Compression tester

[johnstonjohnr]

OK...try not to laugh but, I bought a compression tester at an auction...never been used but can someone tell me how to use it. It came with no instruction. I figured for a couple dollars someone could explain it...without laughing! Thanks :facepalm:

 

[datastream]

Re: Compression tester

Funny you should mention...I got one on loan from a hardware store and tried it tonight. my first time too. It CAME with instructions, but I swear it makes no sense.

Generally, you take out the spark plug and screw the tester hose into the plug hole. Turn the engine over 3 or 4 times and read the gauge. However...

...the one i got came with a valve - like whats in a tires valve stem - screwed into the end of the hose that goes into the plug hole. I could be waaay off, but it seems to me that no pressure is going to go in that hose with that valve there.

Thusly, my compression readings were between 'screwed' and 'junkyard'. I'm hoping I did it all wrong.

 

[tater76]

Re: Compression tester

Depending on what motor and ignition you are running, here is the general way to do it.....

1. Remove all spark plugs

2. Put spark plugs back into the plug wire boots

3. You need to ground all of the plugs to the motor. I use a peice of wire that connects all of the plugs to the engine ground point. (you can make one from one peice of wire)

4. Once plugs are grounded proceed to put the controls inside the boat into forward WOT.

5. Depending upon your particular motor, there may be a neutral interlock switch. It will not allow you to crank the motor while in gear. If you have this, it will need to be bypassed either with a jumper wire or something holding the switch down.

6. When you have all of these steps done, screw the compression tester into the top spark plug hole until it is seated finger tight. Position the tester so you can see it from the drivers seat.

7. Turn the ignition (crank the motor over) while watching the tester. Stop cranking when the needle on the guage stops rising.

8. Record the number, watch for leakdown, then proceed to the next cylinder.

I have found with 30 year old motors 2-4 cylinder, most compression readings fall around 120psi. If you have deviance of over 10 percent between any cylinder you may have issues. Anything under 100 needs attention.

Not all compression testers will give the same results. My cheap Harbor Freight is lower that my Snap-On tester consistently. The big thing to worry about is deviance between cylinders over 10 percent.

 

[Scott Danforth]

Re: Compression tester

I normally just disconnect the coil + wire.

pull all the plugs

throttle open

crank for 5 revolutions read gage, record number

reset gauge by depressing schroeder valve between each cylinder

no more than 10% difference between high and low cylinders is good.

 

[tater76]

Re: Compression tester

Crank until the needle stops rising. There is no set number of revolutions when checking compression. When the needle stops moving up, that is your compression on that cylinder.

 

[jerryjerry05]

Re: Compression tester


Ones a push in(crap) the other's a screw in

The valve in the hose??? It's supposed to be there.This a push in tester? Or a screw in?
It works on the principal that the pressure in, is way more than the valve can stop.
then it goes into the guage and is released when you push in the tip of the valve.

I usually remove all plugs.
Key off.
Jumper wire.
Then screw in the tester on top cylinder or where you want to start.
Then turn it ove till the guage stops going up.
Record the results.
Go on to the others.
Relieve the pressure between cylinders.

If you use the key to turn the motor over,
then you need to ground out the plug wires.

 

[Fishing4Walleyes]

Re: Compression tester

I am taking compression testing 101 here. Could someone please explain the part about grounding out the plugs and especially the part about "if you are using the key to turn the motor over". None of my motors have electric start. I do understand what you mean when you say to ground them out, I am curious about the consequences.

 

[jerryjerry05]

Re: Compression tester

Some motors(most) need the plug wire grounded out so it won't blow the pack.
If the kill switch is in the handle just undoing the switch will stop the plugs from firing.

 

[datastream]

Re: Compression tester

JerryJerry,

Yeah I did it right...the compression was just that bad.

So I cut my losses and got another Force85...that runs...well, I might add.

 

[paultjohnson]

Re: Compression tester

A stone cold engine will give ya lower readings normally then You will get if ya run it up to temp before ya do the test.

 

[RogersJetboat454]

Re: Compression tester

...the one i got came with a valve - like whats in a tires valve stem - screwed into the end of the hose that goes into the plug hole. I could be waaay off, but it seems to me that no pressure is going to go in that hose with that valve there.

Thusly, my compression readings were between 'screwed' and 'junkyard'. I'm hoping I did it all wrong.

The schrader valve is supposed to be in there. It maintains pressure in the gauge so you can read the gauge after you are done cranking the engine over (with out the valve, the pressure generated by the cylinder leaks out of the gauge back into the cylinder). The only time you would remove the schrader valve is if you are doing a running compression test.

 

[jerryjerry05]

Re: Compression tester

What's the results?
What motor?
No electric starter???

 

[birdgod]

Re: Compression tester

This is how I test mine outboard, it is faster this way. Make sure it is in open area outdoor
1) warm up the temp with garden hose.
2) disconnect all spark plug, dont have to ground and thing.
3) WOT
4) put some napkin paper over the cylinder that you are not testing to avoid gas from comming out.
5) jump the starter. Until the guage don't go up any more.

Oops , Jerry already post the same method. My bad.