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

Hola Mike,

Can you please provide more details of the OBD2 set up that you mention on this post. How about removing the computer and cleanning the terminals just in case of a bad contact point!!!

Speedy

I am using a Bluetooth interface device called OBDLink MX available at www.scantool.net. It comes with a free Android app and PC software. The Android app allows the user to define custom dashboard displays. I set up a screen to show both O2 sensor voltages, fuel trim, absolute throttle position, coolant temperature, fuel flow, mass air flow and engine RPM.

I see that scantool.net is having a $100 off sale on their original interface called OBDlink Bluetooth. It has Bluetooth and USB. At $100 off it costs less than OBDlink MX which is Bluetooth only. However OBDlink MX may work better with Android devices since it is a newer product than OBDlink Bluetooth. The scantool.net interfaces seem to be more reliable than the low cost Bluetooth OBD2 interfaces available on ebay.

I removed and re-inserted the ECU a couple of days ago. The connector looked very clean.

Regards,

Mike

I liked your post so well that I posted over in the UK where there are a lot more clutch users:

http://www.volvoforums.org.uk/showthread.php?t=177777

Seems that cleaning the TB, IAC, and throttle position switch helped others.

BTW, my ECM is also not original for whatever reason. Because it is a 1996, it is probably different than yours. Telltale sign was 'GOOD' written on the side! I have no service records regarding a swap.

Hi Klaus,

Thanks for posting on the UK forum, I will keep monitoring responses there.

My ECU definitely has the correct part number for the car. I havent tried another TPS yet. I dont think there are any vacuum leaks because the engine almost stalls (barely idles at about 400 RPM) with the IAC inlet hose plugged. The throttle plate rest position has just enough opening to allow the slow idle.

Possibly 1998 5-speed NA V70s with Motronic 4.4 are so rare that no one ever complained about over-rev during the shift interval. I didnt notice it until you asked in response to my first post about backfire causes. Pre-1998 850s with Motronic 4.3 ECUs may not have the same behavior.

Eventually I will arrange a way to monitor the IAC signal during driving. Then I can determine if my suspicion is correct - the IAC is commanded to full open when the engine is under load at 2500-3000 RPM. Full open on the IAC is probably enough to produce at least 3000 RPM on an unloaded engine. So if my shift point is below 3000 RPM, a momentarily full open IAC would cause the suddenly unloaded engine to reach 3000 RPM. It would be the same as not quickly releasing the accelerator pedal during the shift interval.

Monitoring the IAC signals with the car sitting still in neutral shows this:

1. With throttle closed (reported absolute position about 10.5%) the IAC system regulates correctly at 800 RPM with varying engine loads due to AC, heater blower, headlights, etc.

2. With throttle slightly open (engine speed no more than 1100 RPM) the ECU commands the IAC to be completely closed. This is correct operation because the IAC regulating system is still trying to maintain 800 RPM but it cant because the throttle plate is open.

3. With throttle plate open enough to give 1200 RPM the IAC is commanded to open and the engine speed goes to over 2000 RPM even though the throttle plate didnt open any more. The only way to reduce RPMs is to close the throttle plate slightly so that the ECU-IAC system goes back to the regulating mode described in 1. or 2. above.

Maybe the ECU is working as intended and the IAC behavior has something to do with emissions reduction. Or maybe there is an ECU software problem that wasnt serious enough to correct. There were few 5-speed 1998 NA cars and the IAC system was eliminated by ETM in 1999.

I did fix a small vacuum leak to the air cleaner box a few weeks ago and havent noticed backfiring lately.

Regards,

Mike

I tried another TPS and the IAC system behavior was no different. The original and replacement TPS both check OK with an ohmmeter and there are no trouble codes reported by the ECU. It seems that the ECU on my 1998 5-speed NA V70 is programmed to open the IAC valve when the throttle plate is more than slightly open.

If I can find an ECU from a 1998 NA auto trans V70 I will try it. The only other information I have is that the IAC system does not have strange off-idle behavior on a 1998 XC70 auto trans w/turbo car.

Mike