Vibration at Idle in 700 Series Volvos
If your Volvo has a strong vibration at idle, especially in gear, here's a few things to check.
Exhaust system: Remove the exhaust header pipe support bracket on the passenger side of the bellhousing and see if vibration still occurs. If the vibration goes away the bracket may be bent, or if the exhaust system has had replacement pieces installed, they may be out of alignment. If your car is missing the header pipe bracket, installing one may cure the problem.
Transmission alignment: Loosen the nut on the transmission rubber mount and, assuming the mount itself is not worn out, reposition the tailshaft of the transmission (using a short prybar). Moving the transmission slightly toward the driver’s side, and re-tightening may cure the problem.
Front crossmember: Check to see if the front suspension crossmember shows any sign of being bent. This can occur from the car being jacked up improperly. The crossmember can also be damaged by an off road “excursion." If bent, the rubber bushing below the oil pan may be contacting the engine. Removing the rubber bushing from the top of the crossmember may cure the problem.
Catalytic converter: Remove the catalytic converter and check for a loose catalyst bed by shaking the converter and listening for a "thunking” noise. A loose catalyst bed could cause an increase in back pressure and a exhaust resonance. Note – the flange gaskets will need to be replaced when reinstalling the converter.
Sagging motor mounts: Check to see if the motor mounts are fatigued and settling lower. Many 700 and 900 series models have hydraulic motor mounts which differ from the traditional all rubber mounts. With either style when they deteriorate extensively the engine oil pan will come in contact with the suspension crossmember and make a very strong vibration. If the mounts are only slightly fatigued they will still transmit more vibration to the passenger area than mounts in good condition.
To check whether it is the driver or passenger side mount, you need to simply note whether the vibration is worse in drive or reverse. This is due to the way the engine torques. A fatigued mount will measure less than 70mm or 2 3/4” from the engine bracket to the crossmember bracket.