Vibration in 1993 Volvo 940

Hi, I have a 1993 Volvo 940 that seems to have developed a curious
problem...for about the last three months, it has started to vibrate at
high speeds (60 MPH and over). This all the time now, no matter what
the road conditions are like. My first guess was just the car's
aerodynamics (it is kind of a big box), but if I press the towing mode
button, which disables third gear, the engine revs in about the
4000-5000 rpm range, but the shaking stops.

My local mechanic says it is most likely caused by worn front tires,
but they were replaced three weeks ago and I have noticed no
difference. I don't use premium (91 octane) fuel as the owner's manual
recommends, because of the prices I use regular 87 octane, but that
shouldn't cause the car to shake at high speeds, right?

The owner's manual also says that low power steering fluid can cause
vibrations...I haven't had a chance to check that, but at a free
diagnostic clinic the dealer told me that I should never have to add
power steering fluid and that it was most likely caused by a bad oxygen
sensor -- I know enough about cars to know that that really doesn't
sound like the cause.

Any ideas?

 

From experience of this fault I would immediately inspect the rear wheels
and tyres (tires). It can be as simple as a thrown balance weight and as
complex as a tyre (tire) with a bad carcass or even a buckled rim. Imbalance
over around 65 mph plus is down to rear wheels/tyres (tires)being badly
balanced, whereas imbalance at around 35 mph is usually the front
wheels/tyres (tires).

All the best, Peter.

700/900/90 Register Keeper,
Volvo Owners Club (UK).

 

Need more information: You use the term vibration at the beginning of your
post, then later characterize it as shaking.... if it were shaking does it
manifest itself in your butt, or in the steering wheel? tire balance,
separated belt in the tire.... bushing, tie rods.... alignment? and did the
new tires have any effect on this condition? If you ran the car up in rpms
momentarily while parked what happens?

 

Its probably closer to shaking...you can feel it throughout the entire
car, so it's not just in the steering wheel. The new tires (all four
new) did not change a thing, and they were balanced and realigned as
they should be. When I run the car up in rpms while in park, nothing
signifigant happens. The engine revs, but the car does not move or
shake from side to side as some others do, so it really has me
confused. Thanks for any help you can provide.

 

if you feel the shaking in the steering wheel the "rule of thumb" is to
suggest you focus on the front tires/front suspension.

if you feel the shaking in your butt (through the suspension to the car
frame to the car body to your car seat), then focus your attention on the
back tires/back suspension.

In order of simple-to-difficult and inexpensive-to-expensive:
1. You mention some new tires - don't rule out a bad or "out-of-round" tire
that can't be balanced, including new and old tires.
2. Make sure all tires are balanced (after completing step 1).
3. If problem persists, have front-end-alignment.
4. If problem persists, have steering/suspension specialist inspect and
repair/replace all worn steering joint/bushings, then all all suspension
joint/bushings, then all worn shock absorbers and strut cartridges.

That covers it, from soup to nuts.

 

Thank you all very much...I will do these things until the problem is
fixed. Look for a reply when it is!

 

You said a gear change seems to get rid of it. It could be a torque induced
vibration on the drive shaft. I noticed at certain revs on my over boosted
940 Turbo that the drive shaft would vibrate even though it is generally in
good condition, changing the boost to normal eliminated the problem.

So it could be that one of your drive shaft bearing or couplings is worn.

Another thing to try is putting the car in neutral or lifting off on the
accelerator, changing revs etc.