.


Ron

 

To update and piggyback what Ron said, I have a 90mm LS2 silver blade TB. New in late 2005. Remember it well, because the LS7 wasn't out yet, so it was my only choice. About 4 years ago I took the car to the tuner and asked him if he could lean out the cold start, because I thought it was overly rich on cold start. BIG mistake on my part. I didn't notice a change immediately, because it was early summer (usually over 67°), and it didn't see below 67° until 2 or 3 months went by. I was shocked it didn't start when it was only around 62° outside! So I blamed myself for telling him to change the tune. But I'm starting to think it is something else, like the TB or MAF. Tuner even thought the TAC might be at fault......

 

One other interesting fact. Earlier this summer I had to repair/solder the coolant temp sensor wire. After the repair, I tried starting the car. It was under 67° and had the usual BS trying to cold start it. But once warmed up, it ran great. BUT, after driving it around for 15-20 miles, I shut it down at the local auto parts store. Came out, and instead of starting right up, which it always did when warm, it acted exactly like it did when cold!!! WTF!!! Now I had a car that started horrible whether hot or cold. Back to the tuner, who by now had thrown his hands in the air! He said, MAN, the ECM doesn't just change its own tune, and doesn't just forget it's tune, either. This is nuts!!! So he at least got it tuned again so it starts great when warm, but it was same-o when cold. HELP!!! None of us has ever seen this kind of stuff before......

 

Have you tried tracking it down to specific sensor with heat/cold as mentioned earlier?

 

I am going to try that next. On a decent cool morning when Im not busy, I'm going to isolate the coolant temp sensor. I'll heat just that sensor with either a heat gun or fireplace match, and have the wife get in the drivers seat, and turn the key. Will report results as soon as I get them....