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

What a nice car! I love the manual trannys, except when I don't push left enough to engage 1st and wind up in 3rd.

I would clean the throttle body and the IAC, probably never been done. Take apart the PCV valve, clean it out, and if there is a screen in it - throw that part away. Examine the small vacuum line at the PVC for blockage, replace if needed.

My 1996 needs the same thing done, as soon as the temperature cooperates. It has a tendancy to increase revs when I push in the clutch to change to a higher gear.

As to the exhaust note, yes, mine also does make more noise than the turbo when I give it a lot of gas. I attribute it to a muffler that wasn't hot enough to evaporate all of the moisture and it rusted out a little on the inside. No big deal, as the car is very quiet at any cruising speed.

The SAS valve you are refering to will produce an error code if it fails. The computer opens the valve to pump fresh air into the exhaust stream when the engine is cold in order to help heat up the CAT and burn more raw gasoline. The valve is usually only open for 2 minutes or so after engine start.

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My name is Klaus and I am a V♂lv♂holic

Hi Klaus,

I had not noticed previously but the effect you described is happening. If I accelerate in any gear, then push in the clutch and simultaneously release the throttle, the engine revs increase several hundred RPM in the next 1-2 seconds then drop back to idle RPM.

The 1998 engines have a throttle cable, I confirmed that nothing is mechanically sticking when the throttle is released. What causes the overrev since the throttle plate immediately closes when I release the throttle ?

Can you point me to a good writeup on removing and cleaning the throttle body and IAC ?

The setup looks similar to my 940. I have never had to clean the 940 throttle body - the car has always run well and has about 190K miles now.

Thanks,

Mike

I suspect that the throttle butterfly is not closing completely and the manifold vacuum isn't dropping fast enough. Suspect. I won't know until at least March when my garage temp gets warm enough to lift the hood and do some serious probing. The engine computer will compensate for the addditional air and adjust the fuel delivery, but that takes a few milliseconds.

If you remove the throttle body, there is a thin paper gasket that needs to be replaced. Just spray the TB with TB cleaner, using an old toothbrush for the stubborn buildup.

Because the IAC hose to the TB needs to be removed, clean the IAC with TB cleaner. Just spray it in, slosh it around, and pour out the dirty stuff. Once you hear a lot of metal-metal clacking while sloshing the TB cleaner around, it will be good to go.

Make sure that the hoses are in good shape, inspect the plastic vacuum tree, replace any nipple covers that are badly cracked. Note, the vacuum tree is old plastic an can get brittle with age. There is an O ring at the base, and if you pull out the tree, I suggest finding another O ring to make sure the seal is complete.
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My name is Klaus and I am a V♂lv♂holic