1998 F150 rpm/idle problem

Could be a vacuum leak. Check all the hoses, especially the ones leading to the A/C controls.

Make that 2 for vacuum leak in the HVAC control system.

You can probably find a half-ass-decent vacuum map at the autozone site, but I think they require registration.

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- gpsman

A vacuum leak can certainly cause the issues you report. Additionally, a fluctuating signal from an O2 Sensor can also cause the problems you have. A MAF sensor (mass air flow) can also contribute to the problems you have.

If there is air entering into the intake that the MAF does not know about, then the extra air can speed the idle. If the O2 is reporting a condition that is false, the corrective measures taken by the computer will be wrong, and this can speed the idle. And, the MAF reports the desnity and temp of the intake air and the computer uses the information to regulate how much fuel to deliver. If the MAF is making reports of air quality that vary from one moment to the next, then the idle speed will be affected.

You need a better mechanic, or the same mechanic with better diagnostic equipment. It sounds like he is making guesses that cost you more money than they cost him.

Thanks feedback David. It seems that is the consensus of more than a few people.

Del.

Thanks gpsman. Will be looking into it this week.

Del.

You can probably find a half-ass-decent vacuum map at the autozone site, but I think they require registration.

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- gpsman

Thanks Jeff. I'll be having it looked at again this week. It's nice to have some info to work from.

Del.

You should be getting a Check Engine light for a malfunction that gives the symptoms you report.

The check engine light is REQUIRED to come on when the key is in the Engine Run position -- the last key position before Engine Start, which is also the key position the key returns to after the engine has been started -- for the purpose of a self test. If the light is not coming on with the key in the RUN position, then the light has failed for one reason or another, and you should merely check the stored codes for insight to the problem you have reported. Once you get that problems sorted out, then investigate why the light is not coming on. If the light does come on, and remains on while the engine is running, then just pull codes. The MAF and the O2 Sensor should certainly throw a code if they are the root of the problem you have, the vacuum leak might not -- although the lean condition caused by the MAF should be visible to the O2 Sensors but it is possible that the lean will be small enough to confuse the idle speed and remain within the limits of the O2s.

No check engine light while running and no codes. The light does come on in the start sequence as it is required to. The O2 Sensor was replaced within the last year when it failed. Thanks again for all the help.

Del.