On Monday I wrote about what seemed to me to be unusual knocking after I had my water pump and belts (except timing) changed by my local Volvo dealer. I did not have time to attempt it myself. Anyway. My first drive after the job I noticed what seemed to be excessive knocking. I though perhaps the timing was screwed up or something. I filled my tank up with 93 octane and added the 3/4 quart that was needed to fill the crankcase. The knocking subsided a little but was still there. Responses to my Monday post suggested piston slap and such and that is still possible. Just after my post I did about a 250 mile trip. The car (89 740T)ran fine. Tonight I am sitting in my hotel room after a 110 mile trip. The strangest thing happened on the way here. At about 80 mph which was the average speed I was traveling, I began to hear the knock again. The car seemed to labor and I was suddenly having trouble maintaining 70. I exited and as I reached the end of the ramp, the knocking seemed to stop. So I did a U turn and got back on the interstate and completed my trip without any further problems. WHAT IS GOING ON??? Knock Sensor? Timing? I am totally baffeled! Any sugestions appreciated!

the car having trouble maintaining 70 was probably the result of your knock sensor retarding your timing and causing your car to lose power.

at least that's what i think now. so it's probably not piston slap or anything like that. hmmm...
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Kenric Tam
1990 Volvo 740 base sedan (B230F)
My Volvo 'Project'

So do you think my knock sensor is working? It was so strange that it started knocking as I was accelerating up hill at about 80 and dropped me down to 70. Than as I decelerated on the exit ramp everything seemed to return to normal. It does make sense that the sensor would have that effect. I did call the dealer. They don't think their replacing the three belts and the H2O pump would have affected the timing at all...I am not 100% sure I agree since this just started after that job. I am stll thinking that the sensor might be bad. I suppose it could also be my gas. I have always run 87 octane in her with no problems. My 300 mile trip early in the week on 93 did seem uneventful and I refilled with 87 for my trip yesterday (200 miles). I may try the 93 again tomorrow. The Volvo shop said to bring it in on Monday so they could see the problem. I am still hoping that someone in here has encountered this strange one though and found a fix!

well, if you actually felt knocking and then lost power from it, that would be your knock sensor doing its job of retarding your timing, which would seem to indicate that your KS is working fine.

a faulty knock sensor doesn't cause knocking. either it doesn't detect your knocking at all, or it always detects knocking and retards your timing. i don't believe in any case would a knock sensor be responsible for being the cause of the knock. test your KS with a multimeter capable of 5 MOhms of resistance and see what happens.
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Kenric Tam
1990 Volvo 740 base sedan (B230F)
My Volvo 'Project'

It does sound as though timing is off. Check it with a timing light. I can't imagine the dealer messed with the timing belt, although it is possible the belt slipped a tooth or something.

I thought the same thing about the dealer but this sure is strange timing (no pun intended!) I was thinking I was crazy! Thanks for the post.

If the knock sensor replacement doesn't cure the problem. Try replacing the power stage. I had replaced the power stage in my 745t because of a no start problem. The car started fine, after replacement, but continued to run bad. Replaced almost everthing. Finally took it to a Volvo tech. He found that the new power stage was bad. He put in a another new one, and the problem was solved. He said he had never seen a power stage cause the problems I was having. Usually they either work or don't. But after my experience, it appears that sometimes a bad power stage will cause weird timing problems.
Brad