My 3.8 is idling at 4000 rpm with the intake completely blocked. Removing MAF makes no difference.

This problem started out being poor acceleration. I exchanged plugs and wires...and it has now regressed to running at the rev limit (4000 RPM).

When removing the MAF plug, no difference,
I removed the intake hose and completely blocked the inflow to the throttle body, no difference.

I would think there would have to be an enormous leak to run at 4000 RPM with the throttle body blocked.
I am preparing to remove the plenum for an exploratory search

Solved:
A hole has melted in the intake plenum. Right above the EGR port.

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TBH, I dont totally understand what you mean about blocking the intake and removing the MAF.

What comes to mind for me:
-TB is stuck slightly open (stuck pedal, debris/buildup in the bore). This should be easy to verify by pulling the intake tube off, either the butterfly is closed or it isnt.
-IAC is broken and somehow lodged wide open. Not sure how that would happen, but it's easy enough to check, it's just 2 screws.
-Vacuum leak somewhere - cracked intake manifold, broken vacuum line, etc. I probably lean this direction the most if I understand correctly what you mean by 'blocking the intake'. If you're completely sealing off the entrance to the TB and it's still able to run, air is getting in elsewhere. Just follow each vacuum line and perform a thorough inspection of the intake.

If the last one is the problem, removing the MAF would make diagnostics harder. With the MAF plugged in, the pcm should be reading very little airflow, but with the engine getting air elsewhere, itll idle high and be very lean (extremely high fuel trims). If you unplug the MAF, itll default to SD which will fuel by rpm and MAP load, so fueling will appear decent.

 

^Agreed.

Could be the throttle body stuck open, or the cable being stuck. IAC is another possibility, as is a vacuum leak. I'd check the throttle assembly first.

 

 

That is a new one for me. Never seen a hole melted in the top. Makes me wonder if the exhaust coming through the EGR is way hotter than it should be OR is this just bad luck. Hopefully just bad luck and replace the UIM and off and running again!

 

X2, that's crazy!

I did a bit of googling and did find a video of a similar condition (scary as it was melting through the fuel line too):


They claimed the issue was due to plugged cat problems in their case.