CarTalk.com Blogs Car Info Our Show Deals Mechanics Files Vehicle Donation

Dodge Dakota High Idle

I have a '99 Dakota 4 cylinder. Starts fine but as it warms up or if you gas it the idle stays high. The more you run it the higher it goes. Have replaced MAP senser, throttle position and air bypass whith no effect.( map and high tps voltage only codes shown ) Vaccum problem?

It’s probably a vacuum problem but the source is a bad Idle Air Control Valve. I just replaced the one on my newer Dakota and it was acting the same way. The IAC controls the amount of air at idle for the engine.

I have replaced it already. Guess I’ll check it to be sure it’s working. Thanks a bunch.

rlc1toyz,
You indicated that you replaced the IAC, TP and MAP, you also hinted that you still have the codes. Do you stil have the high map and TPS codes present??? Just because the TPS and MAP were replaced does not mean the PCM is happy with the input readings.

If the TP reads high, the PCM will assume Part Throttle operation and will, as a result Pull the IAC most of the way out. If the IAC pintle is pulled out, the result will be a higher idle. We need to ensure the PCM is seeing the Throttle state as “closed” when there is no accel pedal depression. You would need a scan tool to do this.

I am making an assumption here (That the codes are still present), but i suspect the throttle state would read part throtttle. At this point I would be to check the signal return (Ground) circuit gpoing to the MAP and TPS. If there is high resistance in this circuit it will bias the two sensors high and therefore provide bogus info to the PCM. Signal return circuits again is a grount path, with that said, back probe with a paperclip or other device. Check the available voltage on the signal return circuit with both sensors plugged in when the concern is happening. The Allowable voltage must be as close to 0.0v as possible. (Test with DVOM red lead connected to signal return and black lead connected to negative battery terminal) If you have voltage greater than 0.5 volts, you have a major issue within the signal return circuit.

The MAP and TPS sensor have three wires… One is VRef (Typically a 5 volt power source), the second is the signal sense (provides the input back to the PCM as a variable voltage signal. Any thing from 0.5v approx to close to 4.85-5.0V) The third wire is the signal return which again for all purposes is really a ground so it should be on the ground plane with 0 volts. Typically the Vref is the same color for the two sensors, the same is true for the Signal return. The sensor signal wires will be different colors between the two as they are different inputs to the PCM. (Sorry I can’t remember the exact wire colors)