390m crankcase overpressurizing??? - Ford Forums - Mustang Forum, Ford Trucks, Ford Focus and Ford Cars
LinkBack Thread Tools Display Modes
post #1 of 4 (permalink) Old 02-10-09, 02:49 PM Thread Starter
Registered User
Join Date: Feb 2009
Posts: 2
Question 390m crankcase overpressurizing???

got a head scratcher...any ideas???

I have a 76 f-250 "highboy" with a 390 modified. the engine ran well and strong, just really flat. so we were screwing around with it, tried to put a whole new exhaust on, and ended up breaking 8 bolts off in the heads. long story short, the whole thing snowballed on me. got all new valves, valve seats and seals, shaved the heads and cleaned up the intake manifold. new lifters, rods, springs and a more agressive cam (not wild, but has a nice cam lope to her). new timing chain, and pretty much everything else that needed some tlc was changed. now she runs great, the cam and lifters broke in beautifully. doesn't leak a drop, until i put it in gear. it pumps oil up and out of the top of the dipstick. and a lot, on the 1.5 mile test run, it leaked about 2 quarts out. i thought i snapped a ring, but the compression test was good, along with all the vacuum tests we did. also, the pistons and rings and cylinder walls were suprising spotless, not a mark on them. i'm freaking stumped, especially because it runs so rock steady, even while leaking. any ideas or suggestions would be greatly appreciated.

Last edited by ssgt highboy; 02-10-09 at 02:51 PM. Reason: forgot something
ssgt highboy is offline  
Sponsored Links
post #2 of 4 (permalink) Old 02-10-09, 11:04 PM
Registered User
Join Date: Feb 2009
Location: Blairsville, PA
Posts: 6
Re: 390m crankcase overpressurizing???

did you check for a clogged pcv. if your crankcase ain't gettin proper ventilation, it will force the oil out, no matter how well you built it
jonnydesert is offline  
post #3 of 4 (permalink) Old 02-12-09, 02:25 PM Thread Starter
Registered User
Join Date: Feb 2009
Posts: 2
Re: 390m crankcase overpressurizing???

yeah, no clogs. even with a new pcv valve and the breather off, still does it. starting to wonder if i'll just have to tear it all back apart and rebuild the lower end too. thanks
ssgt highboy is offline  
post #4 of 4 (permalink) Old 02-12-09, 05:57 PM
Join Date: Nov 2008
Posts: 1,153
Re: 390m crankcase overpressurizing???

I'll tell you why it's happening. the engine is developing serious blow by pressurizing
the crank case. there could be a 1000 things that will cause that. You said the pistons and cylindars were spotless ? They had better not be. they should have major cross
hatches and deglazing. Was the bore measured for taper ? Is the bore worn to the max ? How much was shaved off the head ? Anything over .030 you have the shave the manifold. I take it you didn't take the bottom end and pistons apart. This is just a few things that could go wrong. there could be 994 more things. did you cut the ridge out of the cylindar when the head was off ? sometime the piston will move after the
head and gasket are replaced and the piston will hit the ridge and crush the upper and
middle ring land. What is the compression ? Are you sure the engine wasn't 2 qts. over full before you tested it ? Did you get the head bolts mixed up ? there is one bolt that has to go into only one place. You have massive blow by. the engine has to come apart. Without seeing the engine and actually knowing what you did this is all a guess. Here is the lesson learned. A car motor is as close as you will ever get to a perfect machine.
When one part is worn everything is worn. You cannot rebuild the top end without rebuilding the bottom end and the other way around. the exception is if something actually broke and you need to replace one part. bottom line, the bore has to be measured, the pistons have to be measured. Good luck.
chipset is offline  
Sponsored Links

  Ford Forums - Mustang Forum, Ford Trucks, Ford Focus and Ford Cars > The Garage - Tech Forums > Engine Specific Tech > FE (330-428) Tech

Currently Active Users Viewing This Thread: 1 (0 members and 1 guests)
Thread Tools
Show Printable Version Show Printable Version
Email this Page Email this Page
Display Modes
Linear Mode Linear Mode

Posting Rules  
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are On
Pingbacks are On
Refbacks are On

For the best viewing experience please update your browser to Google Chrome