I HAVE ALREADY CHECKED THE 90° ELBOW! with that being said, 2003 4.6 240,000 if I start the truck it will run fine until it's warmed up. The first thing I notice is when coming to a stop the rpms and idle will drop low like it's going to die then jumps back up a little over normal and then evens out (with one loud suckling sound when it rises.) But it does not always do that. The next thing is sometime at an idle it will drop down and do the same thing and also after reving it. It never dies but is idling so low it's chuging , when it dips low if I turn the a/c on it will run it back up to 1200 then come down and hold at normal. Truck runs great while driving, good power no misses or anything just stopping and at idle I have checked all plugs, cleaned mass airflow, cleaned iac valve and have recently replaced the pcv valve. I would also mention there was oil in the 90° elbow and on the pcv valve any help would be greatly appreciated

 

Fuel filter?

 

Would a fuel filter only cause problems during idle or all the time?

 

Just a guess since you had not mentioned replacing it and it is easy and cheap. Another guess would be clogged or bad injector? Again, just a guess/suggestion.

 

No. Pull the check valve out of the booster and plug the vacuum line and see if it runs normal. Also check the plastic tubes and rubber hoses hooked up to the intake.

 

Replace your IAC valve. Cleaning doesn't always address the problem.

 

Yes, I too learned that lesson on my old 89.