1998 350 mercruiser high compression

[Chance1234]

this is a pre purchase Compression test that I had a mercruiser mechanic conducted for me. I'm in MA and the boat is in CT so I was not able to be there for the test but I hired a well know shop to conduct the test. I don't have the exact numbers until later today when I go back down for a power trial but the range of compression numbers were between 178-211 psi. These numbers seem high to me as I have researched on here and normal seems to be around 150-160. The engine has 1100 hours on it according to broker and the gauge. I have a survey from the boat and the survey was done in December of 2015. it states that the engine hours was 1089 at that time. 3 days ago when I looked at it for the first time the guage said 1091. Assuming that gauge still works that would mean the previous owner put only 2 hours on the engine in 9 months. Would this cause high compression numbers? I'll have to talk to the mech that did the test today to see if there was any water in the cylinders. Supposedly the manifolds were done in 2012. Should I walk or run from this sale or buy it. I owe the mechanic $300 already for compression and outdrive inspection so I don't want to put a bunch of money into a boat I don't own. The test was done warm after running on the hose for 20-30 minutes. I know merc states a 30% variation is acceptable but the widley accepted common man range is 15%. Thoughts please. Power trial today at 3pm followed closely by a big decision.

 

[tpenfield]

Usually high compression numbers are due to liquid in the cylinders . . . water . . . maybe oil . . . hard to tell unless you take a look. Did the mechanic have anything to say about the compression numbers? I would hope so.

 

[bman440440]

the factory spec is 150-175... on my vortec 4.3 all 6 are from 168-176... and it is spec'd at the same #'s as the 350 FYI... so it seems like there is something going on with that engine... now I don't know for sure that it is a problem with those #'s but me personally I would not want to risk it and look for another boat... $300 is well spent if it keeps you from dropping 3-5K on a new engine... just my 2c

 

[Chance1234]

the mech only said that it was within spec. I have many more questions for him when I pay up today. howd the plugs look? see any water? etc...

 

[Chance1234]

during power trial today I plan on running WOT for a while although I know it won't make WOT because according to the survey I have it has 26 pitch props on the duoprop and should have 24 pitch. but then I'll try to check for blow by and water in the oil after it has run for a while. after I get the report for the cylinders from the mech I may redo any suspect compression numbers. even my low number is high from what the standard should be.

 

[Scott Danforth]

either his gauge is off, or there is stuff in the cylinders (water, oil, possum etc) like Ted mentioned

 

[JustJason]

Different brands of gauges will read differently. Cheap gauges can read excessively high or low. If there is any question, using another gauge is the same thing as getting a 2nd opinion, and 1 or 2 cylinders should be retested with a different gauge. It won't take that long to retest say the front 2 cylinders only.

 

[harleyman1975]

1100 hrs is getting long in the tooth. Remember a boat is like driving around in low gear with a loaded trailer uphill all the time.

 

[bman440440]

1100 hrs is getting long in the tooth. Remember a boat is like driving around in low gear with a loaded trailer uphill all the time.

I got 3100 out of my last boat with mercruiser 5.7 and sold it still running strong!!

 

[Chance1234]

ok, just got back from "power trial" notice the quotes...

here are the exact compression numbers:

1 185psi 2 193psi
3 184psi 4 202psi
5 178psi 6 200psi
7 210psi 8 199psi

towed it to the ramp, put it in the water, fires right up, go to back off the trailer and we have no forward or reverse. It's like its stuck in neutral and just revs when the throttle is applies. We pull the controls apart and it appears as if something in the control box is jammed up. it will not move over the full range of motion. We played around with that for an hour with the minimal tools we had and then the broker had to meet another appointment. Do the above pressures show anything to you pros? Thank you.

 

[G_Hipster]

This a hot rodded engine? Those numbers seem high for pump gas compatible. I'm with the others, another test with a different gauge is in order. I would test the highest and lowest first, or walk away. Excessive carbon build up from oil consumption.

 

[Pete104]

As said before, you could put another gauge to it but it will probably get similar results with different numbers.
I don't see anything alarming. They are well inside the 15% rule.

As far as accuracy of comp testers. I use Actron comp & fuel pressure gauges. Sure there are more expensive but are they really better?

 

[Chance1234]

so should I pony up another $200 for a compression test and request they use another gauge. I really don't feel like driving 3 hours to stand over there shoulder to make sure.. I'm not a rich guy by any means being in the service so its kinda a tough pill to swallow. especially if it comes out the exact same.

 

[Scott06]

Only time I've seen compression reading that high have been on my 65 GTO, which was like 195 psi post engine rebuild- I suspect this is from having a 10.75:1 compression ratio. Same guage on my merc 3.0 was 145-155 psi. Unless heads were swapped for smaller combustion chamber heads, Most likely the gauge isn't accurate and as stated above I'd try with a different gauge.

 

[Pete104]

so should I pony up another $200 for a compression test and request they use another gauge. I really don't feel like driving 3 hours to stand over there shoulder to make sure.. I'm not a rich guy by any means being in the service so its kinda a tough pill to swallow. especially if it comes out the exact same.

No. There are other issues that are going on with the drive. Let them figure all that out.
I can't say that these guys "aren't" working in your favor. But a close visual inspection could tell if any major engine work had been done in the past. If they aren't offering anything the comp test, walk away.

Thanks for serving :usa:

 

[tpenfield]

Why do you want this boat when it has no signs of being good for you :noidea:

 

[Chance1234]

Why do you say it has no signs of being good for me? Could be a great boat that someone screwed up the compression test. And it needs a new throttle cable. I don't know. That's why I ask questions. So I can learn.

 

[tpenfield]

Why do you say it has no signs of being good for me? Could be a great boat that someone screwed up the compression test. And it needs a new throttle cable. I don't know. That's why I ask questions. So I can learn.

The reason I bring this up is that all too often we see folks focus on a particular boat that has issues and try to rationalize it to themselves, when they may have been better off pursuing a different boat. So, I posted that so that you would not go down that same path, but rather keep your options open.

There is that old saying . . . if something waddles like a duck and quacks like a duck, it is probably a duck.

 

[Chance1234]

I appreciate your thoughts. I'm pursuing all options with this boat mainly because of the hull. I have been a Seacraft owner all my life and never thought I'd look at another hull. This is essentially a seacraft hull with more storage options and its newer obviously then all the 70's era seacrafts that I would look at. The boat in question is a 24 Silverhawk and has gorgeous line and is a fishing machine which is what I would primarily use it for. There are only a very limited amount of Silverhawks on the market at any given time. I think 9 right now around the country. I have completely redone Seacrafts before and I don't have the time or energy to rebuild one anymore. So either I try to find one already done or look at Silverhawks which are also good family boats (with marine head and swim platform) witch my wife and two young kids love.

 

[Chance1234]

Bondo, what say you??