940, Non-turbo, 120,000km

Ocassionally, I hear a pop sound, especially on load. There is no specific pattern, and it does not happen all the time. It comes on whenever it feels like it (Average once a day). It pop just once and then there is a surge like the transmission engages a higher gear. The car runs fine except that idling is a bit erratic, wavering between 750-900 rpm. During such shift in rpm the car shudders a bit.

I've done the following things but the "pop" is still there:

Change plugs (Volvo), and cables (OEM)

Injectors checked by disconnecting the connector one by one and noticing engine shaking quite a bit when disconnected from the connector. I assume this means injectors are okay.

rpm sensor and wires changed

throttle and IAC cleaned. TPS tested okay by turning the throttle slightly and getting the click from the TPS

metal surface of the distributors cap sand-papered, distributor rotor also cleaned and sand-papered.

timing belt quite new with only 25000km

The only other part I suspect is the oxygen sensor. Petrol consumption is noticeably higher and I've done a voltage test on the sensor. Can't say I've got the 0.2-0.7 V reading but I'm not good with the multimeter anyway so I'll have to rule the results out. (I'm getting a new sensor in a few weeks from a friend who purchased it cheaper overseas)

Can the O2 sensor cause backfiring?

Also the tailpipe is of a larger diameter than the original because the mufflers have been changed (from Volvo original to Walker). Other joints fits exactly as before. I don't know whether the backfiring started before or after I've changed the mufflers (center & rear) but I only noticed it some weeks after the mufflers have been changed. I read before that turbo mufflers can cause back pressure. Is it possible that the new mufflers are meant for the turbos? Can the turbos mufflers fit to a non-turbo without modifications? (Just a guess)

What else causes backfiring? Please help. I do not want to end up with something serious and expensive to deal with.

One more thing, petrol filter is 20000km old. Is this a possible cause for the erratic idling or backfiring?

Thanks once more!

Laozong

Laozong: here are a few questions and suggestions (even if they do not cure the problem they will not hurt, and MIGHT help): What grade/octane gasoline are you using (you might want to try the next-higher grade); even though you cleaned the throttle body, as I recall once you remove the TB, there is a small hole, where the throttle was attached, that needs to be cleasned out as well; also, some have mentioned that some TB cleaning materials are not as good/clean as they should be, maybe even leaving small amounts of meatal behind: perhaps a thorough re-cleaning is in order; have you checked the oxygen sensors? Air Mass Meter (IF you unplug the AMM, do it only with the car turned OFF, then run the car without the AMM plugged, see what happens: if you have a harmonic balancer (2-part pulley, with rubber vibration dampener between the uinner and outer pulleys, the rubber might be starting to deteriorate, allowing the pulleys to spin at different speeds, which would affect timing/combustion: paint a straight white mark, from the outer pulley to the inner one, drive your car normally, then look at the mark to see if it is still a straight line: if the mark is now 2 short unaligned marks, that is a possible cause of your problem; Haave you pulled each spark plug out, checked for oil, excessive carbon, etc? Have you thoroughly checked and cleaned the PCV/flame trap system, including the little nipple/connector on the larger PCV hose? Your muffler MIGHT be the problem, but so far I believe that following Chris' suggestions, and mine, might lead you to the answer. Happy New Year (Gung Hay Fat Choy?)--PD.

Dear Pdwyer,

Thanks for the detailed steps I should look into.

I will follow them to the letter and see if there is any improvement. I certainly hope to find what I'm looking for.

Thanks once more and have a Happy and Merry New Year!

Laozong

The fuel pressure regulator could cause that kind of thing to happen. You'd get worse mileage, and with the larger diameter pipes, you can hear a little bit of a backfire, moreso than you would with the normal equipment.

Check and see if you are leaking fuel into the vacuum hose at the fuel pressure regulator. That's one cause for what you've described.

--
Chris Herbst, near Chicago, IL. 93 940, 91 240, 90 240, 88 740, 87 240