Hi Ralph,

I am not entirely sure you are having a problem with old fashion backfire. Backfire is caused when the timing is advance too far back (i.e. the spark plug is set off when the exhaust value is opening, hence the explosion occurs in the exhaust manifold). The symptoms are poor acceleration and a backfire when you let your foot off of the gas. You are saying that you are getting backfire both when you are accelerating and de-accelerating and if I read you correctly when sitting at a traffic light.

Here is what I recommend:

When you take your car to the shop tell the Dealer's Service Rep that you are hearing a noise and describe the noise. DO NOT PRE-DIAGNOSE THE PROBLEM. Say nothing of the backfire. Don't even use the work backfire. Say things like bang, bump and so on. Let the technician diagnose the problem. Keep your description vague but let it be known you are having a problem. This will force the technician to look at everything not just the engine. Let them keep the car for a few days so that they can drive it around to see for themselves. If this does not solve your problem ask the dealer to set up an appointment with a Volvo after Market Sale Representative. He/She can help you get this problem fixed. Don't give up!

My feelings on what your problem can be are listed below:

1) Crank Shaft sensor (Volvo calls it an Engine RPM Sensor) not working correctly. I checked over the weekend on where and how this sensor is mounted. Volvo used a good tough designed on this component.

2) Possible wire/connector problem. Electrical issues are a pain to figure out!

3) Stuck open fuel injector causing too much gas being injected into one cylinder. Which can cause unburned gas making it to the exhaust system and hence detonating.

4) Timing belt not aligned. Jumped a tooth. This will cause the timing to be off.

5) Possible broken muffler/tail pipe hanger (made out of rubber). Causing the muffler/tail pipe to hit drive shaft/body.

6) Broken/missing drive train bolt/nut causing the drive train to hit the body.

7) Broken engine mount (upper only) causing the engine to hit the body (this has happen to me twice).

Let me know what happens.

Good Luck,

Steven---