Engine revving and dashboard clicking (video)

[JohnnyD]

I'm hoping you all can help me get to the bottom of a very frustrating problem. I have a 2000 S80 with 180,000 miles on it, and on a recent road trip the engine started revving erratically, which coincided with a loud grinding, ticking, or clicking noise coming from behind the left side of the dashboard.

After turning off the engine and letting the car sitvfor a while, the car starts normally. I can drive it fine for anywhere from 15min to over an hour. However, at some point, the dashboard starts clicking, and very soon afterwards the engine starts revving up erratically, even at a stop. The car feels like it wants to lurch forward, and the response to pushing on the accelerator is weak and erratic.

Once the car gets to this point, if I shut the engine off, it will not start again. The dash lights come on when I turn the key, but the engine does not turn over at all. However, if I let it sit for a while, the car starts just fine and drives just fine, for a while. The distance I can drive before the problem starts seems to be correlated to how long the car has been sitting un-driven.

Another curious aspect of the problem is the fact that the dashboard clicking seems to be related to the electronics system. The video below demonstrates this: When I activate the turn signal or the emergency hazard lights, the clicking sound stops, but only while the turn signal is illuminated. There are other signs that this is an electrical problem too: When the problem is happening, the LED indicator on the gear selector is off (but it works when the car is not having this problem), and sometimes the overhead ceiling light buttons do not work; again this happens only when the engine revving/clicking is happening and never otherwise. The clicking noise has some mechanical component to it; if I put my hand underneath the driver's side console (down by where the brake pedal is), I can feel a mechanical ticking when the clicking sound happens.

The check engine light is not on, the alert screen displays no errors, and a Volvo dealership found no error codes either.

Here is a video of the problem: https://www.dropbox.com/s/by2qvpvtr2op7 ... 0.mp4?dl=0

In this video I have the car in drive and I am applying light pressure to the accelerator. You can see the revving, though it's mild in this video. If I try to drive it at highway speeds, the car revs erratically between 2000 and 4000 RPM. In the second half of the video you can also hear and see how the clicking behind the dashboard happens in anti-sync with use of the turn signal.

I took it to a dealership and they could not reproduce the problem. Drove it around for over an hour myself and as luck would have it the problem did not reoccur either. Nevertheless the dealer performed a ETM potentiometer sweep test, which it passed just fine, and they couldn't find anything else wrong either. No error codes or anything.

I'm led to believe that this problem is related to the CEM or one of the relays connected to it, because of the electronic symptoms and also because as far as I can tell, the CEM is the only relevant component that is located on the left side of the dashboard.

I thought it might be overheating, so I cranked the floorboard heat full-blast for an hour while idling the car, and used a pillow to direct the hot airflow from the vent directly at the CEM. After this, I took it out for a 90min drive at interstate speeds, but this did not cause the problem to recur. I also briefly entertained problems with the MAF or the throttle sensor but the sweep test and the dashboard clicking seem to indicate that these are not the root cause.

Anyone seen anything like this, or have any brilliant insights on what the cause might be? I have to take a very long drive in it soon and I'm nervous about the problem coming back. Any help is very much appreciated!