1998 Explorer 4L OHV Intermittant High RPM Surge

[tinkering]

I thought I had it all solved but High RPM surging has reared it's ugly head again.
My related threads are here: http://www.explorerforum.com/forums/showthread.php?t=411696
http://www.automotiveforums.com/vbulletin/showthread.php?t=1137487

OK. After getting gas, we started the engine up again, it surged up to 3000 RPM and stayed there. We shut it off and the same thing happened on the second start. The third start it was OK.
We didn't touch the gas pedal at all during all this.

We could feel some intermittent surging while on the highway.

For safety sake it is parked at home until this gets settled.

There is no engine light on.
Might there be codes found with a scan even though there is no engine light on?

I need to get up to speed again with all the sensors and solenoids etc.. Is there an idle speed control solenoid that could be malfunctioning; could the main computer be insane?

Any ideers?

 

[Runnin'OnEmpty]

Tinkering, have you checked the TPS voltage when the
throttle is closed at idle?

One thing that hasn't been mentioned is the check valve
for the brake booster. If it's sticking intermittantly, that
might cause the surging...

However, since the IAC restrictor you installed once before
worked, it seems to be IAC related (or the IAC command).

 

[tinkering]

Tinkering, have you checked the TPS voltage when the
throttle is closed at idle?

One thing that hasn't been mentioned is the check valve
for the brake booster. If it's sticking intermittantly, that
might cause the surging...

However, since the IAC restrictor you installed once before
worked, it seems to be IAC related (or the IAC command).

I will check into this when I get back in a month +/-. Thanks

1. What should the TPS voltage be at idle? Do you check it right at the connector?

2. Where exactly is the brake booster check valve and what does it look like? Is it OK to try one from the wreckers?

3. Where is the IAC command? How do I approach that one?

 

[Runnin'OnEmpty]

The IAC idle voltage should be under 1v; typically it's
about .97v. Your scanner should be able to check it, or
you can check at the connector.

The brake booster check valve is the small black unit
that's pressed into the booster. It has a vacuum hose
running to the intake manifold. They're very inexpensive
and pretty much a generic part.

Probably a poor choice of wording for IAC "command".
I was thinking of the PCM sending a signal to the IAC
to raise or lower the idle speed.