Rough idle problem !!
 

OK, I have a problem that has perplexed me for quite some time. This might be a little bit long because I want to be as descriptive as possible in hopes that someone here can point me in the right direction !

In the past year or so, my '99 24 valve , manual trans truck has developed this rough idle. It has gotten worse as time went on.
During that year ( with the rough idle ) the injectors have been changed out ( with the RV , 50hp injectors) and the injector pump has been changed out with a Bosch certified rebuilt VP-44. I still have the rough idle.
The truck starts perfectly everyday. It idles perfectly until it gets up to normal operating temperature. Then it starts , the ROUGH idle.
There is no smoke out the exhaust when idling.
I have 335000 miles on this motor.
I have tried unplugging the Edge EZ chip for a while on the truck but the problem is still there.

What would cause a rough idle ONLY after it warms up !?
It kind of sounds like one cylinder is dead at an idle , but ONLY after its warmed up.

Here are my thoughts of possibilities.

1. something electronic that is deteriorated and is only affected when hot.( I have unplugged everything I can see and have put dialectric grease on all of the contacts.)
2. I have a bad injector that only malfunctions when its warmed up.
3. The valve lash is not set correctly. I adjusted them about a year and a half ago . I only had to make a couple of very small adjustments. My hopes are that I left one of the lock nuts loose and it went out of adjustment . But again , the problem only occurs after warm-up, if this were the case it would probably do it all the time ?
4. I have a bad valve.
5. I have lack of compression in one cylinder. But why would this happen only after warm-up.

I do have a question . Is there a warm-up mode on these trucks? Maybe there is something that is NOT shutting off after the truck is warmed up ? Like and enrichment setting that hasmt been shutting off or something ? But it would probably smoke then ? I know that there is an intake air heater that operates until the engine is warm . Is there anything else that I am missing ?

I also noted that the hotter the truck gets ( really hot weather when I am hauling a load) the idle problem gets even worse.

PLEASE help me out dudes and dudettes ! I dont know which way to turn other than bringing this thing to Cummins $$$ !!!

Thanks in advance .

Dew.

no guesses ? input ?

I pulled the valve cover today for valve adjustment. Everything was near perfect . Made very tiny adjustments to about half of them.

Is there anything electronic that you all can think of that might cause my problem ?

DEW

Might be a sticking valve when it gets hot. Only off the wall thing I can recall from old days with gassers.

could be ac compressor kicking on and off, mine does it and i was baffled for years

Thanks for the replies. The A/C seems to be working just like it should.

Davey , That is a great thought. A sticky valve could definately do what is happening here.
I have seen some folks here that have used two stroke oil in the fuel . I am not sure what type of problem they were trying to fix. I remember putting ATF in some gasoline motors to free up stick valves. I will read up some on putting some two stroke oil in the fuel.
Thanks again ,

Dew

I put in some TCW3 2 stroke oil. Its running better but didnt fix it totally .Its running a little bit smoother. I only ran it about 10 miles though. I like what it did to my truck .