My car sometime surges in 1st gear. I’ve had it look at and they found no vacuum leaks or anything wrong with the fuel pump. At my mechanic’s suggestion I’ve run fuel cleaner (fifth can now). Any Ideas?
A faulty thermostat that makes the engine run too cool can cause surging.
I would change the thermostat anyway if it’s over 10 y.o.
My top of the head guess would be a bad throttle position sensor.
It only happens when the car is in first gear, hot or cold, and flooring it fixes it. Could it still be the thermostat?
or the bad throttle position sensor?
Honestly you have some good options and again the throttle position sensor was only a guess. Cleaning the throttle body is the cheapest one and if it fixes the problem great. If not, you have a couple of other routes to try.
Another possibility is a small vacuum leak.
At 13 y.o. hoses and other rubber & plastic parts become suspect.
I have put a quarter-million miles on a 2004 325i and it sputtered at around 2800 RPM and surged when I started to let off the throttle, off and on, since it was new. I checked everything I could think of. Fresh oxygen sensors seemed to help, but perhaps that just helped the car to compensate for what the problem really was.
What finally stopped both problems was updating the programming in the ECU to the latest version available for this car. Again, this probably does not fix the problem but rather more effectively allows the car to compensate for whatever the problem really is. The car has run properly for the past 50k miles.