Randomly Revs?

[eponator]

Hey guys, I need a little help diagnosing my cousins truck. Hes got a 91 Mitsubishi Mighty Max with the 2.4L 4cyl and the 5spd manual trans. Don't know much about this truck other than it has lived a hard life!

Now the problem: this thing will randomly rev up for no apparent reason. One minute its sitting and idling just fine then the next it just revs up to about 2K RPM, holds it for a sec then revs down to normal idle. You can never tell when it is going to do it.
Up untill now he says it was more prone to doing it only while cold but the other day apparently it started doing it to him at all times even while warm. As you can imagine, hes a bit concerned safety wise. Having the truck lurch forward while braking is not a good thing nor is it fun randomly jumping off the line with a cop the next lane over.

A full tuneup was done to this car (minus fuel filter which he says was replaced recently, along with the alternator and EGR). My first guess was a vacuum leak so I sprayed the intake and all vacuum lines down with carb cleaner but was unable to detect anything. The truck is not showing a CEL nor was he able to pull any codes at the autozone.
It has been doing this for the past few months now but just got bad enough for him to be very concerned. It's been in and out of many shops and none of them can figure it out so he has finally come to me for help. I'll probably head over there again this weekend and take a better look at it but I wanted to know what you guys thought. Any ideas what could be causing this or other tests to help narrow it down?

[gmctrucks]

TPS sensor.

[stumpuller454]

Or if it has one, the Idle Air Control could be gummed up...

[bradrhale]

i would think that an IAC problem would kill the engine. maybe an injector problem??

[crabtruck]

I seem to recall that truck has an "Idle Speed Control" system which
would make idling poor or rough if the throttle plates were dirty.
The big pain was, if that was the case and you disconnected the battery
for some reason, the truck would not start again.
That's because the ECM would forget it's learned adjustments,
go back to factory settings and the only way to restart it
would be cleaning those throttle plates. Like some kind of booby-trap.

Doesn't seem like the ISC would cause a random surge though,
I think I would start by looking for a loose connection that is
having problems staying solid under vibration.
May want to look for it by wire and plug shaking
to see if you can trip/cause the surge effect.
Then maybe look to fuel delivery.

[eponator]

Not sure myself wether it has IAC or ISC, I'll have to find out when I get there. That was what I was leaning towards myself but I'm just left speculating untill this weekend.

Loose connections giving way under vibration - Definitely possible given the age and condition of the vehicle.

What makes you think fuel delivery though? Unless maybe the electrical related to the fuel delivery? Bad wiring causing surges to the fuel pump? I'd love to hear your thinking behind this and any other ideas any one else may have.

[crabtruck]

If only occurring at idle or no throttle then I would be inclined towards the ISC.
However if it is occurring while under throttle also I would lean toward the TPS.
Other fuel delivery factors could include something causing a pressure build up,
much longer shot. I don't have much else to offer for a mitsu truck.

[eponator]

Originally Posted by crabtruck

I don't have much else to offer for a mitsu truck.

You have offered more than enough Crab thanks for the help!! I have been mulling this over and I am definitely thinking the ISC based on the symptoms I saw. I will have to talk with him to determine if it does this under light throttle or not.

As far as fuel pressure buildup, monitoring the pressure in the fuel lines should give me a good idea of whats going on there during these surges. Well, looks like I have a weekend of chasing faulty electrical connections and pressurized fuel systems, my favorites!! Thanks all for the help, ill post an update with the fix later for those curious.

[eponator]

Well, I finaly heard back from my cousin... turns out the clutch gave out on him over the week and he doesn't know if he feels like dealing with the truck any more!

I may never get to know what was causing this!!!