Mercruiser 350 MAG MPI SKI Inboard Direct Drive Overheating at Idle

[PJCNLV]

I just picked this boat up. It belonged to a business partner who rarely used the boat - total of 51hrs. For a long time, he kept it on a lift at his place on the water in Long Beach, CA. For the past two years it had been in open storage.

Upon retrieving the boat and dusting it off, I opted for a full service that included a new seawater pump impeller. On test after the work, the mechanic reported that the boat would overheat at idle and at 2000rpm while in the water on the trailer with adequate exposure for the seawater intake.

Due to my busy schedule, i opted to take the boat home to Nevada with the problem persisting. Today, I spent some time looking over the cooling system and recording the overheating conditions. The inspections covered:

* Circulating pump. I removed the pump and the back plate and it looks fine. A nice brass impeller in perfect condition.
* All hoses look fine.
* There was a Perko Flush Pro between the seawater pickup and seawater pump. That is no longer needed and will be removed.
* The seawater impeller pulley sans serprentine belt seems firm when turning.

For the test, I rigged up a 40 gallon water barrel with 1.25 drain hose at the base of the barrel to the seawater pump intake direct with constant feed of house water to the barrel. Here is what I observed:

* The barrel is a must because the house water cannot keep up with the seawater pump suction, even at idle.
* At cruising throttle (2000-4000rpm), the seawater pump will drain the barrel in a few minutes.
* At idle the temp slowly creeps up and the alarm sounds.
* If I apply any degree of throttle the motor cools down.

When the motor is fairly hot but, not quite at the alarm state I observe the following temps (as measured with quality IR gun)

* 191 at the side of the termostat housing.
* 164 at the big house off the circ pump to the thermostate housing
* 153 of the small houses from the thermostate housing
* 90 degrees at inlet to water distribution housing
* 90 at the hoses going into the exhaust elbows from the water distribution housing
* 160 degrees on the hoses going to the bottom of the exhaust manifolds
* 180 degrees at the lower distribution manifold downstream of the water distribution housing.

With that said, it looks like the water distribution housing is allowing ample water to the exhaust elbows but is not supplying adequate water to the circ pump. What is this "water distribution housing" for? What is it's purpose? What is inside of it?



Thanks for any and all help.

 

[Bondo]

Re: Mercruiser 350 MAG MPI SKI Inboard Direct Drive Overheating at Idle

Ayuh,.... It's an ole Saltwater boat....

Pull the risers off the manifolds, 'n inspect 'em,....

My guess is they're restricted with rust, 'n crud....

 

[Don S]

Re: Mercruiser 350 MAG MPI SKI Inboard Direct Drive Overheating at Idle

What's the Mercruiser serial number for your engine?
Does your engine have an air actuated drain system?
Is your engine raw water cooled or does it have a closed cooling system with heat exchanger?

One thing that may be a problem is if you have any kind of air leak on the suction side of the raw water pump. At lower rpm, it would suck air instead of water. Check the caps on the flush hose if installed.

 

[PJCNLV]

Re: Mercruiser 350 MAG MPI SKI Inboard Direct Drive Overheating at Idle

What's the Mercruiser serial number for your engine?
Does your engine have an air actuated drain system?
Is your engine raw water cooled or does it have a closed cooling system with heat exchanger?

One thing that may be a problem is if you have any kind of air leak on the suction side of the raw water pump. At lower rpm, it would suck air instead of water. Check the caps on the flush hose if installed.

Hello Don,

• The serial number is 0M398044.
• It has raw water or the standard cooling system.
• it does not appear to have the air actuated drain system.
• I do not believe that there is a leak. The hose from the barrel to the inlet side of the seawater pump is secure and completely unobstructed.

Hello Bond O,

I would also suspect that the risers or the exhaust manifolds are blocked however the temperatures measured externally on the exhaust manifolds are almost exactly the same. With that, I think they would both have to be plugged exactly the same which seems odd.

Do either of you know what this "water distribution housing" does and how it functions? My next work will be to check this unit off of the motor and to also check flow at the drain checks and the manifolds themselves where the hoses attach to the bottom of the exhaust manifolds. Red arrow in image below.

 

[Don S]

Re: Mercruiser 350 MAG MPI SKI Inboard Direct Drive Overheating at Idle

Your problem sounds like a text book impeller with the vanes taking a set bent over. If it was my boat, I would be pulling the pump off and having a look at that impeller and pump housing before doing anything else.
I would also replace the thermostat, especially since it was a salt water boat.

As far as the distribution housing, it's part of the drain system. Since you don't have the air actuated system, and your drawing doesn't show the manual knob to open the valve, I would have to assume you have the 3 point drain system. You remove the blue plug in the distribution housing and the 2 in the raw water pump to drain the cooling system.
This drawing of your system should help explain how it works.

\View attachment Cooling sys.pdf

 

[PJCNLV]

Re: Mercruiser 350 MAG MPI SKI Inboard Direct Drive Overheating at Idle

Your problem sounds like a text book impeller with the vanes taking a set bent over.

The impeller has been replaced with the verified, proper part number. Also, with the serpentine belt off, the pulley is stiff in the proper rotation direction and very hard to turn in reverse.

I would also replace the thermostat, especially since it was a salt water boat.

The thermostat is out of the motor during the tests I detailed.

As far as the distribution housing, it's part of the drain system. Since you don't have the air actuated system, and your drawing doesn't show the manual knob to open the valve, I would have to assume you have the 3 point drain system. You remove the blue plug in the distribution housing and the 2 in the raw water pump to drain the cooling system.
This drawing of your system should help explain how it works.

This is the correct description.

Now what is odd is that per the test I detailed, the inlet line to the housing as well as the outlet to the elbows are cool at @ 90 degrees, while the line to the distribution manifold and onward to the circ pump are hot at @ 170 degrees.

This radical, temperature difference tells me that there is indeed a valve of some type in that housing. I will inspect this housing as well as the flow through the trans cooler next.

 

[Don S]

Re: Mercruiser 350 MAG MPI SKI Inboard Direct Drive Overheating at Idle

So far, you won't check your exhaust, you won't recheck the impeller, you removed the thermostat so none of your tests really means anything, and you have an overheat problem.

I don't know what to tell you.

 

[PJCNLV]

Re: Mercruiser 350 MAG MPI SKI Inboard Direct Drive Overheating at Idle

So far, you won't check your exhaust, you won't recheck the impeller, you removed the thermostat so none of your tests really means anything, and you have an overheat problem.

I don't know what to tell you.

Don, excuse me but, I have added fair comment to each recommendation. I will expand a bit further.

Exhaust: The temp as mentioned in my first post is balanced but high on each side. I will check this component but, it seems unlikely that both clogged at the exact same degree.

Impeller: It's new as mentioned n my first post. Perhaps it still is not sealing properly or, the mechanic mis placed a seal but, I am not ready to go over a new part yet.

Thermostat: Why does a removed thermostat invalidate all tests?

At the same time, no one seems to be able to tell me what is inside the water distribution housing and why the two outlets vary 100%.

 

[Bt Doctur]

Re: Mercruiser 350 MAG MPI SKI Inboard Direct Drive Overheating at Idle

the thermostat slows down the water passing thru a motor to allow it to pick up heat from the engine.

 

[HT32BSX115]

Re: Mercruiser 350 MAG MPI SKI Inboard Direct Drive Overheating at Idle

the thermostat slows down the water passing thru a motor to allow it to pick up heat from the engine.

Yes, and if you have an engine or "transmission" oil cooler or power steering cooler between the raw water pump and the T-stat housing, you may have blockage from impeller fragments or other debris restricting water flow to the engine.

Also, you probably need to verify your temp with a different measuring device to ensure that you don't have a faulty gage, sender etc.....looks like you did that

(as measured with quality IR gun)

I'd also say that Don is trying to tell you that your "NEW" impeller can be destroyed in less than a min or two and may have been. It's worth checking it twice.....

 

[PJCNLV]

Re: Mercruiser 350 MAG MPI SKI Inboard Direct Drive Overheating at Idle

Yes, and if you have an engine or "transmission" oil cooler or power steering cooler between the raw water pump and the T-stat housing, you may have blockage from impeller fragments or other debris restricting water flow to the engine.

I just finished checking the trans cooler. it is clear.

I also removed the "water distribution housing" and it is clear and clean. It is really odd though that it seems to be the thermal wall that it is.

Also, you probably need to verify your temp with a different measuring device to ensure that you don't have a faulty gage, sender etc.....looks like you did that

Yes.

I'd also say that Don is trying to tell you that your "NEW" impeller can be destroyed in less than a min or two and may have been. It's worth checking it twice.....

I remove the hoses at the t-state housing that go to the bottom of the exhaust manifolds and tried to blow through them. Both seem congested. Should they be free flowing or not - perhaps with some restriction?

As mentioned, the impeller has tremendous volume and appears to be intact. It will likely be the last thing I check.

 

[Bt Doctur]

Re: Mercruiser 350 MAG MPI SKI Inboard Direct Drive Overheating at Idle

should be able to blow easily thru those hoses , its a free flow system.

 

[PJCNLV]

Re: Mercruiser 350 MAG MPI SKI Inboard Direct Drive Overheating at Idle

should be able to blow easily thru those hoses , its a free flow system.

Exactly. They are about 90% obstructed with gunk and rust. The risers look fine though.

I will be pricing replacements tomorrow.

 

[HT32BSX115]

Re: Mercruiser 350 MAG MPI SKI Inboard Direct Drive Overheating at Idle

Exactly. They are about 90% obstructed with gunk and rust. The risers look fine though.

I will be pricing replacements tomorrow.

By George, I think you found it!

 

[NHGuy]

Re: Mercruiser 350 MAG MPI SKI Inboard Direct Drive Overheating at Idle

Hey, word on that job...Get a couple of 3/8"-16 bolts and grind the hex ends off of them to use as installation guides. The manifolds are around 40 or 50 lbs and if you try to mount em without the guides you will surely mangle the gaskets. Get them long enough to stick through the manifold holes but short enough to let the manifolds get past the side of the motor to hang em. This also protects your spark plugs from being hit by the manifolds during the installation.
Put the 2 guide bolts in the head, put the exhaust manifold gasket on them, put the exhaust manifold on the guide bolts and slide it close to the motor, put 2 real exhaust manifold bolts in the other 2 holes making sure to catch the exhaust manifold gaskets in the holes correctly, remove the guides and put in the final bolts.
I did the job with some bolts that were a bit too long at 4.5". I almost had to go back and shorten them because on one side I only have about 9" clearance between my manifold and the boat. Whatever you do they need to stick out farther than the real bolts so you can remove em before installing the real ones.
Also don't let them sell you the green paper-like gaskets for the manifold to riser/elbow joint. Get the graphite coated gaskets.

 

[captnvinny]

Re: Mercruiser 350 MAG MPI SKI Inboard Direct Drive Overheating at Idle

Exactly. They are about 90% obstructed with gunk and rust. The risers look fine though.

I will be pricing replacements tomorrow.

any chance you have a pic of the obstruction / problem hoses? I have a very similar problem and am trying to narrow down the cause.

thanks

 

[spdracr39]

Re: Mercruiser 350 MAG MPI SKI Inboard Direct Drive Overheating at Idle

Don, excuse me but, I have added fair comment to each recommendation. I will expand a bit further.

Exhaust: The temp as mentioned in my first post is balanced but high on each side. I will check this component but, it seems unlikely that both clogged at the exact same degree.

Impeller: It's new as mentioned n my first post. Perhaps it still is not sealing properly or, the mechanic mis placed a seal but, I am not ready to go over a new part yet.

Thermostat: Why does a removed thermostat invalidate all tests?

At the same time, no one seems to be able to tell me what is inside the water distribution housing and why the two outlets vary 100%.

You new guys kill me. Don and Bondo are the top notch diagnosis guys on the forum. If you would do any research at all you would know this. Your issue could have been found in the first answer you received but you refused to believe it. Why do people ask questions then refuse to believe the answers ???????????? To all the MODS thanks you for your patience as I could not hang in there like you guys do !!