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4.9 oil leak mystery

Old 03-12-2018, 08:33 AM
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Default 4.9 oil leak mystery

I got this 91 XLT 3 years ago at 65,000 original miles. I put another 30,000 miles on it. The light use over time created a sludge problem. Oil blew back through the breather into the air cleaner and dripped down the fender. Brown sludge made it hard to read the dipstick.

I finally decided to deal with it. I ran a couple quarts of sludge removal products through it an did two oil changes.

Now I have a massive oil leak, like a quart or two in 20 miles.

It seems to be coming form the back. but I can't see a source when the truck is idling.

Yesterday I changed the valve cover gasket. The leak is the same.

I checked the rear main seal via the transmission access cover.

I also ruled out the oil pressure sending unit.

What else is there beside the possibility I screwed up the gasket install?
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Old 03-12-2018, 01:19 PM
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What "products" did you use? Many will do serious damage to engine seals, and should never be used. Only a few are OK, when used according to their labeling.

All leaks appear at the rear of the engine because it tilts down at the back. Follow them forward & up to the source. It helps if the engine is CLEAN, but washing an engine with a severe oil leak can contaminate the oil inside the engine, if water goes in. That's one reason it's important to KEEP the engine clean.


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A common leak on this engine is the side cover gasket.


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Even with a borescope, I don't see how you could inspect the rear main seal with the trans installed. But it's not a likely source if the leak quantity increased suddenly. Have you thoroughly inspected the oil pan gasket?


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Old 03-12-2018, 02:50 PM
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Best to take the truck to a car wash and degrease the engine, then go looking. To lose that much oil, I would suspect a seal rather than a gasket. A bad gasket is not going to leak at that volume unless the bolts are very loose. An oil pan comes to mind.
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Old 03-12-2018, 03:01 PM
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I used a clear thin product specifically claiming to remove sludge. I'm thinking it was mostly kerosene.

I just bought the pushrod cover gasket, I will replace that tonight.

At the bottom of the bell housing there is a cover that exposes the torque converter. Am I correct in thinking a blown rear main seal will put oil over that area?
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Old 03-12-2018, 03:24 PM
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I wouldn't do a thing till I got the engine cleaned up outside, otherwise you could be chasing your tail. Yes the rear seal will leak down the bell housing, so will the rear of the pan. You are fortunate that you didn't waste your engine with the kerosene.
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Old 03-12-2018, 03:43 PM
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Originally Posted by raski View Post
I wouldn't do a thing till I got the engine cleaned up outside, otherwise you could be chasing your tail. Yes the rear seal will leak down the bell housing, so will the rear of the pan. You are fortunate that you didn't waste your engine with the kerosene.
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The product was a "sludge remover" sold by a quality LOCAL auto parts store (we still have those) I don't know what it was, it smelled pretty mild, closer to kerosene that any kind of harsh solvent.

So we've pretty much covered all the places it could leak, right?

- valve cover gasket
- rear seal
- oil pan
- oil pressure sending unit
- pushrod cover

Is that pretty much it?
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Old 03-12-2018, 05:07 PM
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...timing cover, block crack/pores, distributor... Anywhere that oil flows.


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And it doesn't have be from a gasket or seal...


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Old 03-15-2018, 09:18 AM
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Thanks Everyone.
I did the car wash thing. Pretty sure it's the pushrod cover. Rubber grommets were petrified, gasket was brittle in installed sloppily. I got the job about half done before I had to stop.
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Old 03-17-2018, 05:36 AM
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Did that solve the mystery?
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Old 03-18-2018, 11:00 AM
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Well crap. I got it all back together and fired it up and there was a violent knocking sound from the top of the engine. I shut it down and parked it.
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