Engine compression test ?

[bigdog]

Can someone on the Forum explain tgo me, how an engine compression test is performed? For Outboards, as well as I/O's.

I know specific tools/gauges are required, but can you use same tools, to perform both type of tests? Anotherwords, can the same test gauge be used for both type of motors?

This is all new to me, and just want to know the fundamentals, not that I will be perfoming these tests myself. Inquiing minds want to know !

Thanks,
BD

[ishoot308]

Bigdog;

A compression test gauge is simply screwed in where the spark plug goes and the engine is then turned over without starting. The compression is then read on the gauge and you are done. I would assume the same gauge could be used on different types of engines.

Dan

[Just Sold]

Bigdog;

A compression test gauge is simply screwed in where the spark plug goes and the engine is then turned over without starting. The compression is then read on the gauge and you are done. I would assume the same gauge could be used on different types of engines.

Dan

That is correct but you must have all spark plugs out in order to get an accurate reading for each cylinder tested.

[Blue Thunder]

Yup!! What they said!!!

[TomC]

the test is generally done with the throttle wide open - and you need a good, strong, battery for proper cranking during the test

[Jonas Pilot]

That is correct but you must have all spark plugs out in order to get an accurate reading for each cylinder tested.

Would you have to worry about loosing compression with the other spark plugs out? I would think you would do all cylinders, one at a time.

[ishoot308]

That is correct but you must have all spark plugs out in order to get an accurate reading for each cylinder tested.

I am not a mechanic so really do not know anything but the basics but when my outboard was checked last year the mechanic only pulled out the plug on the cylinder he was testing...

Dan

[gtxrider]

Pulling all the plugs lets the engine spin easier and reduces the load on the battery.

[Jonas Pilot]

Pulling all the plugs lets the engine spin easier and reduces the load on the battery.

That's true but will it then give you a false reading at the compression gauge?

[Tired of Waiting]

That's true but will it then give you a false reading at the compression gauge?

NO.

I doesn't matter if all the plugs are in or out with the exception of the one you are testing. The throttle does not have to be wide open. If all the plugs are out the engine will turn over easier and not be as much of a drag on the batt. With the throttel open or closed the cylender will still get enough air to compress.

You do however have to turn the engin over several times. Watch the gage and when it stops climbing the test is done. Read the gage and record. Then compair it with the engine specs.


ToW

[Jonas Pilot]

NO.

I doesn't matter if all the plugs are in or out with the exception of the one you are testing. The throttle does not have to be wide open. If all the plugs are out the engine will turn over easier and not be as much of a drag on the batt. With the throttel open or closed the cylender will still get enough air to compress.

You do however have to turn the engin over several times. Watch the gage and when it stops climbing the test is done. Read the gage and record. Then compair it with the engine specs.


ToW

What if you have "bad" rings on another cylinder?

[NoBozo]

Here's a quote from my 1963 Jaguar XKE Factory Service Manual:

"Compression pressures must be taken with all the spark plugs removed, carburetter throttles wide open, and the engine at it's normal operating temperature."

As stated in a post above, the engine is turned over continuously (coil wire disconnected) until the pressure stops climbing on the guage. Write the pressure down for that cylinder, and move on to the next cylinder. :) NB

EDIT: Just as an example, the 3.8L Jaguar engine with an 8:1 compression ratio should produce 155 PSI. With a 9:1 compression ratio the pressure should be 180 PSI. What's more important is that all the cylinders show similar pressures.

[BroadHopper]

and automomobile restoration is my hobby.

Nobozo is correct. The engine has to be at operating temperature, WOT, coil disconnected, and all plugs removed. You don't want to crank it too much as there is no oil pressure and may cause damage. Crank it just enough to give you a reading.

You will find a different reading on each cylinder. Usually 10% more or less from the factory specification is OK. If the engine was reconditioned and bored, the specification will change, usually lower.

If you really want a status of your engine health a leak test is better. But that is a whole new ball game. :rolleye1:

[MAXUM]

You do not want to leave the throttle wide open, this will flood the cylinders with fuel. Doing so has the potential to wash the oil out of the rings and thus throw the readings off. That is more of a concern the more cylinders the engine has.

What is acceptable really varies, while most folks figure an engine is an engine, there are many different configurations as to how the internals are built and some are more stringent as to what compression range is acceptable including the variations between readings. The manufacturers specs should be adhered to versus a "rule of thumb"

Agreed with broadhopper a leakdown test is a good idea as it tells you more than just a compression test.

[Tired of Waiting]

and automomobile restoration is my hobby.

Nobozo is correct. The engine has to be at operating temperature, WOT, coil disconnected, and all plugs removed. You don't want to crank it too much as there is no oil pressure and may cause damage. Crank it just enough to give you a reading.

You will find a different reading on each cylinder. Usually 10% more or less from the factory specification is OK. If the engine was reconditioned and bored, the specification will change, usually lower.

If you really want a status of your engine health a leak test is better. But that is a whole new ball game. :rolleye1:

years as a factory trained mechanic. The throttle DOES NOT HAVE to be wide open. You can flood the engine amoung other things. ALL you need is some air for the cylinder to compress. That will take several cycles of the engine. That is why you watch the gage and STOP when it stops climbing.

As for no oil preassure and damaging the engine that is false unless the engine is already bad. There is plenty of residual lubricant on the cylender walls and in the bearings to handle cranking revolutions. You are not "running" the engine but just turning it over.

ToW

[BroadHopper]

When I am ready to 'turn the engine' I open the carb and leave it open. Not opening the carb whenever I do a compression check. So there is only one 'shot' of gas. It should be minute as far as damage.

You may be right as far as enough oil residue to keep the motor oiled when you are doing the compression check. As a shade tree mechanic I read in many articles about the damage of turning an engine over with no oil pressure. It is better to be safe.

If you have throttle body or direct fuel injection, when you turn over an engine there is fuel pressure as I can hear the fuel pump running. Does anyone know that the excess fuel will cause harm?

[NoBozo]

Forget the compression test for a moment. With old fashioned carburetters, the engine is cranked over to start the engine. The pistons go up and down and the valves open and close. When the piston goes down..air and fuel is "Sucked" into the cylinder through the intake valve at the correct time....each cylinder in turn. The air/fuel mixture is ignited at the correct time and you're off and running.

Back to the compression test: ALL the spark plugs have been removed. The only spark plug hole that is "covered" is the one with the compresssion guage in it. Turn the engine over with the starter (coil disconnected).

Here's the Falacy about inadvertantly flooding the cylinders with fuel: ALL the other cylinders are OPEN to atmosphere via wide OPEN spark plug holes. The piston going down will be sucking air IN through the ALL the open holes...therefore there will be no..or very little "Suction" at the carburetter throat capable of introducing fuel into the engine via the carburetter jets. :) NB

PS: Fuel Injection "may" be another story..depending on the Type of fuel injection system AND it's computer..??..controls..

BTW: WOT is used more to defeat any "suction" capability AT the Carb venturi, than as a method to supply more air for the test. One cylinder doesn't need much air.

[MAXUM]

Here's the Falacy about inadvertantly flooding the cylinders with fuel: ALL the other cylinders are OPEN to atmosphere via wide OPEN spark plug holes. The piston going down will be sucking air IN through the ALL the open holes...therefore there will be no..or very little "Suction" at the carburetter throat capable of introducing fuel into the engine via the carburetter jets. :) NB

This is true on a naturally aspirated (carbed) engine, as a rule it's a good idea to pull all the plugs when conducting a compression test however it's not necessary per say. If the other plugs are not pulled and the throttle is held open the influx of gas is more of a problem. However there is no benefit to holding the throttle open so therefore no need to do it in the first place.



PS: Fuel Injection "may" be another story..depending on the Type of fuel injection system AND it's computer..??..controls..




Fuel injection is a different story, soon as the ignition is on the fuel pump kicks on providing gas pressure to the injection system. As a former factory trained tech for Toyota I can only comment on how Toyota does things.
Now when throttle is opened - the throttle positioning sensor relays this to the computer which in turn adjusts several systems as a result - including the delivery, open/close timing, and amount of fuel the injectors fire. Since Toyota primarily uses multi port injection each individual cylinder has a dedicated injector that delivers fuel directly into the combustion chamber. Yes it is possible to flood excessive fuel into the cylinders in this kind of setup therefore care must be taken not to over crank the engine.

Throttle body injection systems are much different in that it's a bastardized carburetor, where the naturally aspirated pressure to deliver gas through the jets from the float bowl is replaced with fuel injectors that deliver product directly into the throttle body. This system still is highly dependent on negative air pressure from the cylinders (suction action) to mix and expel the mixture through the intake to the cylinders. Since it is fundamentally less efficient the use of throttle body injection at least in regards to Toyota was abandoned in early 90s not that it was widely used in the first place. With the plugs out of the engine this kind of injection system is less likely to flood the engine.

One way around the flooding problem on a EFI equipped vehicle is to pull the EFI fuse so the injectors won't fire and the fuel pump will not kick on. Of course when you're all done you'll have to clear the CEL (check engine light) or MIL (Maintenance Req'd light) which will illuminate due to error codes from disconnecting stuff.

[NoBozo]

This is true on a naturally aspirated (carbed) engine, as a rule it's a good idea to pull all the plugs when conducting a compression test however it's not necessary per say. If the other plugs are not pulled and the throttle is held open the influx of gas is more of a problem. However there is no benefit to holding the throttle open so therefore no need to do it in the first place..

What you say is essentially correct. I'd just like to clarify the term "Naturally Aspirated" for those less technically inclined than you or I.

Naturally Aspirated has no particular connection to carburetted ...OR.. fuel injected engines.

Naturally Aspirated means the engine sucks its own fuel..naturally ...at atmospheric pressure from the fuel system. It's not pressurized.

The alternative is Turbo Charging....OR... Super Charging. Both methods are pressurized. They "Force Feed" the fuel into the intake system. :) NB

[trfour]

The alternative is Turbo Charging....OR... Super Charging. Both methods are pressurized. They "Force Feed" the fuel into the intake system. :) NB

Not quite, NB.
Forced Induction, being the opposite of, Naturally Aspirated. =, a Turbo Charger, and or a Super Charger are basically Air Compressors.
If they "Force Fed" fuel into a running engine, I would hope that you were waring your proper bomb attire! :)

Here's a video that will help explain. http://www.youtube.com/watch?v=MGdg2Fd2WQY

Terry
_________________________________

[Blue Thunder]

All the OP asked for was how to do a compression test....just sayin'

[NoBozo]

Not quite, NB.
Forced Induction, being the opposite of, Naturally Aspirated. =, a Turbo Charger, and or a Super Charger are basically Air Compressors.
If they "Force Fed" fuel into a running engine, I would hope that you were waring your proper bomb attire! :)

Here's a video that will help explain. http://www.youtube.com/watch?v=MGdg2Fd2WQY

Terry
_________________________________

I guess I wasn't clear enough. The compresssor (supercharger, etc) pumps Air MIXED with Fuel into the intake. I thought that would have been obvious.... Or maybe trfour is just busting my chops..:D :D NB

[ITD]

Ok, here's my 2 cents worth, have the engine warm if possible, remove the spark plugs, open the throttle wide. The function of the throttle plate is to restrict the air flow into the cylinders to maintain the proper air/fuel ratio for combustion. If the throttle is in idle position then air flow is being restricted and the resulting readings could be lower. The proper way to do the compression test is with the throttle wide open so that airflow is unrestricted into the cylinder.

[trfour]

I guess I wasn't clear enough. The compresssor (supercharger, etc) pumps Air MIXED with Fuel into the intake. I thought that would have been obvious.... Or maybe trfour is just busting my chops..:D :D NB

You and I both know that we are not the only ones reading these forum articles. No way would I want someone to investigate only to find that their turbo/supercharger didn't have a fuel line hooked up to it, and add a-line to it~. There would have been a whole new meaning in "Engine Compression test". :D :laugh:

Terry
___________________________

[bigdog]

WOW !

Who would of thought a simple question would generate so much infomration ??? But I guess it really doesn't have a simple answer, many various opinions.

Thanks everyone for your valued feedback, much to digest.

BD