I have been diagnosing an intermittent backfire problem with a recently purchased 1998 V70 NA manual trans car. Sometimes a backfire occurs when shifting. Klaus asked if the engine RPM momentarily increases during the shift intervals when the clutch is disengaged and the throttle is released. The RPMs do increase, unlike any other manual trans vehicle I have ever owned.

The throttle body, IAC valve and flame trap hoses are clean. No vacuum leaks that I can find. The car starts, idles and drives smoothly. No OBD2-DTC codes.

I have isolated the shift interval momentary RPM increase to the IAC system. With the transmission in neutral, slightly pressing the throttle increases RPMs to 1000. A little more throttle and the RPMs go to 1200, then quickly surge to 2000-2200 RPM without additional throttle input.

Blocking the IAC inlet hose with a solid plug eliminates the off-idle surge RPM increase. With the IAC inlet blocked, the neutral/off-idle RPM can be held at any value from 1000 RPM to 2000 RPM with no surge. Also the momentary RPM increase during the shift intervals is eliminated.

So the IAC system is causing the problem, but why ? The IAC valve is clean and the internal vane rotates smoothly. The IAC valve control signals (checked with an oscilloscope) are correct. A replacement IAC valve behaves the same.

I finally realized that the ECU is commanding the IAC to open up at off-idle RPMs > 1200. This was confirmed with a digital voltmeter that averages the IAC Pulse Width Modulated "valve open" signal.

My OBD2 scan setup can display multiple real-time parameters such as RPM, absolute throttle position, O2 sensor voltage, ignition advance, etc. The reported throttle position is stable 10.5% at idle and about 85% wide open. In neutral, an 11.5%-12% throttle position increases engine speed to 1200 RPM and initiates the surge effect. I am beginning to wonder if the ECU firmware is incorrect. I havent checked to see if the ECU part number is correct for this manual trans car.

The other reference I have is my daughters 1998 XC70, but it has turbo and auto trans. It does not exhibit the neutral/off-idle surge effect described above.

I expected the IAC system to be inactive if the engine is off-idle, that is not the case with this car. I dont understand how the ECU can accurately determine an off-idle condition from the throttle position sensor. The TPS reports 10.5% with the throttle closed. In neutral a 12.5% throttle position is enough to produce 2000 RPM engine speed.

Any suggestions are appreciated. Next I plan to check the ECU version and perhaps try a new TPS.

Thanks,

Mike