QuestionQUESTION: Dick, I have a 66 Bonneville Station Wagon with a factory 421. When I bought it, oil would leak out the rear, intermittently, but mostly when the car was driven over 70 on the highway. The motor was rebuilt, but still had the same problem. The motor was removed again, rear main was replaced, crank was checked and everything seemed to check out fine. One suggestion was that I have too much crankcase pressure and to put a rocker arm cover with a breather on both sides. The car is beautiful and is embarrassing driving on the highway with a cloud of smoke billowing from the underside.
ANSWER: The thought about crankcase pressure was odd, because crankcase pressure comes from worn out rings (blowby), so if the rebuild was a good one, and the PCV valve is working correctly, there would have been a vacuum in the crankcase, not pressure.
The oil leak could be coming from anywhere, you have to find the source. These engines have plugs in the oil galleries which could have been left out if the rebuilder made a mistake, or there could be a bad oil pressure sender. Remember that an oil leak will always appear to be coming from the bottom rear of the engine, because the oil will drain down to the lowest point, so the leak could be under the carburetor (in the valley area around the lifters, or even from the front timing cover), but it would still drain back to the rear of the block (the engine is slanted in the car) and then down around the flywheel housing and appear to be coming from the rear of the engine.
The only way to track the leak down is to pressure wash the engine thoroughly, with detergent so as to get all the crud off it, then start the engine and look at all the likely places to find the highest, furthest forward location of any oil. The smoke behind the car is probably from the oil running down onto the exhaust manifold, on one side or the other, so take a careful look at the valve cover gasket - it the valve covers were overtightened, they will leak for sure.
Since the engine was recently rebuilt, you should take it back to where that was done and hold their feet to the fire until they find the leak - it is their fault it is leaking, unless something was done to the engine after they turned it over to you. The engines did not leak when the cars were new.
Dick
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QUESTION: Dick,
There is no crud on the motor and it is coming out and filling the transmission dust cover. I removed the dust cover so the excess oil is not slinging back up to the back of the motor and it doesn't look like Niagara Falls when I stop the car. The smoke is from the oil burning of the exhaust pipes. I now have an oil undercoated car (at least it won't rust). If I drive it easy, it's fine. It just comes out intermittently on the highway. The oil galley plugs are in place. The rebuilder and the machinist are scratching their heads right now as this has been going on for a while. This car was leaking like this before it was rebuilt.
AnswerI doubt very much that I know more than experienced mechanics, since I am only an amateur mechanic myself, but I have to ask - are you sure it is engine oil? The area behind the flexplate or flywheel (depending on which transmission you have) is only accessible to the oil from the transmission - so it would either be red (Dexron), or stinky (Standard Transmission grease) if it were getting in there from the transmission side. So, I assume from what you say that it is engine oil, slinging off the engine side of the flexplate. If this is the case, it sure sounds like the rear main bearing seal is leaking, or somehow oil is getting out in that area. Since it only happens when you drive it at higher RPM, there could be some crack or casting defect at the back of the block, above the normal oil level in the crankcase, so it doesn't leak unless the crank is churning up a fog of oil due to splashing and slinging oil all over the place in there.
I don't know if it is worth the effort, but if you want to, you could pull the engine again, and use the sealer that works on the outside of castings. NAPA sells it - it is a fairly new Permatex/Loctite product that comes in a blue container - I think it is sprayed on. If you have a casting defect that is seeping oil, this should stop it - just keep it off the mating surfaces to the transmission/bell housing so that you don't get it in the way of a tight fit from the engine to the transmission.
One more try on tracking it down - can you see any oil inside the housing on the ENGINE side of the flywheel/flexplate? How about on the block itself. Can you see up there well enough to spot the highest spot that is wet with oil? If so, that should be where it is coming through. If it seems to be wet all the way up to the top of the bellhousing/transmission housing, make very sure it isn't seeping through the joint between the two castings, the oil sender is right up in that area on some engines, it seeps out of the oil sender threads at high RPM and then runs down around the split between the two castings, or right through the joint between engine and transmission, showing up at the bottom looking like a rear main leak - Mopar engines are famous for this, but many also others have the same geography - I'm not sure about 421s - but if the oil sender is located at the back, top of the engine, I'd still take a VERY close look at it! Maybe even replace it - they're cheap - and be sure it's threads are well sealed with a very thin skin coat of "Ultra-Copper" RTV sealant - but don't get any sealant anywhere other than on the threads, and very little there!
Beyond that, I'm out of ideas. Let me know if you ever find it!
Dick
