Compression Test 101

[Jason2tpa]

In a different thread townsend had mentioned a compression test, which had me thinking. If you were to google "how to perform compression test", for every link that provides instruction there are just as many conflicting procedures. Some say ground the plug cap, some don't. Some say warm engine first, some don't. Some say have neutral throttle lever engaged during test, others don't. It's really amazing how one differs from another.

Searching this forums threads, i found a bunch advising to do test but none describing step by step. Perhaps one of you old crusty pros can give us amateurs the end all be all method of performing comp test.

And for the sake of you having to type less, let's assume the lanyard is pulled so engine will not start.

[cpostis]

Here's some info that I use.

[IMG]Scan0024 by Charles Postis, on Flickr[/IMG]

[IMG]Scan0025 by Charles Postis, on Flickr[/IMG]

I do the test on a warm engine, pull the safety lanyard, push the neutral button in, and open the throttle wide open. I do not ground the plug wires as the ignition is disabled by the pulled safety lanyard.

I'm sure you'll get a lot of other recommendations by other members.

[boscoe99]

I have read that a compression test is a teat on a boar hog.

What do you guys think about this?

[boscoe99]

How to conduct a compression test does not appear to be as controversial as how to interpret the readings that result. That is, what should minimum compression be and what should be the difference between the lowest cylinder and the highest cylinder.

Another point of contention has to do with what to do about low or high compression if the motor is starting, idling and running well while appearing to make rated HP. Should surgery be done or should the patient be left alone?

[rodbolt17]

on two stroke a compression test is quick easy and painless.
on a 4 stroke it is a teat on a boar hog.
I cannot tell you how many 4 strokes have passed a compression test with leaking heads,leaking valves and stuck rings due to carbon.
yet the compression was good and the leakage rate above 70%.

the reason I always use my stevens s48 spark tester when checking compression?

it reduces the chance of fire.

ANYTIME you deal with gasoline vapors,especially with EFI or DFI motors, CONTROL the spark.

had a boat fire here in the harbor about 6 yrs ago.
mokanic got blowed out of the hull,owner got blowed out and landed in his backyard.

google colington harbor boat fire.

mokanic was working on a gas hydrolock issue.
pulled the plugs and went to crank the motor over and KABOOM.

I have seen two merc EFI motors fireball.
one I warned the guys and they told me we know what we are doing.
I went and grabbed the forklift driver and said Todd watch this .

the other was a 225 EFI merc that was at the shop. the other tech pulled the lanyard not knowing the customer had bypassed the lanyard.
whoosh.

ALWAYS insure the ign is either disabled OR you can control where the spark is going to occur.

[cpostis]

Looking for input and trying to learn!
OK, how about a leak down test on a two stroke?
From what I've read on internet (good or bad) you place the piston so the rings are above all ports, lock down the crankshaft/flywheel, attach your leak down tester, pump in about 70 psi, and observe the leak rate.

What would this tell you? If the rate is not within tolerances, you may have worn/broken rings, worn cylinder, worn/damaged piston, and/or a leaking head gasket? If is within tolerances, the above would be ok? With this test you can isolate the cylinder with a problem, as stated above?

Back to compression test.
What would this tell you? Could be any of all the above? I did some internet research (good or bad). If one cylinder, you can isolate. What about bad reed valves, bad crank/crankcase seal(s), maybe other causes? All cylinders will read below specs?

So lets say you've done a two stroke leak down test and results were good, yet the compression test failed. Would you conclude the rings, piston, cylinder were all good? Now you isolated the problem to bad reed valves, bad crank/crankcase seal(s), or maybe other causes? If so, you have possibly eliminated the tear down of the motor? You can now concentrate on these possible other causes? Maybe just a seal? Rodbolt told me I didn't have to separate the crankcase halves to replace the seals. I had my powerhead disassembled and placed the seals in the housings, put new O-rings on the housings and connected the crankcase halves around the housings.

Food for thought, tell me what you guys think.

Thanks!!

[rodbolt17]

can a leakdown be done on a two stroke?
yep.
why bother?

unlike 4 strokes you wont see gradual wear and it has no intake or ex valves to leak.
typically any leak on a 2 stroke head allows water in.
water burn you can see when you pull the plugs.

typically when a two stroke blows it is sudden with no warning and compression simply dumps.
two strokes are rather touchy about TDC when trying a leakdown.
DO NOT try to hold the flywheel with a breaker bar.

[99yam40]

compression tests or the leak down on a 2 stroke will not tell you anything about the reeds or crank case seals.
Just that the top end of each cylinder is sealing well or not

[Jason2tpa]

This became a good thread. Learning of rodbolts mechanic getting blown off the boat was somewhat entertaining.

Back to basics, to confirm what IS and what IS NOT necessary for accurate results.
*Neutral button pushed, max throttle while testing??? Is this a must do?
*Engine at normal operating temp prior to testing? Is this a must do? And if so, please explain why a cold engine would give lower psi than a warm engine.

[TownsendsFJR1300]

Back to basics, to confirm what IS and what IS NOT necessary for accurate results.
*Neutral button pushed, max throttle while testing??? Is this a must do? You want as much air going into the cylinder as possible. Keeping the throttle CLOSED is hampering that so yes, it should be @WOT.

*Engine at normal operating temp prior to testing? Is this a must do? You'll get a more accurate reading with the engine to temp BUT if you have a cylinder say at 100, the rest at 200, temp isn't going to make a difference. That cylinder is dead.

And if so, please explain why a cold engine would give lower psi than a warm engine. All the internals aren't to temp, IE piston rings when warm expand slightly (less end gap, less blow by, more compression, etc)

See above..............

[Jason2tpa]

Ok, good stuff. But now one must define engine "warm". Some of these articles I've read state to warm engine on flush, block hot to the touch, tell tale warm.

Well I can tell you, I've had my engine on flush 15-20 mins from a cold start and the block is warm but the TT is cold. Take the boat out on the water under load for 10min then put on flush, TT is warm enough for a cup of tea.

But perhaps that's just with older 2 strokes? Idk, maybe 4 strokes warm faster.

[TownsendsFJR1300]

Ok, good stuff. But now one must define engine "warm". Some of these articles I've read state to warm engine on flush, block hot to the touch, tell tale warm.

Well I can tell you, I've had my engine on flush 15-20 mins from a cold start and the block is warm but the TT is cold. Take the boat out on the water under load for 10min then put on flush, TT is warm enough for a cup of tea.

But perhaps that's just with older 2 strokes? Idk, maybe 4 strokes warm faster.

Generally, IME (and per my OTC compression tester directions), you want compression to be within 10% between ALL cylinders.

If your within 10% cold, it'll be even BETTER when warm...

[Jason2tpa]

Generally, IME (and per my OTC compression tester directions), you want compression to be within 10% between ALL cylinders.

If your within 10% cold, it'll be even BETTER when warm...

So from what I take by reading everyone's responses, the factory compression psi of each cylinder is not of great importance. Rather the cylinders being within 10% of each is.

Rodbolt makes a good point in a different thread:

myself, I would not worry about testing compression unless there is a running issue.

Think I'll put this puppy to rest and follow his advice. Why test something when there isn't a problem.

[cpostis]

compression tests or the leak down on a 2 stroke will not tell you anything about the reeds or crank case seals.
Just that the top end of each cylinder is sealing well or not

That's why I have ?'s in my statements from the net and posted "good or bad".

I am leaning on disagreeing with you about the reeds though. If after the up stroke where fuel/air mixture is drawn into the crankcase, then the down stroke compresses the fuel/air mixture, and the reeds are not seating properly, the air/fuel mixture will escape through the reed. If this is the case, the next up stroke will not have "pre-compressed" air/fuel being blown through the intake port and into the cylinder. My thought are a compression test (not leakdown) should have less of a reading. Less air in, less air to compress, less pressure?

Look at this animation in this link, think of the poppet valve as a reed valve:

Animated Engines - Two stroke

What you think 99yam? How about other members chiming in?

[rodbolt17]

take the reeds out,retest the compression.
you will find it did not change.
however with the reeds out you wont move fuel.
the intake and ex ports will supply enough air at cranking speed to test ring sealing ability.

over the years playing with the two smokes I have found it really does not matter hot or cold or throttle open or closed.

only thing to watch for with the motor warm is a brass tester fitting inserted cold will expand in the threads.
makes it a bugger bear to get it back out.