knock on '05 21v w/ Indmar 325 - boat shopping

[RCD_Wake]

Hi all, This is my first time on this forum -- and I am *hoping* to be a Supra owner very soon (depending on answers to the below ha)...

Hoping for a little bit of help here. I test drove a 2005 21v today with the Indmar "assault" 325 5.7L, 230 hrs per the dash, no maintenance records to speak of. There was a knocking at idle and low RPMs, not sure that it completely went away at higher RPMs / under load, but it was definitely harder to hear (with the compartment closed it was completely undetectable -even hard to hear at idle - with it open it was still tough to hear if the knocking was even happening anything above ~2500 rpm). Based on some research it sounded almost like it could be a rod knocking, but I am a bit too novice to really be able to identify 100%, plus it didn't get louder or worse with high RPMs and the engine under load (including after filling the 1250lb ballast). I've asked the dealer for info/diagnostics on it, but since they're trying to sell me the boat I'm not certain I trust their diagnosis.

The dealer did say they inspected and compression tested the engine and everything checked out, though I have not personally seen the results. The knocking was relatively mild but definitely not something they would miss. On the test drive, the boat ran and performed beautifully other than the knock, even had a very smooth and normal idle (maybe 600-700rpm). It is also in phenomenal condition (garage kept practically show room condition boat), so I'm trying to determine if the knocking is a reason not to buy.

Couple of questions:
- Would the compression test detect something wrong with a rod?
- Could the compression testing detect any kind of OTHER (non rod) knocking problem?
- Are there other common piston-knocking sounds that are not as major as a rod issue?
- Would a rod knocking issue ever be an easy or cheap fix (from the research it sounds pretty major across the board)?
- Any idea on a ballpark repair cost if it IS a rod (considering its not something I am skilled enough to do myself)?

- What other questions should I be asking about this to probe the dealer and help figure out my level of risk?

Thanks for the help!

[RCD_Wake]

One thing I thought I should add --

This is being sold by a Malibu dealership (certified Indmar service center) who says they "certify" all of their preowned boats, which includes the inspection/compression test. I have asked for a formal diagnosis of the knocking by one of their Indmar mechanics before I agree to buy. Not sure if knowing that it is an official Indmar service center changes anyone's thoughts on this.

I am speaking to the mechanic on Monday or Tuesday of next week, would love to hear some feedback before then.

[tg0824SSVGG]

I had a "knocking" that was a bad head gasket -- it was easy to fix (it was under warranty at the time) - but would not have been had I ignored it.

[RCD_Wake]

Thanks for the reply.

Any idea if a bad head gasket would show up in a compression test?

Were there any other symptoms other than the knocking (ie, white smoke, the smell normally associated with a blown head gasket, etc.)?

Nice boat by the way

[RCD_Wake]

Just got the compression readings:

1 - 165
2 - 150
3 - 150
4 - 155
5 - 160
6 - 155
7 - 150
8 - 155

within about 10% but all seemingly on the low side (I believe it should be in range of 150-190, so this is at the bottom)

[michael hunter]

A compression test has nothing to do with a knock. A compression test checks the seal of the piston to the cyl , head gasket and valves. There are many things in an engine that can cause a knock unforgettably most are expensive and will require the engine to be removed and disassembled. Is this boat at a dealer? Will they give you a written warranty concerning the knock? Will they allow an independent surveyor to check it for you? A knock can be a nothing to worry about or a major problem if you have to replace the engine you will spend as much as 10,000 dollars. Make sure you know what is causing it before you buy. Where is the boat?