Blogs Car Info Our Show Deals Mechanics Files Vehicle Donation

1999 Isuzu Trooper

…with an idle-speed problem. When the car is in park or neutral the RPMs rev up to 2000-3000 RPMs; in Drive I can start from a dead stop and gradually get up to 25mph and 2 gear shifts without ever touching the gas pedal. At speed the car is dropping RPMs at random, quick intervals. In the past 6 months I’ve had to clean the Mass Air Flow Sensor twice, which seems like much and I’m wondering if this is a symptom of the underlying problem. Any ideas would be a huge help. Thanks

Why did you clean the MAF, and what were the results?

The Check Engine light was on and I took it to Schucks so they could hook up their computer. The code was for the MAF so I cleaned it and there was some improvement initially and then it went back to the same problem which is now gradually worsening. The check engine light is no longer a problem though!

If the engine is running smoothly at 2000-3000 rpm, then the MAF is probably working okey. Somewhere you have too much air getting into the intake system. I would have the throttle body cleaned paying attention to anything that might be holding the throttle plate open. Also have the mechanic clean the Idle Air Control, its seat, and the idle air passage. If that does not cure the problem you have to look for a vacuum leak that is letting unregulated air into the intake manifold. The concern with a leak external to the air flowing through the MAF is that it would be leaning the mixture and setting a CEL code for ‘system too lean’.

Reply back with the solution to this situation.

I cleaned the Idle Air Control valve and housing and cleaned the throttle body where the plate is. There was some build up of carbon and dirt near the plate. This seemed to help the problem for a few miles but then the same issue cropped up again. Could all of this be from a dirty air filter? Would it be worth changing it out to see if it helps?

Is the problem constant, or not? If it’s not constant, we can stop looking at most mechanical ways of additional air entry.
One area to look is at the throttle plate. If the throttle is hanging up, intermittently, this could be your problem. The throttle, or throttle plate, has a stop, somewhere. It could be a stop-screw on the outside, or the inside, of the throttle body. With the intake tube off, look at the throttle plate, and look that the plate, or linkage, is resting on the stop-screw. Now would be a good time to give the inside of the throttle body a good soaking with Carb/Throttle Body Cleaner (just in case).

The problem is not constant. When I first start the car it will be fine on most starts and then develope after driving a few miles. Sometimes the RPMs drop (from 2000 RPM to 1500 and back to 2000 etc) at around 50-55mph and sometimes at 20-30mph. I can even go a whole trip of 10+ miles without it happening but the problem is becoming much more frequent, 9 out of 10 trips the problem persists. I’ll check out the throttle plate though and see where that takes us. Thanks again for all the help.

Is the engine still surging at idle, or is the problem the rpm drop at speed; or, is it both? If it’s both, I think those are separate problems.
An rpm DROP could be the result of a momentary cut in spark, or fuel.
Has the fuel filter been changed? Have you checked fuel pressure, at idle and under load? Has any maintenance been done on the spark systems?

Have the code scan done again. Ask them to get the freeze frame data. All they have to do is push the FF button. Bring codes and data here (interpretations aren’t needed).

It is doing both things. The check engine light is not on anymore, will doing a code scan still help? As far as I know no maintenance has been done on the spark systems and I haven’t had the fuel pressure checked.