As thread title states, the car has started to stumble (momentary loss of power) repeatedly while cruising down the highway at 60 - 80 mph.

Some background on car and service. The car is bone stock and has 130,000 miles. I am the original owner having purchased the car in 2003. The clutch was replaced about two years and 20K miles ago. The gas tank and filler neck were replaced due to rust (too many CT winters) during the early summer of 2019 and 15K miles ago so the fuel pump was removed from the OEM tank and moved to the new tank I purchased from the local Mitsubishi dealer. The timing belt and drivetrain fluids / service was done about 10K miles ago. Spark plugs done this past summer and fuel injectors cleaned about 2K miles ago. I believe the injector cleaning process was done through additive in gas without removing the injectors. Prior to this work, the car would stumble under high revs when doing some mildly aggressive accelerations from 40-70 mph. After the new plug install and injector service, the high power stumble was gone (until today, now at cruise) and the car ran beautifully. I should also mention, I only run 93 octane.

Car is throwing no codes / no check engine light.

Any ideas of what to check or rule out?

Thanks in advance.

 

Might be a long shot, but do you have the original plastic BOV? (Also known as diverter valve) They are known to leak and cause drivability issues.

Also, does it happen in only 5th gear? Or 4th as well at the same speed? Another possibility could be a boost leak noticable under a high load situation like 5th gear highway speeds.

 

Yes. All original including BOV. I'll have to check if it stumbles in other gears. I was able to get it home going through all the gears once off the highway. My recollection was that this occurred only at higher MPHs on the highway, but I plan on doing some additional diagnostic driving today.

Thanks for the reply.

 

Do a boost leak test, inspect your spark plugs, and what do your long term fuel trims look like?

 

Fuel trims were good, no boost/vacuum leak. When inspecting plugs, one plug wire appeared to be partially disengaged.

Since this was an intermittent problem, I'm going to need to put some miles on to see if that was the issue. It is certainly plausible.

EDIT/UPDATE: Stumble is gone. Poor contact of spark plug wire was the issue.