Q: Random low then high idle jumps

asked by on February 02, 2018
  1. When I start my car in the morning it sometimes starts normal. And sometimes when I start it it'll rev like crazy as if I'm smashing on the gas while I start it.
  2. This just recently started happening. When I drive around 40-50mph I feel a slight almost unnoticable jerk.
  3. When I put my car in park sometimes the rpm will jump and just stay high then settle down after a period of time.
  4. When my car is in drive and I come to a stop it idles really low sitting just above the 0. And it'll vibrate like crazy. Same for when I put it in reverse.

I have to smash the gas a few times to get it to go back to normal. I've changed the fuel pump assembly. A broken fuel line. IAC valve tps, spark plugs and wires. Everything someone at advanced Auto parts would recommend I've replaced. I have down the 02 sensors yet or the map sensor. I've cleaned the throttle body out. One day it actually cut off when I switched from reverse to drive.

My car has 250000 miles.
My car has an automatic transmission.

This may be a sign of a faulty MAP sensor as the vehicle responds adversely to a load put on the motor (i.e. putting it in gear, making turns (which makes use of the power steering pressure switch) turning on AC or engaging the brakes). 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 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 inspect your vehicle.

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