260 Overheating at high rpms only

[888]

[FONT=verdana, geneva, lucida, lucida grande, arial, helvetica, sans-serif]I have a 1987 mercruiser 260hp that pulls hard but overheats only from 3/4 to full throttle. Runs 160 any other time. I have replaced seawater pump have great water flow,matter fact while running on muffs Im getting the same amount of flow from the water inlet hose at the t-stat housing as I do straight from the end of garden hose.I noticed I have excessive exhaust pressure at the thermostat housing outlets. Seems like any hose taken off you get a little bit of water and alot of exhaust. Any ideas? Thanks[/FONT]

 

[Bondo]

[FONT=verdana, geneva, lucida, lucida grande, arial, helvetica, sans-serif]I have a 1987 mercruiser 260hp that pulls hard but overheats only from 3/4 to full throttle. Runs 160 any other time. I have replaced seawater pump have great water flow,matter fact while running on muffs Im getting the same amount of flow from the water inlet hose at the t-stat housing as I do straight from the end of garden hose.I noticed I have excessive exhaust pressure at the thermostat housing outlets. Seems like any hose taken off you get a little bit of water and alot of exhaust. Any ideas? Thanks[/FONT]

Ayuh,.... Sounds like maybe the risers are plugged up on the water side, or the shutters burnt up, 'n fell into the y-pipe, causin' an over all exhaust restriction,....

 

[888]

Thxs Bondo,my thoughts exactly....Now, would ya remove the drive or remove the rubbers and elbows to examine?

 

[Bondo]

Thxs Bondo,my thoughts exactly....Now, would ya remove the drive or remove the rubbers and elbows to examine?

Ayuh,.... The Risers/ Elbows,.... Don't forget New gaskets,...

 

[888]

Ok, I took off the drive and see nothing obvious then i removed the elbows and see no obstructions in the y tube. The exhaust shutters are still intact. I ran engine without elbows and have no back pressure. WTH

 

[NHGuy]

Perhaps your elbows are leaking internally. Block them off, flip them upside down and carefully fill the water sides. Now watch and see if any water appears in the xhaust side. Or to really check them test with acetone, that stuff finds any opening.

 

[888]

Perhaps your elbows are leaking internally. Block them off, flip them upside down and carefully fill the water sides. Now watch and see if any water appears in the xhaust side. Or to really check them test with acetone, that stuff finds any opening.

My elbows are wide open,water mixes with exhaust at the point of exit on riser into the wide open 4" elbow. The only thing I can come up with is your gonna have a small amount of back pressure till the water pressure overtakes it. Air takes the least resistant path which would be backside of riser where hose goes to t stat housing