hot problem no 1

[Dignity]

Recently raised the 150 hp 1991 merc a notch and found the motor temp went up. Had the thermostats replaced and the running temp went down as they were pretty well clogged up. When running the temp is fine but at idle or runnig at 6 knots the temp keeps climbing. In fact when I left Mooloolaba yesterday the temp went to 3/4 mark on the gauge by the time I ran from the ramp to the entrance. Normally it runs at a little under the half way mark. When flushing the motor the water from the cowl is so hot you can't put your hand under it. The water impeller had been replaceds earlier this year so not sure where to go from here.

Are the thermostats the wrong type or has something possibly blocked one or both of them again or is the impeller faulty or is there something more serious going on.


sam

[gone_fishing]

dont know the details of the cooling system of those motors
but when my motor was over heating the thermostat was full of crap also (after replacing and prob still coming back) i needed to remove the head plates and physically clean the gallerys
imo sounds like a flow prob defiinatly recheck thermostats and opening temps and if has been full of rubish a new impeller couldnt hurt
im sure some one knows a lot more than me and will steer you in the right direction
hope this helps any way
all the best

[Dignity]

g_f, how do you recheck the opening temps of the thermostats. The first time after the thermosstats were replaced the motor didn't have a problem but it was this weekend after being out at mooloollaba that it started. There were a lot of jellys of all differnt types in the water and was wondering if I picked something up. I have considered putting a new impeller in. What is involved in taking the head plates off to clean the galleries

[FNQCairns]

Hi Dignity, sounds OK to me but conditional, the thing with guages like those is there are no numbers so it is easy for a person to assume a problem when there is none or even the other way around, this is all stuff you already know.

An O/B temp is a bit like a car, once at full operating temp there is not much of the total heat range left to go upwards before it has overheated, so those guages can move a long way for little temperature changes. Ever driven a nissan patrol and watched it's temp guage? they put all types of thought into ones head

If it were me I would pull the sender from the block re-attach the wires (electronic?) and use a coffee cup with boiled jug water and a thermometer to calibrate what actual temp your dash guage is registering at full, 3/4, 1/2 etc. Then you will be in a position to look for a problem if one exists, I suspect but do not know your engine that somewhere 70s in deg C may be an overheat condition, outboards run cool relative to vehicle engines.

Good luck with it, if you do attemp to nut it out, look forward to hearing what you find out.

cheers fnq

[BM]

Poppit valve may be due for replacement. (pressure control valve, starboard side, below switchboxes).

[Dignity]

fnq - good idea to calibrate the gauge, will do it while it is running too hot. Usually I can put my hand under the stream and it is quite warm but at the moment it is steaming so figure it would have to be around the 70 plus. Surprised tha alarm did't sound which means another investigation.


mv - what does a ppit valve do?

[BM]

poppit valve controls water pressure through the block.

[Noelm]

just be very carefull if you decide to do some internal cleaning, you will soon become an expert on drilling out broken bolts in Aluminium Alloy, and that is a skill that is NOT fun to learn on your own motor.

[ozbee]

check the intake screen for sand grains the same size wedged in .

[FNQCairns]

Another place to look is the impeller as it is behaving like it could be getting slack, it's biggest job is low speed running esp offshore trolling speeds as there is no water presure past the leg to compliment the pump, at dead idle speeds there is buggerall heat produced to combat with a slack pump but the range above and below planing speeds is where it is at for the pump.

Good thing you have the temp guage or you would be in blissfull ignorance still, next boat bit a water pressure guage (after you fix this prob) you will have it nailed straight then!

cheers fnq

[gone_fishing]

g_f, how do you recheck the opening temps of the thermostats. The first time after the thermosstats were replaced the motor didn't have a problem but it was this weekend after being out at mooloollaba that it started. There were a lot of jellys of all differnt types in the water and was wondering if I picked something up. I have considered putting a new impeller in. What is involved in taking the head plates off to clean the galleries

hi dignity
seeing that you didnt have a prob when thermostats replaced but came back i would be checking them again for blockages
if no blockage you can check thermstats by removing them then hanging them in a pot (dont let them sit on the bottom) heat the water and using a thermoator watch to see what temp the thermostats open check a manual on recomended opening temps some do have temp stamped onto the unit
cleaning the galleries im not 100% cause i dont know the motor but breaking bolts is a possibility
just work through the cooling system to check for restrictions as mentioned
all the best

[Dignity]

BM - thanks, not sure what I would do with it once i found it.

Noelm, , bolts were in good order 6 mths ago but then there is always murphys law.

ozbee, intake screen is clean as I checked this first time it occurred

fnq - didn't know you could get water pressure gauges but will now investigate as I don't like the thought of this happening again. Will also now have to check the waqrning buzzer because I would have thought it should have gone off.

g_f thanks for the tip.

Gearing up for fraser next week so wont have a lot of time to play with it but will give it all a try.

[BM]

Dignity, the poppet valve is kind of like a thermostat although its a pressure controlled device rather than a temperature controlled device. Circular in shape, retained by 2 (or 3?) small bolts 3/8 or 7/16 and has a spring and a diaphragm and a couple of other bits inside. You can but a poppet kit and replace yours or at least pull it apart and clean it out and inspect.

To check your overheat follow the wire from starboard head sensor (tan wire with a blue stripe from memory) and it teminates on an insulated screw terminal where it joins another wire that runs to the main loom and the control box to the warning horn. With the ignition on short this terminal block to ground (anywhere on the engine block, you can do it with a pair of pointy nose pliers). The warning horn should sound with a continous tone. If not, you have a warning horn problem. About $60-$80 from memory.

Cheers

[Noelm]

it sounds kind of impossible, but you said all this happened after you raised the motor up a notch! now I see that it could not really be related, but seems a bit of a coincidence? it is also strange how it gets hot at idle, the motor should run cool at idle because of the no load condition, but I guess a weak/blocked pump MAY cause it.

[Dignity]

guys will know more tomorrow as will be giving a good going over, thanks for the info so far


sam