Mercruiser inboard slowly overheats at idle speed - above 1100 rpm OK

I've got a 27' Trojan with a Merc 228 straight inboard - 1986. She runs great except for the fact that she slowly overheats at idle speed. Idle RPM is ~650. In an idle zone, I must shift to neutral and increase RPM to drop the temp and then resume forward motion. The raw seawater pump gets a new impeller yearly. The engine circulation pump has been replaced. The manifolds and risers were replaced years ago and are not showing any major corrosion - I pulled the starboard riser and looked. The hull strainer and sea **** are clean. I'm in fresh water, so barnacles hiding out deep in the passages is not a concern. The thermostat is new and is the recommended 140 degrees. I should also say that when coming off plane, the temp will skyrocket if you don't keep up the RPMs -- much more than the normal bump in temp for a raw water cooled straight inboard. I may be convincing myself that I need to replace the manifolds and risers, but just wanted to know if anyone else has had a similar situation. Thanks!

Re: Mercruiser inboard slowly overheats at idle speed - above 1100 rpm OK

Plumb in a short piece of clear hose on the intake side of the raw water pump. An air leak (you would see air bubbles in the clear hose) could cause low water flow at low rpms, but be ok at higher rpm.

Re: Mercruiser inboard slowly overheats at idle speed - above 1100 rpm OK

Thanks Don, that's a good idea. I did just rebuild the seawater pump at last haul-out and I sealed everything very well. This issue has spread across a few seawater impeller replacements.

I was actually considering an overdrive pulley, but I really think that would be a band aid to the real problem.

Re: Mercruiser inboard slowly overheats at idle speed - above 1100 rpm OK

Sorry Don, I just "got" what you are suggesting.

I have ~24" of Shieldshaust (sp) hose running from the sea **** to the input of the raw water pump. If it was sucking some air I would also imagine that it would be leaking water as all of this is below water level and under pressure. This is a true inboard with the hull strainer amidships.

Re: Mercruiser inboard slowly overheats at idle speed - above 1100 rpm OK

You could also check the seawater pump output. At exactly 1000rpm is should be pumping a minimum of 7.5qts in 15 seconds. That would tell you if the pulley sizes are correct.

Do you have a GENUINE service manual?

Chris.......

Re: Mercruiser inboard slowly overheats at idle speed - above 1100 rpm OK

I know, years ago, I ran inboards, twin gray marines, and had that issue one season. We found it had to do with marine growth on the hull. it was creating turbulance, allowing air to stream under the boat, cavatating around the water strainer.

Re: Mercruiser inboard slowly overheats at idle speed - above 1100 rpm OK

achris said:

You could also check the seawater pump output. At exactly 1000rpm is should be pumping a minimum of 7.5qts in 15 seconds. That would tell you if the pulley sizes are correct.

Do you have a GENUINE service manual?

Chris.......

Click to expand...

I do have the genuine service manual. The raw water pump is the original mercruiser part. The pulley is 6.5" diameter. It's strange, if I just increase RPMs from ~700 idle to ~1000, the temp instantly drops. I may go ahead and try to get a 10 - 20% smaller pulley.

Re: Mercruiser inboard slowly overheats at idle speed - above 1100 rpm OK

I would still look for the source of the problem. the increased flow simply masks the real issue.

you could have a kinked hose, organic fouling in the pump, plugged sea strainer, etc. simply masking the problem by speeding up the pump will come back and bite you later when the small problem grows into a large harry and toothed beast.

Re: Mercruiser inboard slowly overheats at idle speed - above 1100 rpm OK

The thermostat is new and is the recommended 140 degrees.

Click to expand...

Ayuh,.... At the top of the T-stat housing,...
Are there the Check-balls in the lines goin' to the risers,..?? Are they sealing properly,..??

Re: Mercruiser inboard slowly overheats at idle speed - above 1100 rpm OK

One thing to consider is the overdrive pulley is going to take more power to run, and in the end you want the power to the prop, not the pump!