Rebuilt 5.0 - blow by, how much is too much….

Hi folks,
Hoping someone can confirm my analysis or provide additional feedback.

94 GT 5.0
93 Cobra lower and upper intake
99 Explore GT40p heads
76mm TB and 80mm MAF
No emissions
24lb inj
Trick Flow Stage 1 cam

300 miles in to running the rebuild
Misfire #4 and failed cyl load test
Progressive hard starts but drove good
Had a lot of oil consumption
Pulled plugs a and #4/8 caked up bad
Plugs #3/7 somewhat bad
Plugs #1/2/5/6 looked “normal”
Vac leak check using starter fluid and inspected all caps and hose - nothing found
Replaced plugs and ign coil
Runs and sounds great - starts no issue

So obviously not root cause I pulled PVC and hose from PVC to Intake - lots of oil. No oil leaks per se and no smoke in the exhaust and engine passes cyl load test so assuming not rings as the motor was only ~80-90k miles (I didn’t touch bottom end).

Oil pressure is 60-65 psi cold and 35-40 warm - using 5w-20 per Ford spec so I think this is ok and rules out any major bearing clearance issues.

So my feeling is blow by oil (quite a bit) going to intake and since the fitting at the intake is in the rear, it all gets pulled in to cyl 4/8 and 3/7.

Added an oil catch can and going to monitor oil in the can as well as the dip stick. Also will check plugs at 100 miles.

How much blow by is ok on a modified engine like this? Seems the pressure is ok, but should I use 5w-30 or 10w-30 as this may drop the pressure a little?

Thanks

yes screen is in
should also note that the exhaust is catless and doesn't smoke at all minus 5-10 seconds at start-up (only sometimes).

Short block wasn't touched and looked very good - could still see hone hatch marks well and wasn't an issue prior to new top end rebuild.
Heads were machined and rebuilt (upgraded springs) by a shop so can't imagine the valve seals were not done and that the guides at a minimum weren't checked.
I can't confirm this as I don't have a picture nor the original paperwork from the job.

I plan to do a comp and leakdown test at some point soon.
Tryign to avoid removing the intake and valve covers but it may come to that.

AeroCoupe said:

My money is on the valve seals but possibly the guides or a combination. Leak down will tell the story.

Click to expand...

Thx getting a leak down gauge/tool later today and will see if u can clear up diagnosis this weekend.