Unexplained Oil Leak - Ford Mustang Forum
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post #1 of 3 (permalink) Old 3 Weeks Ago Thread Starter
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Unexplained Oil Leak

I am looking for some thoughts about the source of an oil leak on my 86 street/drag hatchback with a 351W. The engine is setup a little loose with ring gaps of .027", .018" and .030" top to bottom, since I race it. Two piece rear mail seal, FelPro one piece oil pan gasket and metal oil rails under the pan bolts. Cam has LSA of 110 degrees. Old school Holley carb and no computer. Rear gears are 4.10s.


Using 30W conventional oil for break in. Oil leak appeared towards the back of the engine, so I thought I might have spun the rear main seal loose. Here's what I found when I got it up on the lift:


Entire bottom of oil pan has a light coating of oil, including the front and back of the pan up to the pan seals. No oil on the sides of the pan, except a little near where the dipstick tube bolts to the side of the pan. All pan bolts are dry, except the last 7/16" bolt from the rear on the passenger side and the rear 1/2" bolt on the driver side which were each holding a drop (neither bolt moved when I tried to re-torque to spec). The bottom of the mechanical fuel pump is lightly coated with oil, but no oil anywhere on the oil filter. Engine block and heads are completely dry. The bottom of the K-member is oily from the center of the engine to the driver side control arm, but dry on the passenger side K-member and control arm. Heavier oil coating on the outside of the C4 inspection plate, the bottom of the engine separator-plate, bottom half of the bellhousing, and transmission pan. Light coating of oil downwind from there. Virtually no oil inside the bellhousing, and no oil on the engine side of the flexplate. PCV valve is full of oil.


Doesn't look like the rear main seal is faulty, since there's virtually no oil inside the bellhousing. Is this just excessive blow-by? Any thoughts as to the source of my leak?


Failure is only the opportunity to begin again more intelligently. H. Ford

1966 A code 4sp Fastback
1976 302 4sp Coupe
1986 351 C4 Hatchback
1990 GT T-5 Convertible
2008 GT Convertible
2011 F100 XLT
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post #2 of 3 (permalink) Old 3 Weeks Ago
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I'd clean up all the oil film and try another road test. It sounds like the oil is blowing back along the block so my guess would be to look at the timing cover seal/gasket. A leak there would be blown to the side by the fan movement and driving would blow it towards the rear.

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post #3 of 3 (permalink) Old 3 Weeks Ago Thread Starter
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Thanks for the tip, Travis. I looked at the timing cover and it is dry, except for the front of the ledge at the bottom of the cover where it meets the oil pan gasket (but not above that). I even used a Q-tip to swab around the front crank seal behind the vibration damper, and it was dry.


However, I may have found the source. I decided to check the torque on all the oil pan bolts, and found three that weren't 10 ft lbs -- the 2nd, 3rd and 4th small bolts from the front on the drivers side. The 3rd one moved a half turn. Doesn't seem like that would let it leak, but maybe.



Unfortunately, I can't test drive the car in the snow.


I anyone has any other things I should look at, let me know.

Failure is only the opportunity to begin again more intelligently. H. Ford

1966 A code 4sp Fastback
1976 302 4sp Coupe
1986 351 C4 Hatchback
1990 GT T-5 Convertible
2008 GT Convertible
2011 F100 XLT
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