What do compression test tell you?

[swick]

I'm a fast learning beginner here. I know compression tests can tell you certain things like if theres a blown head gasket or something. What else can they tell you?

If all cylinders are within 10-15% of eachother this is acceptable right? But what does it mean? What if it is more? What do you do about it?

 

[Lion hunter]

Re: What do compression test tell you?

All internal combustion engine work on the same basic concept. They have to draw fuel into the cylinder, compress it, then ignite it. The compression aids in the ignition of the fuel. If the cylinders have different readings the fuel will not ignite the same between cylinders and the engine will run unbalanced. Ignition is what forces the pistons down. The force of ignition that pushes one piston down pushes another up. If ignition is poor there is not enough power to push the other piston up correctly. So compression tests tell you if the cylinders have the ability to create the same ignition response on all the cylinders. That is why it is better to have low readings that are the same than high readings with one cylinder off.

 

[JB]

Re: What do compression test tell you?

Unlike auto engines, the purpose of a compression test of an outboard is to discover if one or more cylinders are not making full power because of failure to fully compress the fuel/air mix.

A failed/failing cylinder will test much lower than the good cylinders. If all cylinders measure within 10-15% no cylinder is failing.

If adjacent cylinders read low a blown gasket between them is likely, but a single low reading cylinder could be reading low for several reasons. A blown head gasket is probably the least expensive and easiest to fix cause of low compression.

In a very rare case, all cylinders may be failing for some reason common to all such as an overheat causing all pistons to seize, then free up by damaging the cylinder walls of all cylinders. Even then, though, it is unlikely that all will read within 15%.

 

[swick]

Re: What do compression test tell you?

Makes perfect sense, thanks Lion Hunter, thanks JB.

 

[5150abf]

Re: What do compression test tell you?

And it is just a good easy way to keep an eye on how your motor is doing.

I check compression before every season and once and a while during the season just to try and catch a problem.

 

[Silvertip]

Re: What do compression test tell you?

And then there is a cylinder leak down test that is the true measure of an engines health. While a compression test provides a rather "instant" measure of compression, a leak down test shows whether or not the cylinder can actually "hold" compression. An engine may show good compression but exhibit blowby. The cause is combustion gases going past the rings.

 

[swick]

Re: What do compression test tell you?

so how is a leak down test performed, and what do you do if you have blow by?

 

[Silvertip]

Re: What do compression test tell you?

If an engine exhibits blow by it performs poorly because combustion gases are blowing by the rings. On a four stroke it will also quickly contaminate the crankcase oil and oil consumption will likely be very high. The only solution is to rebuild the engine. Here is a good tutorial on leakdown testing and how it's done.
http://en.wikipedia.org/wiki/Leak-down_tester

 

[swick]

Re: What do compression test tell you?

good info silvertip. Was only wondering cause you suggested. But if the compression test comes out alright is there a need ofr a leak down test? I am water testing the boat when I get up to see it. So far everything that I've learned on iboats has been extremely helpful.

 

[Lion hunter]

Re: What do compression test tell you?

I usually do it when I'm doing the compression test. If your doing the test anyway it only takes a few extra minutes. If you've already done the test and there is no reason to suspect a problem I personally would'nt go out of my way to redo it.

 

[Silvertip]

Re: What do compression test tell you?

The leakdown test simulates the process of combustion and hence the true sealing capability of the rings. A compression test does not do that as effectively. It is also far more valuable in determining blown head gaskets as you can hear the air coming out adjacent cylinders.

 

[moomba.schmidt]

Re: What do compression test tell you?

men,scott from scotts marine here.have pwc's and outboards that throw good compression hot and cold all day long and all the cyclinders are wiped out.if you have a motor that has good compression and won't run right and every thing else checks out invest in a head gasket and have a look.hopefuly a leak down test is the best way to be sure.