Hi Just got a quick question. I currently have a 1998 502mpi built to a 540 with around 600hp. The problem I am having is at idle the temps are around 135-145. But as soon as I get on plane and moving any rpm range it starts to run at 160-165. The T-stat is a 140* Anyone have any ideas or seen this before? I need to try to solve this. Thanks Doug

160-165* is fine if it maintains that temp at all rpm ranges.

It is normal for a gauge to show 10 to 15 degrees warmer then what the thermostat is.

why would you even remotely think that that is true ?

Because I see that all the time while reading either a merc scan tool or with a infared heat reading gun at the engine area, And I know there is a 160 degree thermostat installed and then you go sit at the helm and read the temp gauge it is at least 75% of the time if not more always some where around 170-175* if you Are really looking at the gauge for a good reading, That is what I am use too seeing for as long as I can remember on 'avarage' style gauges out there.

so you are suggesting that all the instrumentation is wrong most of the time ? doesn't it strike you as unusual that in modern times, there aren't any gage manufacturers that can calibrate their instruments within 10 % ? or that all the instruments seem to have the same linear error or that none of the thermostat makers know what they are doing ?

and whats really odd , i suppose is , that my boat with thermostats and really poor quality gaffrig gages actually reads what its supposed to as do the boats around here .

and agrees with the scan tool

and the 25 year old stewart warner mechanical temp gage that i use as standard...

i think its absurd and completely unprofessional to suggest to someone that they ignore what their instruments are telling them because of some folklore, voodoo and bullsht.

maybe its just the magnetic influence of the space time continuum in your area. better get your flux capacitor realigned.

stevesxm you might be right I should have said keep in mind your gauge or temp sending unit or wiring might be out of adjustment some place and could be just reading hi as I have seen this in the past And there might not be nothing wrong with the engine it self. Or you might have a cloged up cooling system some place also. Have you ever burned up an impeller in the past, You might have some old rubber parts stuck some place in the cooling system, Like maybe one of the oil coolers or maybe the thermostat. You might also have a blocked exhaust some place. Also this tool in this link will be a grate help as you are looking for you engine temp problems.http://www.harborfreight.com/cpi/cta...emnumber=96451. Also you might be in need of some of these tools also while looking for your problem. http://www.harborfreight.com/cpi/cta...CategoryID=455 Also keep in mind if you have used any kind of sealent to the thermostat gasket with the too little rivet metal rings in the gasket then you might also get a bad reading from the temp sending unit to the gauge because the sender will loose it's ground from the sealent and make the gauge read some times hi and some times low. Also you could have a bad "O" ring or cork type gasket in side the theremostat housing that helps kind holds the thermostat in place and seals it up so to speak. And also the thermostat housing it self could be bad. Also you might need to do a leak down test to the engine to make sure every thing is in working order inside the engine itself. Also here is some good info on what to look for while trying to find out why your engine is getting warm or hot. Well I hope some of this basic info will help you out while trying to find out why your engine is running hot, Good luck to you with it.

Engine Overheats (Mechanical)
Cause Special Information
Engine RPM below specifications at wideopen-
throttle (engine laboring)
Damaged or wrong propeller; growth on boat
bottom;false bottom full of water
Wrong ignition timing Timing too far advanced or retarded
Sticking distributor advance weights
Spark plug wires crossed (wrong firing order)
Lean fuel mixture Refer to “Carburetor Malfunctions” in this section
Wrong heat range spark plugs
Exhaust restriction
Valve timing off Jumped timing chain, or improperly installed
Blown head gasket(s)
A blown head gasket(s) normally cannot be
detected by a compression check. Normally the
engine will run at normal temperature at low
RPM, but will overheat at speeds above 3000
RPM.

Engines that are seawater cooled

: Using a


clear plastic hose, look for air bubbles between


seawater pump and engine. If there are no

bubbles present, install clear plastic hose

between thermostat housing and manifold(s).If
air bubbles are present at a higher RPM, it is a
good indication there is a blown head gasket.
Insufficient lubrication to moving parts of engine Defective oil pump, plugged oil passage, low oil






level.


IMPORTANT:



The first step is to verify if the engine



is actually overheating or the temperature gauge or


sender is faulty

Loose or broken drive belt

IMPORTANT: Best way to test gauge or sender is
to replace them.
Seawater shutoff valve partially or fully closed
(if equipped)
Clogged or improperly installed sea strainer
Loose hose connections between seawater
pickup and seawater pump inlet (models with
belt driven seawater pump only)
Pump will suck air. Pump may fail to prime or will
force air bubbles into cooling system.
Seawater inlet hose kinked or collapsed
Seawater pickup clogged
Obstruction on boat bottom causing water
turbulence
Obstruction will be in front of seawater pickup,
causing air bubbles to be forced into cooling
system
Defective thermostat
Exhaust elbow water outlet holes plugged
Insufficient seawater pump operation Worn pump impeller
Obstruction in cooling system such as casting
flash, sand, rust, salt, etc.
Refer to water flow diagram for engine type
being serviced
Engine circulating pump defective
Also refer to “Engine Overheats (Mechanical)”
IMPORTANT: In addition to previous checks,
make the following checks if engine is equipped
with closed cooling.
Low coolant level
Antifreeze not mixed properly Antifreeze should be mixed 50/50 or maximum
60/40 (60% antifreeze, 40% water)
Heat exchanger cores plugged
Water hoses reversed at the water distribution
block






Refer to water flow diagram in Section 6.
Insufficient Water Flow From Belt Driven Seawater Pump
Cause Special Information
Drive belt Loose, worn or broken
Seawater shutoff valve partially or fully closed
Clogged or improperly installed sea strainer
Loose hose connections between seawater pickup
and seawater pump inlet
Pump will suck air, pump may fail to prime or will
force air bubbles into cooling system
Seawater inlet hose kinked or plugged
Seawater pickup plugged
Obstruction on boat bottom causing water turbulence.
Obstruction will be in front of seawater pickup,
causing air bubbles to be forced into cooling
system

Faulty seawater pump