Intermittent Excessive Engine Vibration at Idle
I’m looking for some input on an intermittent vibration ONLY when idling.
1987 Porsche 924S
At idle, the car will run perfectly smooth, and then randomly transition into a heavy vibration for about 3 seconds before settling back to a smooth idle again. The engine note changes pitch slightly during these vibration cycles.
I found by watching the video you can best see the vibration by looking at the windshield washer fluid in the tank
At the 16 seconds mark you can see the washer fluid in the reservoir visibly sloshing around due to the vibration. Then at the 19 second mark the vibration stops and the engine returns to its normal idle.
The vibration is noticeably worse when the AC is turned on at idle, though the AC was off during the video.
It also happens regardless of engine temp; it’s slightly more pronounced when cold, but it persists even once the engine is fully warmed up.
I’m curious if this is just a quirk of these cars or if I should be digging into something specific. Has anyone else experienced this pattern? Any ideas on what might be causing that short, stumble or vibration?