Engine temp fluctuation Volvo Penta Gsi 4.3L

I took my boat out on the bay last weekend. Nothing strenuous. I noticed that after getting away from the launch the engine temp (according to gauge) jumped up to 180. I cut the motor and looked for something stuck on the RSW intakes on the outdrive but didn't see anything. Put the outdrive back down and slowly motored on. Temp was down around 120, then 160 (where I would expect it), then back to 180... If I would slow down to a crawl the temp would go all the way back down to around 120!?!?!?

I have it on the trailer and I'm going to replace the things on my list to check are impeller and the thermostat. Any other ideas why the temp would vary so dramatically?

Thanks,

Brian

Hey Brian,
Running warm @ higher rpm.
Ok kinda sounds like a circulation issue at first but it really could be a number of things.
Simple to complex...
Think back to any recent repairs. What if?
Verify actual temp with a secondary gauge ie. Pocket thermometer ect. I use a lazer digital type.
Ignition timing/ loose distributer
Ck all elec connections from sending unit to gauge and fix if skeptical. Especially grounds
Loose v- belt to water pump.
Intake screen restriction
Raw water pump defective
Restriction in any plumbing including exhaust elbows
T/stat sticking
That may keep you busy for awhile

I'll vote impeller. Start with the easy stuff and work your way down the list.

Mike

Thanks guys!
I have been reading online about debris/ broken impeller fins lodging in the thermostat resulting in some like what I described, but the quick swing to 180 degrees on the gauge really concerned me.

When I figure this out I'll report back.

-Brian

If you were in the bay could it just be sea junk (kelp, eel grass)? I do a lot of diving in the kelp beds and my D4 hates the kelp - constant issues in there.

Replaced the impeller and thermostat. When I was removing the hoses I noticed a significant amount of what looked like rusty flakes inside the tubing. I could bend the tubes and the layer of this stuff would break up and come out of the tube. At any rate. Took the boat out last weekend to see if replacing the T/stat and impeller made a difference with the overheating under load, and it did not.

I can cruise around the bay all day just above idle and the engine will run at 140-150 degrees. Once I increase the the RMP's the temp goes above 180.

So now I am moving on to the exhaust risers/manifolds.

-B

After replacing the risers and manifolds, it looks like the overheating issue has been resolved. Just to give you guys an idea of what was going on in the cooling system:

After taking a look at the T/stat it made a lot of sense that this would be the culprit...

http://www.bocaboard.com/forum/attac...8&d=1415718620

And after seeing the build up in the water jackets for the risers and manifold, it made sense that this would probably be effecting the cooling as well...

Riser
http://www.bocaboard.com/forum/attac...7&d=1415718618

Exhaust manifold
http://www.bocaboard.com/forum/attac...6&d=1415718617

At any rate, it seems to have been an overheating issues caused by more than one component. Thanks guys for your help and advice.

-Brian