I have a 91 Golf, 1.8L 5 spd manual that I drive 50+ miles per day. I have owned it for 3 yrs and my mechanic keeps it well maintained. The idle acts up mostly during hot weather and calms down a bit during the winter months (Michigan). When I start the car after it sits for a few hours the idle goes up to 2500+/- rpm, down to 200, then to 2000, then down to 400 and so on. It smooths out after it does this 3 or 4 times. Then it runs a little rough and does not idle smoothly at stop lights. I have also noticed a drop in mpg over the last year - about 3 mpg. My mechanic just assures me she is getting old but I think there is still hope because otherwise, there are almost no issues. I bought it with 118K and now just turned 185K. He has done the smoke test on it for vacuum leaks and only found the one that operated my heater valve to direct heat to my feet.
The distributer, plugs, wires…, several coolant hoses and connecters are all new.
Any ideas? My husband and I are not willing to give up yet!..at least I am not.
First, have the fuel pressure checked.
If it’s fine, then check the ignition timing.
If that’s ok too, look at the air flow meter, oxygen sensor, coolant temp sensor and the throttle position switches (in that order).
Thanks - I will print this off for the next visit to the mechanic (he loves it when I bring him lists!)