Thought I would post this to help anyone else digging into the pressure relief valve...
When trolling under 2500 rpm on my BF 225 both exhaust manifolds were running right at 160 F, as per spec. However, when I would run at higher rpm’s the starboard side temperature would drop down to 135 – 140 F. (Years ago I installed external temperature sensors on each manifold so that I can monitor running temps.) Such a temperature variation is not good for the engine! So, after checking everything else, I concluded that the starboard pressure relief valve was likely stuck open. When it is stuck open, it allows more cooling water to bypass the thermostat and dump right back into the water jacket. Thus, the cylinder head temperature is not being fully controlled by the thermostat.
I ordered the replacement parts and proceeded to do the repair.
First, the starboard upper cowling needs to be removed to the extent that you have unfettered access to the pressure relief valve. Looking down on the engine mount case, to the aft, you will see the pressure relief valve (PRV) case bolted to the engine mount case with a small water tube leading into it.
See this link of page 8-4 from the Helm Shop Manual: View attachment Pressure relief Valve.pdf
Remove the water tube. It is helpful to also disconnect the oil return hose from the valve cover in order to better access the rear of the PRV cover. (Note that the port side PRV case does not have a water tube going into it.)
Note that there appear to be three bolts holding the PRV cover in place on the engine mount case – two with relatively easy access along the outside and one tucked way back behind the case. Well I spent over four hours trying to remove that rear bolt, having to grind down a 10 mm socked fitted to a ¼” ratchet so it would fit in that tight space. To get in there I had to remove cowling mount braces, remove the cylinder head oil drain, disconnect the fuel line to the LP fuel pump from its brackets, and cussed a lot while harboring thoughts of doing bodily harm to the Honda engineer who designed the thing. Once removed, the revelation was that the rear-most bolt does not need to be removed in order to remove the PRV cover - it simply holds the two halves of the case together. (I wish that the shop manual pointed that out. It simply says: “Remove the bolts and the relief valve.” One would naturally think that all three bolts needed to be removed.) That is actually a good design, so apologies to the Honda engineer that designed it for my bad thoughts. Bad thoughts are re-directed to the technical writer of that section of the Helm manual.
Once you have the two exposed bolts removed you will likely need to use a 6-in-1 tool or a long handled flat blade screwdriver, to pry the PRV off the mount case. Note that to the rear of the PRV there is a slight gap between the PRV case and the mount case. That is the easiest place to pry off the PRV.
Then remove that third bolt and pry apart the two halves.
I concluded that the likely cause was that the pressure spring had too much lateral play in it and pushed down on the valve at a slight angle, which caused it to stick open. Another possibility is that a piece of debris held the valve open, but I did not see any.
Thoroughly clean the two halves of the PRC case, getting rid of all salt and calcium build up. Clean the area where the PRV case seats into the mount frame, especially the sections where the O rings fit. A Dremel with a steel cone brush attachment works well here.
Reassemble the case - replace the spring, the plastic valve itself, the gasket that the PRV slides through, and the gasket between the two halves. Hold it together with your fingers while you tighten down that pesky third bolt to hold everything in place. Seat the O rings in Vaseline or a heavy grease to hold them in place while re-installing.
Be very careful replacing the re-assembled PRV case, making sure that it is seated properly in the engine mount case and the two O rings are properly seated. Tighten the two anchor bolts a couple turns at a time to assure that the case seats in evenly. In order to get the rear part of the assembly to seat properly, I had to use a long handled flat blade screwdriver to put pressure on the back of the case.
Reattach the small water outlet hose to the top of the case. Reassemble anything else you took apart to get to the PRV case. With the lower unit in a test tank, run the engine with the cowling off to make sure there are no leaks. You’re done.
When trolling under 2500 rpm on my BF 225 both exhaust manifolds were running right at 160 F, as per spec. However, when I would run at higher rpm’s the starboard side temperature would drop down to 135 – 140 F. (Years ago I installed external temperature sensors on each manifold so that I can monitor running temps.) Such a temperature variation is not good for the engine! So, after checking everything else, I concluded that the starboard pressure relief valve was likely stuck open. When it is stuck open, it allows more cooling water to bypass the thermostat and dump right back into the water jacket. Thus, the cylinder head temperature is not being fully controlled by the thermostat.
I ordered the replacement parts and proceeded to do the repair.
First, the starboard upper cowling needs to be removed to the extent that you have unfettered access to the pressure relief valve. Looking down on the engine mount case, to the aft, you will see the pressure relief valve (PRV) case bolted to the engine mount case with a small water tube leading into it.
See this link of page 8-4 from the Helm Shop Manual: View attachment Pressure relief Valve.pdf
Remove the water tube. It is helpful to also disconnect the oil return hose from the valve cover in order to better access the rear of the PRV cover. (Note that the port side PRV case does not have a water tube going into it.)
Note that there appear to be three bolts holding the PRV cover in place on the engine mount case – two with relatively easy access along the outside and one tucked way back behind the case. Well I spent over four hours trying to remove that rear bolt, having to grind down a 10 mm socked fitted to a ¼” ratchet so it would fit in that tight space. To get in there I had to remove cowling mount braces, remove the cylinder head oil drain, disconnect the fuel line to the LP fuel pump from its brackets, and cussed a lot while harboring thoughts of doing bodily harm to the Honda engineer who designed the thing. Once removed, the revelation was that the rear-most bolt does not need to be removed in order to remove the PRV cover - it simply holds the two halves of the case together. (I wish that the shop manual pointed that out. It simply says: “Remove the bolts and the relief valve.” One would naturally think that all three bolts needed to be removed.) That is actually a good design, so apologies to the Honda engineer that designed it for my bad thoughts. Bad thoughts are re-directed to the technical writer of that section of the Helm manual.
Once you have the two exposed bolts removed you will likely need to use a 6-in-1 tool or a long handled flat blade screwdriver, to pry the PRV off the mount case. Note that to the rear of the PRV there is a slight gap between the PRV case and the mount case. That is the easiest place to pry off the PRV.
Then remove that third bolt and pry apart the two halves.
I concluded that the likely cause was that the pressure spring had too much lateral play in it and pushed down on the valve at a slight angle, which caused it to stick open. Another possibility is that a piece of debris held the valve open, but I did not see any.
Thoroughly clean the two halves of the PRC case, getting rid of all salt and calcium build up. Clean the area where the PRV case seats into the mount frame, especially the sections where the O rings fit. A Dremel with a steel cone brush attachment works well here.
Reassemble the case - replace the spring, the plastic valve itself, the gasket that the PRV slides through, and the gasket between the two halves. Hold it together with your fingers while you tighten down that pesky third bolt to hold everything in place. Seat the O rings in Vaseline or a heavy grease to hold them in place while re-installing.
Be very careful replacing the re-assembled PRV case, making sure that it is seated properly in the engine mount case and the two O rings are properly seated. Tighten the two anchor bolts a couple turns at a time to assure that the case seats in evenly. In order to get the rear part of the assembly to seat properly, I had to use a long handled flat blade screwdriver to put pressure on the back of the case.
Reattach the small water outlet hose to the top of the case. Reassemble anything else you took apart to get to the PRV case. With the lower unit in a test tank, run the engine with the cowling off to make sure there are no leaks. You’re done.