RPM Fluctuating/Spiking when Idling in Gear

[extacoma]

Hi everyone, I wanted to ask for everyone’s thoughts on an issue I’m facing:

Timeline

• at 154k miles, replaced motor (due to overheating via failed coolant lines) with a used 145k mile motor (done by Mudrak)
• With new motor, I had driven approx 2k miles with zero issues
• Then I had a string of misfire issue codes being returned. After trying to swap coils myself and codes still returning, I brought it back to the shop and Gary replaced a number of spark plugs and coils. However, we did not check every coil and spark plug. The misfires were fixed and no codes are returned.

The issue

The following issue only started occurring after my above mentioned visit to the shop. Keep in mind the car drove fine for 2k miles before the visit.

• Vehicle in Reverse or Drive
• Foot on brake
• A/C on
• It also seems to be exacerbated in higher temperatures

This results in RPMs spiking/fluctuating harshly between 600-800 (see video). VIDEO:

It doesn’t necessarily affect driving performance (as the RPM fluctuation does not occur when driving, only when idling). Also, no codes are returned.

I am a complete noob when it comes to this stuff, but I started with resetting the ECU, and soon plan to inspect and test all spark plugs and ignition coils. However I’m not sure what direction to go if this does not yield results. I’d greatly appreciate your thoughts on the matter!

 

[AARONTLC]

Some not all of that fluctuation may be your AC compressor kicking on and off as well. Usually when I see an idle like that I go straight to the vacuum lines and then the throttle body. Scan tool might help.

 

[Trapper50cal]

This just started for me as well, only a couple occurrences. very similar. Like it can't decide where it wants to be
while very hot outside and AC on with fan set to low (1) or 2nd position. Not the same as high idle on start up then
settling.

 

[extacoma]

I just went on a 10 hour trip where the engine was on pretty much the whole time.. including driving through Death Valley in 118 degrees. The RPMs even started to become unstable when idling without AC on and even in park.

It does seem that more strain on the engine leads to it being more pronounced. While driving through death valley I got a 2nd cylinder misfire code... we will see if replacing the plug & coil here is related to this issue....

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@AARONTLC - Shop I took it to inspected vacuum lines and found no issues there. Not sure if they checked throttle body.. I will flag that for the next shop that takes a look.

 

[AARONTLC]

I just went on a 10 hour trip where the engine was on pretty much the whole time.. including driving through Death Valley in 118 degrees. The RPMs even started to become unstable when idling without AC on and even in park.

It does seem that more strain on the engine leads to it being more pronounced. While driving through death valley I got a 2nd cylinder misfire code... we will see if replacing the plug & coil here is related to this issue....

----

@AARONTLC - Shop I took it to inspected vacuum lines and found no issues there. Not sure if they checked throttle body.. I will flag that for the next shop that takes a look.

Well, if you have a P0302 I would definitely zoom in on that. I would inspect every spark plug (not just #2). Take a close look at your coils and boots. Coils tend to split right down the mold seam on the shaft. Torque plugs to 18 ft lbs. Good luck!