Stumped on cooling system issue

[dcf1999]

First time running boat this year and we’ve got overheating issues.

It’s a 4.3 mercruiser with alpha 1 drive.

So running it wide open temp climbs and alarm goes off, slow down and you see the temp gauge drop and go to normal. Increase speed and same thing happens. It will also overheat at idle. If it’s run at a certain speed (rpm and I can’t remember offhand what it is) the temp is fine stays consistent and whatever temp it’s at (I.e. gauges starts to climb past normal temp and I slow back to that speed the temp will stay where is at. Not climb or go down). Any faster or slower it climbs.

At at first I thought it was a sticking thermostat because of the temp climbing them speratically going back to normal. So I replaced thermostat and it still does it.

Here are are my observations:

When engine is overheating, hose from seawater pump (in drive) is cold going to tstat housing. Hose going from housing to circulating pump is really hot. I pulled the plug by the bend where the hose goes into circulating pump and water is hot to the point of burning your finger. I can’t remember the temp of the hoses going to exhaust manifolds.

Starboard riser is hotter than port. Pulled riser and not clogged (at least everything looks open. Maybe I’m not looking at what I’m supposed to). I pulled hose from tstat housing that goes to that exhaust manifold and blew in it to see if maybe there was a restriction somewhere in between but that looks open as well.

Noticed when the boat starts to overheat and alarm goes off, the voltage drops to 11 volts. Normal voltage is 13. Could the belt be slipping. Seems unlikely.

Seawater pump in excellent condition. Will replace anyways since it’s apart.

Bad temp sender? Btw, it appears there are two temp senders on the housing. One has a single wire and the other (directly across from it) is a two wire with an automotive type electrical connector on it. In the service manual it doesn’t say what that one is for and only references the single wire sender for the temp gauge. What is the other sender (the 2 wire) for?




Thanks

 

[Bt Doctur]

is this carb`d, injected, fresh water cooled, year,on most T housings there is a temp sender and an alarm switch
High speed overheats are mostly a bad impeller or a bad water pump base sucking in hot exaust gas

 

[dcf1999]

Sorry.

Carb and raw water cooled (no coolant)

boat is an ‘04

it overheats at high speed and at idle. Doesn’t at midrange. When it starts to overheat you can kinda slow down down and you will see the temp quickly drop to normal... as if the thermostat was stuck and it finally opens.

Impeller looks brand new. Like I said, I will replace it with a new kit. I just need to look up the part number for the kit.

 

[HT32BSX115]

Howdy,

Welcome aboard!

First of all, verify that you actually have an overheat. Get yourself a cheap Harborfreight (or equiv) infrared temp "gun" and point it at the T-stat housing and both exhaust risers.......compare temp with the gage.....

In addition to what Dodge indicated above, most overheats are cooling water flow related. That means, there's not enough flow through the engine at higher power output. (or at idle if it overheats at idle)

That can be due to bubbles, a bad impeller or a blocked water intake. Also, if your risers are severely clogged with rust, the over-all flow could be reduced.

You can see bubbles by using a clear hose from the water pickup/raw water pump hose that goes to the T-stat housing.

You can determine a bad impeller or blocked water intake by disconnecting that same hose and allowing it to fill a 5-gal bucket at idle or at 1500 RPM etc. (I think the service manual will indicate how fast it should pump at a particular RPM)

If you have never removed your risers/manifolds to inspect, now is the time to do that.

If it's an older salt-water engine and the block and head cooling passages are very rusty, (which will also reduce flow) it's time for another engine......

Finally,

It’s a 4.3 mercruiser with alpha 1 drive.

Isn't enough information. Tell us the year and serial.....really the serial number... That tells us exactly what you have. Also tell us how old the manifolds and risers are and if you operate in salt.

Regards,

Rick

 

[dcf1999]

I’m at work right now so when I get home I’ll get the serial number. Is there another place on engine where serial number is. It’s used to be printed on the spark arrestor and now it’s rubbed off. I might have it in the original purchase docs.

Never been operated in salt water

i pulled starboard riser and nothing is clogged up. All passages look open with no restrictions.

On that topic, Do i just look at the 4 passages where it meets the manifold and the one that goes out to the exhaust boot? Or are there smaller internal spots in the riser that get clogged up? I.e. do I need to pull out my borascope and do a better inspection of the internals or is it just a quick visual inspection of the riser?

I didnt pull exhaust manifold. I guess I’ll do that next.

manifolds and risers are origional. Boat is an ‘04. I’ll get the serial when I get home.

I have the service manual and I read about the clear hose, just seemed like a PITA to do that. But if I have to i have to.... I think I’m goimg to run a camera down that hose and check everything that way first. Could the oil cooler that’s Inline on the intake hose between the outdrive’s pump and tstat hosing be causing a restriction?

After i put in the new water pump I’ll take it out on the lake and measure the temps of specific components.

Lastly, since the starboard side of engine was a lot hotter than the port, I’d guess if there was a restriction, it would be on the starboard side (manifold / risers)?

 

[dcf1999]

Another thing, when I pulled the riser, I looked into the exhaust manifold (the cooling jackets of course) and notice some sand in there... like someone sucked some up at some point (it’s a family boat so I’m not the only one who uses it... just the one who fixes it). Could there be some sand that is blocking something and if so, is there a way to, I guess, back flush the engine? Or is a little sand and debris not a big deal?

It wasnt a lot, but you could see some settled in there.

 

[danhenke]

I don't have the solution, but I did have a very similar condition last year (3.0L). The tech replaced the impeller, still it existed, flushed the power steering cooler, and most water jacket passages, hoses, etc, got a lot of sand, no help. For whatever reason, he decided to look at the alternator, and what he found was that the bearings were junk, causing the belt to slip, and not turning the water pump properly. Alternator replaced, issue repaired.

Maybe that would help?

 

[dcf1999]

I don't have the solution, but I did have a very similar condition last year (3.0L). The tech replaced the impeller, still it existed, flushed the power steering cooler, and most water jacket passages, hoses, etc, got a lot of sand, no help. For whatever reason, he decided to look at the alternator, and what he found was that the bearings were junk, causing the belt to slip, and not turning the water pump properly. Alternator replaced, issue repaired.

Maybe that would help?

Hmmm interesting. I’ll look at that. I’m going to pull belt and check all that because, as I said, when it starts overheating, the system voltage goes down to 11 bolts and it should be 14ish. I too considered something with belt slippage.

 

[danhenke]

I’m pretty mechanically inclined, but that marine cooling system in NOT my forte. It made perfect sense when he found the issue though, good luck, I hope you can nail the problem down.

 

[dcf1999]

Serial number of engine: 0M3811709
serial number of drive: 0M559823

took off belt and alternated spins freely with no noise. Water circulator pump spins but not freely (you can spin it but it stops when you stop) with no noise. My boroscope wouldn’t go freely down to view the cooler so I’ll disconnect the intake side of it and see if any sediment is in there. I assume it’s just a series of tubes or screen the water goes through to exchange the heat from the fluid.

Exhaust flappers aft of riser are in place and working. I can push each side open using tip of boroscope.

Should I pull the exhaust manifolds and check those for sediment?

Is it it normal for the hose from the tstat housing to the circulator pump to get too hot to touch? If not, that almost tells me that water isn’t getting past circulator pump and could be a restriction in engine or at pump?

 

[dcf1999]

I’m pretty mechanically inclined, but that marine cooling system in NOT my forte. It made perfect sense when he found the issue though, good luck, I hope you can nail the problem down.

I too am very mechanicaly inclined but don’t work on cooling systems all that much. Especially in marine. I’ve rehuilt numerous engines and done transmissions and other car stuff but besides just flushing them out, don’t work on them at all.

 

[Bt Doctur]

You mention a heat exchanger, Alpha 1`s dont have the flow capacity for a heat exchanger.

 

[HT32BSX115]

Could there be some sand that is blocking something and if so, is there a way to, I guess, back flush the engine? Or is a little sand and debris not a big deal?

A little would be no real problem.....a LOT would. If teh thermostat was open when the drive was operated in a lot of sand, it could have deposited fair amount of sand in the engine block. The only way to tell would be to remove core plugs and look.

Before I installed the Mercruiser 454 in my Four Winns, I had the engine on a stand. I removed all the core plugs to thoroughly flush it.

I got a LOT of sand/rust out of the water jacket around the cylinders by flushing with air first and then water. I would have never believed there would be that much debris in there. The 1997 model engine had 120hrs on it when I got it in 2005, and according to the previous owner, had been operated in fresh water (except for once in salt)

Your manifolds (if fresh water only) shouldn't have a lot of rust in them if they were new in 03 or 04.

Try https://www.mercurymarine.com/en/us/parts-and-accessories/parts-catalog

0M3811709.......... There's something wrong with your engine serial number. I think there should only be 6 digits after OM or OW. You can look at the serial number ranges in the above Mercury catalog

 

[dcf1999]

You mention a heat exchanger, Alpha 1`s dont have the flow capacity for a heat exchanger.

I believe it’s the power steering cooler. Its aft of the engine on my boat. After the water pump in the outdrive the hose goes to the power steering cooler then to the tstat housing.

I never knew what it consisted of but I pulled the hose tonight and inspected it. Its just a straight pipe through it. I thought there would be some honeycomb or fins the water would go through but it’s not.

 

[dcf1999]

A little would be no real problem.....a LOT would. If teh thermostat was open when the drive was operated in a lot of sand, it could have deposited fair amount of sand in the engine block. The only way to tell would be to remove core plugs and look.

Before I installed the Mercruiser 454 in my Four Winns, I had the engine on a stand. I removed all the core plugs to thoroughly flush it.

I got a LOT of sand/rust out of the water jacket around the cylinders by flushing with air first and then water. I would have never believed there would be that much debris in there. The 1997 model engine had 120hrs on it when I got it in 2005, and according to the previous owner, had been operated in fresh water (except for once in salt)

Your manifolds (if fresh water only) shouldn't have a lot of rust in them if they were new in 03 or 04.

Try https://www.mercurymarine.com/en/us/parts-and-accessories/parts-catalog

0M3811709.......... There's something wrong with your engine serial number. I think there should only be 6 digits after OM or OW. You can look at the serial number ranges in the above Mercury catalog

I might have mistyped it. 0M381709

im hoping I don’t need to pull the engine and do all that. I did some more poking around tonight and took some pics. I will post them tomorrow.

I am confused my your statement about the tstat being open allowing debris in engine. I want to make sure I got the system flow down... The way I thought the system works is the water gets drawn in by the seawater pump to the tstat housing. From there, some goes to the exhaust manifolds but most goes to the engine circulating pump which pushes it through engine. When the water in engine gets hot, tstat opens and releases water to manifolds/risers and out through bellows out to sea. So no matter what, if tstat was open or closed, whatever the water pump (in outdrive) sucks up would go into the engine right?

 

[HT32BSX115]

I might have mistyped it. 0M381709

im hoping I don’t need to pull the engine and do all that. I did some more poking around tonight and took some pics. I will post them tomorrow.

I am confused my your statement about the tstat being open allowing debris in engine. I want to make sure I got the system flow down... The way I thought the system works is the water gets drawn in by the seawater pump to the tstat housing. From there, some goes to the exhaust manifolds but most goes to the engine circulating pump which pushes it through engine. When the water in engine gets hot, tstat opens and releases water to manifolds/risers and out through bellows out to sea. So no matter what, if tstat was open or closed, whatever the water pump (in outdrive) sucks up would go into the engine right?

I misunderstood on your year model. You probably saw yours comes up as a 1998 model

There might be a fair amount of debris/sand etc in the block.

Also, the way marine cooling systems work, Most of the cooling water bypasses the engine and goes to the manifolds and out through the risers.

As needed, the T-stat opens and allows *some* (enough) through the engine so as to keep it near the desired temp. (140°F or 160°F etc)

If the engine is cool, (or not fully warmed up) nearly all the cooling flow would go to the manifolds and overboard out the risers.

If the engine was already fairly hot (T-stat fully open) and someone were to run the drive into the sand, there could be a fair amount of sand/dirt pumped into the block.

Most would circulate right on to the manifolds/risers and overboard.....but some could settle in the block since the flow through the block is less.....

 

[dcf1999]

Also, the way marine cooling systems work, Most of the cooling water bypasses the engine and goes to the manifolds and out through the risers.

As needed, the T-stat opens and allows *some* (enough) through the engine so as to keep it near the desired temp. (140°F or 160°F etc)

If the engine is cool, (or not fully warmed up) nearly all the cooling flow would go to the manifolds and overboard out the risers.

If the engine was already fairly hot (T-stat fully open) and someone were to run the drive into the sand, there could be a fair amount of sand/dirt pumped into the block.

Most would circulate right on to the manifolds/risers and overboard.....but some could settle in the block since the flow through the block is less.....

Ok I get it now. So, with regards to the circulating pump, it basically just keeps circulating the water around the engine block and when it gets hot (160 degrees), the tstat opens and the circulating pump then introduces cooler water (from the fresh water coming into tstat housing) into engine while the hot water gets released out tstat to risers and out?

If thats all correct, why does the hose (with what should be cool raw water) from tstat housing to circulating pump get crazy hot. I can understand it getting warm but not that hot. I mean there should be fresh water constantly being supplied to that hose. Unless, because that hose is “full”, the cool raw water sort of bypassss it and goes to risers until the tstat opens allowing the cool raw water to enter.

 

[dcf1999]

Here are some pictures of the exhaust manifold (still attached), riser, and water pump.

Im going to replace all the water pump components and put it ball back together and give her a test.

I’m also going to install a clear hose from cooler to tstat housing to watch water flow and do a flow test per service manual.

im also thinking of removing tstat and and drain plugs while running it on a hose to flush out some debris that may be floating around. Can this be safety done (I.e. does the water pump flow enough water to circulate enough water through the system with no more than 2 plugs removed at a time)? I’d assume so sinchebthe plugbopenimgs are small.

Anything else I should look at?

im also going to test the thermostat. It’s brand new but who knows... when I get it on Lake, I’ll use temp probes on multiple parts of engine to tile out gauge.

The gauge and alarm are different sensors on that engine i believe?

 

[dcf1999]

Exhaust manufold

 

[dcf1999]

Risers