Do engine compression readings ever match the book?

[rcx132]

Just tested the compression on my SJ413. It's only done 43'000 miles. The Haynes comic book says it should be 199 psi, min 170 psi. I'm getting 140 psi on all cylinders.

Thing is every engine I've ever tested has always been way below what their books state.

I've had to test my engine cold as it's still not running 100%. I think when hot that adds quite a bit of PSI to the readings.

 

[Pete.]

Do the test with the throttle wide open.

 

[Shox Dr]

You cant do a leak down, or compression test on a cold motor, you’ll always get blow past.

 

[Shox Dr]

Do the test with the throttle wide open.

Whats that going to achieve?

 

[dobbslc]

A battery that's going flat also doesn't help.

 

[rtbcomp]

Take the plugs out of all the cylinders to reduce load on the starter motor.

 

[Pete.]

Whats that going to achieve?

Your not pulling a vacuum between the throttle blade and the inlet valve.

 

[colnerov]

Hi, You also need to watch how the pressure builds up, that can tell you where the problem might be.

Colin

 

[zzr1200]

Your not pulling a vacuum between the throttle blade and the inlet valve.

I agree with Pete throttle wide open allows full air flow into the engine and a squirt of engine oil in each plug hole, just before cranking the engine for each cylinder test, helps sealing the piston rings when the engine is cold.

PS.
Remember cylinder test pressure is for an engine in tip top condition, run in correctly, perfect servicing by the book, as long as the pressures are all about the same and close-ish to the stated test pressure, be happy.

 

[RonA]

As above, if the readings go up with a drop of oil in the cylinder, I would suspect rings/bore worn. If not valves are leaking slightly.
RonA

 

[mike 109444]

How accurate is your meter?!
This image is of my test meter on left (calibrated instrument) and on the right what the customer was using to "check" there endoscopes for leaks! I know it's not a compression checker but does show how inaccurate an uncalibrated pressure meter can be. If I recall many years back they did a test of the air pressure gauges of the petrol station tyre pumps and found a lot of them way out.

 

[Cobbler]

Most of the garage ones are way out. I always do my tyres at home with a known digital gauge. If my car has been serviced I always recheck the tyres & most times there all over the place, so I would expect compression testers to be the same after a few years being slung about into draws.

 

[rcx132]

Mines a cheap one. I've also noticed that it depends where the valve is on the guage. So some have a long tube or extension tube, the presence of which increase the firing cylinder volume and thus decreases the pressure. I bought one that has a valve at the spark plug fitting.

 

[armalites]

I would say it's more important that they are all similar than being exactly by the book.

 

[KemppiFrog]

I would say it's more important that they are all similar than being exactly by the book.

Me too.

 

[8ob]

I would say it's more important that they are all similar than being exactly by the book.

Me too.

Or forget the compression tester and do a leakdown instead

Bob

 

[Johnseat]

Remove all the plugs, connect a decent battery charger and then do the compression test.
If all of the readings are roughly the same but low then I'd say your gauge requires calibrating.
I've done loads with the workshop compression tester and we don't get dodgy results but ours is calibrated at least twice a year.

 

[CompoSimmonite]

Can you borrow another compression tester to confirm figures ?

 

[merryman]

199 psi is over 13 to 1, more like diesel territory. I don't think I have ever had over 140 psi compression on any engine I have tested. No idea how accurate my gauge is but it agreed almost exactly with the gauge on the compressor when I tried checking it with air pressure, because I thought it must be out. Having the throttle open is important, and pouring oil into the bore first will throw your readings out, why do that? Getting even results on multi-cylinder engines gives an indication if one cylinder has a fault, but I work on singles so that doesn't work for me. I find an engine will start if compression is over 90psi and will smoke if the bore is worn. An engine with a burnt valve will have either no or very little compression. That is about all you need to know from a compression test, unless you are building racing engines.

 

[james butler]

to be honest your looking for an even pressure across all cylinders and definately need the throttle wide open when doing the test.
adding oil to each cylinder AFTER the initial test like has been said will tell you if you have worn rings as the compression will substantially increase with oil if worn.