Dear 85oflgirl,

Hope you're well. Few parts interchange between 850 and 9xx cars, chiefly because the former are front-wheel drive and the latter are rear-wheel drive.

The shaking likely is caused by fuel supply and/or electrical abnormalities, that disrupt the normal operating sequence of the engine. I'd look at:
(a) spark plugs, plug wires, and distributor
(b) fuel pressure (should be 43 psi [pounds/square inch]) and flow.
(c) the idle air control valve, mounted on a bracket, below and beside the intake manifold, may need cleaning
(d) the crank sensor could be on the way out: when the signal is lost, the fuel pumps shut-down.

A question: when the car shakes, does the tachometer show a drop in the number of revolutions/minute? Are there any trouble codes (see FAQs for how to read these codes).

The plug wires, if factory-original units, are well past the end of their working life. Time and heat degrade the rubber insulation. The same is true of the crank sensor.

If the fuel pump is failing, then one would expect these shaking episodes to become more frequent. Does the car have Rex-Regina or Bosch fuel management? The Rex-equipped cars have a square coil unit, on the driver's side strut tower (North American models). The Bosch-equipped models have a cylindrical coil mounted on the passenger-side strut tower brace. Rex-Regina models have a single in-tank fuel pump. Bosch models have an in-tank pre-pump and an externally-mounted main pump on the underside of the floor pan, just beneath the driver's seat. Fuel pumps usually last far more than 80,xxx miles.

Hope this helps.

Yours faithfully,

Spook

What weight oil are you using? Some recommend that "high mileage" (1) cars use a heavier weight, e.g. 15W-50. Until the engine is thoroughly heated up that 15W may not be flowing freely. My 960s are sensitive to that, so I run synthetic 5W-30 or 5W-40 in summer.

Another thing to check - at idle you might get an idea of the sound frequency by listening and watching the tachometer. One main bearing would knock at half the rpm. Count fast for 10 to 20 seconds, do a little multiplying. A main bearing knock is often deep and not too loud. Oil starvation might not be limited to just one bearing, you should be able to tell by the sound.

Also, the "right" filter is a Mann 917. I use that one on my 960's and 240's.

Good Luck,

Bob

:>)

(1) Usually mechanic types not familiar with Volvos.