Difficulty with first startup of the day

[pinstripebob]

Hi all!

I'm helping my dad with a kit car he has. It has a stock LM7 5.3 in it with a Magnuson MP112. It has a COS3 flashed, it runs great but it has one really weird annoyance.

The first time you start it up, it runs for one or two seconds then dies. You can do this a few times and it does the same thing, turn on, run for a couple seconds, die.

HOWEVER, if you turn the key on, then off, then turn the engine on it runs and idles great.

If you've driven it around, it will restart just fine. It's only that initial start of the day, or after it's been sitting for quite a few hours. And a double key on will make it turn on and idle just fine.

I have attached the tune and a datalog I got of this first start. I used the V8 software. One thing to note about the log, it shows the IAT is -40C (or -36 or -37, extremely cold). I did a quick scan with V7.5 and the IAT was +20C, close to the ECT. I think this is a V8 software glitch.

Any thoughts would be greatly appreciated. I have tried doubling the initial prime pulse thinking it needed more fuel with key on, but that didn't do anything.

I have attached the tune, log, and what I believe is a stock tune for this engine from the repository.

Thank you!

2001 Chevrolet Silverado Truck Automatic LM7 5.3 Litre (PCM 12208322).tun
2024 12 24 - 1936 Auburn Speedster bad startup log.pld
LS1B Auburn Speedster 12 24 2023 - 1.ctz

[Supercharged111]

Assuming there is no funny business with the IAT, I would start by verifying fuel pressure throughout this whole ordeal.

[pinstripebob]

Can do. We replaced the fuel pump and regulator two years ago. The old pump was super noisy, and the old regulator leaked fuel into the intake. We have a gauge on the rail as well, should be an easy check.

[robof16]

Looks like it dies abruptly, almost like key-OFF? First start lasted about 2 seconds and second start died in less than a second? Spark drops immediately to 6 degrees right after firing on the second start...
B3604 could be a player but I kind of doubt it. Don't rule out spark and idle startup air, anything that has a delay/decay.
Maybe try logs to compare the two scenarios and include IAC. See how spark behaves, narrow it down to air/fuel, etc.
If you have changed the throttle body, IAC area can be a major player. Check TPS reading zero (0.4), etc.

[joecar]

Also check for FPR leak (do you see fuel in the FPR's reference line...?)

[pinstripebob]

I checked the FPR with my dad last night, it is not leaking. It did leak a couple years ago and we replaced it, apparently that style FPR is known for leaking.

My dad is going to check fuel pressure, he needs to put his gauge back on the rail. He'll check it at a few different states and see what he gets.

I need to take a look at B3604 and run through the spark/idle startup tables again. Throttle body is stock, we'll need to double check the TPS is zero but I think it is.