This is for my gen 1 small block chevy engine, but I am going out on a limb that it could happen with any 4 stroke engine. I am totally perplexed and am hoping someone can help me identify the issue before I start tearing things apart again. Please help!

Long story short when doing a compression check I can't build proper pressure with the exhaust rocker arm installed. When the rocker is set for proper valve lash I can't get more than 35 psi out of it (150 is expected from other cylinders). As I slowly loosen the rocker arm I start to build pressure but can't get over 120 psi until the rocker arm stops putting any pressure on the valve. Once there is no more pressure on the valve (zero valve motion) I get 150 psi in short order.

Now for the long story. Just got my car on the road and was shaking it down to look for problems. The engine was built a number of years ago, and I have less than 10 hours on it. The wasn't super confident in the calibration of the chevy temp sender and the Porsche temp gauge, and I think I thought I got it hot on one of my drives. Still ran fine but temp creeped rather high into the red zone.

I noticed my coolant was a nasty brown color, and a couple flushes didn't clear it up noticeably. I thought I may had blown a head gasket so I did a compression test. 7 of the 8 cylinders were ~150 psi, and cylinder #7 (back left) was 35 . I was convinced I overheated it and crushed the gasket creating the leak, so I took the head off to replace the head gasket. I was surprised that my local machine shop said I probably didn't warp the cast iron head so I didn't bring it to them to check it out. Possibly regretting that now.

I cleaned everything up and put the heads and manifold back on. Then I decided to do another compression test and low and behold cylinder #7 still only produced 35 psi with the valve lash set . Freaking out a little I decided to do some trouble shooting which is how I discovered the behavior in the second paragraph.

I will do whatever it takes to fix this, I just want to focus my efforts and leave the engine in the car if possible. Visually things look fine - valve springs, pushrod, lifter, rocker, hell even the valves looked fine when I inspected them.

So this is where I am at and if you have any ideas of what may be causing this I would be grateful.

Josh

solid or mechanical lifters? If all looks fine, valves, train assembly... my suspicions lie with the lifter itself.

Hydraulic lifters. What makes you think its the lifters? Just want to follow your logic so I can better understand.

The engine has been sitting around, with less than 10 hrs on it, and hydraulic lifters can be finicky, particularly when sitting around for any length of time. I don't know if these are rollers or not, but being the only variable in the system, that's what I'd look at first.

You haven't noticed binding with the valve, or the rocker arm, and nothing nutty with the push rod seems to be going on, right? That just leaves one thing left. And lifters are cheap and easy. Was it noisy?

Hope this helps. and, good luck.

Do a leak down test on that cylinder. Check to see if your hear the air leaking through the exhaust pipe, carb or throttle body with butterfly wide open or the oil fill port. Leak down testers are cheap at Harbor Freight.

me too.

Sounds like the lifters are not pumping up. You are setting lash to a deflated lifter. Might be rust stuck in the down position. You need lots of oil pressure to get them un-stuck, maybe more pressure than you have, or just replace them.

If it was built some time ago, they sat without oil inside them for long enough to seize up. You can pull them and soak in a hot oil bath and they might come back to life.

Thanks for the tips. I think I narrowed it down to an issue with the exhaust valve itself. With various combations of the rocker arm fully adjusted, partially adjusted, removed, etc I found that the valve is not sealing in a repeatable way. At one point with a leak down tester installed and the valves closed it was leaking out the exhaust port, then a couple gentle blows to the top with a rubber mallet and it sealed
So off the head came and it is headed to the machine shop tomorrow for a rebuild. Flat tapper lifters are cheap so I think I'll just replace them for added insurance.
The only good thing about this is that it has been too damned hot to drive the car anyways. New goal of having it ready by end of summer

Congrats in advance.