poor idle when cold

[tnoftsger]

Hey all,

i have a 2000 Suburban with the 5.3L v-8. For the last month the idle quality is very poor for the first 60-120 second after the truck has sat overnight. It doesn't show any of the poor idle qualities the rest of the day, only after an extended period (8 hours or more).

I've checked the fuel pressure:

installed a new fuel filter
55 psi running
54 after shutting off
8 psi after 1 hour of being off.
the pressure imediately goes back to 55 when the key is turned "on"
60 psi if you unplug the the vacuum line to the fuel pressure regulator.

i've replaced the spark plugs, only 60k miles on the originals.

this morning it was cold out (45 degrees F), i started the engine, rough like usual, i raised the rpm to about 2500 and held it there. After 10-15 seconds the rpms fell on their own and it almost stalled. I let it idle....
the rpms would fall to 450 and almost stall, then rise to 1200-1500 and return to 450-500 and stumble. it did this for another minute or so and straightened out.

This is an everyday occurance, and is getting worse as the temperatures drop. I scanned the computer and there are not codes. It seems to be part of the choke cycle, but i'm guessing.

does 8 psi after an hour seem like a big drop? Fuel pressure regulator?

maybe the IAC Motor acting up?

any ideas?

Thanks in advance

Tom

[isdave]

I had the same problem and replaced the injectors because they leaking into the cylinder after you turn the engine off thats why your getting that drop in pressure

[tnoftsger]

I would agree, except that at that rate of fuel pressure drop, any leaking would stop after 2 hours. After 2 hours it starts, idles and runs like a top. I'm just not convinced, no offense. The problem is linked to being cold i think.

but i'm into diagnosing before buying. So is there anyway to verify this or isolate which one?

As odd as this sounds, there was good news today. The wife called and said that the "Check Engine" light came on this morning (coldest day this fall here). I'll scan for codes and maybe that will shine some light on the subject.

Any other ideas?

[tnoftsger]

Code registered today was a P0300, Multiple misfire i believe, so i don't think that is helping much to find a solution.

Anybody?

[glenncof]

I'm having same cold idle problem. Actually not really idle, but won't start without tap on acclerator (like it always has over last 100Kmiles).

I have checked the ECT (engine collant temp) sensor at high and low temps, OK.

I have also checked the IAT (incoming air temp.) sensor but the readings seemed flaky.

IAT measured 1.8K ohms at 65F (s/b 3.5K) on vehicle. I finally pulled it out then measured it in an ice bath, boiling water, and room temp. Boiling it got to 200ohms (ok), ice started low like 5Kohms, and room about 2.5K. I cycled it in the boil & ice baths and the room temp improved but ice only got up to 7.5K (spec is 9.4K).

I doubt this is the problem but ordered new part from Napa ~$16.

We'll see in a few days...