1998 7.4L Merc 310 Compression

**semperfido** · 09-12-2012, 02:14 PM

So I am looking at buying a boat with T7.4L Merc 310's 1200hrs but just got the compression numbers back. The numbers are way more than 10% between cylinders. They didn't do a leak down test. Not my mechanic. Anyway hit me with concerns I should have. Are these motors gonna need to be rebuilt now or could I wait awhile. Do I have reason to be concerned? Are there questions I should ask the mechanic and if this is a big problem how much should I take off the price of the boat. I do like the boat so it might be worth my while to rebuild/repower if it needs it but need some advice. Numbers below. Thanks in advance

Port 160 155 Starboard 170 140
130 135 170 130
160 145 175 160
160 165 160 165

**Larry** · 09-12-2012, 02:36 PM

The numbers are just outside of the acceptable range, now you need to now if it is valves or rings. You need to redo the compresion test and drop a little oil into each cylinder first, if the compresion stays the same it is valves, if it goes up it is rings (the oil will seal up the rings a little just to enough to raise the compresion).

Make sure for the test the throttle plate is wide open and the motor should be warm.

**semperfido** · 09-12-2012, 03:05 PM

Larry I agree it needs a wet test but it was not my mechanic so I guess I will need to have it done again so I know what needs to be fixed. I thought it was way out of acceptable range. I always thought it was 10% between cylinders. Is that not the case.

**Larry** · 09-12-2012, 04:48 PM

I would put money on this being a valve issue, not a ring problem, as your lowest compresion numbers are within the center 2 cylinders and this is were water gets in when you have an exhaust failure in the gasket or the casting itself. yes 10% is the delta limits between high and low, but your numbers are not terable (the motor will run with these numbers, but it needs attention).

**semperfido** · 09-14-2012, 06:47 AM

Thanks Larry,

The broker said they were going to run decarbonizer in it and run it out and he thinks this will help. Says this happens a lot to boats that idle around and get carbon build up. I don't see how this would change compression numbers on different cylinders but I am not a mechanic. They are then going to recheck the numbers and let me know. If they come back around the same that would mean I would have to look into engine work and would have to counter that cost on price. Any idea how much a rebuild/repower on these 7.4's costs?