Hi All,

Searched around, but couldn't find anything similar to my specific issue - have been to shops but, can't seem to replicate the issue.

Truck has about 244K on the ODO,manual transmission, runs & drives great. But, when hot, the idle drops < 500 rpm, and I feel like it misses, stumbles. Have not been able to solve the problem - any help would bee greatly appreciated.

To Recap:
1. Starts & runs great when cold. Warm up idle is fine & even idle increase with AC is fine.
2. When hot, idle seems to drop slowly to <500 RPM
3. Sometimes, I can sense a miss

Thanks,
Chris

 

Hi there, I believe its because of the idle speed controller inside the air intake valve part. The idle speed controller,gets cooled by the engine coolant, so when its cool, its all open, when the coolant becomes hot, the idle speed controller almost closes up and reduces the engine rpm. You might need to open the idle speed controller, and re-adjust the spring tension of it by turning the cap that is holding it, to tighten or loosen the spring behind it.Here is a link of a YouTube video about it:

 

Could be a few things, but one really easy thing to check is for even the slightest crack in any of your air hoses going into the throttle body or other vacuum hoses around the plenum (there are plenty). I had your exact symptoms a couple of years ago and there was a crack in the big intake hose that connects to the TB on the bottom of it right where the clamp hit it. So it was not visible and I couldn't even hear it hiss, but when I removed the hose from the vehicle to inspect it was obvious. Ordered a cheapo from ebay that fit perfectly and it idled like a dream at all temps. Worth a thorough look before you start chasing other things. If you don't see anything obvious you could spray carb cleaner or propane/butane around all the hoses and listen for a slight idle up. That will help you find those stubborn vacuum leaks. Don't neglect the TB and plenum gaskets when checking for leaks. If you have removed either and didn't replace the gasket, or if you have never done either, they could have enough of a leak to cause erratic performance.

 

Thanks, I'll look into these...but it's got me wondering - these are for high idle, my idle is low & rough, almost like it wants to stall. I can sense a shutter or miss. seems like these are opposite from a vacuum leak or stuck idle control valve. But it's interesting that it only happens on hot...

 

I hear ya, and see where the confusion lies, but any air unaccounted for by the VAFM can cause very erratic behavior. In my case a few years ago, a crack on the underside of the big tube going into the throttle body caused these exact symptoms you describe. I could not see or feel the crack, only noticed it when I pulled the hose off. It was a crack in one of those ridges. My theory is that I had adjusted the idle over the years to account for that extra air, but as it warmed and flexed the hole sealed off enough to drop the idle down. Once I replaced the hose it was rock steady wherever I adjusted it.
Of course, I have a new engine now and am starting to fight with a similar warm start issue and I've checked all the vacuum lines and air lines and can't find a problem with any of them, so as I said, it can be a number of things on these engines!
Something else you can check on your rig is your fuel pressure. Oreilly and Autozone have a loaner test kit with a M12x1.25 banjo bolt that will fit on the rail just to the right of the fuel pressure regulator, so easy to access. Run through the steps in the FSM for checking the fuel pressure and you can eliminate fuel pump or fuel pressure regulator and you may identify other issues like leaky injectors. It is a pretty helpful test to check some things off your list.

 

Hi All,

I finally fixed this issue, so I'm posting just in case this helps others.

It turns out, I probably had higher idle because of vacuum leaks with cracked lines and hoses and such...once I fixed those, the idle became low on hot. It was workable for a while because I very rarely sat in traffic. But it turns out the idle circuit in my throttle body was clogged. I had to remove the throttle body and really clean out the idle circuit which is separate from the hot idle circuit in my throttle body at least. Once I cleaned that, the idle screw functioned properly to set the idle at hot. Running fine right now. I hope this helps someone who has this issue, because I couldn't find much on a low idle issue when I was searching.

Thanks to all who offered help

 

Got any pics of the specific areas you had to clean? What tools and chemicals did you use to clean?