When I’m driving, steady mph, my car feels as if I’ve just downshifted, or that my foot slipped off the gas pedal. When this happens I’ve noticed that the tachometer drops down from say 20 - 30 to 10 or less! But just momentarily. It happens to a warm or cold engine. It’s going to give me a whiplash! I’ve brought it to my VW dealer. They put it on the computer, and no problem comes up! No lights on my dashboard are on either. This problem is really annoying. I’d like to sell my car but don’t feel like I can sell it with this problem.
Any suggestions for my VW mechanic?
Could be a fuel pump intermittently beginning to fail. You won’t get a check engine light with this. The dealer won’t see it on their computer.
If the fuel pump was beginning to fail it could (not 100%) show up as a misfire or a circuit too lean code. again this is a possibility not 100%.
a general rule is problems at low RPM - ignition, higher RPM indicates fuel system.
A shop should be able to do a test drive with the computer hooked up to figure out the exact cause.
“…my car feels as if I’ve just downshifted, or that my foot slipped off the gas pedal. When this happens I’ve noticed that the tachometer drops down from say 20 - 30 to 10 or less!”
Just to get your terminology correct, it it were downshifting, the tachometer would go UP. It does sound like the engine is cutting out. It could be loss of fuel, as mentioned above, or loss of spark, caused by intermittent failure of a sensor (usually crankshaft position or camshaft position) or some other ignition component.