4.3 merccrusier overheating above 3000 rpm

[Lawnman]

I have a 98 2800 scr with twin 4.3 merccruisers,when going above 3000rpm's the port engine temp alarm goes off. I put the boat back in neutral the alarm goes off. The temp gauge goes to about 200 when alarm goes off it runs about 160 normally. As soon as i put it in neutral temp goes right down. Replaced impeller in out drive over the winter.Engine is fine under 3000 rpm's. Any help would be great.

[mmwjr]

Use a hand held IR temp sensor to verify temps.

No heating issues before impeller was replaced? Could be when you installed the lower unit onto the upper the seal on the water tube is missing or not sealing well. After that clogged exhaust, bad t-stat, ...

[Lawnman]

Originally Posted by mmwjr

Use a hand held IR temp sensor to verify temps.

No heating issues before impeller was replaced? Could be when you installed the lower unit onto the upper the seal on the water tube is missing or not sealing well. After that clogged exhaust, bad t-stat, ...

It was doing same thing before impeller ,manifolds and risers are brand new.I'll check the water tube.

[mmwjr]

t-stat? engine circulating pump?

pull host from t-stat housing that comes from outdrive to verify the raw water pump is flowing water. If so the issue is down stream.

[Lawnman]

Put in a new thermostat over the weekend, finally able to take for a ride today,drove boat about 10 miles over 3000 rpms did not trim port engine temp stayed right on 160. On the way back trimmed engine and the temp began to rise ,trimmed back down and the temp went back to normal,I.m guessing the pickup hose is being pinched when trimmed any thoughts?

[mmwjr]

Pinched, off,...

How old are the bellows? May be time to replace it all.