AppleTech

1993 F-150, 302 - I just finished a full rebuild, including rings and a valve job. Got everything back together and fired up and was having rough idle and poor performance, felt like it was missing on a cylinder. Pulled plugs and found #1 wet. Did a compression test (realizing now I should have done it somehow on the bench!) and all cylinders are 170-185 except 1, which is very low (doesn't register on gauge, but I can hear a hiss when releasing the gauge valve), so basically NO COMPRESSION.

I ruled out the following first:
- Rockers were cycling at proper time
- Valves seems to have full movement as seen from outside
- Valves seem to seat at the same level as the rest (doesn't appear to be stuck)

Once I ruled out any simple fixes, I pulled the head:
- Piston is in same shape, no detonation (barely ran the truck)
- No indication of valves hitting the piston (although I believe it is a N-I engine in this application)
- Valves appear to properly seat - shining a bright light at both sides doesn't appear to push through (No visible openings). About to try the gas test tonight, but both valves look like they are seating fine...

What am I missing here? I'm going crazy because there is no obvious cause of no compression!

bscottie

did you set the preload on the rockers? do you have an odd size rod?

LOCO LAPTOP

Is the head still on or did you take it off already?

AppleTech

Head is now off (I was fairly convinced it was a stuck or bent valve).

All rockers/pushrods are original (and uniform) and went back in the original installation location. I made sure that the rockers did not have excessive preload - tightened them until the rocker had no slack and then torqued to spec, none were more than an additional 1/2 turn. I did loosen the rockers when turning the motor over to see if I had over torqued one but loosening them did not give me compression.

dr_bowtie

clean the head combustion chamber and dry it with brake clean. fip the head so the intake and exhaust ports are facing up.

fill the ports one at a time and check for leaks at the valves. what i do is fill the port with mineral spirits. fill the intake full and watch the combustion chamber and see if it starts getting wet around the valves. if its good then do the exhaust....

when you have low compression you can roll that cylinder so the valves are closed and put an air fitting in the spark plug hole and put air in the cylinder and listen where the air goes..... hiss in the intake... intake valve leaking or not closed... hissing in the tail pipe exhaust valve leaking or not closed. hissing out the valve cover (from crankcase) broken piston or ring package.

Chris_1

iTrader: (1)

Low compression could be rings or valves but no compression at all pretty much has to be a valve not closing.
I would think that even with bad rings you should get something just because the sheer volume of the piston will displace more air than can leak past the sides, so even though it would be low you should still get something.
No reading at all says open valve to me.
Something in the valve train didn't go together quite right is what I would think.
Next time you test that sort of thing and find a low cylinder a wet test would help with your diagnosis.
A few drops of oil in the spark plug hole and a quick compression test will tell you a lot. If there is a sudden noticeable increase rings are a problem. If no change, look at valves or head gasket. You did inspect that head gasket real close, right ?

AppleTech

Will do a leakdown test on the head today with some fluid and let everyone know how it comes out.

Another thing I'm thinking about might be the lifters? Don't they have a spring or something in them that is dampened by oil? Is it possible that it is stuck in the "expanded" position and thus not allowing the valve to seat fully? I remember reading in the manual that there is a tool to pull the rockers up to force oil out of the lifters, but I couldn't find anything in my garage to do that and others didn't seem to think it was necessary, but maybe I should?

EDIT: I'm thinking of a tappet bleed down wrench.

AppleTech

Also... I bought new head bolts (have been run for only a few hours). Are they OK to reuse or do I need to purchase a new set?

Chris_1

iTrader: (1)

From what I've read, they're one time torque bolts. As I understand it they do a bit of a stretch thing at torque and aren't made to do that more than once. Head gaskets are one time things too. Sometimes you get away with things but if reusing things doesn't work, she's all coming apart again - if it doesn't take out the motor when it lets go.
Wait time and more money I know but .....

AppleTech

So the head looks fine - did the gas test and let it sit for several minutes with no leakage. Knowing that I have basically 0 compression and the rings are brand new, I'm starting to lean towards a tappet/lifter not compressing under spring pressure and leaving a valve open. Any thoughts on how I can test the tappets?