Engine compression test ?

[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]

Originally Posted by NoBozo

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.

Originally Posted by NoBozo

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]

Originally Posted by MAXUM

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]

Originally Posted by NoBozo

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
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[NoBozo]

Originally Posted by trfour

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.. NB

[trfour]

Originally Posted by NoBozo

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.. 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".

Terry
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[Blue Thunder]

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