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Intermittent high idle

vehicle 1997 dakota engine 3.9L w/auto

The vehicle exhibits an intermittent high idle. I have replaced the usual IAC stepper motor and checked the tps for proper function, also completely cleaned, inspected and relubed the throttle body and linkage.

According to my Inova 3140, there are no codes and the temperature sensors and tps percentage are reading correctly. Also the battery and charging system are functioning correctly.

I do not have bi directional capability or the ability to monitor any of the pid’s
One other thing is that she also has a slight rough idle, but that could be the 200,000 miles.

Thank You

These are the typical symptoms for a vacuum leak. The rough idle indicates that it is the intake manifold gasket, most other vacuum leaks affect all cylinders equally so the idle is less likely to be rough. Try spraying something like propane, either, carb cleaner, throttle body cleaner or even WD-40 all around the intake manifold gasket, when you come to the leak, something will change depending on what you are spraying. I prefer to use WD-40 myself.

I did mention this is Intermittent, Unlees the vacuum leak is on the downstream side of a vsv I don’t see is how this would be the cause.

What I failed to mention is that the idle is only high at start up and if I turn the key to the off position then restart the engine the idle is normal.

Thank You

If it’s only doing it at start up, than I’d suspect fuel line pressure loss (the line is losing pressure and it’s taking a moment for system to repressuriize and the fuel pressure regulator to stabilize).

No that is not the case/ loss of fuel pressure would cause a hard start or no start condition.
But thank you for the ideas.

Normally that’s true, but a pressure pulse on startup could mislead.

You seemed reluctant for some reason to accept the possibility of a vacuum leak, which could be anywhere past the MAF sensor…or even a leak just upstream of the upstream Ox sensor (an exhaust leak would affect the metering if it were in a spot that causes the Ox sensor to register a higher oxygen level than there actually is, and that would cause the ECU to enlarge the injector pulsewidth). So I thought I’d add a WAG thought.

The reason I didn;t mention the exhaust leak previously is that I’d normall expect an exhaust leak of that type to cause a wandering idle rather than one that settes down.

No maf on this antique. the pressure pulse theory is easy enough to check.
Regarding the o2 sensor and an oxegon rich enviroment I would agree if it were all the time same with the vacuum leak theory but not intermittent and as I explained earlier if the key is cycled it starts at normall rpm.

I am starting to believe that the only way to catch the culprit is to record the labscope readings from the input and output signals upon start up.

Thank You again for the input

Did you figure this out?? I’m having the same problem.

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