2007 Sierra Classic.

As it has gotten cooler out, the first time I turn the key, the truck will start, run rough and then die. If I try to start it again, whether I wait or do it immediately, it fires up fine and doesn't act like anything is wrong. It's worse the colder it is, better when it is hotter out.

I have tried diagnosing the fuel pump/FPR by leaving it on for 3 to 10 seconds before attempting to start and it does not make a difference. Once it is started and running I have no issues starting it the rest of the day.

Fuel pressure was good while running, which leads me to believe it might be injectors leaking overnight..? I did run some BG 44k through it made not much of a difference either way.

Video below of the other day.

https://photos.app.goo.gl/WzVZCHyp8gRGyLfN8

Truck runs fine otherwise. It gets used daily pulling a trailer with lawn equipment and does not idle funny, stumble, or feel down on power. It's basically starting and stopping every 15-60 minutes before being parked for the night.

 

Hook up a fuel pressure guage when you park it overnight, and check on it the following morning before priming it. Bleed-down I believe is normal, or some bleed-down overnight. You may have an injector that leaks after cooling down, and the trucks stalls in the morning because it's flooded. You can also monitor fuel trims when this happens, if you have a scanner with live data.

If it's running really rich, you can try inducing a vacuum leak temporarily, by simply unplugging an intake hose and see what happens...

 

It won't make a difference if you do it that way. The fuel pump prime is timed, it only lasts like 2 seconds, I'm not sure exactly. To test it by trying to prime it before you start it cold, you have to turn the key on for a few seconds, like 3 seconds, turn it off, and repeat a few, maybe 3, times. Besides a leaking injector, a leaking regulator is also a possibility.

 

Great idea on vacuum leak. I tried the flood clear procedure, but it always starts fine 2nd attempt. I'll hook up the scanner and see what it says. I'm going to test cold psi today. First day I've had time, and not taking it anywhere trying to race the sun.

 

Good to know- I'll try cycling the key a few times as well. Wish it was a rail mount regulator....

 

Just tested.

Cold- KOEO- 35, 40, 42psi. Guessing it was a bit higher as it would bleed down right away.

Running cold - 57psi.

It's far enough out of spec I'm going to say pump/regulator?

 

Interesting development.

When I was having the issues, I had started running 91 clear fuel and added BG 44k to the first tank. I believe I had at least two, maybe three tanks in a row of 91 clear where I experience the issue.

The other day I filled up with E10 87, and now the problem has gone away. It has fired up immediately without allowing the pump to even prime. 65* one day, 45* the next.