Compression testing, hot or cold?

[rudeafrican]

All the posts on the forum reccomend doing the compression test on a warmed up engine. What is the reason for that and how different would the results be from a cold engine?<br /><br />I COLD tested the compression on my 'rude 40 hp 1990 model and got 120 and 121 PSI respectively. Would this change up or down when warm?<br /><br />Is this within spec?<br /><br />Thanks

 

[JB]

Re: Compression testing, hot or cold?

Howdy, Rude one.<br /><br />Parts change dimensions when warmed up, and lubrication is assured.<br /><br />I would expect slightly higher psi on a warm engine.<br /><br />What matters is the spread among your cylinders, not the absolute meansurements.<br /><br />As long as all cylinders measure above 100psi and don't spread that measure more than 10-15% you have a good engine.<br /><br />Higher numbers do not indicate a "better" engine.<br /><br />Good luck.

 

[Waterbugtoo]

Re: Compression testing, hot or cold?

Just to ad....<br /><br />Compression checks do not tell the whole story, hot or cold. A cylinder leak down test should be performed on an occasional basis for more accurate diagnoses of internal parts condition.<br /><br />

 

[thosch]

Re: Compression testing, hot or cold?

Can an ordinary shadetree mechanic do a leakdown test? If so, what would be the procedure.<br /><br />Thanks.

 

[Waterbugtoo]

Re: Compression testing, hot or cold?

That depends on how shady it is at the time. LOL

 

[Forktail]

Re: Compression testing, hot or cold?

Sorry, but a cold static compression test is virtually worthless, no matter what the numbers are.<br /><br />Here are some tips....<br /><br />Warm the outboard up to normal operating temperature before compression testing.<br /><br />Remove the wire from the ignition coil. Remove all spark plugs prior to testing. They will be hot, so use gloves.<br /><br />Open the throttle wide open (WOT) prior to cranking. On fuel injected engines disconnect the injector wiring.<br /><br />With the tester installed, crank the engine until the gauge will not move further.<br /><br />Remove the tester gauge and squirt about 2 tablespoons of oil into the cylinder. Use 2-stroke oil for 2-strokes, and motor oil for 4-strokes. Slowly turn the engine over by hand a few times.<br /><br />Reinstall the compression tester and check the compression again. This is a "wet" check.<br /><br />The results of the dry test should be nearly equal for each cylinder and generally not more than 10% of the highest reading.<br /><br />If the wet reading is significantly higher than the dry reading then you have a problem with the rings or cylinder wall. If the wet reading is nearly equal to the dry reading then you may have a head gasket, valve problem, or cracked cylinder.<br /><br />Two adjacent cylinders with low readings usually indicate a bad head gasket.<br /><br />To further diagnose problems you can do a dynamic compression test (motor running). This would be more for the 4-strokes to diagnose intake/exhaust, camshaft, valve train problems, etc.<br /><br />Hope this helps.

 

[vinney]

Re: Compression testing, hot or cold?

You can tell who makes aliving repairing this stuff and who doesnt. Hot/ cold compression is compression. You need so much in the hole to fire the hole.There is book smart and the real world. The two of them dont go together. You need to get and and out to make aliving in this business.

 

[Trent]

Re: Compression testing, hot or cold?

Vinny...Re-read your post..Ya tired? Or drinking? Im stummped?

 

[Forktail]

Re: Compression testing, hot or cold?

Actually book smart and the real world applications do go together....and they make the best combination. <br /><br />Thus cold compression testing vs. hot compression testing will yield totally different results, especially on an engine with problems. Not only will the compression generally rise with a hot engine, but anomalies and % of cylinder differential will change. No good mechanic does cold compression tests. The whole intent of a compression test is to find out if the rings, cylinder, head, or intake and exhaust are leaking, damaged, or performing inefficiently. Without these parts at normal operating temperature, the effort is senseless. The internals of an engine aren't designed to work cold. Heat expands and seals mating parts, it represents normal operating situations, and tests the wear tolerances of the engines design. Just because an engine "fires in the hole" doesn't mean it has sufficient compression to operate as designed. Compression makes power. An outboard which runs with low compression....runs very bad.<br /><br />t.h., I forgot to address your leakdown question....<br /><br />A leakdown test is different than a compression test in that you actually inject compressed air into the cylinder rather than having the engine produce the compression on it's own (by cranking). This requires a source of compressed air and a special gauge(s). Leakdown tests on 2-strokes aren't as common as those on 4-strokes because of their absence of intake and exhaust valves. In fact different leak down gauge tools are used between the two with 2-stroke needing less compressed air to fill the cylinder (some 2-stroke leak down tools have a hand bulb you pump manually). None the less, they are good to do, and generally give a more complete diagnosis than the static compression check alone (as Waterbugtoo mentioned). You can acutally listen for the air leak through the exhaust, carb, or head gasket to find the problem...if there is one.<br /><br />If you've got the tools you can do both the compression and leak down test yourself easily. But make sure the engine is warmed up.

 

[JB]

Re: Compression testing, hot or cold?

This is not worth fighting about.<br /><br />Theoretical facts and "real world" experience are not in conflict. Only the interpretations are.

 

[vinney]

Re: Compression testing, hot or cold?

JB your ok Sir yes Sir

 

[roscoe]

Re: Compression testing, hot or cold?

Sooo... If you are trying to diagnos a non starting engine, you can't warm it up, so there is NO value in doing a comp or leak down test???<br /><br />So we should ignore all the posts that recommend a comp test on an engine that won't fire. Thats a lot of posts.<br /><br />????? So how can you tell if you have a ring or gasket problem, without tearing the engine down?

 

[ob]

Re: Compression testing, hot or cold?

Good point roscoe, And "my" answer is yes, there is some value in performing a cold compression test .Particularly on an engine that will not start when all else seems to be in line.Carburetion,spark,etc.A cold compression reading of say,40 would not likely ,or moreover not,increase to acceptable levels on warm engine and could very well prevent startup.On the other hand,If wanting to track down a potential compression problem as suspect in poor operation the values would be best attained on warm engine as they would then be reading closer to the operational pressures.<br />To answer rudeafricans question:yes, they may come up a bit on warm engine ,and yes your numbers look acceptable.

 

[vinney]

Re: Compression testing, hot or cold?

OB great point. You get as much data as possilble when diagnoising the situation. Hot or cold it doesnt matter. You use your brain and the data and make a concussion. Its you ability to ulilize all the input and make a good diagnosis. The key in business is to make that diagnosis fast and correct. I call it get in and get out. I train my guys to be great trouble shooters. What the books say is always diifferent from the real world. My theory been for years is chuck the book and use your brain. Use the books for specification. If you know how a stator works you dont need to look it up in a book. Unless you dont know.

 

[Forktail]

Re: Compression testing, hot or cold?

Of course it's impossible to do a warm compression test on an engine that won't start!<br /><br />Compression tests and leak down tests are methods to determine why your engine is losing pressure in the cylinder, where that pressure is going, and how much compression you are losing. They are diagnosis for determining why your engine is running less than optimally, if the engine internals are worn, and if it is time for a rebuild in order to prevent more expensive engine damage on down the line. These tests can tell you exactly where the problem is before you tear the engine down. But to do them right, you have to do them warm. <br /><br />Compression and leak down tests are not the first place to go when an outboard won't start. Several other things would normally be checked first. If you are trying to diagnose a "no-start" problem using compression and leak down testing then you would have to do it cold, and hopefully you have already eliminated all other possibilities. An engine can run for a long time in a sad state of affairs on poor compression. They will generally start, however poorly they run.<br /><br />I understood the original poster's outboard did run, and that he did a cold check. If the engine runs, he should do a hot check.<br /><br />ob, I agree with the last part in your last post......except that if when doing the warm compression test, one cylinder comes up to 150 psi and the other stays at 120 psi.

 

[rudeafrican]

Re: Compression testing, hot or cold?

Thanks guys, sorry for the delayed response, had to leave town on urgent business. Makes a lot of sense, all the different input put together. I will deffinately also do a warm as well as a wet test next time out. Luckily I'm not trying to track down a problem, was just curious what my compression readings were.<br /><br />Thanks again.

 

[vinney]

Re: Compression testing, hot or cold?

Fork still waiting for leak down information. The how to. What is the biggest problem when preforming one. real or book

 

[BillP]

Re: Compression testing, hot or cold?

My take on the advantage of a leak down test is it lets you see the problem earlier than when checking with the non leak down compression test. Based on experience owning airplanes this is normal procedure in the aviation business and not so normal in the boating industry.

 

[vinney]

Re: Compression testing, hot or cold?

I love a leak down test and preform them regululary. Fork was going to get me an answer on how he does one. If you do this on a two stoke he would have an answer. There is a difference between reading a book and the real world. My best friend is the smartest human on the planet He has more degrees that any could ever have a couple Drs a couple masters a pe degree engineering he is a walking nerd he is a nerds nerd he is a book smart dud. BUT when comes to apply his knowledge he cant get the job done the two worlds dont go together. He is a lawyer too.

 

[Forktail]

Re: Compression testing, hot or cold?

"Fork still waiting for leak down information."<br /><br />Uhhh....exactly when or where did you ask for this information? I would be more than happy to help....but first you have to let me know. I can't read minds. <br /><br />"Fork was going to get me an answer on how he does one."<br /><br />I was? Again, I didn't know you wanted this? <br /><br />"If you do this on a two stoke he would have an answer."<br /><br />Answer to what? Answers usually follow a question. <br /><br />"There is a difference between reading a book and the real world."<br /><br />vinney, you seem to be hung up here, on the books and all. Be careful. There are members here who are both book smart and have "real world" experience. Rather than frown on that, you should try to learn something from them. That's what these forums are all about. <br /><br />What's up with all this vinney? Are you baiting? <br /><br />This is a good thread vinney. Lets not clutter it with contention. I realize you are a dealer/mechanic. I would love to provide help with any sincere questions you have.<br /><br />BTW, I have already provided some basic information on leakdown tests above.