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Front and rear main seals leaking on rebuilt engine

The engine runs beautifully, but we cannot stop an oil leak. The Machine Shop replaced the front and rear main crank seal three
times and it is still leaking. We took the truck to Ford who put the dye in and checked for the leak with a black light.
Their analysis was that they would START with the oil pan gasket. We then took that info to the Machine Shop who said
they could see the oil leaking from the rear main and front seal again and replaced the seals in it again. THERE IS STILL OIL LEAKING
FROM THE ENGINE. From the same place.

No one seems to know how to fix this problem. Can you please provide some ideas on why we can’t seem to stop the leak,
and ultimately how to fix the problem?


Please tell us the model year

Please tell us which engine you have

I have a solution in mind, but I have to know what you have first

How much is leaking? Does it drip? Does it pour? And yes, what year make and model is it? It is very difficult to confirm a leak on the rear main of most engines.

It might be worthwhile to remove the oil fill cap while the engine is running and fully warmed up to see if pressure is building in the crankcase.

Engine rebuilt and leaks oil out the main seals?

One thing that can cause this is if the ring gaps weren’t staggered enough when the rings were installed on the pistons. If the ring gaps are too close to each other, the combustion gasses easily enter the crankcase building too much pressure that PCV system can’t handle it.


Good comments above. Here’s some other thoughts.

Did they remove the transmission to replace the rear main seal?

Are you certain the shop is using the correct part number for the seals?

A non-functional PCV valve or incorrect routing of the PCV tubing (both the PCV part and the fresh air inlet part) can cause this problem.

Sometimes the area where the seal contacts the crankshaft wears a groove in the crank so that even a new seal won’t seal. Is it possible during the rebuild that the surfaces where the seals go were machined along with the crankshaft?

I had a Ford 302 where it made a groove in the crank…I forgot what they did but we are talking early 70’s here and I am now retired and was in my early 20’s then…The engine shop may have put a sleeve in…but not sure…but they stopped the leak. Getting to old to remember everything. the older you get the faster the days go by…LOL

I knew a guy that put a fresh reman. engine in his truck and had a similar problem of constantly leaking rear main seal. Totally frustrated, he pulled the engine back out and had it looked over. Turns out, the rear main seal bore was off center to the crankshaft centerline. The reman company had messed up when line boring the main caps, and cut them off center. They had to give him another engine.

the engine is a 4.0 sohc with 150,000 miles the timing chain tension broke was reason for rebuild.

After about 400 miles the leaking oil reaches the exhaust system and stinks when you step out of the truck and could be a fire starter.

The work was done by a local shop, old school style not a reman.

The seals they used 2nd and 3rd time were Ford parts

I will check the pvc system this week to see if its building up pressure.

Would the seals stop leaking if the pressure drops to lower level or would new seal need to be installed ?

It’s a 2002

There’s still a lot of unknowns about the process that was used but with the mileage I tend to agree with keith about oil seal wear grooves on the crankshaft. If that’s the case, it’s something that should be remedied during the overhaul.

What I don’t get is why a shop would repeat this 4 times without pausing to take a deep breath and wonder why as this is all very labor intensive.

There’s also a technical bulletin from Jasper Engines about 2002-2004 Ford 4.0 engines leaking oil from an oil filter adapter flange and being mistaken for a rear main seal leak.
Replacing that adapter gasket should be part of an engine overhaul if the assumption, right or wrong, is made that the shop just changed the oil filter only.

At some point a shop would find it more profitable to throw in the towel on that engine and install a factory rebuilt and be done with it. The time wasted chasing this problem can be more costly than the price of good crate engine.

I could not see what vehicle the engine is in. My work truck is an 03 ford, not sure what engine, there was a tsb for head gasket as millings could cause the head gasket to leak and drip oil on the exhaust manifold. Very stinky. No recall, so we put in a plate to allow the oil to drip somewheres else. Leaking oil can blow to a lot of different spots, hope this helps, if not please IGOR this post.


I can do something about hump!!

Possible worn main bearings; causes play in crankshaft-replacing seals won’t
help much.