Intermittent Overheats 2007 BF225, but only when in 100gallon dunk tank

Have yet to get an alarm on the ocean, but when I return home I have a 100+ gallon tub that allows me to run and flush both motors at same time.

The intake screens are about a foot under water and the hose stays on to keep the tub fresh.

Outboards are both trimmed level.

Impellers, thermostats, high and low fuel filters, plugs and fluids all changed a week ago. Same thing happened with the replaced parts.

Once the tub is full I start them up and just let them idle.
If a minute or so goes by and no alarm, then there won’t be one.
Howevever, about 30 percent of the time one of them will alarm and overheat.

Tell tale is strong on both. It seems like a blockage or air lock, but it’s so random.

On the ocean....no alarms in 30 trips or so.

I have the shop manual. Anyone know of any trouble areas that might be prone to obstructing water flow?

Always something, but I’ll take this if this is the something.

At idle you should get between 10 to 15 psi ,on the water your water presure increases in the motor due to an increase in rpm when this happens the poppet valve opens too the the motor stays cooler.

When the alarm goes off get a pyrometer and take the temp of the motor and compare it to the opening temp of your thermostatats in hot water .

I'm guessing an air block. As soon as you start the engines, rev them up a bit to, say, 1500 rpm to try to clear the air block, then back down to idle and see if that fixes the problem.

Would be interesting to see if on the ocean you just left them to idle and didn't rev them up the same happens? When we fitted Bobs nose cones using a tank we needed to submerge the leg much more than we did without to get the tell tale stream going. I just assume the impellers have no self priming capability and need water full in them to do anything. Maybe you have a slight blockage that traps air sometimes in the impeller. I assume the tank is empty and you fill it with the engines trimmed down - if so i wonder if the motion of trimming down into the water when on the ocean clears this...... Weird both are suffering with exactly the same symptom.

Chawk, I just did a flush with a little rev at start and no alarms. Maybe it is just air lock. Will use that procedure and see if that solves.

RTant, I think you are right that trimming down into the ocean water is different enough that it avoids an air lock. I will try an extended idle only next time at the ramp.

Here’s a pic of the tub. The motors get set into it and trimmed as you see. Then it is filled and hose left on.
Actually.....if it is an airlock, then why steady tell tale?

Good point... plot thickens. I can only assume you have an obstruction and the warmer tank water is enough to result in a raised temperature triggering the alarm when the cooler sea isn't? From memory the water out the tell tale is warm but not burning hot ie i can swipe my hand under and often do... which is a bit stupid perhaps.

Rtant, you've got a scientific based analytical mind. Like it.
Chawk has obviously been doing this for some time and knows his stuff. Hondadude too and I think he's a Detective in real life.

I'll always be working on something on this boat. Glad I found this forum.

Now for a deep dive study into the cooling water flow of these motors.
Talk soon.

This forum would be significantly weakened without chalkman and hondadude and others. I have saved meny thousands of pounds (I'm in the uk) thanks to this forum. It's top notch and i try and add my little bits to help out. Keep us up to date with your discoveries.

If you have the Helm shop manual, look on page 8-2. It's confusing, but there appears to be a fairly narrow passage where water is supposed to come out of the top of the cylinder head to the thermostat. That would be an ideal place for an air block. If so, no water would go into the exhaust, which in turn would set off the alarm. Once the t-stat opened, then the air block would be relieved.

The tell tale comes off the water relief valve - starboard side. See page 8-4 in the Helm manual. So you could have an air block, yet your tell tale would still work fine.

I looked it over in the Helm shop manual. Agree, it's confusing, but gave me enough to try to help trace the path on the outboards.

It certainly does look like a giant air lock just waiting to happen if the passages in the heads don't get the hot water to the thermostats.
Will check it out this weekend.

I had an air lock problem too its all sorted out now the motor pisses right over the fence now . as the block fills up it should push the air out.