7.4 MPI overheating

[Lucrative]

Mercruiser 7.4 MPI Gen VI (1998-2000 I think)
www.boats. net/parts/search/MercuryMarine/0L609594/2705/parts.html <- link to my engine.

I have replaced the exhaust manifold, risers, themostat, t-stat housing, impeller.

Engine will stay at 160 on the hose. When I took it out, it would begin to climb over 160 when I go 20 mph or so. It will stay cool running at about 15mph. I think the only thing I haven't replaced at this point is the engine water pump. Not sure where to go here other than the engine water pump.

 

[DeepBlue2010]

I just read a thread yesterday with the same issue and finally it turned out to be the engine water pump. I would start replacing it if I were in your shoes.

 

[Lucrative]

Yeah, I might as well, as far as I know it's not been replaced and it seems like the only option left for me.

Is there a water flow diagram somewhere for my engine? I am pretty new to this, but I have rebuilt the top of the engine and still do not fully understand the flow of water after it gets to the t-stat housing.

 

[skippy2235]

This sounds like an air leak in the out-drive pickup tube.
Pickup tube to the impeller should be air tight.
When on muffs you are pushing water up the tube, hose pressure.
When driving in water, you are sucking water up tube. Exhaust is also on the outside of tube. If not air tight air / Exhaust will get in tube and overheat.
When driving faster that will sometimes force water up tube like on the muffs.

 

[MarkSee]

Yeah, I might as well, as far as I know it's not been replaced and it seems like the only option left for me.

Is there a water flow diagram somewhere for my engine? I am pretty new to this, but I have rebuilt the top of the engine and still do not fully understand the flow of water after it gets to the t-stat housing.

Not sure if you have fresh or raw water cooling but start here and see if you can follow the arrows....fresh water a few pages further...

http://boatinfo.no/lib/mercruiser/manuals/mercruiser23.html#/852

Mark

 

[DeepBlue2010]

The water circulation will vary depends on what cooling system do you have (raw sea water, 1/2 closed, closed etc) In general, while the engine is cold at start and up until it reaches operating temperature, the thermostat remain closed and doesn't allow any cooling water from outside to cool the engine. In effect, the engine water pump keeps circulating the same coolant (water or anti-freeze depends on the system) inside the engine. The reason for this is to bring the engine to operating temp ASAP. Cold engines are not efficient. After the engine reaches operating temp, the thermostat opens and outside cold water is allowed to cool the engine either by circulating directly through the water jackets (raw water systems) or to extract the heat from the anti-freeze in a heat exchange (closed systems).

 

[alldodge]

Is there a water flow diagram somewhere for my engine?

Do you have open or closed cooling?

 

[Lucrative]

I have an open cooling system

 

[alldodge]

Have you checked your power steering cooler to see if it is clogged?

 

[Lucrative]

Alldodge

Yes, I took the hose off the tstat housing that runs from the oil cooler and took off the hose at the impeller and ran a hose through it. Seemed to be flowing fine past the oil cooler, fuel cooler and power steering cooler.

Are there any other locations that could be clogged that would cause these symptoms?

 

[DeepBlue2010]

Try to reverse flush the system. Disconnect the out hose from the seawater pump and the in hose from the thermostat housing. attach a garden hose to the latter and see what comes out of the end of the hose that supposed to be connect to the seawater pump

 

[alldodge]

Alldodge

Yes, I took the hose off the tstat housing that runs from the oil cooler and took off the hose at the impeller and ran a hose through it. Seemed to be flowing fine past the oil cooler, fuel cooler and power steering cooler.

Are there any other locations that could be clogged that would cause these symptoms?

Yes what Deep said. Need to check the side of the cooler closest to the pump. You could have pieces of the hose stuck in the cooler tubes and water will pass right through the other tubes not blocked. Remove the hose from the cooler end coming from the raw water pump and look inside, even with a camera on your phone

 

[Lucrative]

I'm confused. I thought I did what you guys are describing already. I removed the hose from the t-stat housing and then took the other end off the impeller housing. I put the hose into the end that connects to the t-stat housing and ran it. This should flush out anything stuck in the oil cooler, fuel cooler, and power steering cooler, as they all run in line consecutively, right?

 

[alldodge]

I'm confused. I thought I did what you guys are describing already. I removed the hose from the t-stat housing and then took the other end off the impeller housing. I put the hose into the end that connects to the t-stat housing and ran it. This should flush out anything stuck in the oil cooler, fuel cooler, and power steering cooler, as they all run in line consecutively, right?

Don't think you have all three but it may flush the items out but not always. While the water from the hose has volume, it does not have enough to make sure all items have been flushed out. This is the reason I would look into the ends to make sure they are clean

 

[Lucrative]

I have all three and they run right after one another. Power steering, then fuel, then oil before it gets to the tstat housing. I see what you're saying about looking at the ends now. I'll give that a try, but I'm leaning towards this being something else. The only thing I'm really looking for would be impeller parts, right? I'll post back in a few hours with what I find.

 

[FuelishBehavior]

I'm having a similar problem with mine. I am going to pull then engine anyway to put in a new engine and the first thing I plan on checking is where the pick up hose comes through the transom. There is a term called "Bravoitis" (google it) where the hole the hose runs through corrodes and causes the hose to squeeze and crush/break the plastic insert that holds it in place thus causing enough water restriction to overheat the engine. You might just check your flow at that point if you can get to yours on your boat. I talked to a Mercruiser mechanic and he indicated that it's a common problem for salt water boats like mine.

 

[FuelishBehavior]

"Bravoitis" possibly. Google it. I would also pull the hose fitting where the intake hose comes into the back of the transom. If it's a salt water boat like mine, corrosion causes the hose to crush the plastic insert that secures the hose and causes a water restriction.

 

[MarkSee]

Merc has since updated the design, at least my 2000 7.4 mpi (and probably many other models) where the plastic insert on the transom side is no used and there is a new water hose with attached plastic fitting that goes through the transom and into the pocket cover.

I just replaced mine water hose with this new style setup.

Mark

 

[kcfromohio]

I'm in the same boat! (pun intended)
My 4.3 v6 is giving me problems too.
Been working on it for a week.

 

[Lucrative]

Ok, so I located a good bit of sea grass clogged in the oil cooler. My other coolers don't have a strainer on them, so it pretty much lets anything through, but the oil cooler had grass jammed in the strainer pretty good. Thanks for that suggestion, as I have back flushed it on two occasions and this did not come out. I definitely know how and where to look for a clog next time. I hope that's all that was wrong. We shall see.

Now, next question/problem. I've also taken off the engine water pump to inspect it. It appears to be in good shape. When I took it off though, I had some pretty rough looking water/sludge come out of the block. Black oily water that seems like it has been sitting in there forever. Is this an indicator of anything in particular?? Also, it seems like I should somehow flush the block out and I dont know how to do this.

Can someone instruct me on how to flush out the block?