thank You dave, lots of good info there . I was not aware of the idea of blowing in through the oil filler cap to detect a leak.
Ive done the belts and front oil seals on this engine before.
Ill check if the flame trap which looks like a plastic plug full of 1 mm or so holes and is located wehre there is very little room for hands under the intake manifold. If I find that plugged with balck goop it might tell me the story.

yes Ill clean the pan area. I had a flame trap come loose on my 240 ( same engine) it looed like an oil pan leak.

I have a feeling that if it's the pan I'd see sort of a slow drip but maybe not such oil loss so Im fearing it's the rear main oil seal. I of course want to determine this is the case before I go pulling out the transmission.

I usually use 10W 30 regular oil not synthetic. maybe 20W 50 is ok since its a bit older.

I previously had issues with a stuck injector adding about a teacup full of raw gas into cyl 3 upon each shutdown. It took me while to find and solve, so maybe the oil had some gasoline that worked past the pistons and it may be polluted , that might be bad for the seals too.

Ive never tried the seal swell stuff , I'm not sure how much faith to put into that but is there type or brand that you think might help?


the rear main seal rides on the crankshaft. I thought I might be able to reposition it laterally, just slightly, if the crank is worn there where the seal makes contact.


I have another similar 740 that I bought for parts and drove home, it have very little maintenance and was pushing the dipstick out, and some oil, so I think that one has a serious case the plugged flame trap issue.

I assume if you remove the oil filler cap it should have suction.. would it then blow if the crankcase is going high in pressure? im trying to think if there is any airpath between the rocker cover area and the sump or if these are two different "containers"
Im trying to think how the apparent pressure in the "rocker box" can be used as an indication of engine wear or other issues.

the compression is 150 or so on all cyls..

It seemed much worse after the oil light popped on for a sec when cornering and I found it down to the line and added oil. maybe it would leak less if it's down 1/2 a quart?
If it's the pan gasket I think it's not full up to the mating line so in that case it would only leak there at least any considerable amount when the oil can slosh. I dont think a leaky pan gasket can cause a constant drip when not being run. Ill check all the bolts and the oil separator bolts and make sure the flame trap is cleared as a first step.. . I had a leak in my 240 and it was only a loose oil separator.. It looked like a pan gasket leak.

the top half of my engine and around the timing belt and distributor is clean.