Compression test On 08 mag 350 dts

Coz

Active Member
Oct 9, 2018
443
Boat Info
2008 Sundancer 310
Engines
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.
 
Looking for instructions on how to do the test. Service manuals online are for older models. Also acceptable values.
Looking for instructions on how to do the test. Service manuals online are for older models. Also acceptable values.

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
 
I assume you should disconnect the coil?
I wondered with no fuel or ignition if the pcm would even allow a crank cycle.
 
I assume you should disconnect the coil?
I wondered with no fuel or ignition if the pcm would even allow a crank cycle.
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.
 
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
 
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.
Did you try just using the start/stop buttons?
 
Did you try just using the start/stop buttons?
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.
 
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
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
 
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

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...
 
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

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.
 
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...
I don’t see your numbers in the other thread.
 
I don’t see your numbers in the other thread.
  • 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
 
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.
 
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?

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.
uc


Here's my harness:

uc
 
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.
uc


Here's my harness:

uc
That's exactly what I was thinking. Thanks.
 
if you can,t open the throttle i have my doubts about the results !
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.
 
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:


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.
 

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