I just purchased a 1997 850 sedan. When the car goes over rough road surface, I heard a knocking sound like something is hitting the bottom of the car. Where and what should I look for. On normal surface, the car runs quiet and smooth.

What should be the idle rpm? Mind is 950. Is it too high?

 

Check the front suspension. Could be endlinks or spring seats.
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+1.

The idle speed should be 850 plus/minus a fraction. 950 seems a bit too high, I would visit a localAutoZone or other parts shop that provides free OBD-II diagnostic service and see if there are any codes stored in the ECU. If not, it may be a matter of adjusting the throttle cable;the throttle valve could be slightly open. Or it could be the idle air valve. Moderator Tech could pinpoint the problem.

Tech (+anyone), would a vacuum leak cause high idle like this?


JPN

 

The idle should be about 850 RPM.

Does the noise sound like from the front or rear?
If the front check what was listed above.

If it sounds like the rear check the rear sway bar at times they will break as well.

 

I forgot about the rear sway bar. We had one of the clamps let go on ours a few weeks ago. Turns out on of the bolts had backed out. Weird....

 

First thank youvery much for all the replies.

My mechanic is a Ford expert. I am not sure if he is good at Volvo. He first found the Air Guidewas loose. He fixed it for me. There was less knocking noise. Next he found a plasticcover under the A arm is broken. He removed it as many cars have no such cover. Nowthe problemimproved but on bumps. I tried to stand next to the front wheel and cannot hear the knocking sound. It ismost noticedfrom inside the car. Really sounds like the air guide i****ting the car bottom. I will ask my ford mechanic to check the endlinks and spring seats on my next oil change which is about 400 miles to go. Should I change to synthetic oil?

It there any recommendation on a Volvo mechanic near Irvine, S California?

 

^Highly doubtful it's your endlinks or spring seats. If it were those two items, you'd describe the noise as clunking (if it's the spring seats). If it's the endlinks, it'll be more of a rattle. A easy, but not great test is to pop the hood and twist the rubber around the strut bolt. If it spins freely, most likely your spring seats are dead.

Most likely, the noise you are describing is not your spring seats/endlinks, but they go bad very commonly so it's very possible that you might find out they have gone bad in your inspections.

What you could do is get under the car, and look if you can find any scrapes and possibly locate your problem that way.

What is your mileage? And is your car a turbo?

 

My car is non-turbo with 98K. My mechanic already raised the car and inspected the bottom. He said it is not the shaft that he worry most. Hethinkthe caris safe to drive and not suggest to inspect teh suspension. May be it seems ok to him.The friend that sold me thecar is now in Hong Kong so I have no chance to ask for the history of the suspension. I know he replaced it about 1.5 years including the top part that can be seem from under the hood.

 

The spring seat wear cannot be inspected unless removed. But if you did not hear anything while stood next to it, it is probably fine.

Did the Ford tech inspect the bottom thoroughly? Were you there when he did the inspection? Ask him to see if you can see the bottom of the car while it's on a hoist. Try shaking the exhaust pipe to see if there is a looseness and hitting the peripheral structure.

If it's non-turbo, you wouldn'tgain much benefit by using synthetic oil. Name brands' 10W-30 would suffice. Make sure to use OEM filter (MANN) or other high-quality filters that have check valve in them.


JPN

 

My mechanic do checked the bottom and think the A arm cover is the culprit. To inspect the suspension needs more work. I want to contact my friend who sell me the car to get the history of the suspension and then make the decision to replace the suspension or not as the labor is about the same.