Oops! EDIT: I meant COMPRESSION TEST in title. For those who don't want to read the below...the question is "Will I do more damage than good running a Compression test when I THINK I see rust in one of the cylinders (it's crud of some sort, just like the top of the pistons).

My 390FE "ran when parked" a billion years ago...or so I'm told. Quite awhile ago, when I bought the truck I filled all cylinders with Marvel Mystery Oil before I turned it over by hand. After soaking for a number of days, it HAND cranked easily.

Fast forward to now. All the accessories pulled. Just the block, heads, intake manifold. I used an extension to spin the oil pump and tested the pressure - all good (40 PSI?). Went to hand crank and NOTHING. Locked up. My best guess is that even though it sat outside fully tarped, with rags stuck in every opening it still got moisture in there locking the rings.

I wanted to run a compression test and then a leakdown test before I pull the manifold and heads. My reasoning was to get a sense of how it operated when the previous owner was using it. It was parked for a reason presumably but I bought it through a third party who couldn't provide any details.

So if I'm running the starter on it, no plugs, just running those pistons to do my tests, am I going to score those cylinder walls up? I would think "yes"...and it negates the value of such a test. But if the lack of combustion (with plenty of oil via my priming of the pump before testing) would allow a test, I'd love to run them.

Thanks. JP

 

Well if you cant turn it overby hand I would not try with the starter as you can pull the top off pistons from stuck rings.
Fill the holes with a 50 / 50 mix of ATF and acid tone. Let it sit to work in a few days and try turning by hand till it breaks free.

At that point I would not worry about a compression or leak down if you plan to pull it apart as you will see just how bad the walls are with rust / pits when apart.
After what it just went through I dont think either test will come out good and just be a wast of time.
When apart you can measure the holes to see if you need to go for larger pistons & rings or if a hone and new rings will work.

Or if you dont care once you can turn by hand just run it as I still dont think the tests will really show anything till the rust is off the walls.
Dave ----

 

Thanks Dave. That makes sense.

I will say that I filled each cylinder last night with MMO. This AM, 5 of the 8 were dry. And three cylinders were holding some oil still. Thinking aloud here, but isn't this a crude leakdown test of sorts too? If something as viscous as oil will completely drain in under a day, my rings must be pretty worn. So I'd think. ... but just postulating.

JP

 

If it turns over run it and see what happens. it's not going to hurt anything that's not already damaged.

It's amazing how rust you can get away with in a cylinder as long as it's just spots. I've seen walls that looked pretty bad and the engine ran just fine and didn't use oil.

 

But the last he posted it would not turn by hand so that tells me the rings are rusted to the walls.
If he was to use the starter to spin it over if atop ring was stuck and the piston on its way down can pull the top ring land off the piston.
This is not good and could hurt more things.

Remember you have ring end gaps. MMO is not that thick and will run past the ring end gaps so I dont think that is a good sign of a good or bad motor with out more checking / running.
But if you are going to pull the heads anyway then why do the extra work of a compression or leak down test?
Heads off measure the bores and go from there.
Heck pull the motor and check the bearings and the piston ring grooves.
If the bores measure good, ball hone, new rings and bearings, valve job to the heads and you got a junk yard rebuild motor.

Yes I did skip over a lot you treat this just like a machined motor with measuring the crank for the right bearings and if it needs machining.
Cleaning the block so you can eat off it before putting it together etc.
Dave ----

 

Thanks Guys. Appreciate the input. The engine is already pulled from the truck, so I will pull manifold and heads and take a look. I just wanted to get a sense of it's operation ideally before any work. And yes, stuck at the moment, but I don't think truly seized. I will see I guess!