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:

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

[slacker_53]

I'll bet my money on the fuel pressure regulator, easy check, just pull the vacuum hose off and if you get raw fuel, it's bad.

Fuel pressure regulator is located midway, above the left (driver's side) valve cover. Vacuum line attaches from the top, regulator is held in by a clip, you can change it yourself with about $60 and literally 2 minutes time. Watch for the little 'O' ring, make sure the old one comes out before the new one goes in.

Bet this fixes your problem, good luck!

-slacker

[tnoftsger]

i've pulled the vacuum hose of the regulator, no raw fuel...Turned the key on, while the vacuum hose was off, no raw fuel....Pulled the vacuum hose off while running, pressure shoots up but, no raw fuel.

I thinking temp. sending unit. But i'm having trouble getting my Volt/Ohm meter leads in it while it's on the truck. But i might replace both of them just for good measure. I just hate throwing parts at it until i get it. It's pretty expensive way to do it.

Thanks

[Ghost916]

I have some of the same issues, could a bad temp sendinf unit cause a rough idle?, my rough idle is only at a stop, my Idle sits at just 500RPM is that normal, I feel its a lillte low.

[Captain Ron]

Originally Posted by Ghost916

I have some of the same issues, could a bad temp sendinf unit cause a rough idle?, my rough idle is only at a stop, my Idle sits at just 500RPM is that normal, I feel its a lillte low.

There seems to be a common issue with those motors having a bad gasket and manifold leaks. It will be noticed during cooler weather. Sometimes you will also loose coolant when it is being inhaled into the motor through a manifold crack or bad gasket. Once warm the crack closes and the symptoms are less. Chevy knows about this but does not admit to the design flaw, you need to have it fixed with your $$.