Compression check

**sfmill** · 08-18-2018, 10:04 PM

Hey everyone first post been lurking a couple years.
Just purchased a new Bosch compression tester because i am in the process of selling some Mercs. I did a compression test on the motors after getting them warm, removing plugs and turning key.
I keep reading about a wot compression test. What is the benefits of doing that and what is the proper way to perform this?
I have also been reading about grounding plugs and a remote starter. Will doing this also make a better compression test?
Thanks in advance.

**RSWORDS** · 08-18-2018, 10:13 PM

Wide open throttle allows the cylinders to fill with air for a proper reading. It's not about the numbers being (although they will) it's about the test being accurate.

If you are not using a remote starter and are using the key switch you should ground the plug wires or pull the emergency lanyard to keep from possibly damaging the ign system or worse get causing a fire from the mixture coming out the spark plug holes.

**sfmill** · 08-18-2018, 10:41 PM

Thank you, i am curios what the #s difference will be.

**rude tim** · 08-18-2018, 11:21 PM

Wide open throttle will not change the number, it only allows it to reach that number faster. As said above it is not about how much compression, it is about even numbers across all cylinders and a bench mark as time gets put on the motor.

**RSWORDS** · 08-19-2018, 08:18 AM

Originally Posted by rude tim

Wide open throttle will not change the number, it only allows it to reach that number faster. As said above it is not about how much compression, it is about even numbers across all cylinders and a bench mark as time gets put on the motor.

I've had some motors (especially 4 strokes) that change a HUGE amount.

**Chaz** · 08-19-2018, 09:18 AM

Originally Posted by rude tim

Wide open throttle will not change the number, it only allows it to reach that number faster. As said above it is not about how much compression, it is about even numbers across all cylinders and a bench mark as time gets put on the motor.

Kind of hard to draw in air and squeeze it ... with the door shut.

It's really all about the first three pumps anyway. And when they are really sealed up you will notice that you get most of it (75%) in the first swing. The majority of the left over in the second and the rest in the third.
If your the kind that grinds on the starter till it sounds like your runnin a saw mill , waitin on that last five pounds to come up .. either your testing method is flawed , or your motor is as wore out as I am ...

RSWORDS

I've had some motors (especially 4 strokes) that change a HUGE amount.

RS , Ifin they won't let ya do it the right way , loosen the valve lash and squirt a blob of 90 wt gear oil down the hole ... should come up UUUGE ..

**H2OPERF** · 08-19-2018, 09:54 AM

Agree with chaz 100%.... do same number of pumps on all cyls

**powerabout** · 08-20-2018, 04:31 AM

Originally Posted by RSWORDS

I've had some motors (especially 4 strokes) that change a HUGE amount.

yep, open throttle is 4s thing

**Chaz** · 08-20-2018, 06:33 AM

Originally Posted by powerabout

yep, open throttle is 4s thing

Yes , we have a 4s easons resort here in Palm Beach . Not sure I'd go full throttle there , but I could show ya some biker bars ya could get away with it ...

**KIRCHNER** · 08-20-2018, 10:15 AM

I THINK if you use a remote starter on starter and leave key off, u dont have to ground plugs or worry about fuel because elect system is off. i cant find 2 gauges that read the same! dont use a harbor frieght for sure. I HAVE to retract my negative comment about the harbor f. gauge. had it checked out its right on. put 150 to it read 150! question i have is the hose holds 7 cc volume, is that air become part of the chamber air being measured? say your testing a 30cc head for psi, is the gauge seeing it as 37cc??

**RSWORDS** · 08-20-2018, 08:23 PM

Originally Posted by powerabout

yep, open throttle is 4s thing

Nope. Makes a difference on 2 strokes too

**RSWORDS** · 08-20-2018, 08:24 PM

Originally Posted by KIRCHNER

I THINK if you use a remote starter on starter and leave key off, u dont have to ground plugs or worry about fuel because elect system is off. i cant find 2 gauges that read the same! dont use a harbor frieght for sure.

Correct

**powerabout** · 08-20-2018, 08:28 PM

Originally Posted by RSWORDS

Nope. Makes a difference on 2 strokes too

did the manufacturers ever care enough to say that in a manual, nope.

**flabum1017** · 08-20-2018, 09:31 PM

Originally Posted by powerabout

did the manufacturers ever care enough to say that in a manual, nope.

From a Mercury outboard manual:

Compression
All Models – Using a fully charged battery,
throttle shutters wide open and cylinder block
warm – 110 – 135 psi (753.3 – 924.5 kPa)
Variance between cylinders should not exceed
15 psi (102.7 kPa)