740 Knock

[Bryan vz]

My 1990 740T with 250K mi has a rough idle that smooths out at 1500 rpm (in
"park" in the driveway). When under load (whenever it's in gear) it has a
BAD knock that renders it undrivable. Any experience with this?

Bryan vz
Kentucky USA

 

[Half a Bee]

Using good fuel, right ? OK, so maybe it's the knock sensor gone kaput.
The car periodically tests just how efficient it can be by
advancing the spark timing.

How you car thinks:
Can I detect knock ? No ? OK, then advance the timing.
Can I detect knock ? Yes? OK, retard timing.

If the sensors are not working (or some part of the advance mechanism is
stuck) then knocking is guaranteed. Smooth no-load operation at 1500
rpm is just a symptom of the flywheel rotating fast enough that you
don't notice the knock.

Time to visit your friendly Volvo dealer.

Chris