I was working on a rough idle several days back. Replacing old vac hose I thought stopped it, but I think the weather just turned warm at the same time. Now the temp dived again and the rough idle is back.

It acts like a vac leak. How could cold cause this?

 

When the temperature gets colder parts contract. After the engine warms up does the rough idle go away? A loose clamp or hose may loosen further when cold.

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the problem does get better as the engine heats up

 

I'd say clean your IAC on the back of the throttle body elbow- looks like a silver electric motor can with a plug on it. Clean that, throttle body and MAF sensor to see if that helps. Otherwise, those things are good to do anyways.

 

Have replaced IAC, cleaned MAF, replaced small vac hoses, retightned clamps on TB pipe, replaced PCV valve.

In addition I have also called it ugly names and kicked the tires a few times. The latter did not help, but I felt much better afterward.

It's the cold issue that confuses me. I wonder if there could be a crack in somedthing that opens in the cold and closes when warmer.

 

I've never experienced it (yet) but maybe a plugged EGR? Pretty sure a code would come up pretty quick for this though. How are the spark plugs? Fuel filter? injectors? I'm at a loss, just fishing!

 

A vacuum leak will cause your symptoms. Cold engine, start it and using carb cleaner spray all the vacuum lines and intake manifold joints. When the idle changes you have found the leak. According to the year of your truck you have a composite intake manifold. The odds are that it is leaking where it connects to the heads.

Regards

Jean Marc Chartier

 

can a intake manifold leak only show up in very cold weather, then go away when it warms up?

 

the temp dropped from 40's to 25 and the miss at idle came right back. I can't find this stupid leak

 

after a summer of running perfect the temps have dropped again and the miss is back. feels like my options are:

A) Someone here gives me a awesome idea of where to look
B) I move to the Gulf Coast
C) I shoot it

hopeing for options A, not sure which of the others I would prefer

 

Weather has absolutely nothing to do with it. Thought I would let you know.

 

What is your exact idle rpms cold and @ operating temp. You have to be precise if you want correct info. So write it down, next time you think of it.

 

check the IAT and EGR . either could cause this problem

 

When the outside air temp drops below 50, the idle bounces from 300 to 700 every one to two seconds. When the temp is above 50 it sits rock solid at around 650. When the outside air temp is around 50, warming the enging up will make the rough idle go away. When the outside air temp drops into the low 40s or 30s, the rough idle is there not matter how long the engine has been running. You have to use both feet at a stop light to keep it from dying. Off idle it runs great not matter what the engine or outside air temp is.

 

A couple of other things to look at are your maf and thermostat.....