Mercruiser Temperature Creep as RPM's Increase

Hi All, New to the forum, though I've lurked many times and have read a number of threads over the past few years.

Details: 1990 22' Searay Sundancer, 260hp, 5.7 Mercruiser (Quadrajet carburetor), 750 hours, good compression), Alpha 1 Gen 1 leg, freshwater cooled, antifreeze runs through the manifolds, sea water through the risers, boat operated in Pacific Northwest salt water.

Symptom: as RPM's increase (specifically between about 2500-3500) the temp rises from about 165-170 to close to 200. If I stop the boat, put into neutral but increase engine rpm, temp comes down to 165-170 ish.

Work/solutions done to date:

• ohmed out temperature sensor - within spec, tested gauge - tests full scale and 0 scale as it should
• replaced impellor in leg
• after looking into leg further replaced leg with new Sterndrive Engineering leg (issue with leg was not related to overheating, but it does come with yet another new impellor)
• tested engine thermostat in pot of warming water- opened fully as it should
• replaced transom seals and hoses - confirmed that sea water hose is not crimped/restricted
• checked hose from transom to power steering cooler, hose clear
• power steering cooler clear
• flow checked seawater pick up per the manual (don't recall specific numbers as this was done last year, but water flow characteristics were within manual specs at set RPM)
• rodded out heat exchanger - clear
• replaced engine water pump
• inspected manifolds and risers a couple times, looked fine... replaced them anyway with new Mercruiser units
• pulled heads, checked head gaskets, rebuilt and re-installed
• carb rebuilt - runs well (not lean, if anything, the plugs are a bit sooty)
• checked for any obstructions in all hoses, all are clear
• used infrared thermometer around engine, temps are slightly lower than the gauge indicates, but are close enough that I would say the gauge is indicating what is going on.

I'm a car guy with a pretty well equipped shop, I build hot rods as a hobby so I'm comfortable working on engines and mechanicals. I'm obviously missing something here. Any thoughts out there?

guessing the water flow from the Alpha 1 is not sufficient for full system cooling. Almost all use engine mounted raw water pumps with 1 1/8 or 1 1/4 hose

SEI impellers are just as bad as Sierra in an Alpha one, the rubber impeller will slip on the brass hub and cause the temp to creep. Replace the pump with an OEM pump.

Docksidemarineservices said:

SEI impellers are just as bad as Sierra in an Alpha one, the rubber impeller will slip on the brass hub and cause the temp to creep. Replace the pump with an OEM pump.

Click to expand...

Thanks for the feedback...the first impellor was replaced with an OEM last year...it had no impact. But it will be easy to install it back into the SEI leg.

Bt Doctur said:

guessing the water flow from the Alpha 1 is not sufficient for full system cooling. Almost all use engine mounted raw water pumps with 1 1/8 or 1 1/4 hose

Click to expand...

Thank you for the feedback...is there a standard/typical retrofit kit that is being used? It seems all the parts for the full system cooling, hoses, routing etc. are original (or original replacement). The boat is almost 30 years old (though I know little to none of the history), do you think it likely that this has always been an issue with the cooling system?

An alternative solution if this is the issue might be to route sea water through the manifolds instead of engine coolant, but I thought that was a lesser system and would add salt water corrosion to the manifolds as well. it would however confirm your theory/guess as a short term experiment.

a crank or engine mounted pump and seacock will cure it. Most engines with engine only fwc can get by with the Alpha pump, full systems cant.Take care of 2 28 Coastals with small blocks (5.7) both run the Bravo type engine mounted pumps , 1 raw water cooled and 1 fresh water cooled . Ne problems with temps.
A customer has a volvo 5.7 that kept eating the crank mounted raw water pump, A cured it by doing this

Bt Doctur said:

a crank or engine mounted pump and seacock will cure it. Most engines with engine only fwc can get by with the Alpha pump, full systems cant.Take care of 2 28 Coastals with small blocks (5.7) both run the Bravo type engine mounted pumps , 1 raw water cooled and 1 fresh water cooled . Ne problems with temps.
A customer has a volvo 5.7 that kept eating the crank mounted raw water pump, A cured it by doing this

Click to expand...

Thank you for this, sounds like a plan...what would I need to do with the pump in the alpha 1 leg?

Typically when temps increase under load it would indicate a loss of flow or flow with a lot of bubbles. Bubbles would most likely come from exhaust.

Your flow test may have had adequate water volume but was it free of bubbles? Only way to test is to use a clear hose in place of original from transom to heat exchanger.

The Exhaust flows over the impeller housing in outdrive in a standard system (not thru hull exhaust). If gaskets are torn, installed incorrectly or the thick squared edge o-ring is not sitting on top of impeller housing (at vertical drive shaft) then air can be introduced to water. Other way would be for an obstruction causing air bubbles from hull to be sucked in thru outdrive intake. Is there anything installed on transom within the 5-6 inches from center of hull under the transom that could cause turbulence (bubbles)?



As far as the current system being insufficient, although it may be less sufficient than a engine mounted impeller, it did function properly for many years as is. So either you figure out what is the issue or install newer and more efficient.


Even though you tested the sender. the question is does it read correct value and increase steady or does it get to normal temps and then within seconds or so it gets over 190-200?

will it continue to rise above 190-200 if you keep running or stay at 190-200ish?
I ask because I chased a high running temp issue once where the sender failed only at normal temp range, would run at or around 190-200ish all day. To high normally but once sender was changed (after testing to diagnose in hot water with ohm meter on sender) temps stayed at norm, 160-170.

you can plumb it to use as an emergency water source, use it for live wells, simply direct it overboard thru a hull fitting , or just cut the hose at the gimbal housing and cap the end inside the boat

kghost said:

Typically when temps increase under load it would indicate a loss of flow or flow with a lot of bubbles. Bubbles would most likely come from exhaust.

Your flow test may have had adequate water volume but was it free of bubbles? Only way to test is to use a clear hose in place of original from transom to heat exchanger.

The Exhaust flows over the impeller housing in outdrive in a standard system (not thru hull exhaust). If gaskets are torn, installed incorrectly or the thick squared edge o-ring is not sitting on top of impeller housing (at vertical drive shaft) then air can be introduced to water. Other way would be for an obstruction causing air bubbles from hull to be sucked in thru outdrive intake. Is there anything installed on transom within the 5-6 inches from center of hull under the transom that could cause turbulence (bubbles)?



As far as the current system being insufficient, although it may be less sufficient than a engine mounted impeller, it did function properly for many years as is. So either you figure out what is the issue or install newer and more efficient.


Even though you tested the sender. the question is does it read correct value and increase steady or does it get to normal temps and then within seconds or so it gets over 190-200?

will it continue to rise above 190-200 if you keep running or stay at 190-200ish?
I ask because I chased a high running temp issue once where the sender failed only at normal temp range, would run at or around 190-200ish all day. To high normally but once sender was changed (after testing to diagnose in hot water with ohm meter on sender) temps stayed at norm, 160-170.

Click to expand...

Thank you for your input...it is all very much appreciated. When I checked the sensor, it was linear in its readings. As the water temp came up the ohms changed smoothly and within the expected range. The temperature gauge slowly increases and decreases as the engine temperature rises and falls (as confirmed with infrared thermometer).

I've had the temperature gauge showing near 210 before I chickened out, backed out of the throttle and then let it run at 2000 rpm in neutral to let it cool back down. Frankly, I don't know how hot it would get if I kept it working.

I did not have a clear hose on the system during the flow test, so can not attest to whether there were any bubbles. I can add that last year when I installed the first OEM replacement impeller and housing, both the old and new pumps had the square shaft seal, and all the gaskets and plates were stacked correctly and replaced with new gaskets. Neither had an impact on the engine temperature. This spring, I replaced the leg complete with an SEI unit that has a factory installed impeller and housing and again, no impact on the temp.

I do have a transom mounted transducer for my chart plotter. It is 12" to the starboard side of the hull center line (or about 10 1/2" from the water intake on the leg). Is that too close?

I agree with you on the presumption that this set up must have cooled sufficiently in the boats previous 29 years. But also admit that I don't know the history. The previous owner had the boat for a very short term and seemed somewhat deceptive and evasive in his responses to questions about the boat. He purchased the boat from the estate of the original owner. The boat had been in storage for a number of years while the original owner battled illness until his demise.

Again, I am grateful for all of your inputs while I ponder and plot my next plan of action to resolve the issue.

12 inches is fine,

Over heat is typically three things,

Water in, water out, control (thermostat, housing)

So if you rule out water in and water out then it has to be control.

do your elbows stay warm to touch when at 160-170?

You said you did thermostat test, did you see what temp it was when thermostat opened?

What is the thermostat set point? 140 or 160? This is critical.
If someone slipped in a auto thermostat in (and yes I have found a 196 degree auto used before) this could be your issue. The spec is 140 or 160 and I would have to look later when at home. Most closed cooling use 160.

Other concern is hose routing....are you sure it is correct.

Water path from impeller, did you look up at upper ger case when lower was apart where copper tube goes? Did you pinch rubber that copper tube fits into?
Is the plastic part that copper tube fits into in good shape? If melted that would cause flow/exhaust bubble issue.

at this stage I say air bubbles in incoming water flow knowing what we know up to this point.

kghost said:

12 inches is fine,

Over heat is typically three things,

Water in, water out, control (thermostat, housing)

So if you rule out water in and water out then it has to be control.

do your elbows stay warm to touch when at 160-170?

You said you did thermostat test, did you see what temp it was when thermostat opened?

What is the thermostat set point? 140 or 160? This is critical.
If someone slipped in a auto thermostat in (and yes I have found a 196 degree auto used before) this could be your issue. The spec is 140 or 160 and I would have to look later when at home. Most closed cooling use 160.

Other concern is hose routing....are you sure it is correct.

Water path from impeller, did you look up at upper ger case when lower was apart where copper tube goes? Did you pinch rubber that copper tube fits into?
Is the plastic part that copper tube fits into in good shape? If melted that would cause flow/exhaust bubble issue.

at this stage I say air bubbles in incoming water flow knowing what we know up to this point.

Click to expand...

Thank you again kghost (Jack) for taking the time to respond. As I recall the thermostat was a 160...it is a Marine thermostat. I had it's replacement in hand last year when I tested the original. On the test, it began opening ever so slightly below 160 (about 150-155 as I recall) and was fully open around 165-170...the water temp in my heated pot may have been heating up faster than the thermostat could react thanks to me wife's induction cooktop...but we won't tell her about the little science experiment in the kitchen...lol. As the water temp came down, the thermostat remained fully open as the temp dropped below 160 when it started to close slowly. After the original tested out correctly, I re-installed the original.

Yesterday, when returning from the buoy to the trailer (yet again!!) I ran it up to the near 200 degrees on gauge and then pointed my infrared thermometer around the risers and manifolds...the risers were around 170-180...the manifolds were 200-220 (hottest towards the center 2 exhaust ports). Both manifolds and risers are brand new Mercruiser with less than 1 hour run time.

Hose routing, I think is correct based on the manual showing coolant and raw water paths (but let me add that I would love to hear that they are wrong and the simple fix is to correct them!)...plus all hoses are Mercruiser numbered and pre-bent.

• Raw water goes from transom, through power steering cooler (port side of engine), up to the bottom chamber port side of heat exchanger
• bottom chamber about 6" inward of the starboard side of the heat exchanger attaches to the engine coolant/water pump
• upper starboard side of the bottom chamber of the heat exchanger goes to the center leg of a Tee, with each of the 2 outer legs of the Tee going to each side riser
• 2 lines off the thermostat housing each go to the bottom of each side of manifold
• top of each side manifold goes to each side of the top chamber of the heat exchanger
• there are also 2 additional lines going to the hot water tank, one originates at the water pump, and the second goes to the top of the intake manifold.

The copper tube from lower gear case to upper slid/slides nicely in the upper gearcase guide/sleeve. Between the old and new gearcases, changing raw water pumps, checking and rechecking the o-ring and gearcase gaskets, etc...I've had the leg off probably more than 1/2 dozen times and have looked closely and been extremely careful at the various interfaces while trying to sort this out. I have never found anything out of sorts on any of the disassembly/re-assembly efforts (save for once I found the orange o-ring had come a bit out of it's groove...thinking I found the smoking gun...I carefully used adhesive on the re-assembly only to find no change...that was about 4-5 leg-offs ago.

I forgot to mention that because the manifolds are engine coolant cooled, the gaskets between the risers and the manifolds are solid in order to isolate the 2 sections.

I will review manual diagrams based on your description later, just pulled a 10+ hr day at work.

One other thought, where is the sender located? Thermostat housing or intake manifold and is it threaded in with no bushing/reducer?


Later

kghost said:

I will review manual diagrams based on your description later, just pulled a 10+ hr day at work.

One other thought, where is the sender located? Thermostat housing or intake manifold and is it threaded in with no bushing/reducer?


Later

Click to expand...

Explain how temp sender location can effect cooling/overheat condition. Lol

kghost said:

I will review manual diagrams based on your description later, just pulled a 10+ hr day at work.

One other thought, where is the sender located? Thermostat housing or intake manifold and is it threaded in with no bushing/reducer?


Later

Click to expand...

Thermostat housing...no reducing bushing(s).

Hope this works...this is basically how my system is configured except instead of the belt driven raw water pump, I have the Alpha 1 pump in the leg which may not be adequate for full freshwater cooling.

I've been going through a number of searches looking for a kit for the conversion, but have not been able to find one. It looks like this may be a pump I could use/need (Mercruiser 46-807151A12) but it doesn't inlude the pulley, have not been able to determine what size/part number of pulley would be required (is there an automotive equivalent?). I assume I will need to make a bracket for it to bolt to the block (looks to be the 2 mounting holes alongside the timing chain cover), needs to be adjustable for belt tension, there needs to be room for the mechanical fuel pump (as this is what I have, whereas it seems most list they are for electric fuel pumps), and then there is the whole matter of popping a hole in the boat for a seacock, high speed scoop/strainer (not sure which one to use?), whether there is a need for an inline strainer? Where is the best spot for the seacock? ...oh yeah, and this all in a very tight bilge.

kghost said:

12 inches is fine,

Over heat is typically three things,

Water in, water out, control (thermostat, housing)

So if you rule out water in and water out then it has to be control.

do your elbows stay warm to touch when at 160-170?

You said you did thermostat test, did you see what temp it was when thermostat opened?

What is the thermostat set point? 140 or 160? This is critical.
If someone slipped in a auto thermostat in (and yes I have found a 196 degree auto used before) this could be your issue. The spec is 140 or 160 and I would have to look later when at home. Most closed cooling use 160.

Other concern is hose routing....are you sure it is correct.

Water path from impeller, did you look up at upper ger case when lower was apart where copper tube goes? Did you pinch rubber that copper tube fits into?
Is the plastic part that copper tube fits into in good shape? If melted that would cause flow/exhaust bubble issue.

at this stage I say air bubbles in incoming water flow knowing what we know up to this point.

Click to expand...

Hi Ace, i don't recall OMC ever having an issue whereby adding a secondary water pump was needed to cool the engine! BHAHAHAHAHA

Looking in the manual I found this first.



Thermostat spec is 143 or 140 as we all use these days............



Picture of component order for thermostat housing


Here is the same basic flow diagram but with heat exchanger in back. A bit easier to follow flow maybe.


So based on first image it looks like the 160 degree is incorrect. You should swap it out to a 140 degree thermostat to start.

Confirm parts in thermo housing are in the correct order.

Confirm all hoses are routed per diagram.

See what happens after that.

I also reread your last post, you have the hot water option, I didnt upload that image, I will later need to get ready for work now. Not sure that would have any influence on issue unless it was slowing down flow causing the overall circulation to be slowed.

kghost said:

Looking in the manual I found this first.

View attachment 20726

Thermostat spec is 143 or 140 as we all use these days............


View attachment 20727
Picture of component order for thermostat housing

View attachment 20728
Here is the same basic flow diagram but with heat exchanger in back. A bit easier to follow flow maybe.


So based on first image it looks like the 160 degree is incorrect. You should swap it out to a 140 degree thermostat to start.

Confirm parts in thermo housing are in the correct order.

Confirm all hoses are routed per diagram.

See what happens after that.

I also reread your last post, you have the hot water option, I didnt upload that image, I will later need to get ready for work now. Not sure that would have any influence on issue unless it was slowing down flow causing the overall circulation to be slowed.

Click to expand...

Just wanted to add that I've not abandoned the thread and all the advice here. We have guests (guests that I had hoped to take fishing and for a tour around with the boat...but that's not going to happen until this is resolved), so the time I have available to work on the boat will be limited for the next week. I will get back to it, and will post the results and outcomes. I did scan the flow schematic you provided and it is exactly the way mine is configured. Thanks again for your inputs.

So, if I decide to convert the full freshwater cooling system to a 1/2 system (removing the exhaust manifolds from the equation), does anyone know if there is a hose kit available for the conversion? Obviously I would need to change the type of gaskets between the manifolds and risers from the style that stops crossflow to the type that has the crossflow.

Kghost (Jack), once our guests are gone, I will again pull the thermostat and confirm what rating it is (as I said above, I was relying on my memory, as i performed the thermostat test last summer. I do remember that it performed as it should, just can't remember 100% what it was rated for..and my memory isn't what it used to be!).

Thanks again, Tim

Duplicated post in error...sorry about that.