rapidly overheating at wot

[natwales]

5.7 merc. alpha 1 raw water cooled.

New manifolds + riser + circ pump & impeller within last year.

Runs steady at 140 deg at any other rpm other than extreme WOT. My tach seems to be reading about 4800, maybe 4900 at the time it begins to overheat.

Don't run this fast too often, but it concerns me a bit. Is the overheating a normal situation if the engine is running at too high rpms or am i probably looking at a minor exhaust restriction?

 

[cobra 3.0]

Re: rapidly overheating at wot

What do you call "overheat"?

 

[natwales]

Re: rapidly overheating at wot

200+ I throttled back and didn't let it get any higher than that.

 

[Crownie2]

Re: rapidly overheating at wot

I'm no expert on this but could be some kind of cooling restriction. Was the old impeller in good shape when it was changed? Also could check the new one to make sure it looks good...

Bob

 

[natwales]

Re: rapidly overheating at wot

The old impeller was in perfect shape last time it was changed and I've never run the new one dry so i doubt that is my issue. I can't check for sure until I am hauled out this fall.

I did some careful trials and here is some more info. I'm stumped.

1000 - 4100 rpms, steady temp at 140 which is what my t-stat is rated at.

4100 - 4300 rpms temp climbs to 175 - 180 and remains steady.

Somewhere around 4300 rpms+ temp keeps climbing to above 200 and I have to throttle back to below 4000 rpms for it too cool back down.

Again, all new exhaust, thermostat, circ pump, impeller last season so I really want to beleive its not a restriction in exhaust.

No meaningful marine growth around the intake on the outdrive.


I do have a squeaking altenator belt. Could this really be the issue though?Is it slipping just enough to cause lack of output from circ pump needed at top of rpm range? The power steering belt, which also comes off of the circ pump pulley is fine.

Are these symptoms of a bad head gasket? My understanding is that would also cause overheat at slightly lower rpms as well.

Can anyone suggest any other places to look?

thank you,

nat

 

[crazy charlie]

Re: rapidly overheating at wot

Adjust the belt properly and pull the stat out for the day and see what happens.Start with the easy things first.Charlie

 

[Blk-n-Blu]

Re: rapidly overheating at wot

Charlie, Check out the responses in this post:
http://forums.iboats.com/bbBoard.cgi?a=viewthread;fid=24;gtid=1149437
8)

 

[natwales]

Re: rapidly overheating at wot

I removed tstat and installed new belts ( I never realized how bad my old ones were until I saw what new ones look like). I cleaned the outdrive and checked all hoses for crimping. I still overheat at 4500+ rpms.

What do I look for next? I have read in other forums about installing clear hose and looking for bubbles which could indicate a blown head gasket, however due to the fact that this overheat only occurs at the very high end of the rpm range and is fine otherwise I feel like that is unlikely the problem and is not worth testing. Is that a reasonable assumption?

Could this be an internal blockage and what can I do about that?