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Mercruiser 175 getting hot under load

flathead41ford

flathead41ford

New member
I’m leaning towards manifold or risers but not sure how to test them without taking apart.

Boat is an 87 Sundancer 270 with a pair of 175s. Fresh water, alpha 1 gen 1. We took our maiden voyage for this year July 3 for fireworks. The late start for us was frustrating then the boat getting hot added to it. Get to Lake Erie, WOT to get on plain and buzz around for a bit, starboard engine heats up quick. Go back to idle and creeps to 195-200 and starts dropping. Put it in neutral give it some rpm and temp drops fast to 145ish. Removed t-stats and “road test”, same outcome (but got cooler when back to idle). Pull boat and change impellers (3 years old), impellers looked great but changed anyway, same outcome. Water feed flushed both ways, with good flow, when impellers were being changed. No visual hose kinks or anything obvious.

I’m thinking the exhaust may be starting to get bad inside and restricting too much. Age of manifolds and risers unknown although this engine was replaced ant some point and is supposed to have around 250 or so hours. I have owned the boat for 5 years. Boat runs fine with no funny noises or signs or water in the cylinders. Fires fast, cranks nice, no misfires, good power (for what it is) and quiet. It’s a little cold blooded but that has always been the case. Just has to high idle a couple minutes so it doesn’t have a little bog. Engine water pumps changed (quicksilver just to be preventative) when I did gimble bearings and bellows in 2022.

Outside of removing risers and looking inside, is there a way to check for restrictions? This heats to those temps in 10-15 seconds at WOT and will come back down within a minute. When I came in yesterday, I kept engines at 1800-2000 rpm and it creeped to 150ish while the other was 125ish. Probably would have kept creeping but time didn’t allow for more low rpm testing. No wake zone at 1100-1300rpm port is around 120/125 while starboard in the 130/135 area. Both stay steady in the no wake zone. Before overheating issues, engines would stay in the 150 area steady with 145 or 142 (can’t remember for sure) t-stats while cruising. T-stats still out while diagnosing.

Thanks in advance.

EDIT, engines are Chevy 4.3 V6. Sorry about that.
 
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flathead41ford said:
I’m leaning towards manifold or risers but not sure how to test them without taking apart.

Boat is an 87 Sundancer 270 with a pair of 175s. Fresh water, alpha 1 gen 1. We took our maiden voyage for this year July 3 for fireworks. The late start for us was frustrating then the boat getting hot added to it. Get to Lake Erie, WOT to get on plain and buzz around for a bit, starboard engine heats up quick. Go back to idle and creeps to 195-200 and starts dropping. Put it in neutral give it some rpm and temp drops fast to 145ish. Removed t-stats and “road test”, same outcome (but got cooler when back to idle). Pull boat and change impellers (3 years old), impellers looked great but changed anyway, same outcome. Water feed flushed both ways, with good flow, when impellers were being changed. No visual hose kinks or anything obvious.

I’m thinking the exhaust may be starting to get bad inside and restricting too much. Age of manifolds and risers unknown although this engine was replaced ant some point and is supposed to have around 250 or so hours. I have owned the boat for 5 years. Boat runs fine with no funny noises or signs or water in the cylinders. Fires fast, cranks nice, no misfires, good power (for what it is) and quiet. It’s a little cold blooded but that has always been the case. Just has to high idle a couple minutes so it doesn’t have a little bog. Engine water pumps changed (quicksilver just to be preventative) when I did gimble bearings and bellows in 2022.

Outside of removing risers and looking inside, is there a way to check for restrictions? This heats to those temps in 10-15 seconds at WOT and will come back down within a minute. When I came in yesterday, I kept engines at 1800-2000 rpm and it creeped to 150ish while the other was 125ish. Probably would have kept creeping but time didn’t allow for more low rpm testing. No wake zone at 1100-1300rpm port is around 120/125 while starboard in the 130/135 area. Both stay steady in the no wake zone. Before overheating issues, engines would stay in the 150 area steady with 145 or 142 (can’t remember for sure) t-stats while cruising. T-stats still out while diagnosing.

Thanks in advance.
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What engine is the 175- is this the 3.7L 4 cylinder or 4.3 V6 ? Check the PS cooler for blockage. Got a serial number.

Do you have separate supply to elbows and manifolds? Mine is much later but has a check ball system to restrict supply to elbows after thermostat opens and feeds hot water out of block to manifolds

You are correct it is a water supply or restricted flow issue
 
Dieter said:
What engine is the 175- is this the 3.7L 4 cylinder or 4.3 V6 ? Check the PS cooler for blockage. Got a serial number.

Do you have separate supply to elbows and manifolds? Mine is much later but has a check ball system to restrict supply to elbows after thermostat opens and feeds hot water out of block to manifolds

You are correct it is a water supply or restricted flow issue
Click to expand...
From what you are saying about yours, it sounds to be the exact same as what I have. Springs and check balls don’t seem any different between the 2 engines so my assumption is that isn’t the issue.
 
Bt Doctur said:
I`d pull the risers and look there first
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Never had one apart. Do these normally cooperate and come apart or do they tend to break bolts. If they tend to be frozen, is there a way to test with them still together?

I’m a mechanic in the salt belt of Cleveland OH so exhaust work at a location far away from my torches has me considering alternative ways if possible.
 
It all depends on the care given and the environment used....

If they are the original manifolds and elbows, it may be a PITA....if they were separated when the engines were replaced, the rust and corrosion may be minimal...

No test that has a factory blessing other than visual inspection...
 
Pulled the elbows and they didn’t look the best. Exhaust manifolds looked pretty darn good. Replaced elbows with new quick silver and still getting hot. It does seem to take longer to get hot though so it is better, but not fixed that’s for sure. Manifolds not showing any heavy scale on the ports like the elbows did. I tried to post photos here from my phone when I pulled them. But, they were apparently too large to load. Back to the drawing board.

I should add that I took it out for a while longer and rode around at low speeds keeping the temperature below 180. Starboard engine is definitely hotter to the touch than port so I’m pretty confident it isn’t a gauge issue. Before I never let the gauge stay above 145 for any length of time so they felt the same to the touch.
 
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Please clarify, "Fresh water" cooling.

Two types,
1. Raw water, No heat exchanger.
2. CLOSED COOLING, Heat exchanger with antifreeze.

The use of the word Fresh Water is not good to use as it is missused all the time.

Manuals Say CLOSED COOLING VS RAW WATER.
 
Boat is raw water cooled. Sorry, I didn’t realize that the alpha 1 gen 1 was ever used in closed cooling. Fresh water was referring to it being used in a fresh water lake, not salt.
 
Issues causing high temps.

1. Low water flow in- restriction in incoming path, (note upper gear case where copper tube pushes in. it is a S path and can be clogged by rubber boot tube fits in if not installed properly- pinched) (seen this many times)
2. Low water flow out - typically elbows clogged.
3. Exhaust gas getting into cooling water system. Head gasket typically.
4. Thermostat not working properly very common and rust scale floating T stat housing assembly (fresh water use or not)
5. Sending unit. May read properly but spikes when it gets warm. (Chased a overheat for three days once and it was the sender in the end. When I was new at this)
 
1. When doing impellers, feed line flushed standard and reverse flow on both engines. No restrictions noticed. The seal you mentioned was checked as well. I had that issue on this boat when I first got it so it was the first thing I looked at when I dropped the lower units. My fingers were crossed, but it wasn’t the same issue as the past.

2. Elbows replaced with new Quick Silver. Originals looked crusty but not plugged. Having never been inside marine manifolds, I wasn’t sure what too much restriction “looked like”. Replaced to be safe and same outcome. Although, it doesn’t do it as fast, now it’s about 30 seconds to touch 200 compared to 10-15 before. Almost zero flakey scale or swelling in the manifolds. I removed restricted gaskets and used new restricted gaskets when new elbows were installed.

3. No hot or cold start up issues showing signs of water making its way into the combustion chamber and no water in the oil. It can’t be ruled out, but for as fast as it heats up, I would expect to see other signs if it was a head gasket or a crack somewhere.

4. Thermostats removed while diagnosing the heating up issues.

5. Engine is noticeably hotter to the touch after low speed testing keeping the gauge at the 170-180 area compared to the port engine that is running at 120-130 in the same operating conditions (t-stats out in both for comparison reasons). I do have a summit mechanical oil pressure and temp gauge set at the shop for testing cars and I haven’t brought it to the boat to verify temp with a mechanical gauge. But the noticeable difference in touch leads me to believe that the starboard engine is defiantly getting hotter than port. Both elbows “felt” to be roughly the same temp. I guess I could swap sending units for that matter and verify that way.

Later last night, I told my wife I should have trimmed the hot engines out drive all the way up (not trailer) to see if it made a difference. When the boat was out of the water and I pulled the lower units, I flused that line with the outdrives all the way up (trailer). I’m now wondering if that hose is crimping when in the fully down position.

After the impellers were changed, a friend pointed out that the starboard engine was “moving a lot of water” compared to port when I put it back in at the ramp. I mentioned that that was the one that was getting hot. I did look back and there was definitely more “agitation” in the water but to be honest, I never studied what it looked like back there in the past. I have no clue what it looked like before this issue.

This weekend I will try to get back down there and physically look inside the power steering cooler just to be safe. As well as have someone else drive the boat to see if something goofy is happening to a hose while under load. Probably wouldn’t be a bad idea to swap sending units as well. I will bring my infrared temp gun too. Thanks for the help!
 
A kinking intake hose at drive side would show up as a restriction in. Reving engine may mask it. So trimming drive up til trim limit stops drive up, should allow water to flow normal if kinked when down all the way.

If engine is physically hotter then other then the sender is most likely ok.

Only other possibility I can think of is a bad on engine circulating (water pump) issue. Very rare as marine water pumps have one direction bronze impeller vs auto are typically bi-directional with stamped steel impellers. Not knowing if yours is Marine vs Auto style would be good to know... auto type could fail if used with only lake water as the stamped steel mounting could fail due to corrosion....

Hard to say more at this point
 
kghost said:
A kinking intake hose at drive side would show up as a restriction in. Reving engine may mask it. So trimming drive up til trim limit stops drive up, should allow water to flow normal if kinked when down all the way.

If engine is physically hotter then other then the sender is most likely ok.

Only other possibility I can think of is a bad on engine circulating (water pump) issue. Very rare as marine water pumps have one direction bronze impeller vs auto are typically bi-directional with stamped steel impellers. Not knowing if yours is Marine vs Auto style would be good to know... auto type could fail if used with only lake water as the stamped steel mounting could fail due to corrosion....

Hard to say more at this point
Click to expand...
I should have added that as well. Both engine water pumps were replaced in 2022/2023 when I did bellows and gimble bearings. As far as I knew they were original to the boat so I replaced them before I put it in the water for the 2023 season just to be safe. Pumps were Quick Silver. I was having the over heating issue in 2021 which ended up being that seal you mentioned at the upper end of the copper transfer tube in the out drive. I didn’t want to deal with overheating anytime in the near future and I had no records of when the gimble bearings and bellows were replaced. So I just did it all. Unfortunately, here I am overheating again.

With the elbows changed I’m thinking a low flow issue may be more possible than I had originally thought. I was thinking restriction in the exit was the issue and not an impeded intake. With the new impellers and no noticeable restrictions on that end is where I was moving my diagnosis towards bad risers or something along that line. But, with both elbows about the same temp to the touch tells me that both banks are exiting at about the same temp. I don’t remember if I changed that little “S” hose where the outdrive goes into the transom. And I don’t remember what it looks like in its “down and up” form. I know I bought those hoses, and I know where they would be inside of the boat if I didn’t install them. I’ll look for them as well this weekend.

One bonus is that I finally replaced the water plug in one manifold with a petcock valve like I have been meaning to do the last couple times I winterized it. So I guess that’s a plus, lol.
 
What little S hose?

I was referring to the location where the copper tube pushes in at the upper. The plastic part it goes into is S shaped inside. This allows the tube to go straight up as the hole in the upper cavity is about and inch or so inward on the hoising.

Also, The fat square edged Oring that sits on top of impeller housing was put in place, correct? This deters exhaust from getting into impeller housing.

Also if any of the impeller housing gaskets are torn, this also will allow exhaust to be pulled into impeller housing.
 
kghost said:
What little S hose?

I was referring to the location where the copper tube pushes in at the upper. The plastic part it goes into is S shaped inside. This allows the tube to go straight up as the hole in the upper cavity is about and inch or so inward on the hoising.

Also, The fat square edged Oring that sits on top of impeller housing was put in place, correct? This deters exhaust from getting into impeller housing.

Also if any of the impeller housing gaskets are torn, this also will allow exhaust to be pulled into impeller housing.
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The seal you were mentioning is good. It was cocked or balled up when one engine was doing the same thing in 2021. If I remember right there was like a 4 bolt little housing that came off and it was a larger cylinder type seal. Right where that transfer tube slides into the upper housing of the outdrive. Replacing that in 2021 fixed the issue. I did not pull the upper unit apart so I have no clue of the condition of the tube inside of there.

I had to google the “S” hose I was referring to to get the proper name. It’s the gimble water hose. I believe it moves, or flex’s, every time the drive is moved up or down and when steering.

That square o ring is in place. I did not just change impellers, I did the housing as well with all new gaskets. Also o rings between lower and upper unit. I bought the complete housing “kit” and it’s Quick Silver. No change from before these were replaced or after. Impeller was my first thought so I did it no matter what since they were also done in that 22/23 off season. Some have told me to do them every year and others said every 5-7 years. Not sure what is accurate as far as doing those for preventative maintenance.

I use dish soap when putting the impeller in and as lube when re assembling.

Also, I was told this has updated exhaust bellows by fellow dock members. The exhaust is not attached to the outdrive with a hose clamp like what comes in the kit I had bought. It’s just open and the tail end, kinda oval shaped, and touches the outdrive when it’s all the way down. At least that’s how I remember it. They said to leave them and they were a better choice. They have been on the boat since I bought it in 2020. No clue as to when they were installed.
 
Exhaust bellows has nothing to do with issue.

The water hose comes in two forms. Straight and shaped.

Now if you did not buy or replace with Qicksilver/mercruiser, The length could be long and this would cause kinking.
Did you replace this hose recently? Before the new over temp issue started?
Not installing an Oem hose properly can caise it to be "to long" and possibly kink.

The newer Shaped hose which is more like an S shape, in my opinion is the better one to use as it fits and installs easier.
,
 
kghost said:
Exhaust bellows has nothing to do with issue.

The water hose comes in two forms. Straight and shaped.

Now if you did not buy or replace with Qicksilver/mercruiser, The length could be long and this would cause kinking.
Did you replace this hose recently? Before the new over temp issue started?
Not installing an Oem hose properly can caise it to be "to long" and possibly kink.

The newer Shaped hose which is more like an S shape, in my opinion is the better one to use as it fits and installs easier.
,
Click to expand...

All parts I have bought for the boat are Quick Silver. I bought the gimble hoses for sure but do not remember if I replaced them when I did the bellows in 22/23. They are formed I’m sure of that. I did not do them in the past couple weeks when this started acting up. I know where they are in the boat if I did not install them. I will be checking tonight/tomorrow to see if they are in the “boat parts compartment” of the boat. These hoses are the ones I was talking about previously, seeing if changing the trim will make a difference in the cooling. I’m also wondering if the extra turbulence that my buddy saw behind the starboard engine while idling is a sign of a ripped hose. I have minimal history from the prior owner, he did not save paperwork. I have no clue if or when they have been replaced. If I didn’t do them with the bellows, they have not been replaced since I bought the boat in 2020.

I would love to post photos of it but it seems my phone camera pictures are “too large” to put here. That way I could show the condition of the elbows, what these hoses look like, etc.
 
There are some tests you can perform. Not saying its easy based on running two engines but here are tests for Incoming water flow from drive.

You will need a single hose and attach at the thru transom fitting (incoming water supply from outdrive) This is the inside of boat, of the water hose we hve been speaking of that may be kinked on the outside at outer transom.

Merc has a flow spec based on gear ratio and I have copied it from 1985-1988 service manual.

I would recommend a CLEAR vinyl hose to do two tests at once.
Test has to be done in the water (clean water)
These two tests will either show an issue or end discussion of incoming water flow issues meaning No issues or Yes issues.

test 1. Bubbles in water supply test. Clear hose connected directly to the input location at on engine water pump. With a clear hose you can see if there are bubbles in the incoming water supply. a few are normal but a lot is not normal.

test 2. Flow rate. At 1000 rpm.
Below is a chart in the service manual. The flow rate is based on gear ratio.







Alpha flow rates by gear ratio.jpg
 
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kghost said:
There are some tests you can perform. Not saying its easy based on running two engines but here are tests for Incoming water flow from drive.

You will need a single hose and attach at the thru transom fitting (incoming water supply from outdrive) This is the inside of boat, of the water hose we hve been speaking of that may be kinked on the outside at outer transom.

Merc has a flow spec based on gear ratio and I have copied it from 1985-1988 service manual.

I would recommend a CLEAR vinyl hose to do two tests at once.
Test has to be done in the water (clean water)
These two tests will either show an issue or end discussion of incoming water flow issues meaning No issues or Yes issues.

test 1. Bubbles in water supply test. Clear hose connected directly to the input location at on engine water pump. With a clear hose you can see if there are bubbles in the incoming water supply. a few are normal but a lot is not normal.

test 2. Flow rate. At 1000 rpm.
Below is a chart in the service manual. The flow rate is based on gear ratio.







View attachment 35905
Click to expand...

Great info, thanks!!!! When I flushed that feed I didn’t see a leak after I thought about it so I guess a ripped hose is low on the possibilities. I don’t know if I will have much time to spend on it this weekend as some other things have come up. I will post here again with what I find. Leaving posts like this dead without an answer doesn’t help others so I will keep posting what I do, find and what the fix is. Thanks for all the help!!
 
Bringing this back up. Tested water flow from the impellers. I ended up checking where the hose goes into the engine. I did this to both engines. Good engine was done first and was under what is specified. It put out just over 2qts in 15 seconds. Did the engine that got hot next and it put out roughly 4qts.

Since I never ran the thing at a higher idle in neutral, I thought I’d give it a try to see how the exhaust manifolds felt after some time. I set both engines at about 2k RPM and went back into the cabin to work on some upholstery I’m doing. After 25/30 min I felt the exhaust manifolds. Both engines showing around 120 degrees (still no t-stats). Port engine (the one not overheating) had luke warm manifolds to the touch. Starboard engine (the one giving me trouble) had a luke warm left manifold and a hot right manifold. Not burn your hand hot, but hot enough that my hand wasn’t comfortable after 5 or so seconds.

I decided to pull the down pipe on that manifold to check the exhaust flapper (I don’t know the technical term). Only half was there. I chose to pull the boat for the season and pull more apart, hoping to find a piece lodged in there. So I pulled the other downpipe and I saw the same problem, half gone. Nothing could be seen in the tail pipe or by looking down. Brought my scope from the shop and ran the camera down the “Y” pipe. All clear on both sides.

Now I’m speculating that even though I’m showing zero signs of water going into the combustion chamber, I have a hairline crack in an exhaust port or exhaust manifold. I removed the exhaust manifold from the side that was getting hot and have it set up on my vice so I could fill it with water. I saw nothing after a couple minutes but left it full and will check back when I get to the shop tomorrow.

If the manifold holds water, I guess I’ll pull both heads and check for a restriction in the block from rust and have the heads checked for cracks in the exhaust ports. With clean oil, I’m fairly certain the block is crack free. I don’t know how high water stays in the engine when it is shut off. I would assume it drains to the water level of where the boat sits in the lake? If that is the case, and I do have a crack in the head, maybe that’s why I don’t have any issues with water entering a cylinder.
 
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