Excessive crankcase pressure
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Joined: Apr 2016
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From: Southeast MO
Excessive crankcase pressure
So I bought a nice 71 f100 with 70K miles on it and after working some bugs out of it it run great. The problem is it blows oil out of every orifice. So I checked (and replaced) the PCV valve and vacuum line.
*Vacuum gauge shows healthy engine
*Compression test (130-138 with 1 cylinder at 122) not ideal
*Leakdown test (with homemade tester) 7-10% shows healthy engine
*With air applied and piston at the bottom of compression stroke, if I rotate the engine so the valves are closed you can hear some air at the valve cover. When I rotate the engine so the exhaust valve opens it sounds like a hurricane at the valve cover.
*I start it in my shop every morning and it does not smoke!
Is it a compression problem or some kind of exhaust system problem?
Any input would be appreciated.
*Vacuum gauge shows healthy engine
*Compression test (130-138 with 1 cylinder at 122) not ideal
*Leakdown test (with homemade tester) 7-10% shows healthy engine
*With air applied and piston at the bottom of compression stroke, if I rotate the engine so the valves are closed you can hear some air at the valve cover. When I rotate the engine so the exhaust valve opens it sounds like a hurricane at the valve cover.
*I start it in my shop every morning and it does not smoke!
Is it a compression problem or some kind of exhaust system problem?
Any input would be appreciated.
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Joined: Jul 2002
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From: Long Island USA
Is there a breather on the valve cover opposite the PCV valve?
It could be a clogged exhaust... but if there's a breather on the valve cover, and it was building too much crankcase pressure, it would come out the breather and not blow oil out ... where is it coming out?
Is that a 360? If so, that compression sounds about right in general
It could be a clogged exhaust... but if there's a breather on the valve cover, and it was building too much crankcase pressure, it would come out the breather and not blow oil out ... where is it coming out?
Is that a 360? If so, that compression sounds about right in general
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From: Southeast MO
Thanks for the reply Filthy Beast.
My vacuum gauge is steady at idle (19.5") and at 2000 rpm so I'm thinkin my valves are ok. The oil galleys appear to be clean and open.
I'm thinkin either my oil control rings are sealing and my compression rings are bypassing enough to pressurize it or I have a leak from my exhaust system into my crankcase
My vacuum gauge is steady at idle (19.5") and at 2000 rpm so I'm thinkin my valves are ok. The oil galleys appear to be clean and open.
I'm thinkin either my oil control rings are sealing and my compression rings are bypassing enough to pressurize it or I have a leak from my exhaust system into my crankcase
Cargo Master
Joined: Jan 2016
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For fun you could get the engine up to operating temp, plug the holes in the valve covers, and then rev it to 2500. If the oil dipstick jumps up, you have a lot of blow by. I haven't seen it but have heard of it happening.
Your compression numbers could be artificially boosted by oil getting on the piston tops.
Your compression numbers could be artificially boosted by oil getting on the piston tops.
Logistics Pro
Joined: Mar 2005
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From: tujunga, calif
I ran into a similar situation on my original 352, the crank case pressure kept getting higher and higher as time went by and started leaking oil everywhere. There was no smoke from the exhaust and compression in every cylinder was perfect. We pulled the engine and went ahead with our plans to make it a 390. We couldn't find anything wrong when we tore it down. When we assembled the 390 my son was cleaning up the intake manifold and pulled the shield on the bottom side that covers the exhaust cross over port we found a big hole in it.. So the exhaust pressure was entering the crank case from there. This was an aluminum manifold, don't know if the cast iron ones ever did this? Just a thought....
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From: Southeast MO
Thank you all for your replies. I think hiball3985 may be on to something. I suspected that the intake manifold gasket had blown at the exhaust crossover so I pulled the intake and replaced the gasket. It almost immediately started leaking at the back rail, valve covers, breather tube. I didn't pull the shield off the bottom (wasn't sure how to put it back on)
Is it possible to block those exhaust crossover ports off? Would that be a good idea? Or am I looking at replacing the intake
Is it possible to block those exhaust crossover ports off? Would that be a good idea? Or am I looking at replacing the intake
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From: Southeast MO
Thanks Krewat. Yes it is a 360. According to my vac gauge the exhaust is not blocked. If I connect my breather tube to to the air cleaner it will fill it up with oil. I have it disconnected with a rag tied over it to catch the oil. It also leaks at the back of the intake, and the valve covers.
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Logistics Pro
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From: tujunga, calif
I'm pretty sure they make a gasket to block it off. From past experience on a few other engines that they were used on they must be for short term racing as they were burned through on the two street engines I tore down.. Maybe some one else has had better luck.. Some people just make a small metal piece to put in there. Maybe you could fill the exhaust port in the intake with some kind of epoxy?
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From: Long Island USA
Never seen a gasket that blocks the exhaust crossover, but I haven't really touched an FE in 20 years to be honest.
I used pieces of sheet metal, and high-temp RTV to seal them in place, but it was a brand-new manifold so if it didn't hold, it wouldn't hurt anything anyway.
Epoxy might be the best route. Or if it is indeed the intake, just replace it.
I used pieces of sheet metal, and high-temp RTV to seal them in place, but it was a brand-new manifold so if it didn't hold, it wouldn't hurt anything anyway.
Epoxy might be the best route. Or if it is indeed the intake, just replace it.
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From: Southeast MO
Once again thanks for the input. I picked up new gaskets and talked to the Napa parts guy about it. He was not able to find gaskets with blockouts installed in them. He said to just put sheet metal inserts in. I'm going to try that and replace to gaskets later this week and I'll keep you posted
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From: Southeast MO
I pulled the intake and heat shield and found a fairly large hole in the bottom. I cut some block outs from the old gasket and sealed them with rtv. After letting the rtv cure over night I drove it about 30 miles. It still has some drips that could be residual oil or the rear main. I'll try some Lucas products.
Thanks for all the help. This is the best forum
Thanks for all the help. This is the best forum
1st Gear
Joined: Aug 2018
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The hole was about half that size before I tapped it with a hammer.
I just came across this forum and glad I did. I also have a 1971 F100 that had started getting more and more crankcase pressure not long after I had done a total rebuild, oil coming out of dipstick hole and also going into the air filter via the breather. I was really scratching my head with this one, compression being even and all, I was seriously thinking I would have to strip the engine down again. I had used the original cast intake manifold after taking to my machine shop and having them acid washed prior to painting, because of that and the fact the metal plate covers the crossover there was no indication of damage. After reading hiball3985's comments I removed the manifold and noticed black soot charring on part of the crossover cover plate, I removed the two rivets hold the plate on by tapping anti clockwise with a sharp chisel just enough to get a vise grip on them and they came out easily. And sure enough a hole burnt through the crossover. So it appears this is somewhat of a common trait with the FE's. So just putting my 10c in and confirming hiball3985's diagnosis to be on the ball.