sound like an air leak?

[swynx]

For awhile now I've been trying to fix a high idle problem on my 94 4.0 5speed When I got the truck it ran poor. I put a new fuel pump in it and a new fuel pressure regulator. That fixed the problem with it not starting under a 1/4 tank. And dieing after high rpm runs. Gained alot of performance back.

I've replaced both 02, the iac, Tps, and the other usual that are described in each of these threads.

I notice these 3 things. On start up it'll idle under a grand like 900ish. If I hit the gas it won't ever idle under a grand, like 12-1300ish. Pushing the clutch in repeatadly will lower the rpms. Above 3 grand it makes a sound like a Turbo car. I wanna say that's normal egr stuff, but Idk.

Hopefully those are good clues.

 

[swynx]

Bump

 

[RonD]

With engine cold remove tension on serpentine belt, so no accessories will turn, makes engine bay very quiet.
Start engine, some dash light will come on, that's OK.
Listen for any air leaks.
Also without fan blowing it makes it easier to spray carb cleaner or starting fluid around to find a leak.

Cold engine can run without water pump for a few minutes.

The 4.0l had a tendency to get lower intake/head gasket air leaks, the torque on these is only 18ft/lbs

 

[1994Ranger$$$pit]

Turbo sound= sounds like your fan clutch is stuck.

 

[swynx]

It never over heats. I've used both a torch and Carb cleaner I can't find any air leaks. If i let it sit for 15 seconds or so the idle will go down to 900. Defiantly has a higher more out of control high idle before the 02 sensors kick in.

 

[RonD]

Plug the air tube(air cleaner end) with a towel while idling, and serpentine belt off.
If it stays idling you have an air leak, block most of the air flow should stop the engine

The IAC valve and computer should run cold idle above 1,000rpms
As engine warms up idle should slowly drop until it is at 750, lower with manual trans

The Computer watches the CPK(crank position) sensor to get the current RPM.
And the IAC valve has some room to compensate for a small air leak.
So don't think of this setup as a static system, it is dynamic, computer "sees" the RPM, and opens or closes IAC valve to set warm idle at 750, air leak or not.

I would disconnect IAC valve wires, then start warm engine, see what the idle is.
If it is still high then IAC valve was closed all the way so air leak.
If it drops to 500 and/or stumbles and dies then computer is opening the IAC valve for a reason, to set the high idle