Overheating mercruiser 3.7l lx

[FosterFish]

Ok, I know this has been covered but I haven’t found the thread that can give me the info I need to solve the issue. So here it goes... I had a pair of these engines completely rebuilt, one of which works perfectly. The other however has run hot from the first turn of the key. New riser, new out drive, new impeller, new carb, new cap,rotor, ect. you get the picture, I threw the bank at it with the parts gun blazing til the barrel warped. It gets up to about 150 in les than 5min at idle. If you put it under load the temp will spike as high as you let it and it is next to impossible to cool down after it goes up. I removed the thermostat and it still does the exact same thing on the same timeline. It will do this even if I hook my garden hose directly to the inlet on the heat exchanger(which is brand new). I have also tested for combustion gasses in the coolant and that was negative as well. Am I missing something. Oh yeah, my working engine will sit at 116f all day at idle without the stat installed. Also... I’m really hoping Don S will chime in on this. Thanks

 

[1000 Islands]

Have you confirmed that the gauge isn't lying, aka used an infrared thermometer?

 

[FosterFish]

Have you confirmed that the gauge isn't lying, aka used an infrared thermometer?

Yes sir, I’m only reading the temp with an ir gun

 

[Scott Danforth]

150 F is almost to temp

What is the max temp it gets up to?

If it gets over 180f, then you have an issue

If so.....

Did you check the heat exchanger? New doesn't mean good. Did you buy it new from Monitor or Champ (the only two original manufacturers)

Bypassing the raw water pump did two things. It verified it's not the raw water pump, and it smoked your impeller

Did you check the circulating pump impeller?

116F is too cold. The motor needs to be at 160

 

[Scott Danforth]

I’m really hoping Don S will chime in on this. Thanks

Not without a seance, Don has been gone for many years now

 

[FosterFish]

150 F is almost to temp

What is the max temp it gets up to?

If it gets over 180f, then you have an issue

If so.....

Did you check the heat exchanger? New doesn't mean good. Did you buy it new from Monitor or Champ (the only two original manufacturers)

Bypassing the raw water pump did two things. It verified it's not the raw water pump, and it smoked your impeller

Did you check the circulating pump impeller?

116F is too cold. The motor needs to be at 160

I removed the out drive before starting the engine with hose on the hx so the impeller is fine. I know 116 is too cold I was just doing different thing on both engine to gather a frame of reference. I wanted data to compare one engine to the other in a variety of situations. As far as check it, what do you mean exactly?
Really sorry to here Don is no longer boating with us. Defiantly lost a wealth of knowledge there.

 

[FosterFish]

Not without a seance, Don has been gone for many years now

And yes, if I put the engine under load it will go above 180. I just got a head back because of that

 

[FosterFish]

And yes, if I put the engine under load it will go above 180. I just got a head back because of that

150 F is almost to temp

What is the max temp it gets up to?

If it gets over 180f, then you have an issue

If so.....

Did you check the heat exchanger? New doesn't mean good. Did you buy it new from Monitor or Champ (the only two original manufacturers)

Bypassing the raw water pump did two things. It verified it's not the raw water pump, and it smoked your impeller

Did you check the circulating pump impeller?

116F is too cold. The motor needs to be at 160

i can’t remember the manufacturer of the hx it was aftermarket but it did cool the other engine fine.

 

[FosterFish]

Thanks for jumping on this so darn fast too. I love this forum

 

[FosterFish]

I removed the out drive before starting the engine with hose on the hx so the impeller is fine. I know 116 is too cold I was just doing different thing on both engine to gather a frame of reference. I wanted data to compare one engine to the other in a variety of situations. As far as check it, what do you mean exactly?
Really sorry to here Don is no longer boating with us. Defiantly lost a wealth of knowledge there.

As far as checking the circulation impeller, what I have done was installed a clear tube on the return side of the HX to verify flow and it seemed to have good flow.

 

[FosterFish]

I feel like the heat is starting in the exhaust manifold but only because that is the first place to get hot. Could there be something obstructing flow through there?

 

[Scott Danforth]

I feel like the heat is starting in the exhaust manifold but only because that is the first place to get hot. Could there be something obstructing flow through there?

You will need to check

 

[Dubed]

Do you have the #8 manual for this system? It has troubleshooting section

 

[Dubed]

Cooling

 

[kenny nunez]

Is the pressure building up in the engine where coolant starts to blow out from under the cap of the thermostat housing?

 

[FosterFish]

Is the pressure building up in the engine where coolant starts to blow out from under the cap of the thermostat housing?

I don’t let it get hot enough to do that

 

[Eric 3.7LX Alpha 1]

If it's climbing to 150º in 5 minutes, you aren't moving water, either coolant or seawater.

So far you bypassed the seawater hose running through the transom, and voltage regulator on the seawater side. Pop the seawater hose off the heat exchanger on the outlet side of the heat exchanger and see how flow is with the engine off. (Top hose aft end). It should be exactly the same flow as on the inlet with the garden hose going into it. If you have a power steering cooler inline check both sides of it also.

Post #14 from Dubed has a nice troubleshooting chart attached. As you read through the possible causes, you've eliminated a few already. Some to be checked further are: Blockage in heat exchanger, (both seawater and coolant side) hasn't been confirmed yet, blocked exhaust elbow; hose fitting in exhaust elbow installed too deep.

Forget the fact that some of these parts are new, anything is possible, you could have a rag, or piece of shipping material in the heat exchanger for all you know.

You have the 3.7 model which is the newer version. What are you running for an exhaust manifold? Aluminum or cast iron?

 

[FosterFish]

If it's climbing to 150º in 5 minutes, you aren't moving water, either coolant or seawater.

So far you bypassed the seawater hose running through the transom, and voltage regulator on the seawater side. Pop the seawater hose off the heat exchanger on the outlet side of the heat exchanger and see how flow is with the engine off. (Top hose aft end). It should be exactly the same flow as on the inlet with the garden hose going into it. If you have a power steering cooler inline check both sides of it also.

Post #14 from Dubed has a nice troubleshooting chart attached. As you read through the possible causes, you've eliminated a few already. Some to be checked further are: Blockage in heat exchanger, (both seawater and coolant side) hasn't been confirmed yet, blocked exhaust elbow; hose fitting in exhaust elbow installed too deep.

Forget the fact that some of these parts are new, anything is possible, you could have a rag, or piece of shipping material in the heat exchanger for all you know.

You have the 3.7 model which is the newer version. What are you running for an exhaust manifold? Aluminum or cast iron?

I have the aluminum exhaust manifold.

 

[kenny nunez]

Since you have 2 engines one way to find whether the drive or the engine is causing the problem is to cris cross the drive to engines cooling hoses with some auto heater hose. I know this may sound weird but it is easy to do.