Okay, I think this is a new one. Today on a long highway drive I noticed something strange with the oil pressure gauge (74 450slc). Usually pegged at 3 bars at highway speed, I was amazed to see it was reading between 2 and 3 bars, depending on throttle position. Goes down to 2 bars when floored, and back to 3 when the throttle is lifted completely. Temperature, oil level, are ok. Oil pressure reads about 1 bar at warm idle. Car is running very well otherwise, hope I'm not doing something destructive driving it with these readings.

Why would oil pressure ever drop while accelerating and rise again when coasting (vacuum related)? Anybody seen this one before?

That's a mechanical gauge, so I expect the tube is either crimped by engine movement or cracked, so that when you accelerate it leaks.

Check the tube from the firewall to the fitting on the engine (rear of cylinder head, I think). The braided cover tube to the right head is the coolant temp line.

There is no way acceleration will lower engine oil pressure....

I'd think the engine and transmission mounts probably need to be replaced as well.

Peter

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1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!

Motor mounts are only a few months old, actually. Did those together with the subframe mounts, that was fun.

Didn't seem plausible that oil pressure would drop under acceleration, I agree. I don't know where the oil tube connection to the motor is, but I'll look at the back of both heads. I have seen the temp gauge sensor on the passenger side head, is the oil tube usually on the other side?

I thought that perhaps the pressure drop might actually be vacuum-related. Could crankcase ventilation drop so much that the crankcase pressure rises and somehow affects oil flow in the engine?

thanks, Peter.

1) Have you changed the oil recently? If so what brand of filter?

2)Any unusual top end engine noise? You may want to remove valve covers, adjust valves if apllicable, and ensure the oiler tubes are in place over the camshafts.

I believe if the oil line was comprimised at or near the cylinder head you would notice a rather large puddle of oil accumulating under the car. As well the oil would find it's way to the exhaust system which I believe would create some noticeable smoking at stop signs.

Just my .02