Merc 7.4l water mixing with oil & low compression

thormx11

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Hello guys, I just purchased a new project for the winter so I am sure I will have a lot of questions along the way.

The boat is a 1997 Maxum 2800 SCR with a carbed 7.4l/bravo 3. I bought it knowing it had an issue or bad engine. Was told water was mixing with the oil. I never tried to crank it, I first pulled it out of the boat and drained probably 30-35 quarts of white oil/water milkshake. I was told it ran good on muffs for about 30 minutes until the milkshake oil started coming out of the breather tubes. Not sure how long the boat has sat with this stuff in the engine, was told it was about a month ago when ran.

I pulled all of the spark plugs and did a compression test. The cylinders on the port side (bank 1?) showed about 110 psi on cylinders 1 and 7 while cylinders 3 and 5 showed 65-75 psi. Cylinders 2,4,6,8 all showed roughly 90-110 psi. All of these numbers seemed low

I have been reading a lot of other forums online with similar issues and people say most of the time it is the exhaust manifolds/risers that are the problem. But I would think that most of the water would be pushed out of the exhaust valves unless maybe the rings are worn and causing the low compression and allowing water to seep by?

I am not sure which direction to head next, should I pull the heads off or go a different route?
 

alldodge

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You need a another motor.
There is a guy on offshore only selling used 502 drop in ready motors
 

thormx11

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Man that is not what I was wanting to hear! There's no way I could afford a 502. Most likely I will end up buying a reman 454 longblock. I also am going to need to be buying a bravo 3 upper which will cost as much as the longblock.
 

Scott Danforth

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you bought the boat knowing you need a motor

a motor that was probably run in salt and killed by lack of exhaust manifold maintenance

you may just need a head job..... wont know until you do a careful teardown

now pull the drive and the motor

then tear it down, and either fix what you got, buy a motor, or build a motor.

Here is a thread on my motor build if you want some light reading

https://forums.iboats.com/forum/eng...rds/10101672-bbc-build-don-t-tell-the-admiral
 

thormx11

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The drive was already off the boat when i purchased. The gears were already out of the upper drive and clearly shot. I have not started to tear down the outdrive yet, I am starting with the engine. I had already pulled the engine last weekend so I will probably tear the heads off and take a look at everything. The boat came out of NY and as far as I can tell probably spent its whole life in the salt water.

If it comes down to it I will put the boat back together and just float it in my slip for next summer if I can't get it up and running. I have a runabout that I can keep until I am sure I will be able to have this boat going.
 

tpenfield

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Probably a cracked engine block, but you could always do a cooling system pressure test to be sure.

As to your question about water in the cylinders . . . you will get some blow-by and water in the oil, but the volume of milk-shake that you described seems like a lot of water.

The cylinders that are very low on compression may have bent connecting rods if the water resulted in a hydro-lock situation.

I agree with the others . . . you probably need a new engine block. 454/7.4 blocks are a bit pricey. I would go for a long block, unless you can verify that the cylinder heads are in good shape . . . probably not if the boat has been used in salt water its entire life.

I assume it is a Gen 6 block, being a 1997 boat. You will also be buying new intake and exhaust manifolds/risers. The good news is that you can convert to a full closed cooling system and never worry about salt water.

you might be able to find a used 7.4L Mercruiser to drop in, but those can be hit or miss,

I hope you planned for enough cash to see this through. . . wrench turning can take you only so far . . . Merc 7.4 and Bravo combo are the priciest of the bunch to replace.
 

Scott Danforth

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a core gen 6 out of a truck or a suburban is between $350 and $400. a good machine shop can rebuild it for about $1500-$1800

if your motor is the 310hp or 330hp version, you have a full gen VI L29, then you are set, the same long block as the truck. I would recommend the comp cams XM284-HR cam for the L29 motor. it will bump the hp up by about 40-50hp and the torque up a great deal.

if you have one of the higher HP motors, you have the 088 heads on the motor. those will cost a bit more to locate the heads which are rectangular port.

in that case, your looking at about $2500 to do your long block. you still need new exhaust manifolds, a new circulation pump, and a new intake manifold to dress out the bobtail.

if you look around, you can find full motors in both a 454 or a 502 for about $2500-$300
 

alldodge

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All Bravo drives (1, 2 and 3) use the same upper. The gear ratios are the only things that change.

Look at OSO swap shop and if need be post in the wanted section. There are a few guys there like Sick Stigler which most times have good used stuff at decent prices. The 502 in many cases cost about the same as the 454
 

thormx11

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Thanks for all the info guys, I will probably start looking into getting a reman longblock. I believe it is the L29 engine 330hp (serial# 0c602211) The boat will stay in freshwater now so I will probably stick with raw water cooling and use the money for other things.

I had already planned on new exhaust, but how come I will be needing a new intake manifold as well? Is that something that is not reusable or is it probably trashed too?
 

alldodge

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The L29 was 1998-2000 and 7.4
The serial number you listed shows as a carb motor

Salt water flows thru the intake and out the thermostat housing
 

thormx11

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Yes mine is a carb motor. So would mine be gen V instead then?
 

Scott Danforth

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Thanks for all the info guys, I will probably start looking into getting a reman longblock. I believe it is the L29 engine 330hp (serial# 0c602211) The boat will stay in freshwater now so I will probably stick with raw water cooling and use the money for other things.

I had already planned on new exhaust, but how come I will be needing a new intake manifold as well? Is that something that is not reusable or is it probably trashed too?

because the stock cast iron intake manifold will most likely be rusted beyond usable because your boat was a salt water boat with questionable maintenance.

nothing special about the manifold. its an oval port carb manifold. if your in fresh water only, you could run an aluminum intake like the RPM air gap and simply epoxy coat the raw water passages. the down side is once the aluminum mixes with any brackish water, it will start to corrode.

or simply toss on a heat exchanger and run a half system since you will basically have a new motor at this point.

there are also marine specific versions of the aluminum manifolds with bronze lined water passages. these are offered by Edelbrock and Sierra. or you may find them with PCM or Volvo cast on them. they all come from Barr (casting house for Edelbrock and Mercruiser and Volvo, etc.)
 
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