when cold - long crank to start & rough idle

[mudhunter]

Hello everyone!! I'm somewhat new to this site and it looks like I need some help. Here's what I have: 2002 Tahoe 4.8L with 170,000 miles. Over the past few weeks I've noticed when it gets really cold at night (teens or 20's), and I go to start in the A.M., it cranks a while before starting. When it does finally start, the idle is super rough and surging. It does this for a few minutes, then smooths right out and everything is fine. It ONLY does this first thing in the morning. Sometimes I get a flashing MIL, then goes out. Sometimes it comes on and stays on; usually code P0300. When it's above 30 or so degrees out, no problems. I'm the 2nd owner and everything is stock. I've cleaned MAF sensor, TB, IAC, new air filter, O2 sensors, cats, and plugs. I've Seafoamed it. The fuel pressure is where it's supposed to be and compression as well. Checked all the plug wires, all within specified resistance. I've sprayed around the intake for leaks, nothing. [/SIZE]
I've been reading through other's posts trying to find something that I may have missed, but have come up empty. Any thoughts or advice would be GREATLY appreciated.

 

[rockenthebowtie]

Have you checked for a leaking fuel pressure regulator ? Remove the vacuum line thats connected to it to see if theres any fuel coming out if so replace fuel pressure regulator

 

[bladenbullet]

you said the fuel pressure is where it is supposed to be....when?...is it maintaining fuel pressure overnight when static?...

the flashing mil is an indication of a catalyst damaging misfire or raw fuel running into the exhaust...obviously from the rough running....

have you checked the coolant temperature reading before starting the vehicle to make sure it is the same as ambient temperature?....a faulty temperature sensor or circuit could be creating false temperature readings and effecting cold running...

throttle body clean?...including iac passage?...

 

[mudhunter]

Have you checked for a leaking fuel pressure regulator ? Remove the vacuum line thats connected to it to see if theres any fuel coming out if so replace fuel pressure regulator

Yes, I have checked that. No problems there. Thank you.

 

[mudhunter]

you said the fuel pressure is where it is supposed to be....when?...is it maintaining fuel pressure overnight when static?...

the flashing mil is an indication of a catalyst damaging misfire or raw fuel running into the exhaust...obviously from the rough running....

have you checked the coolant temperature reading before starting the vehicle to make sure it is the same as ambient temperature?....a faulty temperature sensor or circuit could be creating false temperature readings and effecting cold running...

throttle body clean?...including iac passage?...

I have only checked fuel pressure while running, not overnight. I'll have to check that. Should there be any drop in pressure overnight?

I have not checked the coolant temp sensor. How do I go about testing that?

And yes, TB completely cleaned, inside and out.

Thanks for the help!

 

[mudhunter]

This morning 46 degrees; fired right up and maintained a smooth idle.

 

[remauto1187]

I have the same problem with my girlfriends 2003 5.3L. My 2003 5.3L is more stable but is not 100% when it gets much below 30 degrees outside. Hers I attribute to poor maintenance from the previous owners. Ill have to run through hers when spring gets here and then mine too.

Her Tahoe LT was "arrested" for transporting heroine! No Kidding! It was seized from the owners' boyfriend while he thought is was a good idea to have heroine in the vehicle in Ohio. Bought the Tahoe off a Sheriff Dept. that had it listed on Ebay for the whopping sum of $3100 and drove it home!

 

[bladenbullet]

fuel pressure should stay up overnight....it may bleed back some but any pressure in the line will mean there is a solid column of fuel that can come to full pressure the instant the pump engages at key on....

if the fuel bleeds back to no pressure and possibly bleeds back into the tank to an empty line full pressure may not happen at key on (limited fuel pump on time) and you will run lean on start up....this will have a larger effect in colder weather where rich running is necessary....

the pressure should be at spec running...when the engine is shut down the pressure should stay up....if it drops off it is bleeding out somewhere....most frequent is the fuel pump check valve....possibility of a bleeding injector but you would probably experience rough running after short shut downs (running into a store quick) and the like...

there are other possibilities but these are things that need to be checked....

 

[rockenthebowtie]

Kinda wondering once outside temp drops low enough to condense the fuel enough to cause low line pressure after all it will be warmer from the running of the vehicle once you park it over night temp drops fuel condenses

 

[bladenbullet]

as long as theres a column of fuel it will pressurize at key on....when it bleeds back into the tank the short pump on event isnt enough to get full pressure in the line and the engine will stumble....the cold exaggerates it because the demand for a rich mixture is so high...

 

[mudhunter]

ok, i do get some lean codes when this happens. so its kind of making sense. no problems with short trips. like i said, its just in the morning, when its cold overnight.

 

[rockenthebowtie]

You know even though you sprayed around the intake doesn't necessary mean there not leaking sometimes you actually need to look at fuel trim scan data, another thing is dis you spray when the vehicle was cold or warm, if when warm everything is expanded and sealed a little better might want to try it when its cold

 

[mudhunter]

You know even though you sprayed around the intake doesn't necessary mean there not leaking sometimes you actually need to look at fuel trim scan data, another thing is dis you spray when the vehicle was cold or warm, if when warm everything is expanded and sealed a little better might want to try it when its cold

did it cold.

 

[mudhunter]

Well, my neighbor stopped by with his Snap On scanner. Turns out it IS the IAS. It was reading -40F. And and after some searching online, this is a common reading when they go out. Unfortunately, it's built into the MAF. Now I gotta get one of those. Glad I work at a parts store; good discount!

 

[rockenthebowtie]

Cool, thanks for following up on your finding hopefully it can help someone in the future when looking up threads, most dont even have the common decency like you.. good luck

 

[bladenbullet]

sweet....

 

[2004ChrisTahoe]

Well, my neighbor stopped by with his Snap On scanner. Turns out it IS the IAS. It was reading -40F. And and after some searching online, this is a common reading when they go out. Unfortunately, it's built into the MAF. Now I gotta get one of those. Glad I work at a parts store; good discount!

What is an IAS ?