Poor Idle, Cold Engine

[TallPaul]

My 2000 Chevy 454 v8 motorhome has an idle problem that shows up in in cold weather. On cold start the idle will not stablize but fluctuates between around 500-600 and around 1000. At the low end it will start to run rough, then speed up. Once speeded up it drops back until it starts running rough and repeats the cycle. This cycle probably takes only about 3 to 10 seconds. Once the engine gets to a certain level of warm, it idles fine. Idle on this motorhome is 600 rpm. It only happens in colder weather. I have experienced it in the 30s (F) and below, but not in warmer temps like 50s and up.

The motorhome has 15,500 miles and is a 2000, so it had sat a lot over the years, but otherwise runs great.

Is there any value to disconnecting the battery neg cable for a few minutes then reconnecting to reset the computer?

Seems like maybe that thing called an idle air control could be malfunctioning? If cheap and accessible I could throw a part at it and see if it works.

 

[CBDFrontier06]

I had an 94 Eclipse and an 86 Civic do this and was told that my first step should be a throttle-body cleaning (which they said clears it up 99% of the time, but if it continues, a new idle air control valve was needed. That's where I'd start.

 

[mrsilv04]

Is that engine "old school" enough that it has one of the hoses extending up from one of the manifolds to the air intake, so that it can draw warmer air off of the manifold when the engine is trying to warm up?

If so, is the hose there, and is the damper in the air intake snorkel functioning correctly?

 

[Tosh]

Originally Posted By: TallPaul
Seems like maybe that thing called an idle air control could be malfunctioning? If cheap and accessible I could throw a part at it and see if it works.

If it has one, it might just need cleaning. The glop in it is stickier when cold, so the problem's worse then.

 

[TallPaul]

Hmmmm, wonder if I can hose the intake with intake cleaner, or will it mess up the air flow sensor? Probably really needs mechanical cleaning. But for 15,500 miles can it really be that dirty? Stuck damper seems to make more sense.

 

[Curious Kid]

That could be it, but doesn't the idle air solenoid draw air that is also receiving PCV gases...could be blow-by residues from that labored big boy.

Do a web search for idle air valve cleaning - I can't recall any procedure for mine (been luck in that area so far).

Good luck.

 

[mechtech2]

How about a left handed thought...
Maybe the gas that is in the motorhome is summer gas, and now it's winter. This will cause vaporization problems, for sure. Reid vapor pressure is adjusted for different seasons, along with a few other things in gasoline.
Whaddyuh think?

 

[TallPaul]

Interesting mechtech2. We filled up in September, then it did this idle problem on Thanksgiving (overnight to 15F!), after that trip, I topped up, about half a tank, so now I have half summer gas (early Sept) and half winter gas (later Nov). Unfortunately, won't get that out for a long while, or is there an additive I can throw in to correct it? Still, didn't seem to improve with the 50/50 summer/winter mix. Alternately I can just drive it. Am going to Florida late January (wife signed up to participate in the Tigers Fantasy Camp) and will see how it does on way down and back. Will get last fill up in colder climate. Chances are it won't be a no run situation anyway. I can always keep my foot on the pedal until it warms up.

BTW, I didn't think EFI needed or had those heat riser tubes.

 

[Curious Kid]

That is an interesting possibility mechtech2 - such awareness.

 

[Merkava_4]

Originally Posted By: TallPaul
BTW, I didn't think EFI needed or had those heat riser tubes.


I had one on my old school 1978 7.0L Cadillac. It was sporting a Rochester 4MV carburetor.

 

[alanu]

coolant temp sender can possibly be out of range (correct specifications but wrong time). 02 sensor may be within "working" parameters but not at the appropriate time. Gummed up hotwire in a MAF sensor. Sooty idle air control valve.

You assume your in open loop when the engine is cold but I'd get the computer scanned. You definitely need fuel enrichment at cold temps.

 

[TallPaul]

Well, my first thought was to take it in and that probably is the best bet for me. Too many variables. Let the professionals handle it.