Leakdown Test vs Compression Test? Help with a used boat.
 

I have a friend buying a Maxum 19ft bowrider with a Mercruiser 4.3. The boat only has 50hrs on it and has never been stored in the water. It looks new. I took a test ride on it and it drives fine. Do you think a Leakdown or Compression test is needed? and if so, which one? The marina wants to charge $125 for the compression test and $150 for the Leakdown. Im not sure why the Leakdown would cost more since I didnt think it takes any more time.
Thanks

Compression test is static and does the combustion area only... where a leak down test is dynamic and tests the entire intake thru exhaust plus the combustion area. Opt for the leak down test and they do take more time compared to a static compression only test.

Leakdown test. You can have good compression numbers and still have a crap engine. Leakdown test will tell the real story.

I paid for the compression test and it tested good. I noticed a lot of blow by and ran a leak down and discovered 2 bad cylinders. Engine had to be rebuilt. By the way mine had 50 hours and garage stored. Problem was it sat for long periods without being properly prepped

Thanks All. Does anyone know what numbers Im looking to hit with a Mercruiser 4.3?

Compression can be fudged a little by how many compression cycles you run it through .I good tight seal will shoot the pressure up pretty high on first stroke .Typically four compression strokes in each hole is what should be done. You can hear it compress and watch the needle rise on each compression stroke. If you just keep spinning it to the highest number it is not the best way and may show decent compression even when it is somewhat low ..Leak down is the way to go and does take longer. Not only will it tell percent of leakage, but also where it leaks ..intake ,exhaust or rings/cylinder

Is 4.3 a vortec motor

I think its this motor.

http://www.mercurymercruiser.com/mer...odel=4.3%20TKS

what year

Sorry,
Its an 08, the original owner took delivery in 2010 as a left over. Its an inexpensive 19ft Maxum bowrider with 50 hrs. Looks new. Ran fine during the test this past weekend.

Without looking it up I think it should be around 175-180#'s ..If i'm wrong i'm sure someone will chime in soon

Boat that size with the 4.3 should run 45 - 47 mph with a light load.

Leak test is a percentage, your fine if 15% or less. The 4.3 is pretty durable I put 2500000 hard miles on one and checked it before I sold it and the numbers where nearly perfect almost no leakage and compression were all within 2% of each other.

Leak down test are a %.
http://www.carcraft.com/techarticles...akdown_tester/

4.3 Merc listed the compression pressure 180 psi in late 80 & 90's service manuals. Late 90's and 2001 manuals, they Merc listed it as a min of 100 psi. Anyways for whatever its worth.

Also a leak down test is not 100% full proof. One example, I have seen some that shown around 8 to 12 % leakage and a compression test of 65 psi. Broken valve spring and when you put compressed air in the cylinder it helped push the valve tighter against the valve seat to help seal it. IMO both are important but not 100 % full proof.

The 4.3 / 262 CID is still not as relieable as a 5.7L 350. We see some valve problems from them and the carb intake manifold has it own drain plug to drain water out for winterization. Many people and shops miss that drain plug on the carb intake as it easily freeze breaks. Also very weak lifter valley more so on the port side will crack very easy from improper winterization.

The V-6 marine engines hate to be over reved if you do not have a rev limiter incorporated in the ignition system.

IMO I would not rule out a vacuum test for testing the internals of any motor. Marine MPI engines can have a tad lower vacuum reading of 15 hg or 16 hg even from a brand new motor - the reason is because of the IAC system incorporated in the intake. The old norm for a good motor at idle was 17 - 21 hg for a vacuum test and basically is the still norm for carb motors. .

Motor you are correct about the specs of 180 psi but as I said Merc changed it to a min of 100 psi for the current 4.3 's in their listings of the service manuals. the thing about compression testing is to have close to even #:s across the board and IMO 140 and up would be fine for a V-6. If I had to pick 2 test only, I probably would do a vacuum test or a leak down and a full blown lake test.

I was under the impression that the 4.3 was the same engine as the 5.7 with two less cylinders.

Thanks all. Can someone simplify, am I looking for a result of 140 per cylinder? Or is the number in percentage form?

They are having the test completed tomorrow. So is the final answer the leak down % should be 8-12%? Then its ok to buy the boat?

Thanks