Revving while accelerator not engaged?

[furt]

2000 Jeep Wrangler, manual transmission. Check Engine light came on 2 weeks ago, but then went off and has not been seen since.

When the engine is running and I am not pressing the gas, the engine revs way up to about 2500 rpm, and stays there. This happens both in gear and in neutral. It's been that way for 3 days now.

Ideas?

[Machine Elf]

A major leak in the intake manifold? Operating in closed-loop, the ECU will attempt to compensate for the unmetered excess air by injecting extra fuel, so that the net effect is about the same as pushing the gas pedal down slightly on a healthy engine. The CEL is probably on because the ECU recognizes that the compensation is excessive and indicative of a major air leak.

Tip: visit an auto parts store and get the trouble codes read for free. Report back here what the codes are.

[jz78817]

yeah, vacuum leak. Chrysler engine management systems almost universally are speed-density, and will happily add more fuel to account for the extra air being taken in through the vacuum leak.

Originally Posted by Machine Elf

A major leak in the intake manifold? Operating in closed-loop, the ECU will attempt to compensate for the unmetered excess air by injecting extra fuel, so that the net effect is about the same as pushing the gas pedal down slightly on a healthy engine. The CEL is probably on because the ECU recognizes that the compensation is excessive and indicative of a major air leak.

nitpick: a speed-density system calculates fuel need based on the manifold pressure and air temperature. so technically a vacuum leak is still metered air.

[Machine Elf]

Originally Posted by jz78817

yeah, vacuum leak. Chrysler engine management systems almost universally are speed-density, and will happily add more fuel to account for the extra air being taken in through the vacuum leak.

nitpick: a speed-density system calculates fuel need based on the manifold pressure and air temperature. so technically a vacuum leak is still metered air.

I'll confess that I don't know which vehicle brands/models use which sorts of air measuring systems. But if this engine calculates fuel need based on manifold pressure/temperature, why might the CEL be on?

[Gary T]

Originally Posted by Machine Elf

I'll confess that I don't know which vehicle brands/models use which sorts of air measuring systems. But if this engine calculates fuel need based on manifold pressure/temperature, why might the CEL be on?

The computer monitors LOTS of things, for example I would guess idle speed out of normal range if the light were staying on. In this case it's conceivable it had something like a misfire at first due to the vacuum leak, or the fuel trim went to its limit for a while, which has since been mitigated by the computer's adaptive strategy.

[jz78817]

Originally Posted by Machine Elf

I'll confess that I don't know which vehicle brands/models use which sorts of air measuring systems. But if this engine calculates fuel need based on manifold pressure/temperature, why might the CEL be on?

because the manifold pressure is outside of the expected range based on the throttle position and engine RPM. I'd wager a guess the CEL is on due to something like "P0068 MAP/MAF - Throttle Position Correlation" set in the PCM.

http://www.obd-codes.com/trouble_codes/

[Fubaya]

He said the CEL came on then went off two weeks ago and hasn't been on since.

If it comes back on, he can skip the trip to have the codes read and put the key in the jeep and flick the ignition on-off-on-off-on-off-on then watch the odometer. It will flash the engine codes like P0442...P0443... then say "done" and show the normal odometer.

[GreasyJack]

It seems like if it's idling super-high but there's no CEL currently lit, it would have to be a throttle linkage (or possibly cruise control) issue, right? If it were a vacuum leak, wouldn't the higher manifold pressure without any throttle engagement trip the CEL?