BF 175s overheating

Have a pair of BF 175's (same block as 200 etc). They will overheat and go to safe mode when you try to throttle up. Fine at lower rpms. Initially it was just the port engine and if it alarmed you could shut it off, restart, and then it would be OK and you could cruise around at 4000rpm etc happily. Come back to idle for a period, or shut off, and the process would start again. Sometimes it might alarm a couple of times needing to be restarted before it came right.

Thought sticky thermostats so pulled them and found one jammed open. Other one was OK in hot water. Replaced the stuck one and still the same issue. Thought that maybe the 'good one' might have been opening too slow so changed that out as well, and did both thermostats on the other engine at the same time. Again one looked OK and one was jammed open. Went out for a test drive and both engines alarmed and went into safe mode as I tried to get on the plane. Almost identical timing. Shut off and restarted a couple of times but couldn't get past the faults now.

Checked codes and definitely an overheat issue. Both engines initially had one stuck open thermostat and the alarm would go away after a restart. Now with 4 x new thermostats the alarm won't clear.

I am thinking that it could be the water pumps (impellers) although the pee streams are very strong still. Impellers are probably about due now. Other thought was maybe the pressure relief valve being blocked.

Any thoughts about that or other ideas would be welcome. Boat lives in the water so need to lift it to check the impellers which I am arranging to happen.

Typical cause of this issue is a rotted out oil pan where the water tube is inserted with a rubber grommet which leaks excessively drastically reducing water pressure.How old are your motors?

Engines are 2007 and 2000 hrs but the powerheads were new around 1000hrs ago. They run beautifully other than these alarms. What are the other oil pan symptoms?

Sorry, I misread your post, issue I am referring to is overheat at idle and fine at higher rpm, opposite to what you are describing. Are you sure you have the correct thermostats, should be 60 degree C? It would be useful if you could have someone run Dr H and see which sensor of the three is causing the alarm.

It seems strange that both engines are doing the same thing, and it's equally strange that you have one T-stat stuck open on each engine. When you pull the boat, the first thing I would do is remove the intake screens on the bottom of the leg, just above the prop drive. Those screens can definitely get restricted by marine growth.

I could be the pressure relief valves causing the problem, but that should not show up at idle, or low rpm's. Supposedly, it only kicks in at higher rpm's.

Pull fault codes again. A "6" fault is the Engine Coolant Temperature (ECT.) That sensor sits in the V of the block. A code 24 is the overheat sensor 1 that is on the top of the port (I think) exhaust manifold. A code 25 is the overheat sensor 2 that is on the top of the starboard (I think) exhaust manifold. I may have those reversed.

The ECT sensor is the one the ECU relies on to control various other engine functions. If your engines are truly running well, then I would suspect one of those on the top of the exhaust manifolds. First check the wiring and the connectors. They can become easily corroded. Then measure the resistance between the two pins coming from the sensor. They should read between 1,000 to 3,000 ohms at ambient temperature, say 75 degrees F.

Thanks guys. I had the Honda man down with Dr Honda and he confirmed that they were overheat alarms. I didn't know though to ask him for the details. Th alarms are not at idle. It is only when you hit say 3000+rpm trying to get up on the plane. They idle happily. I am suspicious about the water flows (maybe the PRV's?) as with the stuck open thermostats the engines would eventually allow you to get up and stay going. With the new (OEM) thermostats it seems that you are going to hit the alarm every time you start to speed up.
This started with the Port engine intermittently but pretty often. Then the starboard engine did it occasionally but getting more regular. New thermostats and it is every time now. When running they start well, idle well, run steadily at the normal 4000rpm cruise but will get into the high 5000's quite happily. Fuel burn is normal. Overall no sign of anything other than the engine getting hot as you start loading them up. Not using oil, oil is clean, charging is good, props are perfect, hull is good etc etc

We will pull the boat soon so I will check the PRV's (bottom rear of mid section?) and also get the Dr Honda man to get more detail on which sensor. Will check the intake screens first though as suggested thanks.

Intermediate update. Boat comes out for service in a couple of days. I took the thermostats out and went for a run. Engines ran perfectly. Hopefully just tired water pumps from some sand going through them or similar.

Probably just blocked and best cleared by blowing compressed air into the water jet whilst running the engine. The knock sensor is hard to get to but it can be done if the intake plenum and the fuel vapor separator are removed.That way you can dos connect the valley harness and inspect it. Either way this is not a quick fix.

Sorry,reading too many threads, ignore the bit about the knock sensor

Just going through the engines now. Pulled the exhaust manifolds and there is salt etc built up around the anodes which look like they should be changed. That would have been affecting the water flow to some extent. Poppet valves had some bits of salt/sand etc around them so they might have been affecting things too. Impellers are OK, but will get changed anyway. Pump housings are not as good as they could be so will change those. At this stage I suspect that the engineer who has done the last couple of services may not have been as thorough as he could have been. Fortunately he isn't available this time so the boat owner allowed us to use a different engineer instead of his 'cheap' one. Will update next week when boat is back in water for a sea-trial.

Final update - engines are perfect now. Pulled and cleaned manifolds. Replaced water pumps. Replaced thermostats. Cleaned poppet valves. Which item was the main fix I can't say but I am suspicous of the manifolds which would have definitely been reducing the water flow This was all done at around 1000hrs.

Aliboy, can you give a detailed description on how you got to the "poppet" (i.e. pressure relief) valves. I've never done that myself, and would like to check them the next time I pull my boat - which may be today given the Tropical Storm Michael is coming this way. Thanks.

I had he mechanic do the work this time as we are into our busy season now. I have cleaned the 'poppet' before though so can write a basic description. Had a similar problem around 6 or 7 years ago and cleaned the Poppet then as a precaution, although in that case I think that was the fix. They seem to be pretty reliable though so not a normal maintenance item like on some of the Yamahas etc.
Start by removing the curved covers around the base of the power head. Looking from the back the Poppet is just to left (port side) of centre at the back of the engine. It is a bit tucked away so access is a bit fiddly without being impossible. First time I did it we had to back the boat up to the dock and do it on the water as I couldn't get a haulout arranged in time. That made it a bit more awkward but it was quite doable like that. From memory there are 2 x 10mm bolts to undo and the valve can be removed and the casing + valve cleaned. Sorry that is a little vague but once you get the plastic cover off and spot the valve it is a simple job to unbolt the top cover and then clean everything.

Got it. Thanks.