Heating up at idle

[dvd]

I had a situation where my water flow seemed restricted and I was using more bypass than usual. Thanks to the forum I was advised to remove the nipple at the back of the exhaust manifold and clean inside. I did this and I removed some pieces of rust and gunk and it worked fine for awhile. Now it seems to be running cool enough but when I come to an idle the temp goes up. Not to bad but before when I idled it would cool down rather than heat up.

Any thoughts as I am getting ready for my annual cruise to SF bay and I would of course like to avoid problems.

Any help would be appreciated. DVD. Btw I am raw water cooled.

[brewgyver]

Your root cause may be either reduced flow or heat transfer issue. The passages can scale up, even in fresh water, not to mention silt. You're in the delta, right?

I would check my impeller first, to make sure it's intact. If it is, you can check total flow with a bucket test. Run the engine to reach normal operating temp. Close the intake thru-hull, remove that end of the intake hose and stick it in a five gallon bucket of water, start the engine. At or close to operating temp, It should empty the bucket pretty quickly. If you have good flow and still overheating, probably time for an acid rinse, or more.

If you don't have good flow, check all the usual suspects.
Start at the thru-hull for your raw water intake. Does it have an external grate? Don't forget to check the hose connection at the thru-hull. I had a problem that turned out to be a cracked hose barb adapter, which wasn't bad enough to flood the bilge, but did allow air to suck in.

Then check all of the fittings: pump suction, pump discharge, block inlet tee, t-stat housing fittings, manifold fittings...
Do you have a raw water strainer?

[JOHN COOKSON]

It is normal to get a little bump up in temperature when a warmed up engine is brought to idle.
When a thoroughly warmed up engine is brought to idle not as much water is flowing through it because the pump is not turning as fast. As a result the engine, which is still hot, is not cooled as effectively as it was when more water was flowing through it at higher RPMs.

TRUE GRIT

[Dave Neptune]

dvd, what John said.

I have on may an occasion stated that to discern the actual running temp of a "raw water cooled" engine is to run at cruise and then shut down in a minute or so, IE rapidly. About 5 minutes after shutting down turn the ign back on (don't forget to turn back off) and read the current temp. This is the "core temp" that the engine is running at before the copious amount of RAW cold water carries away the heat!!!!! I see around a 25~30 degree rise on my boat from the 120 range when doing so. This means the engine operating temp is around 145 not the number of 120 on my gage. The running number on the gage is the "cooling water temp' not the engine operating temp~ESPECIALLY on RWC engines.

Dave Neptune

[dvd]

It makes sense except this seems to be a new condition. I'm pretty sure it would tend to cool down when I idled down in the past. It doesn't heat up a lot but goes up to about 180. If I give some throttle it cools back down quickly. Maybe I don't need to worry about it.

[JOHN COOKSON]

Originally Posted by dvd

It makes sense except this seems to be a new condition. I'm pretty sure it would tend to cool down when I idled down in the past. It doesn't heat up a lot but goes up to about 180. If I give some throttle it cools back down quickly. Maybe I don't need to worry about it.

I run 145* to 150* at cruise RPM with a bump up to ~ 155* when I idle back when the engine is fully warmed up on my RWC engine.
IMO you're cooling system is marginal. You'er close to overheating at your cruise RPM. I say this because
(A) The temp spike is a new condition - it used to cool down quickly in the past (B) the temp goes up to 180* (C) It cools back down quickly when you give it some throttle.
If you haven't done one lately I would recommend an acid flush.

TRUE GRIT