Losing 15% compression on cylinder1

[sambo2]

Hello everyone. I'm fairly new to engine work and would appreciate some feedback.

My 4g64 had oil starvation and spun some crank bearings during a long trip (burned oil much faster than normal) and I got a replacement engine. The replacement engine unfortunately loses about 15% of pressure in only one of its cylinders during a leak down test. I hear hissing the loudest from the exhaust valves.

I suspect poor valve seating (but would appreciate your thoughts). I'm planning to pull the head and lap the exhaust valves on the leaking cylinder to try to address this issue. Is this worthwhile or should I just throw it into the car and run it until I can afford a more thorough repair? How long may an engine run with a weaker cylinder?

I love this car but don't have the time or money at the moment to do a better overhaul. Thank you for your time

 

[waltah]

I think the first question is how bad is a 15% low leakdown test. I haven't done that test and don't know: Maybe someone who has can educate us?

Then I wonder if this is a random pull junkyard engine in which such an issue isn't surprising or a better grade that should have some warranty.

And finally, what's the downside if you put it in the car? If the engine is otherwise sound it ought to run and the one-cylinder leakage may actually fix itself -- rust on a valve that was open in the stored condition, for example.

It seems to me that if the engine's otherwise okay the worst you're looking at is pulling the head when you have a few hundred $ for a valve grind and gasket set.

 

[Dericsh]

I worked in the shop for a different brand and their spec for leakdown was 20%.

The only thing that kinda throws me off here is that it’s only experienced on one cylinder. I don’t think I would hurt to inspect the valves at least and make sure you’re not just dealing with some rust or something hopefully easy.

 

[sambo2]

Thank you for the replies! I talked it over with some local gearheads and the consensus is to just run the engine.

I got it from a dismantler who stated that it had run and passed smog before being removed from the car. Surface rust also makes sense since it had been in storage for months. What I should have checked first is the valvetrain; there was a lot of play in it meaning the hydraulic lifters must have emptied out over time. Cylinder 1 lifter may have gotten stuck preventing full closure.

I'm hoping to get it running in a couple days. I'll let y'all know how it goes!

 

[sambo2]

Update on the engine:

It runs great! The 15% leak was reduced to 1% after running for a few minutes, and the other cylinders had nearly 0% leak (tested at 50 psi). Glad that I didn't try to lap the valves. I think it was from being dry, maybe some surface oxidation/rust, and loose lifters.

There were a few valves that had hydraulic lifters that didn't fill with oil and created a bit of valve tapping. This was resolved by using some motor flush and doing an oil change.

I'm really happy with how this project turned out. The only remaining issue is that engine coolant temp circuit is reading high. I will figure it out soon.

Thanks for your time fellas!