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I have a Mercury 150 XR6 that has had a problem with overheating.

The engine will overheat at...
I have a Mercury 150 XR6 that has had a problem with overheating. The engine will overheat at anything above idle, but will cool back down once on plane and running about 45-50 mph. Reduce speed and it heats up again. This problem has become progressively worse over a two year period. I have replaced impeller three times and housing twice. I pulled the exhaust plate between the heads to check for any blockages, none found. I installed a pressure gauge at the brass fitting on top of the block, this is where a hose runs down to the poppet valve. The pressure gauge shows 4psi at idle in the lake, drop it in gear and it drops to 2 psi. As I open throttle, pressure increases to 7-8 psi and never changes, all the way to WOT. If I run in the 3500-4000rpm range the temp climbs. At about 4500rpm, the gauge starts going back to normal. Verified cylinder head temps with infrared thermometer at 185 degrees when gauge was reading 3/4 of the way to hot. I cant find a water flow diagram for this engine to help me any further, but if I am only reading 7-8psi at the top of the block, wouldnt this be a problem with the impeller, housing or the tube running up to the engine. What could be blocked ar leaking between the pump and this fitting on top of the block behind the flywheel. I may be way off base here, but I was trying to think the water flow through and see if it was neccessary to remove the heads to check for any obstruction there? Puzzled??
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Answered in 4 hours by:
8/1/2010
Marine Mechanic: tom, Marine Mechanic replied 12 years ago
tom
tom, Marine Mechanic
Category: Boat
Satisfied Customers: 117
Experience: 32 yrs exp. certified Johnson/Evinrude, Mercury outboards , also omc/cobra sterndrives
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Hello I'm Tom, I'll be happy to help you,if you have follow-up questions, just return here. I think you are on the right track, there must be a reason the water pump is not building pressure, you should be seeing a least 20 lbs pressure at 4500 rpm, but if you are sure the impeller and the housing are in good shape, there must be another cause of the pressure loss. On the top of the water tube there is a seal or a rubber grommet, if this is not sealing correctly or somehow caused a blockage of the tube, it can restrict water flow, the gaskets in the adapter plates could be at fault, as well as the water tube itself, if the tube is bent or has been shortened in any way and does not seat properly into the seal on the waterpump housing, there can be a loss of pressure, also I have seen the tube have a hole corroded or rubbed in it. The bad thing about a problem of this nature is that after you have resolved that the obvious things are not the problem, then pulling the powerhead to inspect the adapter and base are sometimes the only way to go. First, I would remove the gearcase and pull the pump down, check the wearplate for grooves, check under the wearplate for any blockage, then I would look inside up in the exhaust housing for anything obvious, I find that a small mirror reflecting the sun up into the housing is better than any flashlight I have, you can then try a water hose directly on the water tube with the thermostats out and see if you have a good water flow, then with the thermostats back in you should be able to build some pressure with the water hose, if you can then the pump is still the problem, if not then its time to pull the powerhead and check the adapter plates, I am enclosing a diagram of the water tubes location(#28), note the seal I mentioned(#35) I will be happy to help if you have more questions on this, thanks-----------Tom Diagram of Mercury Outboard Parts 1999(NNN) NNN-NNNND DRIVESHAFT HOUSING AND EXHAUST TUBE Diagram

Edited by tom on 8/1/2010 at 1:43 AM EST
tom
tom, Marine Mechanic
Category: Boat
Satisfied Customers: 117
Experience: 32 yrs exp. certified Johnson/Evinrude, Mercury outboards , also omc/cobra sterndrives
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Customer reply replied 12 years ago
Tom, Thanks for your answer, that was what I was looking for. So I take it that you agree that if I only show 7-8psi at the top of the block that it must be a problem straight down from there, basically eliminating any blockage in the head at this point? I have the exhaust plate removed and dont have the new gasket so I cant pressurize from the water tube yet. I am curious though after talking to a local mechanic, he advised that the running temperatures on an outboard in the texas heat were often around 185 - 190 at the cylinder head and that the alarm does not sound until around 204 degrees. Is this correct info? If my heads are below 200 degrees am I within normal. For some reason I thought it was supposed to be much cooler. Someone removed the factory sending unit on the starboard side and replaced it with an after market sender and temp gauge. Wondering if this thing has been fooling me all along into thinking its too hot? There are way too many variables in this one and its driving me nuts, thanks for your help
Marine Mechanic: tom, Marine Mechanic replied 12 years ago
It's possible that the gauge is a little off, but there ought to be more water pressure, you should also be sure the poppet is functioning properly, you can see if you can locate an infra-red temp gun and get an accurate reading with that but I'm betting there is cause for the elevated engine temps, the local man is right on the temp sender, I know that elevated water temps that we have in the south make water pressure a must, I would try to get the head temps down below the 190 range if possible, but as I said with our surface water temps bumping the 90's sometimes thats hard to do, I still think more water pressure will be the ticket, there has to be a reason that the engine will not build more pressure than that, I would also see that the water pressure gauge is good, try a little air to see if it moves freely, don't peg it out though, thanks---Tom

Edited by tom on 8/1/2010 at 2:53 AM EST
tom
tom, Marine Mechanic
Category: Boat
Satisfied Customers: 117
Experience: 32 yrs exp. certified Johnson/Evinrude, Mercury outboards , also omc/cobra sterndrives
Verified
tom and 87 other Boat Specialists are ready to help you
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Marine Mechanic: tom, Marine Mechanic replied 12 years ago
Hello again, I noticed this morning that you accepted on this question twice, you don't need to do that, I will request a refund for one of your accepts, thanks -------Tom
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Customer reply replied 12 years ago

Hi again, I have the new gasket for the exhaust plate and a new water pump kit. Going to try everything you mentioned above tomorrow evening. One question though at this point, and if I need to ask this as a new question, please advise. I decided to test the warning buzzer for the temp. and discovered what seems to be something wrong. This '99 engine was installed on an older boat, '87 Stratos. When I look at the parts diagram it shows that in the starboard head, there shoud be a temp sender. This is a little device with a spring on it that fits in a dead hole in the head. The wire is tan with blue stripe and runs to a junction box, where another wire comes in from a little black box on the port side of engine. (oil warning maybe??) Then the diagram shows a temp sensor on the port head. This device has a ground wire that grounds right beside the dead hole for the sensor and it is a small bullet shaped device that fits in another dead hole. This one is hooked up to a wire that does not appear to be part of the wiring harness and runs directly to the dash gauge. Shouldn't the sensor go to the overheat alarm and the little device with the spring (sender) go to the gauge? Or what was the original wiring for these two? Im wondering if someone has hooked up the wrong device to the gauge. I dropped the device with the spring into boiling water and it never sounded the alarm. Is the spring device(sender) in the starboard head supposed to go to the alarm or to a gauge?? Thanks in advance for any help and I will followup about the results of the water flow problem.

 

This is port side and goes direct to gauge

graphic

 

This is starboard side and goes to warning buzzer, but buzzer only goes off if wire is grounded, not in hot water.

starboard head

 

 

Marine Mechanic: tom, Marine Mechanic replied 12 years ago
Hello again, you don't need to open another question as long as the subject is the same, the pictures were great, the wiring looks correct, the sender will not activate when you submerge it in hot water because it needs to be grounded to complete the circuit, you can heat it and touch a bare place on the block with it and it should activate, it has a vernatherm type material that swells and makes contact with the body inside and completes a circuit to get an alarm. One thing more on the cooling system,check the poppet valve and the cavity under it for any problems, I would replace the poppet as a precaution, it's not a very expensive part and can be the culprit in pressure or cooling problems, make sure that nothing is in the block behind it, and that the valve is not sticking in the seat, thanks--------Tom
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Customer reply replied 12 years ago
I forgot to mention that I did rebuild the poppet valve last week. It made no change. I have a theory I would like your opinion on. After sitting and staring at this thing for a while, I noticed that at some point in time, someone placed silicone on the lower unit divider plate that separates the exhaust from the water pump area. I have never applied silicone to this area as someone told me long ago it was not neccessary. The silicone that is left is jagged and not uniform. Is there any possibility that there is enough exhaust blowing into the water pump pick up area to cause flow problems and low pressure. I guess I am throwing darts at this point, but something has to click soon. Thanks, XXXXX XXXXX to put things back together.
Marine Mechanic: tom, Marine Mechanic replied 12 years ago
Hello, the silicone that you mention should not be in the water circulating system, unless excessive ammounts were used and some got through the cooling system and were pushed into the motor or adapter, or there is a leak of exhaust as you suspect, -------Tom

Edited by tom on 8/2/2010 at 6:28 PM EST
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Customer reply replied 12 years ago
OK, just an fyi at this point. Installed exhaust plate back on motor. Installed new impeller and insert, even tho less than 6 months old. The only thing different is that I installed silicone on the divider in the lower unit that seperates the exhaust chamber from the water pump area. Dropped boat in lake with no expectations and gauge instantly read 6psi. Touched throttle and jumped to 10psi. WOT almost 20psi. Temp gauge barely jumped over mid point and went right back down. I guess the guy that told me 3 years ago not to worry about silicone on the lower unit needs to be shot. Thanks for all your help with this.
Marine Mechanic: tom, Marine Mechanic replied 12 years ago
Hello again, thats good news and great water pressure, thanks again----Tom
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