Tuesday, March 12, 2013

Dakota Truck: "Idles up and down"

Engine idles up and down

It feels like giving gas and letting it off.
Also it feels like car not taking gas while driving.

"RPM goes UP and Down"

It's very likely that the truck thinks you are actually accelerating and letting off when you aren't.

The RPM fluctuation symptoms , rpm up and down you have described are usually caused by an erratic signal from the throttle position sensor (TPS). The tps throttle position sensor is mounted on the left side of the throttle body and tells the powertrain control module (PCM) how far the throttle is open.

Throttle position sensor location:

The left side of the vehicle is the left side if you were sitting in it. Assuming you are in the US the left side of the truck is the driver's side. We want the throttle position sensor.

dodge throttle position sensor

It sounds like you are having two common symptoms that happen with an erratic TPS signal. When the sensor gets worn it will start to create small voltage spikes in it's signal. The PCM will see these voltage increases and think you are opening the throttle when you actually aren't. When this happens at idle it will cause the idle to go up and down because it thinks you are opening and closing the throttle.

When you're driving at a steady speed the torque converter clutch will lock up and give a mechanical lock between the engine and trans, this drops engine RPM and slightly raises fuel mileage. When it sees the voltage increases from the TPS while the converter clutch is on it thinks you are starting to accelerate to either speed up or pass, so it unlocks the torque converter. This will cause the RPM's to jump up and then settle back down as the voltage drops back to normal.

The TPS is held on with two Torx screws and is easy to replace.

The acceleration problem could be related to the TPS. If your "check engine light" has come on I would recommend first seeing what obd codes are set, which you can do with a built in feature or by using a scan tool or code reader. Most large autoparts stores like Autozone will read fault codes for free. If there are performance related codes set then they should be diagnosed and repaired and hopefully that will take care of the acceleration issue.

It sounds like you may be experiencing a hesitation from a bad upstream oxygen sensor, which threads into the exhaust before the converter. If the acceleration problem is happening during warm up or operation in colder weather it may be happening because of a weak oxygen sensor heater. If the built in heater doesn't bring the sensor to operating temperature and keep it there it can cause a very lean fuel mixture, hesitation, and sometimes some backfiring on acceleration.

Upstream Oxygen Sensor Location

Dodge Upstream Oxygen Sensor Location

This will help.

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