compression tester

[scothorn]

Where is a good place to buy a cheap but good compression checker, if there is such a thing. I am searching for a good used boat, and want to be able to check the compression myself. Thanks in advance,
Scott

 

[svdk]

Re: compression tester

I got mine from the local autoparts store. It came with several adapters for different size plugs (standard & metric). It was about $30, but there were others that were cheaper. Check to see if a store near you has a tool lending program. The store I shop, has a free tool rental policy that I have used in the past. This would have been the cheapest route, but I don't mind buying tools.

 

[Tacklewasher]

Re: compression tester

Cheap at the auto parts store. Just make sure it is a screw in and not the kind you have to hold in place. Impossible to get consistent readings with those.

 

[rickdb1boat]

Re: compression tester

Don't know where you're located, but Walmart has them for $20.00...

 

[Dennis1958]

Re: compression tester

I have heard people talk about testing the compression but Ive never really understood what that means? Can someone explain why and what your testing and what the reading should be? I guess Im asking for compression 101 please.

 

[Silvertip]

Re: compression tester

An engine has to suck in a fuel/air mixture, compress it (squeeze it) and then at the proper time ignite it to create the explosion that forces the piston down, turning the crank which spins the propl Compression occurs when the piston is on its up stroke. At this point its working just like an air compressor. With the spark plug removed, you spin the engine over, and read the pressure developed in the cylinder with a -- you guessed it -- a compressioni tester. On most engines all cylinder will rarely read exactly the same but they should be close. Within 5 - 10% will be ok. As for the actual reading, most manufacturers don't even publish compression readings. But if you have 90 PSI or more the engine can be assumed to be ok internally. That does not mean the electrical and fuel systems are ok. It is advised that you hear the engine run and if possible do an on-water test run.

 

[jtexas]

Re: compression tester

3 things your motor needs for internal combustion: fuel, spark and compression. Compression makes heat...compress anything and it will get warmer. Piston compresses the fuel/air mixture inside the cylinder and the spark ignites it. Without good compression, you can't have good ignition.

A compression gauge screws into the spark plug hole and shows PSI inside the cylinder when you turn the engine over with the starter.

Compression results is the best indicator of the condition of the internal engine omponents, other than disassembly for visual inspection.

Good compression is when the lowest cylinder is upwards of 100 PSI and within 5% to 10% of the highest. Difference between cylinders is more important than actual numbers.

 

[Dennis1958]

Re: compression tester

What symptoms will your motor have if you have low compression or total failure?

 

[rickdb1boat]

Re: compression tester

The symptoms of low compression can mimic other problems with the motor. A compression test will show you that the motor is in good or bad overall condition internally. It is the first step before getting involved with an engine of unknown quality. In other words, why put money trying to repair an engine that is worn out or has major problems inside(Pistons, Rings, Etc). I doubt this is the case with your motor, but it is never a bad idea to run a test on it. If your motor has real low compression or low compression on one or more cylinders, it can fail to run at all or may run rough and have a significant loss of power depending on the degree of wear...

 

[clint]

compression tester

Pep-boys. I purchased mine the other day and it was the best $20 bucks I've spent. It has a rubber boot which you push it into the cylinder port opening when cranking and get the cylinder reading instantly. To re-set you push a pin and releases old reading and is ready for next. I also rigged up a on-off switch and with 2 wires and aligator clips, I attach to my solinoid and starter terminals and I stand right in the back of the motor and crank it with the made up switch and use the hand held compression tester. With all plugs out it is faster than screw in types. If you need information on the on-off switch e-mail me clntewood3@aol.com Good Luck!
Clint

 

[scothorn]

Re: compression tester

thanks for the info, never thought about walmart, I knew auto parts carried them but wasn't sure what the prices were. Thanks again

 

[jtexas]

Re: compression tester

Use a fully charged battery.
Warm engine up to operating temperature.
Disable ignition by pulling the emergency kill switch or use jumper wires to ground the spark plug leads, to avoid damage to the ignition system. If you use a remote start switch like Clint, you can just leave the key in the "off" position.
Do each cylinder 2 or 3 times to be sure you get good reading.

Be aware that if you connect your remote start switch between the solenoid and starter terminal it carries a lot of current - enough to hurt or at least heat your jumper wires. Use a heavy-duty momentary starter switch and heavy-gauge cable. Or try activating the solenoid by connecting your switch between the main solenoid post (where the battery cable is connected) and the small solenoid post coming from the boat wiring harness (usually has a yellow/red wire).

A gauge that presses into the hole is good as long as the rubber boot can hold the pressure, if it leaks it'll distort your readings.

 

[StratosRob]

Re: compression tester

Make sure of one thing before you ever turn the motor over for the test.That is make sure you use the right adapter because the wrong one will screw in to far and make contact with the piston.We dont want that. Use your spark plug to help choose the right adapter.