How do I perform a compression test with the motor out of the car?

edit: scratch that, there's no tranny attached so I can't do a compression test, what about a leak down test? How do I go about that?

 

not really much you can do... I suppose a leakdown is doable, and would be the best way to check a junkyard engine, especially if it's sat a while. (sometimes the lifters will lose their prime and keep the valves from opening until oil pressure builds, so a compression test might not be accurate) To do a rudimentary leak- down test on an engine that isn't in the car, connect each cylinder to a source of air with all valves closed. listen/ feel for air at the exhaust and intake- shouldn't be any. Not a whole lot you can do to check rings this way, but at least you'll know the top end is sound. Also- if the source vehicle is a wrecked car... should be okay. Too bad you're in Milwaukee- One of my jobs is technical advisor/ equipment mechanic for a salvage yard. There's a '97 T-5 engine with 130k that I know ran really well. The car was in a pretty bad accident. (I thoroughly look over everything European that they get, so I know what's available for my customers' cars, if they need something that is best obtained from a yard or are on a budget and can't afford new)

 

install the compression tester and give the crank 5-10 turns.

not sure why a trans would have anything to do with compression.....

 

Because you need the starter and the crank sensor from what I've read, with out a trans you have to turn it by hand. Which I guess is doable, but I'd rather do a leak down test because I don't feel like cranking the motor countless times and it sounds like a PITA.

 

I dont think you could turn it fast enough to get a good reading. Unless you can find a way to mount the starter well enough to crank the engine over the leak down will be the best you can do.

 

Yeah, the leak down seems easy enough. Do they make something that you put into the spark plug holes to pump air in that seals it? And I don't have an air compressor either, so you think I could like use a really good tire pump? I dunno, just trying to make lemonade out of pears here. haha. I'm sure the compression is fine, I'm not too worried about it, the oil was clean and the motor is really clean, plus only 104k miles, just something that I would like to know.

 

You need a compressor, there will always be some loss past the rings. Cylinder being tested needs to be at top dead center on compression stroke. I use a compression tester and take the valve out of it, just like a tire valve in the bottom and a pressure gauge inline. 100 psi line pressure, holding 90 psi constant at motor, 10% leakage as an example. Need a starter to compression test.

 

Meh, too much. I don't have access to of that sort of stuff anyways, seeing as how the motor is in my parking garage and that's where I'm going to be doing most of the work on it. I am planning on cleaning the motor up this weekend and remove remaining accessories from it, winterize it, maybe I'll do a leak down, but I'm sure the motor is completely fine.

 

A high CFM compressor and a good impact will turn it faster enough.

 

The best indicator of an engine that has been sitting a while(or any engine) is the spark plugs. Are the plugs caked with oil and sludge? If the plugs show normal wear, most likely the engine is in good shape.

I bought a 91 Mercedes 300E wagon for $400 that had overheated and blown the head gasket. I had the head planed, did the valve job myself and found low compression on 3 cylinders which was due to the overheating. The plugs showed normal wear and running condition so I knew the compression rings and oil rings were good before the overheat problem.

After I got the engine back together, it missed and ran quite rough. 3 cylinders measured above 160psi and the other 3 were 60-90 psi. I pulled all the plugs and soaked each cylinder with a 1/2 cup of auto trans fluid and let it sit for 4 days. After the 4 days, I cranked it over for a few seconds to blow out the trans fluid and then put the plugs in and started it and let it warm up and after 20 or so minutes, it ran perfect. The next day I put about 10 miles on it and then re-did the compression test. Now all cylinders measured close to 165psi. No more smoke or rough running.

 

Nice. I'll check the spark plugs this weekend. Pretty sure there wasn't any problem with over heating or anything. I know this isn't anything official, but the throttle body is super clean and when I looked into the intake manny it was super clean too. So I know there wasn't any oil going through the PCV system and being recirculated back into the fresh air intake.

 

It doesn't have to be an overheat situation. Using a crappy oil can cause a sludge buildup to partially stick oil and compression rings.

ATF fluid is an excellent solvent and is an old secret to cleanse gunk from piston rings.

 

Oh, ok. Well there's nothing I can really do about that now anyways since the motor won't be going into the car until spring. In in the mean time I'll fog it, and come spring if compression is questionable I'll have to try that.

 

The motor I put in from the auto wrecker 2 weeks ago wasnt running, had volvo plugs in it and turned over by hand easy. I took my chances and put it in after calling them butchers for cutting everything when they pulled it out. I did crank the engine with the coil wire grounded untill I had oil pressure before starting it.

 

So to work on it I'm going to purchase an engine stand, anyone know best mounting points? I figure where the transmission mounts. And anyone know what size bolts I would need?

Thaaaaaanks.

 

Got an engine mount today. Anyone happen to know what size bolts I need to mount the engine to the stand via where the transmission mounts?

 

I would guess a 10mm diameter shank with 1.25 thread pitch bolt.

 

Huh, now that's what I call an answer. Thanks! I'll try to confirm that today when I pick some up.