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Tan, MerCruiser Certified Technician
Category: Boat
Satisfied Customers: 10899
Experience:  40 yrs. experience with complete engine repairs both inboard/outboard and MerCruiser Certified Technician.
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I have a 1983 wellcraft boat with 170 4 cylinder mercruser

Customer Question

I have a 1983 wellcraft boat with 170 4 cylinder mercruser motor that has heat exchanger. Have had it out several times but it overheats every time have changed impeller water pump. And checked exchanger for any blockage. Only thing I can think is head gasket. It runs really good at first then around 200 degrees it shuts down and I have to wait for it to cool off before it will start again. Help I am stumped
Submitted: 1 year ago.
Category: Boat
Expert:  Tan replied 1 year ago.
Hello. My name is***** you loosing coolant? We need to do a compression test. The compression must 100+ psi with no more than 15% difference.
Customer: replied 1 year ago.
Can't do compression test right now have not bought the tester yet . Just trying to get an idea of where to look next. Yes I am losing coolant. It's all over them motor when it overheats. Boat won't overheat until I'm in water testity on trailer and everything is fine
Expert:  Tan replied 1 year ago.
If you have a blown head gasket the compression test will show low compression in one cylinder. We need to pressure test the cooling system. See if you have an internal leak not visible. When you inspected the heat exchanger did you inspect every tube was clear? The exchanger is a series of tubes that the salt water runs through. If any are clogged it can cause over heating. We also need to do a water pump volume test to verify the cooling system is getting enough water to cool the coolant. Here is the test that needs to be performed. Do these tests/checks and post back with findings.
Expert:  Tan replied 1 year ago.
To pressure test the cooling system you can rent a radiator pressure tester. The tester connects to the cap # ***** on the reservoir # ***** where the coolant cap is located. Post back with findings.
Customer: replied 1 year ago.
I have a ratiator tester my dad repaired radiators for years have used it with no outstanding problems on motor. All tubes on heat exchanger were clear took it off flushed and boiled it then checked it again flush and boil were just a precaution in case I missed anything.
Customer: replied 1 year ago.
Also not running in salt water fresh water only where I live
Expert:  Tan replied 1 year ago.
Ok we need to do the water pump volume test. You probably have the 1.65 gear ratio. But to be sure look on the top left side of the drive. The ratio will be stamped in the drive.
Expert:  Tan replied 1 year ago.
Drive ratio location.
Customer: replied 1 year ago.
Yes it is a 1,65 ratio but none of that helps. I need to know where to look for my next problem. I told you it has new pump and new impeller. I need an awnser of where to look next an f as far as I can see it is a head gasket. Please tell me if I'm wrong
Expert:  Tan replied 1 year ago.
I told you what you need to do. A compression test. If you just guess and remove the cylinder head and that's not it you wasted your time. Did you open the link to the water pump volume test? What do you not understand about the compression test and the water volume test? The water volume test will tells us if you have a water delivery issue. We have to do testing to find the problem. This is a process of elimination. Assuming can cost allot of money and possibly be wrong.
Customer: replied 1 year ago.
Well here is what I haveD done to motor. I have dropped lower unit and put new impeller in put new raw water pump in motor. Checked and boiled heat exchanger to make sure I cleaned it good. Everything runs good on trailer. Get on water and it goes to hell fast.
Expert:  Tan replied 1 year ago.
That is why we have to do the tests I posted. What you have done is fine and well but were not the problems. If you have a kinked hose between the drive and the transom assembly the water pump can't deliver enough water causing an over heat issue. If the water tube in the drive has a hole in it the engine will get exhaust gases in the cooling water causing over heat. The compression test will verify if you have a blown head gasket. If the head gasket is blow it can cause exhaust gases to get in the cooling system causing over heat. Now the chances of the head gasket being blown are slim. The pressure test of the cooling system eliminates this as a possibility. But I like to do compression test to verify the strength of the engine. How much pressure did you apply to the cooling system?
Customer: replied 1 year ago.
Ok I'm not stupid basically this is a Ford four cylinder motor with a couple things changed to make it a Boat motor. I have checked everything but head gasket. I Will do compression test. But do you agree that I need to change gasket it is after all an old boat
Customer: replied 1 year ago.
I put around 40 psi to the system with radiator test
Expert:  Tan replied 1 year ago.
Ok with that test we can eliminate the head gasket. Changing the head gasket can't hurt. I suspect that you are getting exhaust gases in the raw water side of the cooling system. This would only show up when the boat is in the water when there is back pressure on the lower unit exhaust. The water tube # ***** and water tube grommet # ***** are places for exhaust gases or a restriction of the grommet where the water tube goes into the cover assembly # *****:// If the cover assembly # ***** has ever gotten real hot it will have pin holes and cause over heating.

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