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  1. #1
    Redrum's Avatar
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    Question MSX 150 Compression Test

    Okay, so I know how to do this on a car with a carburetor, but I would just like an idea on this thing if there is anything different I should know. I searched around the forums but have only found references of people running the test, not instructions on how to do it.

    - Make sure the engine is dead cold.

    - Run the test with the other cylinder I am not testing having its spark plug installed.

    - Make sure the battery is fully charged (It's an AMG battery I bought last year)

    - Should I disable the fuel system by pulling that brown connection? Will the throttle body work when that is unplugged?

    - Hold the throttle wide open while cranking and crank the engine for about 5 seconds.

    Anything else?

    Thanks.


  2. #2
    ripcuda's Avatar
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    Cold or slightly warm engine... shouldn't really affect the readings. If the engine has sat forever... I'd probably want to run it shortly to ensure everything's "lubed up". But if the engine ran recently... then you should be fine doing it cold. I've done it both ways.

    Definitely want a healthy charged battery.

    Insert the lanyard.

    I like to pull the fuel pump wiring connector on top the fuel tank. This ensures the pump doesn't prime and even if the ECU triggers the injectors... there's no fuel being injected. In theory, when you pin the throttle while cranking... the ECU is supposed to NOT fire the injectors... so as to help clear a flooded engine... but I pull the pump plug just for peace of mind.

    Remove both coils and both spark plugs.

    Insert compression tester lead into the first spark plug hole.

    Pin the throttle (squeeze it to the handle... "wide open").

    Press the start button and allow engine to turn over until the compression gauge needle stop increasing... usually a couple three seconds... (then let go of the start button)

    Read the gauge.

    Relieve air from the gauge. Remove gauge lead and put in 2nd hole.

    Repeat.

    Some like to do two readings on each cylinder and average them. You're looking for 125-140psi in each and even across them both.

    Cheers!

  3. #3
    Iantryinit's Avatar
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    Does holding the throttle wide open make a big difference? Or just for easier cranking.

  4. #4
    Redrum's Avatar
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    I believe the theory behind opening the throttle is it allows air easier entrance into the cylinder to be compressed for more accurate reading.

  5. #5
    Click avatar for tech links/info, donation request K447's Avatar
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    Quote Originally Posted by Iantryinit View Post
    Does holding the throttle wide open make a big difference? Or just for easier cranking.
    Well, on normal engines with mechanical throttle, holding it wide open during compression test provides a consistent process. Throttle closed just means it takes more cranking for the engine to build up the pressure since it is working against a restriction in the air intake.

    And if the throttle plates are really fully closed then the engine is trying to suck in a vacuum and compress it. Whatever number your compression gauge reads is going to be greatly variable.

    Throttle wide open during testing means the cylinder gets fully filled with atmospheric air pressure, which then gets compressed. Just like when the engine is running at full throttle (but at lower RPM, of course).

  6. #6
    Redrum's Avatar
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    Okay we'll I guess I goofed it up. The instructions on the pressure tester said to let the engine get to operational temperature so I let the ski run on e hose for 3-5 minutes.

    Anyways, I tested them and got 128 psi on the front and 125 psi on the rear. Seems a bit low, but since I did not follow exactly what you stated I think I okay. I'll check it again at my next oil change and see if it has fluctuated at all.

  7. #7
    ripcuda's Avatar
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    It's fine to test with a warm engine. 128#-125#... does seem a tad on the low side... but is even enough... so I wouldn't worry about it for now.

    I can't seem to recall... did you rebuild this engine, Red?

    Cheers!

  8. #8
    Redrum's Avatar
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    No on the engine rebuild.

    Rip I got the ECU in the mail. I might have to redo all the steps of hooking it up. Everything seems okay, but it won't go above 7300 rpms.

    I think I found out why my engine is getting all wet. I trout it was the through hull bearing and replaced that. Turns out the section of the intercooler where the water flows through has a bad seal. All the screws are tight so I guess I am going to have to take it off and put a new seal in. Sucks as I just had it out last year. I hate trying to work on the damn IC.

    Also I am getting hesitation when I hit the gas while cruising to reach full power. My waste gate moves freely, but I am thinking I have a bad wastegate solenoid. It did this before I put in the reflashed ECU. I think I will pick up a new one and see if that fixes it.

  9. #9
    ripcuda's Avatar
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    Hey Red... that ECU in my ski rips up to ~7700 rpms depending on air temps and such... so you're not quite up to full power yet.

    So the large intercooler O-ring around the end where the fresh water hoses connect is leaking? Bummer... those ICs are a bear to get out with the engine in place. Do you think your loosing boost at all? If your leaking boost... you won't get max RPMs. The engine needs that boost to push it that high. I had this issue once where I keep having intake hose leaking issues... until I got the clamps put on right.

    Do you have the new style wastegate solenoid (on bracket above manifold off side of valve cover)... or the old style (attached to side of turbo intake scroll)? If you have the old style... yeah... I'd replace it. Although... when they go bad... they seize and keep the ECU from controlling boost levels... so your gonna get a check engine light for overboost more than likely.

    Cheers!

  10. #10
    Redrum's Avatar
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    I am not sure if I am leaking boost. All thew major intake lines are very tight, but the small vacuum lines might need to be all replaced... I am not sure on that and I don't have a set-up to pressure test it.

    I have the new style wastegate solenoid, but I have seen some posts with these going bad. I know testing them with a voltage is not enough as it can be affected whether it has pressure on it or not.

    When putting in the ECU I was following the steps of slowly opening, holding, then letting off the throttle. I think I was doing it too slow though as during the second time I was half way to wide open and the butterfly valve stopped responding at all (you can hear the high pitched noise of it). That is why I am thinking maybe it did not get done properly.

    I am in the middle of moving right now so no time to work on the ski.

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