Q: Low idle stalling out

asked by on December 13, 2016

The Jeep will idle real low like 200 rpm then it may stall out or jump back up to 1000 rpm and has a rough idle when it goes back to normal. Only does it when the jeep is in drive while sitting and idling with foot on brake. it doesn't do it when in reverse, neutral or park. It has been doing it more frequently for example instead of once a week it is more like once a day I do drive it everyday. There are no vacuum leaks the throttle body has been cleaned and the IAC has been changed. Any suggestions on how to repair would be greatly appreciated. My car has 89000 miles. My car has an automatic transmission.

My car has 89000 miles.
My car has an automatic transmission.

This suggests that the low idle problem may be related to a faulty MAP sensor since the symptoms you describe point to the low idle issue when the engine is under a load when applying the brakes at idle. The Manifold Absolute Pressure (MAP) sensor senses engine load and generates a signal that is proportional to the amount of vacuum in the intake manifold. The engine computer then uses this information to adjust ignition timing and fuel enrichment. When the engine is working hard, intake vacuum drops as the throttle opens wide. The engine takes in more air, which requires more fuel to keep the air/fuel ratio in balance. When the computer reads a heavy load signal from the MAP sensor, it adjusts the fuel mixture to slightly more rich than normal so the engine can produce more power. The computer will then retard (back off) ignition timing slightly to prevent detonation (spark knock) that can damage the engine and decrease performance. This may also be caused by other closely related components such as a dirty or failing mass air-flow sensor, a faulty throttle position sensor or potentially a vacuum leak. I would recommend having an expert from YourMechanic come to your location to diagnose and replace your MAP sensor.

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