help help help overheating Mecrusier

[spomey]

1998 Merc 5.7 with Alpha one outdrive. The boat overheated on its first use. I changed the impeller, and it overheated again. I took it apart impeller looks good but i am going to replace it again anyway. I don't know where to turn to troubleshoot. I just realized it looks like my motor uses both an impeller and a mechanical water pump on the front of the engine.

How do you tell if the water pump needs replaced?

I understand the thermostat is inside the unit on top of the engine that has 4 hoses connected to it. which hose goes to what? I want to loosen the one from the outdrive and check for flow, then check flow through the thermostat. (probably just replace it)

Could someone help me identify which hose is which and what to check? I am fairly capable at mechanics and I am pretty sure the first impeller swap was clean, so i suspect it is something else.

fresh water boat - no salt water.

I am pretty sure there is a water pump behind the large pully. HELP!

 

[Scott06]

Start by seeing if you get the proper water flow up from the drive. First attachment has flow test should get 3-4 qts in 15 seconds up up to thermostat housing. Is the hose that comes from transom housing up to thermostat housing.
second photo is your cooling system diagram. Shows water flow. If you have right amount of water coming to housing look for blockages. If not enough water check ps cooler on flywheel cover. it can get clogged.

did you use a quality impeller ? Oem is best for impellers . Did you forget the key when installing impeller?

water pump on block is a circ pump circs cold water from thermohousing into block up and out heads back to thermostat when thermo is open.
two small lines to manifolds need to pass water at all times to cool exhaust system regardless of what thermostat is doing .

 

[Scott Danforth]

With any new to you used boat the first thing to do is all the maintenance the prior owner didn't do.

Pull the drive
Inspect the gimbal and u-joints
Replace the bellows and raw water pump
Check the alignment
Change the gear lube

Change the engine fluids and filters

 

[tpenfield]

That one (small) picture does not really show much. If the impeller disintegrated there are probably bits and pieces further along in the cooling system flow. You'll need to find/flush those pieces. There are usually a couple of oil coolers along the intake path before the incoming water gets up to the thermostat housing. Those are usually the places to check for clogs.

 

[Scott Danforth]

with the drive off, back-flush the line from the T-stat housing to the gimbal bellhousing. this fill flush out bits of impeller and other debris from the PS cooler.

 

[dubs283]

Agree with others about checking water supply, flow/volume at the thermostat housing and checking for blockages/back flushing the water supply line/cooler(s).

Yes your engine has a water pump mounted on it, known as a circulating pump. They fail much much less than a sea water pump. Easiest way to see a circ pump failure is water present at the weep hole, bottom side of the pump housing

Am curious to know under what circumstances the engine.is overheating. On the trailer running on muffs at idle? In the water under load? What load/rpm is the overheat occurring? Are the risers cool when the overheat occurs?

 

[spomey]

Start by seeing if you get the proper water flow up from the drive. First attachment has flow test should get 3-4 qts in 15 seconds up up to thermostat housing. Is the hose that comes from transom housing up to thermostat housing.
second photo is your cooling system diagram. Shows water flow. If you have right amount of water coming to housing look for blockages. If not enough water check ps cooler on flywheel cover. it can get clogged.

did you use a quality impeller ? Oem is best for impellers . Did you forget the key when installing impeller?

water pump on block is a circ pump circs cold water from thermohousing into block up and out heads back to thermostat when thermo is open.
two small lines to manifolds need to pass water at all times to cool exhaust system regardless of what thermostat is doing .

Thank you so much for the input, it is exactly what I was needing, My compatment is so tight I cant even see the hose coming thru the transom.

What is a PS cooler? I will look for it.

I used a cheap impeller I suspected they were all made at the same location and marketed differently. I did use the keyway. I will take stuff apart today!

 

[spomey]

Agree with others about checking water supply, flow/volume at the thermostat housing and checking for blockages/back flushing the water supply line/cooler(s).

Yes your engine has a water pump mounted on it, known as a circulating pump. They fail much much less than a sea water pump. Easiest way to see a circ pump failure is water present at the weep hole, bottom side of the pump housing

Am curious to know under what circumstances the engine.is overheating. On the trailer running on muffs at idle? In the water under load? What load/rpm is the overheat occurring? Are the risers cool when the overheat occurs?

The engine overheated both times at he lake, after I replaced the impeller (the old one was intact) I noticed the "up tube" had a melted spot and the adapter to the pump was slightly melted but everything fit very tightly and I checked it over carefully I would have replaced it but no parts were available locally.
Overheating occured while boating at slow speeds, and would heat much faster at high speed. after the impeller change I left the boat on the trailer and set the throttle at approx 2500 rpm and it took 5-10 minutes to overheat, both time I got smoke from the engine area but couldnt find a source.
No water leakeage anywhere that I could see in the engine compartment.

 

[Scott Danforth]

Thank you so much for the input, it is exactly what I was needing, My compatment is so tight I cant even see the hose coming thru the transom.

What is a PS cooler? I will look for it.

I used a cheap impeller I suspected they were all made at the same location and marketed differently. I did use the keyway. I will take stuff apart today!

Power Steering cooler

And no, the cheap impellers are made out of junk, and the OEM impellers are made out of decent rubber. They are manufactured by two different companies or more. With the cheap ones coming out of Chinese knockoff companies

 

[ROY WILLIAMS]

The engine overheated both times at he lake, after I replaced the impeller (the old one was intact) I noticed the "up tube" had a melted spot and the adapter to the pump was slightly melted but everything fit very tightly and I checked it over carefully I would have replaced it but no parts were available locally.
Overheating occured while boating at slow speeds, and would heat much faster at high speed. after the impeller change I left the boat on the trailer and set the throttle at approx 2500 rpm and it took 5-10 minutes to overheat, both time I got smoke from the engine area but couldnt find a source.
No water leakage anywhere that I could see in the engine compartment.

so you inspected it of the water out of the drive shaft around it , did a lot of the exhaust water ..
then the engine is a head gasket leaking it inspect the cylinder pressure ..

 

[Scott06]

Power steering cooler is a shell and tube heat exchanger bout 8-10" long black tube hoses on either end back on the flywheel cover. Water supply comes up from outdrive to PS cooler, then up to thermostat housing. That's why if you check water flow there you can narrow issue down.

if the water tube was melted it was run hot and with no water cooling the exhaust things melt...

The OEM merc impellers last a long time. I changed mine out last year after 6 seasons looked new. A couple years back changed my neighbors with him - was original to the boat ten years old same thing looked new. Not saying to not change them for ten years , just buy the oem as they are very good impellers. I think the alpha gen 2 impeller kit (impeller and gaskets) is like $55

 

[spomey]

Start by seeing if you get the proper water flow up from the drive. First attachment has flow test should get 3-4 qts in 15 seconds up up to thermostat housing. Is the hose that comes from transom housing up to thermostat housing.
second photo is your cooling system diagram. Shows water flow. If you have right amount of water coming to housing look for blockages. If not enough water check ps cooler on flywheel cover. it can get clogged.

did you use a quality impeller ? Oem is best for impellers . Did you forget the key when installing impeller?

water pump on block is a circ pump circs cold water from thermohousing into block up and out heads back to thermostat when thermo is open.
two small lines to manifolds need to pass water at all times to cool exhaust system regardless of what thermostat is doing .

so I used the schematic and I am not getting water to the thermostat housing. I removed all the hoses and nothing. So I will remove the lower drive unit again and put in quality impeller parts. but i kindof think either I have it assmebled incorrectly or backwards or something. do you have a recommended assembly proceedure?

 

[Scott Danforth]

do you have a recommended assembly proceedure?

The one in the factory service manual

 

[Scott06]

Since there is no water coming up from drive would back flush down that hose to see if u get water flow in reverse direction. If you dont there is a clogg somewhere.
get the merc manual for your drive will see if i can do s9me screen shots of impeller work.

 

[Scott Danforth]

Did you replace the whole water pump assembly or just an impeller?

I recommend the whole pump kit

 

[Scott06]

this is part one of two. This guy is s legit mechanic who used to be a moderator on this forum should have link to part 2 when u watch this

 

[Scott06]

Did you replace the whole water pump assembly or just an impeller?

I recommend the whole pump kit

on a gen two unless housing is scored there is no need. The stainless housing is much more robust than the earlier plastic ones. Merc has a kit with impeller baseplate and gaskets for about $55. Unless it was run dry forever thats all you need. Many places to buy but this is the oem kit merc and quicksilver are the same .

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[karayj]

With any new to you used boat the first thing to do is all the maintenance the prior owner didn't do.

Pull the drive
Inspect the gimbal and u-joints
Replace the bellows and raw water pump
Check the alignment
Change the gear lube

Change the engine fluids and filters

That’s a great tip, but wait what about the guy that doesn’t have $2000 to drop on maintenance what does that guy do?

 

[karayj]

I like the backflush idea I know you got that’s on the old dry that stop debris in all things getting in there and clogging stuff up but you never know

 

[karayj]

so I used the schematic and I am not getting water to the thermostat housing. I removed all the hoses and nothing. So I will remove the lower drive unit again and put in quality impeller parts. but i kindof think either I have it assmebled incorrectly or backwards or something. do you have a recommended assembly proceedure?