Just bought a 1998 non-turbo V70 with manual trans, 114K miles. A backfire noise (distinct pop sound from under the car) occurs occasionally when shifting. It happens randomly perhaps 1 out of 20 times on the 1-2 or 2-3 shift. I have not found any way to force the backfire to happen.

The dealer I bought the car from was unable to duplicate the problem but is willing to fix it if we can figure out what is wrong.

Except for the backfire the engine idles and runs smoothly.

So far I have inspected all the vacuum hoses I could see and didnt find any leaks.

There is a metal valve mounted above the exhaust manifold. It appears to inject air into the exhaust. A purple vacuum line runs from that valve to an electrically controlled valve mounted near the top of the radiator. I wonder if part of that system could be malfunctioning and causing air to be injected when it shouldnt be.

Another possibility could be a small exhaust system leak. The exhaust system appears to be all original. There is a noticeable growl when accelerating that is louder than the exhaust note on my daughters 1998 XC70 (however her car has a turbo). The dealer checked the exhaust and didnt find any leaks but still I wonder. Could a small leak ahead of the cat cause occasional backfire ?

Thanks,

Mike