Compression test On 08 mag 350 dts

Discussion in 'General Maintenance/Repair Questions' started by Coz, Jul 7, 2020.

  1. Coz

    Coz Member

    249
    Oct 9, 2018
    2008 Sundancer 310
    Merc Mag 350 DTS w/Bravo III
    Looking for instructions on how to do the test. Service manuals online are for older models. Also acceptable values.
     
  2. stephenm27

    stephenm27 Active Member

    441
    Nov 3, 2009
    Milton, GA
    Boatless. ISO 2010+ 330 Sundancer w Axius
    Remove all spark plugs, remove fuel pump relay (or unplug fuel pump) to prevent fuel flow while cranking. Screw compression gauge into a plug hole, turn engine over to capture pressure readings. Repeat with each cylinder.

    I used the start relay (on the manifold behind the throttle body) as the point to crank the engine—alligator clips with a simple momentary toggle.

    I was told all values should be above 100, with no variance greater than 10% between min and max values.

    This thread covers my experience:
    http://clubsearay.com/index.php?thr...on-start-compression-test.98136/#post-1133922
     
  3. Coz

    Coz Member

    249
    Oct 9, 2018
    2008 Sundancer 310
    Merc Mag 350 DTS w/Bravo III
    I assume you should disconnect the coil?
    I wondered with no fuel or ignition if the pcm would even allow a crank cycle.
     
  4. stephenm27

    stephenm27 Active Member

    441
    Nov 3, 2009
    Milton, GA
    Boatless. ISO 2010+ 330 Sundancer w Axius
    Good catch...

    Yes, I pulled the wire going to the ignition coil as I didn’t know if running it with no load would damage it.

    When you look at the start relay, you can decipher which contacts you need to jumper to cause the starter to crank. The PCM luckily isn’t involved since you’re doing the jumpering of the 12v signal directly to the starter.

    This was my first compression test and it was easier than it sounded.
     
  5. Mig22

    Mig22 New Member

    12
    Apr 24, 2016
    Enjoying life on the water
    2000 - 340
    2001 - 340
    Current 2001 - 380
    8.1 S Horizons
    Typically numbers found between 135ish-165ish the motor is still considered tight. Below you may want to investigate further, for example maybe a cylinder or two is on the low it’s possible the head needs a refresh or a ring job...I have also seen some motors with 110ish across all cylinders and still had a decent run life. Identifying variances and potential reasons is key.

    When performing the compression test take the boat out for a drive first and put some good heat into the motor specifically the exhaust. I have seen several times some mechanics just warm the engines in a slip or on muffs for 7-12mins; then poor readings result. Why because boat motors, unlike cars, have water cooled exhaust systems that intake water to cool and when not heated properly it tends to impact heat in the motor block and the compression readings of the cylinders accross the engine.

    I’ve always test in the following manner:
    1.) Heat motors with a good cruise
    2.) Test in order 1-8
    3.) Heat motors again with a good cruise
    4.) Test again but in reverse order 8-1
    5.) Compare testing results and retest any cylinders which have a number that is 12-20% below your highest number.

    Two other important factors when testing:
    (1)make sure the oil in the engine is clean (testing after a oil change is ideal)
    (2) Testing with throttle at WOT
     
    Chris-380 and joeyleggz like this.
  6. Coz

    Coz Member

    249
    Oct 9, 2018
    2008 Sundancer 310
    Merc Mag 350 DTS w/Bravo III
    Did you try just using the start/stop buttons?
     
  7. stephenm27

    stephenm27 Active Member

    441
    Nov 3, 2009
    Milton, GA
    Boatless. ISO 2010+ 330 Sundancer w Axius
    I did not. I wasn’t certain how quickly you can stop the pre-programmed crank cycle so I elected to use the start relay. If you use the start stop buttons, the PCM is involved and in control more than I wanted it to be.
     
  8. Coz

    Coz Member

    249
    Oct 9, 2018
    2008 Sundancer 310
    Merc Mag 350 DTS w/Bravo III
    Getting plugs out with a hot engine is gonna be a bear.
    Can’t open throttle on dts. Not sure how that affects the results
     
  9. stephenm27

    stephenm27 Active Member

    441
    Nov 3, 2009
    Milton, GA
    Boatless. ISO 2010+ 330 Sundancer w Axius
    I had the same concerns. In the end, I did my test cold (boat on trailer) without throttle manipulation. My numbers are in the other thread but suffice to say they were all valid. If you have marginal numbers, then perhaps consider these other factors...
     
    Espos4 likes this.
  10. Mig22

    Mig22 New Member

    12
    Apr 24, 2016
    Enjoying life on the water
    2000 - 340
    2001 - 340
    Current 2001 - 380
    8.1 S Horizons
    It can be a warm job for tight spots - gloves and long extensions are helpful. Unplugging most modern throttle body connectors allows the throttle valve to default to a limp angle/mode which in turn is enough to allow for good airflow during a compression test.
     
  11. Coz

    Coz Member

    249
    Oct 9, 2018
    2008 Sundancer 310
    Merc Mag 350 DTS w/Bravo III
    I don’t see your numbers in the other thread.
     
  12. stephenm27

    stephenm27 Active Member

    441
    Nov 3, 2009
    Milton, GA
    Boatless. ISO 2010+ 330 Sundancer w Axius
    • Cylinder 1: 152 PSI
    • Cylinder 2: 150 PSI
    • Cylinder 3: 143 PSI
    • Cylinder 4: 152 PSI
    • Cylinder 5: 150 PSI
    • Cylinder 6: 145 PSI
    • Cylinder 7: 150 PSI
    • Cylinder 8: 154 PSI
     
  13. Coz

    Coz Member

    249
    Oct 9, 2018
    2008 Sundancer 310
    Merc Mag 350 DTS w/Bravo III
    Thanks Stephen. Can you clarify how you jumped the start relay? I'm looking at a parts diagram and looks like there are 3 relays mounted behind the throttle body. Those are plug-in relays, so if I take the relay out, I should be left with the plug with female spade ends correct? I could take 2 pieces of wire, crimp on male spade terminals and insert into the plug. I'm guessing you used the wires that connect to the common and NO on the relay?

    I'm gonna try the start/stop pushbutton first, but with the fuel pump and coil disconnected, I'm not sure it will crank.
     
  14. stephenm27

    stephenm27 Active Member

    441
    Nov 3, 2009
    Milton, GA
    Boatless. ISO 2010+ 330 Sundancer w Axius
    Disclaimer: I'm having to do this from memory as my boat is currently inaccessible.

    There is a series of relays mounted atop the harness bracket located on the intake manifold. Mine happened to be still labeled. They are:
    • trim pump up
    • trim pump down
    • fuel pump
    • start
    • (can't remember)
    The start relay is the one you want to remove and jumper. Jump terminal 30 to terminal 87.
    [​IMG]

    Here's my harness:

    [​IMG]
     
    Chris-380 likes this.
  15. aerobat77

    aerobat77 Active Member

    605
    Aug 12, 2014
    germany
    2006 240 DA Sundancer
    5.0 MPI / Bravo III
    if you can,t open the throttle i have my doubts about the results !
     
  16. Coz

    Coz Member

    249
    Oct 9, 2018
    2008 Sundancer 310
    Merc Mag 350 DTS w/Bravo III
    That's exactly what I was thinking. Thanks.
     
  17. Coz

    Coz Member

    249
    Oct 9, 2018
    2008 Sundancer 310
    Merc Mag 350 DTS w/Bravo III
    Stephen's results were good. I also called Marine Max and talked to their head service guy, he said the throttle body position was irrelevant. I did read an article on-line that said the idle air circuit should provide plenty of air for a crank cycle given that is much lower rpm than idle.
     
    stephenm27 likes this.
  18. Coz

    Coz Member

    249
    Oct 9, 2018
    2008 Sundancer 310
    Merc Mag 350 DTS w/Bravo III
    Stephen, did you unplug the wire between the coil and distributor cap or did you disconnect the coil from the wiring harness all together?

    Did you have the key on? Wondering if the star relay has power with the key off.
     
    Last edited: Jul 9, 2020
  19. aerobat77

    aerobat77 Active Member

    605
    Aug 12, 2014
    germany
    2006 240 DA Sundancer
    5.0 MPI / Bravo III
    ok . then it might work !
     
  20. scoflaw

    scoflaw Well-Known Member

    Aug 10, 2011
    cape cod mass, cape coral fl
    1999 Powerquest legend 260 sx
    502 mpi Bravo 1
    [​IMG]

    Attach this to your slave or starter solenoid and you won't have to activate the starting system on your engine. Works with the key in the off position.
     
    Chris-380 likes this.

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