Ok at 20 mph, too hot at 22?

SloBurn

SloBurn

Active Member
May 30, 2013
842
Greenwood Lake, NY
Boat Info
1994 270 Sundancer. 7.4L Merc. Tow with a 2006 Dodge RAM Hemi
Engines
340 HP Merc 454 c.i.
I've got a "new this year" impellor, two year old manifolds and risers. She only goes in the salt 2 days a year and gets flushed at the ramp when pulling out. But she runs right at 145 up to 20 mph, a little faster and the temp goes up to the upper range line on the dial. Any suggestions? I do all my own mechanical work, BTW. I'm thinking maybe a block full of mud??
 
Did you check thermostat or it could be plugged block do you have sea strainers or are they iOS
 
I have pulled the thermostat and it looks clean, dunked it in hot water and it opened right up. No sea strainers, and not sure what an iOS is...?
 
By "iOS" he's asking if you have outdrives, commonly referred to as I/O (Inboard/Outboard)
 
Was the old impeller in one piece? Any chance the water is restricted from the drive?

I know 2MPH isn't much but the RPMs it takes to exceed 20MPH may be the breaking point for a system that has limited water flow.
 
drop the lower and check the impeller for separation from the hub. low speed works, high speed the rubber dosent turn.
 
Check the by-pass valve if it exists in that motor. It's items 3 through 6 in this diagram off a 5.7L. The spring on mine was all clogged up with mussels preventing operation and caused an overheat condition. I don't remember now if it was a low or high speed overheating but I'm thinking the symptoms were very much like an impeller failure.

 
mwph said:
Was the old impeller in one piece? Any chance the water is restricted from the drive?

I know 2MPH isn't much but the RPMs it takes to exceed 20MPH may be the breaking point for a system that has limited water flow.
Click to expand...

Good point. Great point! The raw water intake hose may look OK from the outside, but be fatigued and collapsing at a certain RPM due to the increased suction/increased demand for cooling water. It's not a salt water thing, it's a wear thing. I've read about here and elsewhere.

Before re you tackle it, try running without a T'Stats. If you get similar results, mwph may have narrowed it down a lot for you.
 
play dough said:
Check the by-pass valve if it exists in that motor. It's items 3 through 6 in this diagram off a 5.7L. The spring on mine was all clogged up with mussels preventing operation and caused an overheat condition. I don't remember now if it was a low or high speed overheating but I'm thinking the symptoms were very much like an impeller failure.
Click to expand...

I had a similar issue with a 7.4, the ball parts of items 3 through 6 were stuck in the housing. Freed and cleaned. :thumbsup:
 
i had similar symptoms as you with my boat soon after i bought it.....after replacing everything else in the cooling system i decided to replace the water circulation pump on the engine.....that solved the problem....there was no water dripping from the weep hole in the old pump and no visible signs that anything was wrong with the old pump....when i removed the old pump i could see that the vanes had worn to the point the pump could not move enough cooling water through the engine at higher engine rpm's.....

cliff
 
Every time I've pulled the impellor, it looks almost new, so that's not it. I've played with the ball valves, tightened, loosened. That really only seemed to effect the low speed temps. When I stretched them out, she ran hot at low RPM's. Set them back and now runs hot at high RPM's. I have never had a water circulation pump fail, unless there was water coming from the weep hole, even on some of my cars with well over 100K miles. This boat only has 680 hours on it. Maybe I'll pull it over the winter-not many days left for boating here in NJ, and I can keep it cool for that long. Thanks everybody!! I'll keep banging my head against it.
 
My circulation pump only had around 400 hrs when I changed it....maybe it has something to do with the depth of the water the boat is used in....my boat came from central Florida and was used in shallow fresh water small lakes....it may be possible the water pump sucked in a lot of sand from these shallow lakes and caused the premature wear of the circulation pump vanes....

just a a guess, but changing it out did fix my problem.....

good luck...

cliff
 
So I decided to pull the thermostat again and take a look at it, looked OK, but after closer inspection, it appears as though the rod that runs through the center of it may not have been seated exactly in the center of the bar that holds it in place, so after jabbing it with a screwdriver many times to try to get it centered, the boat now runs UNDER 145 degrees, from the lower range line on the dial, up to 145, whereas before it would run from 145 right up to the upper range line on the dial, ???. So now I'm running cool again!
 
SloBurn said:
So I decided to pull the thermostat again and take a look at it, looked OK, but after closer inspection, it appears as though the rod that runs through the center of it may not have been seated exactly in the center of the bar that holds it in place, so after jabbing it with a screwdriver many times to try to get it centered, the boat now runs UNDER 145 degrees, from the lower range line on the dial, up to 145, whereas before it would run from 145 right up to the upper range line on the dial, ???. So now I'm running cool again!
Click to expand...

Just replace the thermostat (and gasket), will cost you less than $20 and will work a lot better than just "jabbing it with a screwdriver many times"
 
Yup, time for a new thermo, although jabbing it with a screwdriver was an interesting repair. Risers are only 2 years old and run in fresh water.
 
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