Oil leak while running

[rmv42158]

My 1991 Wagoneer I just bought leaks oil out the rear main seal while running it not while sitting. Compression is 140-100 in random cylinders. Local machine shop says this motor is notarious for pushing oil out seal when compression is low. Anyone heard his before? $2300 to rebuild the motor which seems pretty good and may just do it for peace of mind but not something I was planning on while buying it. It

[joe]

Granted a 40psi variation is not a good sign for overall eng health. Pretty much indicates a tired motor needing attention soon, but... hows it perform for your current needs? If it's running good enough for your needs/expectations but just the RMS is puking oil. Replace the RMS and see where you're at for leaks. AMC motors are famous for laking RMS. On the plus side it's an AMC 360 motor(NOT CHRYCO) with a two part RMS so no need to pull the trans etc. It won't help your tired comp readings but should stop the leak for now. Spend the $20 for the seal kit, get lotsa shop rags, hand cleaner etc and RTFM on replacing the seal before committing to a $2,000+ shop repair.

[ZackN920]

"Notorious for pushing oil out the rear main seal if compression is low"? WTF? Can't say I've heard that before. If it has major blow-by, it should be pushing the blow-by through the oil breather/oil cap. and It would be putting (little) pressure on all seals, not just the rear main. Check or just replace the pcv too.

How many miles on it?

[FSJunkie]

Most leak out the rear main, even after being rebuilt. Rarely do they not leak at all. The question is how much leak is too much.

I generally don't like adding more than one quart of oil every 3000 miles.

[gophman]

Mine is doing my tits in! 3 main seals and still drips every 3-5 seconds when hot idling, you can drive 300 miles in a day and I'll level stays on max, drive a week or two short journeys and it will need a top up... Next time I change I'll pull the motor just in case it's a lifter gallery plug etc.

[letank]

"Notorious for pushing oil out the rear main seal if compression is low"? WTF? Can't say I've heard that before. If it has major blow-by, it should be pushing the blow-by through the oil breather/oil cap. and It would be putting (little) pressure on all seals, not just the rear main. Check or just replace the pcv too.

How many miles on it?

X2, WTF... check the inside of your air cleaner enclosure for oil , a good indication of blow by.


check the crank for fore and aft motion a sign of damaged thrust bearing that can be fixed.... but the engine has to come out... so a rebuilt is not a bad idea...



all over compression can be many things, like dirty engine oil, bad compression can be increased with a quart of ATF in the engine oil... It turned my 60psi cylinder to 110psi.... but the 2 cam lobes were chewed up



$2300, installed is a good price, make sure that you have all the details done on the rebuilt, your core, or one of the known vendors... camshaft (and which one) and cam bearings, crank turned, valve guides.... come to mind... what type of warranty...


EDIT: who did the compression measurements? the machine shop that needs to make boats payment or yourself?

[babywag]

EDIT: who did the compression measurements? the machine shop that needs to make boats payment or yourself?

Yeah...
Also total BS on pushing oil out RMS due to low compression.

I'd do some good research on this shop before giving them your business.

[FSJunkie]

Poor piston ring seal will push more blow-by into the crankcase, but the crankcase should not build pressure unless the filter gauze in the oil fill cap is clogged up.

Getting the rear main seal to seal completely on AMC engines is hard because you cannot offset the seal halves from the parting line of the main cap. The seal has tabs that prevent that. It is just about impossible to get a total 100% drip free seal without removing the crankshaft. The only way I've been able to get them 100% drip free is by applying Permatex #2 to the outside lips of the seal halves, to the faces of the seal halves where they meet each other, to the top and bottom of the rubber seal tabs, and along the edge of the main cap.

You have to watch the oil pan gasket. A leaking oil pan gasket looks a lot like a leaking rear main seal. The oil pan gasket is actually four pieces and they tend to leak where the pieces meet together unless sealant is used at those points. I use good old Permatex #2 where the gasket ends meet each other and paste the gaskets to the block with wheatherstrip adhesive so they don't shift around as the pan is installed.

Tricks I learned by working at the shop that builds more high performance AMC engines than anywhere else. Their engines do not leak...at all...from anywhere.

[Carnuck]

Clogged PCV and the gauze on the other inlet hose being a blockage instead of a crankcase air filter causes pressure buildup that forces the seals so tight that they burn up or shrink away from the mating surfaces so oil can get past. The pressure seeks the weak link to escape.
Someone was putting a vacuum pump on the valve cover breather to draw the pressure down to stop the leaks, like a vacuum cleaner sucking the oil back in.

[rang-a-stang]

Sometimes the rear of the intake manifold or valve cover gaskets will leak, dribble down the block and look a lot like a RMS leak, too.

[babywag]

a real rms seal leak will pee/leak visibly while engine is idling.
vc or intake usually will take a bit to show and back of heads/block/tranny will be gooey.

best way is still UV dye to pinpoint true oil leakers.

degrease & pressure wash it, toss in some dye.
the glow will lead you home.

[ZackN920]

a real rms seal leak will pee/leak visibly while engine is idling.

Holy BLEEEP!? Really?! That much?! Heck, I thought at worst they'd be an "occasional drip" type of leak.

[Carnuck]

I had the oil pressure sending unit on my Mexican 4.5L where it didn

[babywag]

Holy BLEEEP!? Really?! That much?! Heck, I thought at worst they'd be an "occasional drip" type of leak.

Yes and they'll be an oil slick coating the bottom of truck.
I've owned a lot of these things...
Only time it was really a rms they were wet with oil all over underneath from pan to rear bumper.

Of course they could leak less, but a bad one is very obvious.

[Locked and Loaded]

Check your PCV system. Should have about 4 psi of vacuum.