7.4 Mercruiser overheat

[D Bell]

You may be getting air in the system before the pump. That would explain the no water at idle and water flow at 1,000 rpm.

As Bill mentioned……it is something simple. The pump could even have a crack in it.

Just did the clear vinyl hose trick:

at idle - nothing
at 1,000 - gurgling flow
at 2,000 - good flow but bubbly

Air ingestion or blockage?

 

[PlayDate]

Where did you insert the clear hose?

There is one way to settle this today. I wouldn't be jumping up and down to do it on a 340 but I'm a big guy. Pull the pump off the working engine and put it on the troubled one.

That will stop the guessing (me included) and set you on the right direction.

One of two things will be true:

The troubled engine will flow water perfectly.
The troubled engine runs the same as it did before the pump swap.

If it runs the same.......you have a blockage in the system after the pump or something is blocking the thru hull (more on that in a moment).

At this point the clear hose is inconclusive. The clear hose should have water flowing through it at idle. At a 1,000 rpm the hose should be full and no bubbles. At 2,000-3,000 rpm small bubbles will appear from time to time.

It is possible a blockage on the thru hull could be the problem. The easiest way to tell is to remove the strainer caps for both engines with the valves closed. Then open the valves to see if they overflow the same rate. If they don't there is a good chance something has clogged up the thru hull. BTW I think this is unlikely to be the case because the engine would be overheating across the entire rpm range.

 

[D Bell]

Where did you insert the clear hose?

There is one way to settle this today. I wouldn't be jumping up and down to do it on a 340 but I'm a big guy. Pull the pump off the working engine and put it on the troubled one.

That will stop the guessing (me included) and set you on the right direction.

One of two things will be true:

The troubled engine will flow water perfectly.
The troubled engine runs the same as it did before the pump swap.

If it runs the same.......you have a blockage in the system after the pump or something is blocking the thru hull (more on that in a moment).

At this point the clear hose is inconclusive. The clear hose should have water flowing through it at idle. At a 1,000 rpm the hose should be full and no bubbles. At 2,000-3,000 rpm small bubbles will appear from time to time.

It is possible a blockage on the thru hull could be the problem. The easiest way to tell is to remove the strainer caps for both engines with the valves closed. Then open the valves to see if they overflow the same rate. If they don't there is a good chance something has clogged up the thru hull. BTW I think this is unlikely to be the case because the engine would be overheating across the entire rpm range.

Thanks for the info.

I inserted the clear hose on the incoming side of the Trans Cooler - nice, easy and visible. Prior to that, I only have the 1 1/4 flexible hose, the impeller, the wire rigid hose and the through hull. Everything else has been eliminated. They are all down where I don't fit.

I'm calling in for help to pull the pump and hoses. I have a spare pump in hand and can swap it out.

Would a partially broken impeller be causing this? I know a cracked casing will. Should I hear the vacuum sucking?

Wouldn't blockage have limited flow "all the time"?

 

[PlayDate]

Thanks for the info.

I inserted the clear hose on the incoming side of the Trans Cooler - nice, easy and visible. Prior to that, I only have the 1 1/4 flexible hose, the impeller, the wire rigid hose and the through hull. Everything else has been eliminated. They are all down where I don't fit.

I'm calling in for help to pull the pump and hoses. I have a spare pump in hand and can swap it out.

Would a partially broken impeller be causing this? I know a cracked casing will. Should I hear the vacuum sucking?

Wouldn't blockage have limited flow "all the time"?

My money is on the pump and specifically the impeller. I have seen them spin on their hubs and behave exactly they way yours is behaving now.

Change the pump and impeller with one you trust.

A blockage creates resistance that does not go away. However, increased pump pressure (higher rpms) pushes more water down the other open paths which can cool the engine well enough to keep the temperature in line.

I don't think it is possible to hear a pump problem over the exhaust/belt sounds of the engine running.

 

[D Bell]

The sage is over and shame on me!

I purportedly bought an OEM Mercruiser rebuild and repair kit (2) from an on-line wholesaler (name withheld until I hear direct from them). Failed miserably. The tines have turned to mush and remnants of rubber is cooked (not shredded, but melted) all over the sides of the body. The impeller is a hard plastic D flange key, a cheap imitation.

The OEM impeller I bought from my local dealer yesterday (and installed in the old original body) has a brass D insert key. System is running perfectly now and the entire (raw and closed) cooling system on both engines is as clean as a whistle. Now just waiting on the Starboard side to go bad (wallet needs a break).

I learned a lot. This boat is a precursor to a Trawler for the Great Loop. I have learned another system and am moving farther into what I need to know for the future.

Thanks everyone for the help! Much appreciated.

 

[PlayDate]

The sage is over and shame on me!

I purportedly bought an OEM Mercruiser rebuild and repair kit (2) from an on-line wholesaler (name withheld until I hear direct from them). Failed miserably. The tines have turned to mush and remnants of rubber is cooked (not shredded, but melted) all over the sides of the body. The impeller is a hard plastic D flange key, a cheap imitation.

The OEM impeller I bought from my local dealer yesterday (and installed in the old original body) has a brass D insert key. System is running perfectly now and the entire (raw and closed) cooling system on both engines is as clean as a whistle. Now just waiting on the Starboard side to go bad (wallet needs a break).

I learned a lot. This boat is a precursor to a Trawler for the Great Loop. I have learned another system and am moving farther into what I need to know for the future.

Thanks everyone for the help! Much appreciated.

No reason to feel bad......you solved it. Congratulations.......I like happy endings.

 

[Scott215]

I'm glad your good to go

 

[D Bell]

Thanks all. I called out the shop and they refunded the kit costs (both).

I should have checked the parts first. I know what OEM is!

…..and I learned so much in the hunt.

On to the fresh water pump. My original is 2.8 gph and starting to leak. Buddy suggested a 5 gph would fit and an easy swap.

Thoughts?

 

[PlayDate]

Thanks all. I called out the shop and they refunded the kit costs (both).

I should have checked the parts first. I know what OEM is!

…..and I learned so much in the hunt.

On to the fresh water pump. My original is 2.8 gph and starting to leak. Buddy suggested a 5 gph would fit and an easy swap.

Thoughts?

Check the amp draw.

 

[D Bell]

?????? Wasn't expecting that. Reasoning?

 

[PlayDate]

?????? Wasn't expecting that. Reasoning?

Sometimes doubling the capacity of the pump translates into an almost doubling of the amp draw. Your current pump (2.8) draws 7 amps. Doubling that puts you at 14 amps. At that point, wire size and fuse size for the pump matter since you don't want to pop fuses/breakers or heat up the wire.

 

[Scott215]

Just check the amp draw on the pump I don't think it'll be a big deal check it anyway and see what they want for inline fuse

 

[D Bell]

Thanks. After thinking overnight and having owned a very old house, additional water pressure can't be good for a 23 year old system. I think I’ll stick to the 3 gph range and gain more amp efficiency with a newer model. Trying to better understand battery needs as we like to stay out.

 

[94SD370]

There is no downside to pulling the heads to see what is going on if you are doing the work. If the valves are not sealing......the numbers for compression as well as leak down will be all over the place.

I wouldn't read anything into those numbers at this point.

I may have found the cause of my overheating. Through the process of pulling the head, I found the riser elbow inlets to be partially obstructed by gasket material. Picture below.

Also, isn’t there supposed to be a flapper valve on the riser elbow?


*** disregard. I think the gasket covers are supposed to cover those holes. I have closed cooling. The hunt continues…****

Still working on the head, chasing the low compression issue.

 

[PlayDate]

Those pieces look okay.

 

[94SD370]

Those pieces look okay.

I’m curious, the exhaust manifold gasket on the (boat) left side shows signs of leaking. Could a leaking exhaust manifold gasket cause an overheat?

Also worth mentioning, I don’t believe these manifolds have ever been changed. I was able to remove both sides with ease. I was expecting the bolts to be seized, and was prepared for at least one to break. Some bolts I was able to back out with little resistance, others a little effort with a breaker bar. Maybe they were not torqued correctly? I just expected that to be much more difficult. FWIW, I did soak the bolts in Kroil for most of the summer. Lucky?

 

[PlayDate]

I'm speculating that you mean it was leaking exhaust not fluid. In that case not likely the source of your problem.

On another subject......has it been losing antifreeze in the problem engine?

 

[94SD370]

I'm speculating that you mean it was leaking exhaust not fluid. In that case not likely the source of your problem.

On another subject......has it been losing antifreeze in the problem engine?

Exhaust. I believe so, but it’s hard to tell. I’ve had it apart so many times this summer. Each time I lose some. I ran out of time today, I will be back at it mid week. Hoping to pull that head and see if it has a story to tell.

 

[94SD370]

I'm speculating that you mean it was leaking exhaust not fluid. In that case not likely the source of your problem.

On another subject......has it been losing antifreeze in the problem engine?

Got the head off. Cylinder 6 is the one with low compression. It appears (to me) the gasket is blown between 6&8.

 

[94SD370]

Those pieces look okay.

A look at the cylinders. I am concerned about the scoring on #4. My compression numbers (except 6) are in the 100-120 range. I’m wondering if I can get some more life out of this one with just a new head gasket. I was going to get the heads redone. Maybe it’s not worth it at this point. This is my first time doing this, is this a punt situation?

The circle on cylinder 8, is coolant. I’m not sure if it leaked down there during the disassembly, or if it is from the blown head gasket.