poor idle when cold

Hey all,

i have a 2000 Suburban with the 5.3L v-8. i know that this is the Silverado forum, but the problem isn't model specific i don't think and i will post over in the Suburban forum as well. 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 have scanned the computer and there are no 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

i have the same problem with my 99 silverado z-71 4.8l and its driving me crazy. it does'nt give me a code so i'm stuck. it cranks over longer, alot longer. then it barely starts and runs ruff for about 1 to 2mins. then it smoothes out and runs great. if anyone knows wht would cause this please tell me.
thanks
stumped in indiana

gray99silz71, I would put money on your fuel pressure regulator. Check it out, I would guarantee it's bad.

-slacker

i've pulled off the vacuum hose and there is no raw fuel in it. I even pulled the hose off while running and the pressure shoots up, like it should, but no fuel coming out. Is there a better way to test it. I'm going to get my vacuum/pressure tester tonight so i will see if it will hold pressure and what the pressure is while running.

Thanks

check the coolant temperature sensor

I would have to agree with Slacker 53 , these are classic symtoms of a bad FPR. Your testing method is a new one for me, but does not mean it is not effective. I usally would hook up a 100 psi test guage and observe bleed down, or pinch the fuel return line with some needle nose vise grips (just a little) then keep tightening in small incriments until problem goes away.

what is the best procedure to check the fuel pressure regulator. All of the simple test don't indicate that it is bad. I just don't want to start replacing parts until i find it.

Thanks

Alright, put a vacuum gauge on the pressure regulator and pulled 20 psi. It held the 20 psi for 2 hours with no loss. I don't think it is the Fuel pressure regulator.

I guess, i'll have to take off the coolant sender to check it. I can't get my meter's probes in there with in on the engine. Oh well, probably time to change the coolant anyway.

Any other ideas? Anything else i could check. It still doesn't throw any codes, so the computer isn't helping much. But in the mornings it is like starting a carburated car with no choke.

Thanks

I have a 99 5.3 that does the same thing. I've changed lots of stuff on it, but its no use. I havent checked my fuel pressure but the whining little bastard isnt 30k old and whines just the same as the day it was put on. I changed my tps sensor, iac valve/sensor, manifold pressure sensor, nothing works. My problem is in cold weather the engine cranks, followed by high regular idle, then a couple big choking exhaust notes followed by a big rev to about 1500 rpm back to the choking 4 cylinder lope and almost stall. 3 minutes down the road and the rest of the day its fine. if you find anything out please let me know ive been suffering with this for 2 winters now and it blows. I live in texas so summertime starts arent that noticeable.

gduck,
have you replaced the temperature sender? Someone mentioned it and i can't get in there to test it. I might just replace it, the coolant needs changed anyway.

Solved it!!!!! Intake gasket leaking. Replaced them and it runs perfectly on cold starts. I assume that the leak was small and nearly sealed when I got some heat in the motor and things expanded, even tho it has the composite intake.

Thanks for everyone's help.

Where is (or are) the intake gasket(s)? And how did you detect that it (they ) was or were leaking?

Where is (or are) the intake gasket(s)? And how did you detect that it (they ) was or were leaking?

I used a vacuum gauge to determine that they were leaking.

If you remove the plastic cover, you'll see a black plastic composite intake plenum. It has 10 small bolts holding it to the heads (5 on either side). The gaskets go between the black plastic plenum and the shiny aluminum heads.