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Thread: Compression test

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    Default Compression test

    Is 145-150 psi on a compression test gauge normal for an engine with 119,xxx miles on it?
    1976 GHLS30-036614
    1946 Willys CJ-2A

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    Registered User Kurbycar32's Avatar
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    That seems low but the goal of a compression test is to be consistent. If you get within 5% of each other across all pistons you are generally in good shape. My 260z instruction manual says that compression should be 171-185psi. You should repeat the test and do every cylinder. Remember to let the motor crank (not start) a few rotations. Its supposed to build up to and hold 180psi, not make it on one throw.
    Early 1974 260z
    https://sites.google.com/a/thecomputerrehab.com/260z/

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    The variety of testers and the various hoses and adapters that come with them can have a large effect on the numbers. I've measured 180 psi without an adapter and 120 psi with. The volume of the gauge and the hose and any fittings "count" as combustion chamber volume when the measurement is taken. More volume results in a lower pressure reading.

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    Are you supposed to hold open the throttle flap when you check compression? I'd like to check mine too.

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    Registered User Kurbycar32's Avatar
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    Quote Originally Posted by siteunseen View Post
    Are you supposed to hold open the throttle flap when you check compression? I'd like to check mine too.
    Nope its not necessary and may actually flood the motor with fuel.
    Early 1974 260z
    https://sites.google.com/a/thecomputerrehab.com/260z/

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    I think that purpose to opening the throttle blade is so that the closed intake manifold vacuum doesn't affect the pressure readings. After one cylinder pulls a vacuum and closes its intake valve, the next cylinder to open its intake valve will be pulling on the previous cylinder's vacuum. Less air to pull in will be less air to compress leading to lower pressure readings. In theory, I haven't practiced it.

    On an EFI car there won't be any fuel to flood with if the main distributor wire is pulled,or if a remote starter is used. The EFI injects based on pulses from the spark events. No spark, no fuel.

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    I have the trigger remote starter that clips on the starter bypassing the switch and the fuel injection. Sorry I didn't mention that. I was thinking the flap in the mouth of the throttle body would let more air in and increase the compression reading.

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    Registered User LeonV's Avatar
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    Quote Originally Posted by Kurbycar32 View Post
    Nope its not necessary and may actually flood the motor with fuel.


    Yes, throttle must be WOT. I've disconnected the fuel lines before to do this, but in reality, with the pedal floored and the starter cranking the engine, there is no way it'll have enough of a vacuum signal to draw enough fuel to start, let alone flood the engine. All plugs out, WOT, and you're good to go.

    FWIW, EFI cars usually engage a "flood-clear" mode if you floor the throttle while cranking, meaning injector PW goes to zero.

    EDIT: I see that the OP has a 280Z. Refer to L-Jet EFI guys above for why "flooding" is not a problem.
    Last edited by LeonV; 11-06-2012 at 10:38 AM.
    2/74 260Z

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    My bad, I was referring to carbs on the flooding thing. With the fuel pump running, the motor cranking and no spark its flooding time on a carbureted vehicle. I still dont see how its necessary to open the throttle on a FI motor though. I did a compression check on a 2003 saturn 3 weeks ago, the motor has plenty of air coming in to build up compression without operating the throttle and thats the same state as a normal startup.
    Early 1974 260z
    https://sites.google.com/a/thecomputerrehab.com/260z/

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    Registered User EuroDat's Avatar
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    My 280Z has between 160 and 170psi with 131k miles on the original motor.

    The General rules I follow for all engines I have tested:
    1: Run the engine to operating temp and then remove all spark plugs.
    2: Hold the throttle full open. That makes a differance by preventing vacuum buildup.
    3: Non EFI; Don't keep pumping the gas peddle between tests. If its a carby model the fuel can dilute oil around the piston rings, which can lower the compression reading.
    4: Crank engine 6 to 8 revolutions per cylinder test (always do the same amount of revolutions).

    The figures you get depend on several thing. Compression ratio, age of the engine, NA or turbo all have an effect. All the figures should be are around 5% of each other.

    EuroDat

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    Quote Originally Posted by Kurbycar32 View Post
    My bad, I was referring to carbs on the flooding thing. With the fuel pump running, the motor cranking and no spark its flooding time on a carbureted vehicle. I still dont see how its necessary to open the throttle on a FI motor though. I did a compression check on a 2003 saturn 3 weeks ago, the motor has plenty of air coming in to build up compression without operating the throttle and thats the same state as a normal startup.
    A carburetted vehicle will flood, if you keep the throttle closed. At WOT, the vacuum signal is not sufficient to draw out an appreciable amount of fuel.

    It's SOP, throttle must be wide open in order to get proper, repeatable readings.
    2/74 260Z

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    I don't think there is much risk of flooding regardless of the throttle position. I don't think it matters.

    The carbs supply fuel proportional to the air flow and inversely proportional to the venturi area. If you've got the plugs pulled, then you'll have minimal air flow. The only cylinder that would be pulling any air in through the carb will be the one currently under test. The other five will be huffin-n-puffing through their spark plug holes.

    I'm thinking that even if you do manage to pull any fuel into the one cylinder under test, it'll dry out as soon as you move to the next cylinder?

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    Quote Originally Posted by EuroDat View Post
    Don't keep pumping the gas peddle between tests.
    throttle open or closed does not matter with fuel ratio. Its the plunger that keeps discharging extra fuel for acceleration. That happens ever time you pump the gas peddle. Engines with downdrafts are effected most because gravity helps the fuel flow to the engine. Sidedrafts have less trouble because they dont have the velocity to pull the fuel into the engine when cranking.

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    Quote Originally Posted by Captain Obvious View Post
    I don't think there is much risk of flooding regardless of the throttle position. I don't think it matters.

    The carbs supply fuel proportional to the air flow and inversely proportional to the venturi area. If you've got the plugs pulled, then you'll have minimal air flow. The only cylinder that would be pulling any air in through the carb will be the one currently under test. The other five will be huffin-n-puffing through their spark plug holes.

    I'm thinking that even if you do manage to pull any fuel into the one cylinder under test, it'll dry out as soon as you move to the next cylinder?
    I agree, I don't think flooding is a concern. I should have said, keeping the throttle closed (on a carburetted engine) while doing a compression test will suck more fuel into the cylinder. Whether that floods the cylinder or not depends on how long it takes the operator to perform the comp check.

    Quote Originally Posted by EuroDat View Post
    throttle open or closed does not matter with fuel ratio. Its the plunger that keeps discharging extra fuel for acceleration. That happens ever time you pump the gas peddle. Engines with downdrafts are effected most because gravity helps the fuel flow to the engine. Sidedrafts have less trouble because they dont have the velocity to pull the fuel into the engine when cranking.
    EFI and round-top SUs will not care how many times you pump the pedal between tests. Triples, 4-barrels and other carbs with accel pumps will...
    2/74 260Z

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    Different guages will give you different readings. The important data from your test is how close the numbers are together. I remember I got 185-190 on each cylinder l28 flat tops.

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