This is related to a previous post in February. My original problem was occasional backfire on 1-2 shift of a 1998 V70 manual trans, non turbo.

Klaus asked if the engine momentarily keeps reving when the clutch pedal is pushed and throttle is released. Indeed it does, instead of RPMs decreasing as I release the throttle, engine speed increases several hundred RPM for 1-2 seconds, then drops back as the next gear is engaged.

I checked the throttle cable and linkage, it is not sticking. Also removed and cleaned the throttle body and IAC valve. All the vacuum lines look OK, the flame trap and connecting hoses are not clogged. When the throttle pedal is released, the throttle body butterfly immediately closes all the way. So how can the engine speed keep increasing for 1-2 seconds ? The ECU must be momentarily commanding the injectors to keep delivering fuel even though the throttle body butterfly is closed ?

One other symptom: With the car at rest and trans in neutral, it is very difficult to find an accelerator pedal position that will hold the engine speed at 1500 RPM. Slight pressure on the accelerator pedal causes engine speed to increase from idle, but it usually overshoots to 2000-2500 RPM.

Could the throttle position sensor on the throttle body be partially defective ? Check engine light is off. Throttle response and engine smoothness are OK during normal highway driving.

Thanks,

Mike