My 88 240 (b230F) is at the mechanic however they're unable to find "the gremlin". I'm wondering if anybody has any brainwaves/thoughts. Details are...

Intermittent very loud clattering under power as revs build past 3000rpm, surging heavily. Sounds a lot like it has no oil, however the oil is plentiful and the oil filter is ok. The mechanic says the oil is fine.

Mechanic has had the car for a couple of days and says that it seem to run fine and then suddenly will begin clattering and blowing huge amounts of smoke for a while, and then return suddenly to normal again.

For about a month it has been surging faintly under power, mostly uphil in second as the time under revs is extended and it's more noticeable. I live in a very hilly part of town so a 300foot hill is an everyday feat. It has felt quite sluggish for about 1 month, as though I'm driving with the hand brake partly on.

Odo is 135,000kms (not miles). No fault indicator lights coming on. Spark plugs/leads/plugs/rotor button replaced last month. Car starts first time, idles well, sounds like it's brand new when running cold.

I'm wondering if it's an intermittent coil failure or if maybe the oil pump is worn? Or it it's time for a 1 way ticket to the wreckers...

 

Manual I presume? Maybe clutch/flywheel disintegration.
Nose the car against an immovable object, like a big tree, and push.
Listen for noise. Locate source.
Use gauge to test oil pressure.
Ed

 

Thanks Ed7 - the car is an auto.

It turns out the issue is timing related, in that each cylinder is firing normally and at the correct moment, but then at higher revs under accelleration it is also firing a second time, hence the loud clattering and lots of smoke.

It drives well when not being rushed along, so I'll just avoid steep hills and drive it gently now until it dies completely.

 

I'd look into harmonic balancer separation which may affect the timing like you describe.
A B230f with 135k km should have very few serious issues and a long life left...

 

Your explanation makes no sense in that the ignition system is unable to fire more than once per cycle.
lev reminded me of a timing problem that doesn't happen very often. Though harmonic balancers do come apart with age, it wouldn't affect the timing. The timing gear behind the balancer may have sheared it's key and shifted the timing. The key is integral with the gear.
Ed

 

my 745T started behaving like you said, it turned out it was a head gasket that was blown between cyl 3 and 4. It was bad enough clattering that I parked the car within a couple miles of it first happening and had it towed to the shop.