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135HP Mariner Overheating

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pepsea

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"I have a 1996 135 Mariner V6

"I have a 1996 135 Mariner V6 outboard and it overheats only when engine is working hard, ie, trying to get up on plane. Temp OK up to 2000 rpm, overheats between 2000-3500 rpm, then temp comes back to normal when above 3500 rpm. I have replaced both thermostats, poppit valve assembly, all hoses after the water pump, water pump kit, and pulled both heads and replaced head gaskets. Heads and cylinders very clean, no corrosion. Tell tale is normal. Same problem when running without thermostats. Any suggestions as to where to go from here? Thanks."
 
"Leo, what do you mean by over

"Leo, what do you mean by overheating?

The gauge says so? you are getting an overheat alarm? steam coming out of the tell-tale? tell-tale feels hot?

What temp is the gauge reading when you say it's overheating. Between 140-175 would be considered normal...."
 
"Graham, engine overheats betw

"Graham, engine overheats between 2000-3500 rpm's, at 180 plus degrees. It has gone as high as 220 before I caught it the first time. 1200 to 2000 rpm's, temp is 150-165. Above 4000 to 5500 rpm's, temp drops to normal, around 145. I have checked the temp gauge and both sensors with a infarred temp meter and is OK.
Appreciate your feedback and any further suggestions. Thanks. Leo"
 
"That really sounds like a bad

"That really sounds like a bad poppet, but you said it was replaced.

The thermostats look after the temp below about 1500 rpms or so and at full throttle the poppet is bypassed.

Might be worth pulling the poppet again and looking for a blockage or crud right in that area because that seems to be the focal point with the system working as it should otherwise..."
 
"Thanks, Graham and Bandit for

"Thanks, Graham and Bandit for the info. Only the diaphragm and gaskets were replaced. Mechanic said poppit and spring OK and was not replaced. Will do the spring over the winter since springs loose its copression over time."
 
I am not sure why people think

I am not sure why people think that springs " loose compression over time " ----- Not many people that I know replace valve springs on a 20 year car because the springs are shot.
 
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