390 Crankcase overpressure
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390 Crankcase overpressure
Recently rebuilt 360, upsized to 390 with crank. Has excessive pressure in crankcase. Heavy blow by through valve covers. Checked compression, have 95psi +/- in all eight cylinders. Not nearly the power it should have. Terrible gas mileage. Stock exhaust manifolds, Holley 750. Ignition timed at 8 degrees. New distributor with pertronics. I've about decided the cam is not properly timed. Intake staying open too deep into the compression stroke. Also considering headers to reduce back pressure. Otherwise, runs fine. Any suggestions would be appreciated.
Fleet Mechanic
Joined: Oct 2004
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From: clinton,tn
i hate to tell you this, my 390 has 120 lbs in all eight . sounds like you are getting blowby pressure passed your rings.i use 2 pvc valves hooked up together in each valve cover, works and runs very strong.and 1 more thing,welcome to FTE .
Last edited by 1949f1; Dec 5, 2005 at 03:41 AM.
Fleet Owner
Joined: Jun 1999
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From: Creedmoor, North Carolina
72Fonco Welcome to FTE 
The best there ever was.
How many miles on the engine now. I don't know what kind of rings you used but they don't sound like they have seated. Do you have a PCV valve in and hooked up? Upgrading the rotating assembly shouldn't be a problem.
Did you build it yourself? Have someone with building experence talk with you about the steps taken. The 750 is part of the bad gas milage. It is too big for that engine and is dumping gas out on the road through the tail pipes. Go look at how sootie they are. Header are best for engines turning 5,000 rpms and greater, if you have a good flowing dual system it should be ok for a daily driver.
good luck
John
The best there ever was.How many miles on the engine now. I don't know what kind of rings you used but they don't sound like they have seated. Do you have a PCV valve in and hooked up? Upgrading the rotating assembly shouldn't be a problem.
Did you build it yourself? Have someone with building experence talk with you about the steps taken. The 750 is part of the bad gas milage. It is too big for that engine and is dumping gas out on the road through the tail pipes. Go look at how sootie they are. Header are best for engines turning 5,000 rpms and greater, if you have a good flowing dual system it should be ok for a daily driver.
good luck
John
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Guessing less than 3K miles on rebuild. My son's truck. Generally only driven to and from school, about 4 miles each way. Barely has time to warm up good, so I suspect the rings have not fully seated yet.
Have dual exhaust. No pcv valves, just valve cover vents, both sides, to air filter. I actually see bubbling from the valve cover bolts, even with both sides vented.
If I go in to reset the cam/crank timing, any suggestions as to a setting that works best. This is a 72 F100 SWB on a 78 Bronco 4WD drive train. No heavy off road use (though my teenage son would prefer otherwise), and not looking for street racer. Just good, responsive performance.
Appreciate the warm welcome by the way.
Have dual exhaust. No pcv valves, just valve cover vents, both sides, to air filter. I actually see bubbling from the valve cover bolts, even with both sides vented.
If I go in to reset the cam/crank timing, any suggestions as to a setting that works best. This is a 72 F100 SWB on a 78 Bronco 4WD drive train. No heavy off road use (though my teenage son would prefer otherwise), and not looking for street racer. Just good, responsive performance.
Appreciate the warm welcome by the way.
Fleet Owner
Joined: Jun 1999
Posts: 24,552
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From: Creedmoor, North Carolina
Dittos on installing the PCV.
P= Positive
C= Crankcase
V= Ventilation
Ya gotta have it installed and vacuum pulled into the carb base preffably.
John
P= Positive
C= Crankcase
V= Ventilation
Ya gotta have it installed and vacuum pulled into the carb base preffably.
John
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I'm just thinking that the disconnected PCV is not going to cause low compression. All cylinders down on compression at the same percentage leads me to believe that either the valves are not opening and closing at the optimum time or there is a ring seating problem.
I would suggest doing a leak-down test and determine if there is excessive bleed down into the crankcase. If there isn't I'd check the marks on the timing gears.
Someone could have even installed the wrong gear set.
Who rebuilt it??
I would suggest doing a leak-down test and determine if there is excessive bleed down into the crankcase. If there isn't I'd check the marks on the timing gears.
Someone could have even installed the wrong gear set.
Who rebuilt it??
Fleet Owner
Joined: Jun 1999
Posts: 24,552
Likes: 74
From: Creedmoor, North Carolina
Originally Posted by instig8r63
I'm just thinking that the disconnected PCV is not going to cause low compression. All cylinders down on compression at the same percentage leads me to believe that either the valves are not opening and closing at the optimum time or there is a ring seating problem.
I would suggest doing a leak-down test and determine if there is excessive bleed down into the crankcase. If there isn't I'd check the marks on the timing gears.
Someone could have even installed the wrong gear set.
Who rebuilt it??
I would suggest doing a leak-down test and determine if there is excessive bleed down into the crankcase. If there isn't I'd check the marks on the timing gears.
Someone could have even installed the wrong gear set.
Who rebuilt it??
John
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