2001 4.0L surges at idle.

[jdsjgreen]

I'd appreciate any help here and thanks in advance!
The engine runs like a top including including smooth idleing until.....10 seconds to a couple of minutes after the engine idles (when warm) it very suddenly begins to surge 2-3 hundred rpm up and down. A little throttle smooths it out and it continues to drive just fine. I get codes indicating random misfires and lean conditions on both banks when this happens. Here's what I've done to try and cure it so far.

Complete tune up including new plug wires.
New air and fuel filters.
New mass air sensor.
New oxygen sensors (both the 2 upstream and 1 downstream of the cat conv).
Idle airflow sensor.

I don't believe (although I'll take arguments) that I have a vacuum leak or manifold leak as I believe it would never idle smoothly.

Any Ideas?

 

[BrooklynBay]

Welcome to this forum! I've moved your thread into the 2001-2005 Sport Trac section. What are the code numbers? Did you check the TPS?

 

[swetrid]

Could be rotted pcv hose

 

[jdsjgreen]

Havn't suspected the TPS but I suppose that's next.
The codes are P0171, P0174 and P00300 (although the 0300 code for random misfires has not popped up the last couple of times after I reset the CEL)
I think the PVC hose is ok but I'll take a really close look at it to be sure.

Thanks!!

 

[swetrid]

It rotts out right at the intake on the pass side. Another possibility coulld be intake gaskets.

 

[jdsjgreen]

Well, swetrid, you are a genious! It wasn't easy to see because there was a cut that appeared to be made by some tool that couldn't be seen without a little flexing but it was the PVC intake side fitting. Thanks so much for your help. I looked at that hose a dozen times and didn't see it and probably wouldn't have looked again without your urging. Thanks again!!!!

 

[BrooklynBay]

Thanks for the update!

 

[jdsjgreen]

Brooklynbay - I'm new at the forum so thanks for your assistance and guidance!