1996 Merc 150 HP Temp climbs at low RPM loads

[bobdec]

Have had boat for two seasons, replaced impeller when I bought boat used and this has always happened. When running above normal 700 RPM idle speed of 4-5 MPH and going up to 10 MPH w/o planing at 1200 RPM, the engine temp starts to climb. Last year I had two ocassions to tow boats less than a mile or so to the docks at slower speed (1200 RPM). Both times the engine started to get up into the 200* F mark when it normally runs 160* . Also if I run for extended periods of time 20-30 mins at 1200 at 10 MPH , sort of digging in but not planing it starts getting hotter. The engine will cool quickly back to the 160's when I go back to idle RPM or increase RPM and get it up on a plane. Is this normal, boat is a 20 ft Proline walkaround. Since thermostats control temp at lower water pressure before the poppets open , could T-Stats be a problem. My next step is to get a pressure gauge and see what pressure is at 1200 RPM..

 

[Chris1956]

Re: 1996 Merc 150 HP Temp climbs at low RPM loads

I had that issue on my Mercruiser. Turned out that the impeller had it's bottom chewed up.

When you replaced the impeller, did you replace wearplate and both gaskets? If not, that can be the issue as well.

 

[bobdec]

Re: 1996 Merc 150 HP Temp climbs at low RPM loads

Yes...Sorry, I should have been more detailed. Since the boat was purchased from an individual and came w/o any maintenance records I did a complete boat & engine maintenance overhaul prior to launching, complete lube, electrical condition visual, compression check, plugs, fuel filters, water pump, prop spline lube, driveshaft lube, new main battery and added 2nd battery & switch, it had a bad voltage regulator so replaced them both. And I replaced the complete WP assembly with a Sierra 18-3319 kit. Currently the engine starts great, idles smooth (for a 2 cycle) and jumps smoothly up to 5200 w/o and problems. Personally it seems to me it's not getting enough water flow at lower RPMs. The teltale does not have a strong stream like I see on some newer outboards, but I don't have a '96 150 to compare it to. Service manual says 1-3 PSI of water at 0-1500 RPM, does not seem like a lot to me. Wondering if this is just the way it should be or if I have a real problem.

 

[Dukedog]

Re: 1996 Merc 150 HP Temp climbs at low RPM loads

Looks like ya mighta missed tha poppit. Check it. Also check your T-stats. Temp climbs? How high does it go? Are ya gettin' an alarm?

 

[oldman570]

Re: 1996 Merc 150 HP Temp climbs at low RPM loads

The motor will heat up some as most 2 cycle motors are not designed to be run at low RPM's with a heavy load for extended periods.As the manual states the lower water presure for a New impeller is around 4LBS. and will be less for a worm impeller, at idle and up to17 to 20 Lbs. at WOT. The prop pitch also will make a diffrence as the higher the pitch the more the motor has to work. With less pitch The higher the motor RPM will be and the more the water will flow to cool the motor. When towing a boat I have found that if extra wieght from the boat being towed is placed in my boat it helps keep my motor temp down. The same will happen when just putting around and plowing water rather than just going slower or getting the boat on plane the backing off just enought to keep the boat on plain. JMO
Oldman570