454 overheating speratcly

[kworman]

"454 mercruiser, is overheatin

"454 mercruiser, is overheating I can run for a hour and idle throught the no wake zone all the way to the dock and it will overheat just after I hit the dock only sometimes, somtimes it will overheat shotly after leaving the dock somtimes it will overheat after running for twenty minutes,
The first time I had a overheating problem it turned out to be the impeller so that was replaced and everytime I have a problem that is one of the first things I check. I have checked the raw water system from the sea water inlet all the way to the sea water out let I have replace my strainer installed larger diamater hoses, replaced the oil cooler, impeller and housing, maniflods and risers, I have also replaced the thermostat twice the first time it ran beutifly at 160 and one day it just jumped up to 190 and was slowley climbing higher That day the impeller got replaced and I tested the thermostat. which had gone bad (160 that was opening at 190) so I replaced it with a 140 and it has ran fine until now when I cracked the impeller housing I replaced it thismorning and took the boat for a test run and ran for about ten minutes at 3600rpm and it ran perfect at 175. back at the dock after tieing the boat up I checked the temp guage and it was on its way up so I ckecked the risers they were running cool so I removed the heat exchanger cap and the temp started dropping from 195 to 160 in about 30 seconds. I cant fugre this darn thing out so ANY help would be appreciated."

 

[guyjg]

"Kyle:

"I removed the h

"Kyle:

"I removed the heat exchanger cap and the temp started dropping from 195 to 160 in about 30 seconds."

By your statement, it appears that you relieved built up pressure in the cooling system by letting trapped air out which allowed immediate increased flow of coolant. Have you pressure tested the cap? Is there an air bleed in the cooling system? Are your FW cooling hoses collapsing when the engine starts to overheat, specifically the cirulating pump hoses? Have you ever used a radiator cleaner in the FW cooling and flushed the fluid and refilled? Since you are in AK, the coolant mix can be 60:40, 60% antifreeze and 40% distilled water. Do you have thru hull exhaust? If not, have you inspected the exhaust flappers? They may be malfunctioning and causing a restriction to the exhaust. Good luck."

 

[Pat DeChello]

I have a brand new only 8 hour

I have a brand new only 8 hours mercruiser 260 engine which yesterday over heated after maybe 5-10 minutes. I shut it down and was towed to port. It now turns over very hard and won't start any advice.