slow speed overheats/ fast speed cools down

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hl

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I was riding with my wife(she does not like speed) yesterday at 20 to 25 MPH when the overheat alarm started. Slow to 1200 RPMs and it stops. This continued until we decided to take it home.I just had the water plump replaced last May by a professional. Thinking it might be the thermostats I started to replace them and found there was no thermostats under the casing. I put two in. This motor has been running great for the last 2 months.After replacing the thermostats I started fast 40 to 50 MPH with no alarm. Found that speeds of 20 to 25 the overheat alarm will go off. I let it cool down and ran 40 plus MPH with no alarm. Run slow overheats / run fast cools down. This is an 1985 XR4 pushing an 18 foot Ranger. Please give advice
 
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saumon

saumon

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Re: slow speed overheats/ cools fast speed

Re: slow speed overheats/ cools fast speed

hl said:
I was ridind my wife yesterday at 20 to 25 MPH when the overheat alarm started...
Click to expand...

You probably mean "with your wife", no? ;)
 
C

Chris1956

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Re: slow speed overheats/ fast speed cools down

What is your RPM at WOT? Perhaps the prop is too tall for that slower speed?

On another note, my old Mercruiser would overheat at idle, but worked perfectly at 1500 RPM and above, up to WOT. Turned out to be a partially chewed water pump impeller. Just the very bottom of the impeller was torn up by sand/gravel that I ingested.
 
W

westexasrepublic

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Re: slow speed overheats/ fast speed cools down

oil injection or water pump. tell the guy who installed your water pump see what he says/
 
j_martin

j_martin

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Re: slow speed overheats/ fast speed cools down

Possibilities:
1. poppet valve. If it pees at idle, it's not the poppet valve.

2. Water pump, base gasket or grooved base plate not r-r'd.

3. Overload. T-stats missing indicate that previous clueless owner had trouble also. What's WOT rpm. I'm thinkin' the PO maybe removed the advance/stabilizer module, advanced the timing, and has it overpropped. Mid speed your timing would be fully advanced, and the engine overloaded.

With the timing advanced, you should prop it so WOT is around 6000 rpm.

4. Head gasket leak. Squeaky clean plugs and piston top, or low compression are clues.

5. What's your water pressure at different rpms, say idle, 1000, 2000, 3000, 4000, WOT. If you don't have a water pressure gauge, you're missing the single most useful cooling system diagnostic and monitoring tool you could own.

hope it helps
John
 
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sschefer

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Re: slow speed overheats/ fast speed cools down

I've seen this a couple of times and for no apparent reason. The problem ended up being a Merc Manual issue. The newer impellers are cavitating at low speeds if you follow the manual and lightly grease the cups during installation. Leave the grease out and the problem goes away. Check with your mechanic.
 
H

hl

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Re: slow speed overheats/ fast speed cools down

It runs about 5700 RPMs at 52 MPH(GPS). Pressure gage reads 17 PSI at 3000 to 4000 RPMs. I have little knowledge about the prop suggestions. I will chect the timing later this week. looking in the spark plug the piston seems to have a dark color. Thanks for the advice.
 
H

hl

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Re: slow speed overheats/ fast speed cools down

I talked to the guy who put the plump on and he did use grease when installing the impellers.
 
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hl

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Re: slow speed overheats/ fast speed cools down

Also forgot to ask about the water temp comming out of pee hole. After the overheat alarm stopped. I felt the water comming from the motor. Guesting it was not as hot as the water comming from my shower when completely turned to hot; which is set at 130 degrees.
 
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sschefer

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Re: slow speed overheats/ fast speed cools down

hl said:
I talked to the guy who put the plump on and he did use grease when installing the impellers.
Click to expand...

I'll call it a long shot but it could be the cause. The only way to find out is to pull them and clean them up. Like I say, it's a long shot but it has been happening in more than one that I've seen lately so I think it's worth checking out.
 
j_martin

j_martin

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Re: slow speed overheats/ fast speed cools down

While you're at it, make sure there's a bead of sillycone on the divider plate between the exhaust side and the shaft side of the LU.
 
saumon

saumon

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Re: slow speed overheats/ fast speed cools down

sschefer said:
I'll call it a long shot but it could be the cause. The only way to find out is to pull them and clean them up. Like I say, it's a long shot but it has been happening in more than one that I've seen lately so I think it's worth checking out.
Click to expand...

Instead of the manual recommended grease (wich is waterproof), i put a very thin film of dishwashing soap, wich is water soluble, inside the w/p housing to prevent rubber burning at initial startup. Worked great so far.
 
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CharlieB

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Re: slow speed overheats/ fast speed cools down

saumon said:
Instead of the manual recommended grease (wich is waterproof), i put a very thin film of dishwashing soap, wich is water soluble, inside the w/p housing to prevent rubber burning at initial startup. Worked great so far.
Click to expand...

I like that idea!

The Volvo manual is very adamant about using glycerin to lube all new impellers.

Dishwashing liquid is always around and far cheaper than buying glycerin.
 
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sschefer

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Re: slow speed overheats/ fast speed cools down

saumon said:
Instead of the manual recommended grease (wich is waterproof), i put a very thin film of dishwashing soap, wich is water soluble, inside the w/p housing to prevent rubber burning at initial startup. Worked great so far.
Click to expand...

I like that idea! :)
 
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