Cold Start Rough Idle

[legalscot]

Brand new to this forum. A friend of mine suggested I ask about a problem I've been having on my 00 GMC 1500, 4.8 v-8.

 

Here's the problem:

When I start the engine after it has set for a while it has a rough idle. It's like something is not telling the engine that it needs to advance the idle because it is cold. After it heats up there is no problem and it idles just fine.

 

It's never stalled out, and it will bump the idle rpm up when it gets to stumbling for a little while on a cold start, but then it drops back down.

 

I've taken the Idle Air Control Valve out and cleaned it. I also checked it per the Haynes manual. It had 4 terminals and I had 28 ohms resistance across the first 2 terminals and 28 across the second two. The Haynes manual suggest that it should be 40-80 ohms, so it's a little low. Could this be it??

 

Thanks for any help,

Bart

[RyanZ71]

Cleaning the throttle body sure helped with my rough idle problems, that and getting rid of the iridium plugs and putting Rapid Fires in.

[snoman]

Cleaning the throttle body sure helped with my rough idle problems, that and getting rid of the iridium plugs and putting Rapid Fires in.

 

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He might check the temp sensor that ECM uses to monitor engine temp because if it is out of spec, the ECM will not know it is cold.

[legalscot]

Cleaning the throttle body sure helped with my rough idle problems, that and getting rid of the iridium plugs and putting Rapid Fires in.

 

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I cleaned the IAC valve and just the front of the throttle body; I didn't take it off. How hard is it to get the throttle body off? Looks like the accelarator and cruise control cable is the hardest to figure out how to take off. I did clean some carbon off around the butterfly, not a huge amount, but some.

 

I also put Iridium plugs in when I bought it a few months ago but it had the cold engine idle problems before and after. NAPA said to not change the gap on the Iridiums even though they were set a 040 and the engine calls for a gap of 060. They said it had something to do with them being iridium and you weren't suppose to change their gap.

 

Thanks, I'll try both the throttle body and the engine sensor.

[legalscot]

Cleaning the throttle body sure helped with my rough idle problems, that and getting rid of the iridium plugs and putting Rapid Fires in.

 

<{POST_SNAPBACK}>

 

 

 

 

He might check the temp sensor that ECM uses to monitor engine temp because if it is out of spec, the ECM will not know it is cold.

 

<{POST_SNAPBACK}>

 

 

 

 

Is that sensor called tne "coolant temperature sensor"? The only sensor I can find in Haynes that seems like it would send the ECM the engine temp would be that sensor, which is mounted to the block.

 

Thanks

[snoman]

Cleaning the throttle body sure helped with my rough idle problems, that and getting rid of the iridium plugs and putting Rapid Fires in.

 

<{POST_SNAPBACK}>

 

 

 

 

He might check the temp sensor that ECM uses to monitor engine temp because if it is out of spec, the ECM will not know it is cold.

 

<{POST_SNAPBACK}>

 

 

 

 

Is that sensor called tne "coolant temperature sensor"? The only sensor I can find in Haynes that seems like it would send the ECM the engine temp would be that sensor, which is mounted to the block.

 

Thanks

 

<{POST_SNAPBACK}>

 

 

 

 

 

There are 2 on engine one for gauge and one for ECM. You want one near water outlet to radiator. One on head is for gauge.

[RyanZ71]

I sprayed carb cleaner into the throttle body (engine off of course) wiped it up, sprayed it in again, wiped it up, propped the thing open for an hour to let everything evaporate, put everything back together, started it up, and it ran good.