PWagon

Over the weekend I wanted to compression test a 5.3 I just recently bought. The engine is not running (sitting in my carport) at the moment. My question is this. Can you get an accurate compression test with removing all spark plugs, testing one cylinder at a time, and by turning the engine over with the starter only? Or, do you have to check the compression only when the engine will start and run?

C4 LS1

yes you can get an accurate test by just using the starter with the plugs removed.

fox12303

iTrader: (7)

If you find any issues with the compression, do a leakdown test before you take anything apart. This will give you a better idea of what to look for during teardown.

rotor vs. piston

iTrader: (3)

That's actually how you should do a compression test. The engine shouldn't run when you're testing.

Old Geezer

iTrader: (3)

Throttle body blocked wide open. Crank for the same # of "hits" on each cyl.
Find any really low, shoot a spritz of oil in the cyl, and re test.

PWagon

Thanks guys for the advice. Unfortunately my engine is shot to sh#t. The damn thing has about 65 psi per cylinder. Now I'm just wondering if I should mess with rebuilding it or just getting another one from a salvage yard. The problem is I'm affraid I'll get another one that's worn out.

fox12303

iTrader: (7)

Did you oil the cylinders? I'd spray all the cylinders with PB Blaster and then oil. Turn the motor a few times and spray again. Let it sit over night and retry the compression test. Seems odd that they would all read low. How long has the motor been sitting and do you know any history prior to you purchasing it?

i r teh noobz

iTrader: (4)

If the lifters and stuff aren't pumped up, perhaps the valves didn't open all the way. The rings also need oil to seal.

A weak battery or closed throttle plate can cause a low reading. I'd double check if they're all that low. 65 psi is barely enough to run on.

rotor vs. piston

iTrader: (3)

If they're that consistent I would think something else is giving the low number. The sign of a bad engine is usually the readings are all over the place. It's not doom and gloom, if it were me I'd continue to go forward and budget for another engine IF that one doesn't run once you have it finished.

PWagon

If I do a leak down test on the engine, how do I know what are acceptable values on the gauges? I've seen some gauges on the Internet that have green, yellow, and red ranges. Is it really that easy to determine if the leak down is acceptable or not? Or, do I need to keep the leak down within a certain percentage range? I'm considering buying an OTC 5609 leak down tester ( ). Thoughts?

oange ss

iTrader: (50)

8-12% would be a good start, dont freak out if it's more as the engine is cold, results will improve once you test the engine when warm

PWagon

Well, I just placed the order on Amazon. The leak down tester will be here on Friday. I'll take pictures and post my results. Thank you guys for your help in educating me on this.

Image98

iTrader: (1)

I always thought a compression test was done with a closed throttle. I guess i was misinformed.

Which is the correct method open or closed?

fox12303

iTrader: (7)

An engine is like an air pump, with the throttle open it can pull air into the cylinder faster than with it closed. This would probably get a higher result with less cranking time. Typical lifters have .120" of travel, so if they are not pumped up this could result in the low compression across all cylinders. Maybe get some Marvel Mystery Oil to top off the crankcase, a good battery charge, pull the valve covers and crank til oil comes out all 16 pushrods. Then do your compression test again. Squirt the Mystery Oil into the cylinder too. What is this stuff made of anyway?

usdmholden

It's a mystery...

i r teh noobz

iTrader: (4)

Open. Closed will give a false low reading.

unioncarpenter84

iTrader: (22)

so after u get ur cylinder pressure what do u divide it by??

Old Geezer

iTrader: (3)

referring to a leak down test, no math involved. Read the gauge.
USE CARE when doing a leak test. Shooting a 100psi into a cyl can/will cause the engine to rotate!!
The valve train has to be loosened to do the test.
Once the rockers are off, and the air is injected, there may be a high leak at a valve. B4 you condemn that cyl, take a rubber mallet and whack the top of the valve a couple times. This will snap it open and slam it shut. This should re-seat it, and the leak will subside. [Assuming the valve is not toast.]

Some folks check the leak with the piston at TDC.

oldgoat69

For best accuracy test at TDC and BDC. Old Geezer is right about the valve trick if there is not a rust ring on the seat. Try the mallet method several times before you go too far. Sometimes once is not enough.Oil in the cylinder will also help to create a better seal.

PWagon

Well, my leak down tester came in on Friday, and I checked all cylinders this weekend. I saw a really strange thing when 50% of my cylinders (on both heads) were leaking air at the exhaust valve area. Yes, I turned the crank over and made 100% sure that both the intake and exhaust valves were as-closed-as-possible, and air still leaked at the exhaust valve. I could maybe see one or two being plausible, but 4 of them?!?!? Do you guys have any suggestions on why I would be seeing this on so many exhaust valves?

Some cylinders had between 10 to 21% pressure loss (pictured below). The leaking exhaust valves leaked 70 to 80%.

Do you guys see any problems I might have just having the heads rebuilt and NOT rebuilding the bottom end (pistons, rings, etc)?