7.4 Mercruiser overheat

[94SD370]

I have a 1994 7.4 with about 850hrs. It overheats after about 10-15mins of run time. Engine RPM does not seem to matter. I never let it get over 190 degrees before shutting it down. Sometimes it will climb to 200 degree after shut down. I have put a laser thermometer on several parts. The thermostat housing (sensor) is 190, same as dash gauge, riser elbow 90 degrees, same as water at outlet. Manifold is 220 degrees. This is closed cooling and in fresh water. I have changed the impeller, cleaned the sea strainer, pulled all the hoses, cleaned out the tubes in the heat exchanger. I pressurized the heat exchanger to 19 PSI and it held that until I released it about 15 minutes later. I checked the oil cooler and no obstructions. Water flow out of the side of the boat has a very slight hesitation in flow, but others have said it is normal. I could not pull the large tubes from the riser elbows without removing the riser. I stopped there.

On the coolant side, I replaced the thermostat. Did a coolant flush. I have cracked the bleeder on top of the thermostat housing and verified the circulation pump is moving coolant. I cannot however get it to a point where there are no air bubbles in the coolant. I tightened every coolant hose. I also have the same intermittent bubbles in the heat exchanger. I bought an exhaust gas detection tool and pumped the air out of the heat exchanger while running. It did not detect (change color of the test solution) exhaust gas. I am wondering where the air is coming from and if that is my problem. I suppose it could be not enough exhaust gas to be detected? But still be a head gasket. I am waiting to replace the manifolds because of fear of snapping a stud. If I do that now, likely the heads will have to come off, and that is a season ender for me. My next step is a compression test. After that manifolds. Any other ideas? Or should I put down the shovel.

It’s worth noting that I also have a rattle (the marble sound) at Idle. I changed plugs, cap, wires, etc.. It did not fix the sound. The mechanic at our marina says that noise is normal for a boat of this age and related to worn springs on some type of plate on the transmission.

Thank you in advance.

 

[Golfman25]

Last “simple” fix might be to change the water feed hose from the thru hull to the pump. Sometimes they can collapse internally restricting flow.

 

[Scott215]

Are you sure water pump impeller is good

 

[94SD370]

Are you sure water pump impeller is good

I replaced the raw water pump impeller. I don’t know how to check the circulation pump (coolant) impeller, other than to see if there is pressure at the bleeder to cause a geyser like effect. I suppose I could pull it, but at that point, I might as well replace it. I think it is moving coolant, I just can’t be sure if the flow is correct.

 

[94SD370]

Last “simple” fix might be to change the water feed hose from the thru hull to the pump. Sometimes they can collapse internally restricting flow.

Good idea, I will look into finding a new hose. That is a long one, so I understand how it could collapse under pressure.

 

[Hoplite808]

Could it be a plugged up heat exchanger from growth or debris that's preventing the raw water side from providing enough water to the closed cooling side to exchange sufficient heat?

 

[94SD370]

Could it be a plugged up heat exchanger from growth or debris that's preventing the raw water side from providing enough water to the closed cooling side to exchange sufficient heat?

I don’t think so, but I don’t know enough about it. I did take the heat exchanger apart and used a dowel to clean out each tube. I pressure checked it (from coolant side) and made sure the inlet and outlet rubber seals were good and free of debris. I think I did everything I could. Is there more?

 

[Korkie]

Did you consider the thermostat may be getting stuck?

 

[Scott215]

Thermostat is a good easy thing to check. Thanks korkie

 

[Yendor]

Am curious what degree thermostat is called for in a '94 (and what is actually in yours). Mine is a 2000 and obviously has a 170 in it. From a cold start, you can literally watch the gauge gradually climb to about 172, then drop to about 140ish when it opens and then repeats over about a four minute span.
Mine does not have a closed cooling system, so that would admittedly make a difference. In any case, I would not expect a closed system to use a higher temp 'stat though. Maybe try putting a 160 in it, if for no other reason than to note what difference it makes compared to what you have? Hopefully you just have a bad thermostat. Over the years, I've seen quite a few bad out of the box. Which is why I go out the way if need be, to get a Stant.

On the collapsed hose note, I might try putting a (garden) hose straight to the raw water pump. If that did or did not make a difference, might save you the time and expense of changing out that hose if it isn't contributing to the problem.

 

[PlayDate]

I have a 1994 7.4 with about 850hrs. It overheats after about 10-15mins of run time. Engine RPM does not seem to matter. I never let it get over 190 degrees before shutting it down. Sometimes it will climb to 200 degree after shut down. I have put a laser thermometer on several parts. The thermostat housing (sensor) is 190, same as dash gauge, riser elbow 90 degrees, same as water at outlet. Manifold is 220 degrees. This is closed cooling and in fresh water. I have changed the impeller, cleaned the sea strainer, pulled all the hoses, cleaned out the tubes in the heat exchanger. I pressurized the heat exchanger to 19 PSI and it held that until I released it about 15 minutes later. I checked the oil cooler and no obstructions. Water flow out of the side of the boat has a very slight hesitation in flow, but others have said it is normal. I could not pull the large tubes from the riser elbows without removing the riser. I stopped there.

On the coolant side, I replaced the thermostat. Did a coolant flush. I have cracked the bleeder on top of the thermostat housing and verified the circulation pump is moving coolant. I cannot however get it to a point where there are no air bubbles in the coolant. I tightened every coolant hose. I also have the same intermittent bubbles in the heat exchanger. I bought an exhaust gas detection tool and pumped the air out of the heat exchanger while running. It did not detect (change color of the test solution) exhaust gas. I am wondering where the air is coming from and if that is my problem. I suppose it could be not enough exhaust gas to be detected? But still be a head gasket. I am waiting to replace the manifolds because of fear of snapping a stud. If I do that now, likely the heads will have to come off, and that is a season ender for me. My next step is a compression test. After that manifolds. Any other ideas? Or should I put down the shovel.

It’s worth noting that I also have a rattle (the marble sound) at Idle. I changed plugs, cap, wires, etc.. It did not fix the sound. The mechanic at our marina says that noise is normal for a boat of this age and related to worn springs on some type of plate on the transmission.

Thank you in advance.

So was there a point where everything was okay with this engine or is this a new boat to you?

Most of the time.....it seems that we are in the middle of story....not the beginning.

For example if the engine worked fine and then you replaced the impeller....that would most likely be an air leak at the raw water pump. It doesn't take a big leak to cause the impeller to cavitate and not produce enough water to cool the heat exchanger.

Not too long ago we had another thread where the OP had paid a ridiculous amount of money chasing a heating issue when a previous impeller issue created a debris block in a hose the mechanic didn't bother to check. http://clubsearay.com/index.php?threads/sea-water-pumping-issue-25k-in-so-far.113338/

The raw water side is normally where heating problems arise. That said on the coolant side......the thermostat and the water pump are also likely candidates. The engine is 29 years old so either side could be the issue.

In regards to what you mechanic said about the "marbles" that is likely the transmission dampener plate which does have torsion springs on it.

 

[Scott215]

Collapsed hose is a common issue

 

[94SD370]

So was there a point where everything was okay with this engine or is this a new boat to you?

Most of the time.....it seems that we are in the middle of story....not the beginning.

For example if the engine worked fine and then you replaced the impeller....that would most likely be an air leak at the raw water pump. It doesn't take a big leak to cause the impeller to cavitate and not produce enough water to cool the heat exchanger.

Not too long ago we had another thread where the OP had paid a ridiculous amount of money chasing a heating issue when a previous impeller issue created a debris block in a hose the mechanic didn't bother to check. http://clubsearay.com/index.php?threads/sea-water-pumping-issue-25k-in-so-far.113338/

The raw water side is normally where heating problems arise. That said on the coolant side......the thermostat and the water pump are also likely candidates. The engine is 29 years old so either side could be the issue.

In regards to what you mechanic said about the "marbles" that is likely the transmission dampener plate which does have torsion springs on it.

 

[94SD370]

I am three years into this boat. It ran fine without any issues prior.

I did replace the impeller, which was fine btw, however I did find rubber in the heat exchanger from a previous failure.

 

[94SD370]

Am curious what degree thermostat is called for in a '94 (and what is actually in yours). Mine is a 2000 and obviously has a 170 in it. From a cold start, you can literally watch the gauge gradually climb to about 172, then drop to about 140ish when it opens and then repeats over about a four minute span.
Mine does not have a closed cooling system, so that would admittedly make a difference. In any case, I would not expect a closed system to use a higher temp 'stat though. Maybe try putting a 160 in it, if for no other reason than to note what difference it makes compared to what you have? Hopefully you just have a bad thermostat. Over the years, I've seen quite a few bad out of the box. Which is why I go out the way if need be, to get a Stant.

On the collapsed hose note, I might try putting a (garden) hose straight to the raw water pump. If that did or did not make a difference, might save you the time and expense of changing out that hose if it isn't contributing to the problem.

I do not know much about thermostats. The guy at the boat store said I need a 140 thermostat for my motor, so I went with that. Both motors typically run at 160, not sure if that makes sense for a 140 stat? My temp gauge gradually climbs to 160, and never moves or fluctuates beyond that. I did not consider that the brand new thermostat could be bad.

 

[Scott215]

New parts can be bad out of the box. For t stats I stay with merc only. Tried another brand and came with crappy gaskets and would not seal completely

 

[PlayDate]

I am three years into this boat. It ran fine without any issues prior.

I did replace the impeller, which was fine btw, however I did find rubber in the heat exchanger from a previous failure.

Did it start happening after you did some work or is it a problem that just showed up one day. Something that just shows up is like a thermostat or an impeller .....it just quits working and the engine heats up.

I'm less likely to believe you have a closed cooling problem since you replaced the thermostat and pressure tested the system. Also it is unlikely to be a head gasket when it holds 19 psi for 10 minutes or so. The bubbles are a curiosity though....I assume you bled the air out of the thermostat bled when you replaced the coolant. The closed side doesn't create air and when properly bled.....only coolant should come out the bleeder.

As to doing a few more diagnostics on the raw water side.....replace the raw water hose to the heat exchanger with a clear vinyl one. That will show pretty quickly whether you have cavitation (air leak) at the raw water pump.

 

[Pirate Lady]

Cmon guys, 2 years ago on here I broke the internet with a full summer discussion of my 7.4 B1 overheating saga.
One word… BRAVOITIS!
OP, look it up. Its your problem. You can spend all summer chasing it like I did or ask a mechanic to fix it and be done in 2 days.
When I finally replaced every dam part related to cooling from June 5 to end of September, i gave up, took boat to mechanic who pulled outdrive and in 15 minutes found the problem.
Find the thread, or call a good mechanic and tell him BRAVOITIS. Its a known disease.
Your call.

 

[PlayDate]

Cmon guys, 2 years ago on here I broke the internet with a full summer discussion of my 7.4 B1 overheating saga.
One word… BRAVOITIS!
OP, look it up. Its your problem. You can spend all summer chasing it or ask a mechanic to fix it and be done in 2 days.

Are you having cocktails for lunch? He has a 1994 370 Sundancer with V drives. The last thing he needs is Bravoitis.

 

[Scott215]

Dope!