Outboard water pressure gauge questions

[over60pirate]

I have a 1987 25hp Yamaha 2 cycle, and want to install a gauge to warn of overheating.
Some people recommend a water pressure gauge, instead of a temp gauge. As I understand, they t into the pisser line.

If the thermostat fails, I think the water pressure at the pisser, would be the same. The water pump will still be pumping water. Right or wrong?

How are temp gauge senders attached, if there is no place to screw them in?

Open to ideas.

 

[Bob_VT]

Re: Outboard water pressure gauge questions

2 methods.....

Drill and tap into the water jacket

or

There are temperature sensors that attach with a Z bracket so they contact the head of the motor.

Water pressure gauges could indicate weeds built up on pick up or plastic that blocks the pick up and the pressure drops. Pressure drops are an earlier indicator of overheating then a temperature gauge.

 

[over60pirate]

Re: Outboard water pressure gauge questions

If the water pressure tube is t'ed into the pisser, do you know the amount of pressure? I have a line on a gauge, and want to knopw the renge pressure range needed.
Thanks

 

[Silvertip]

Re: Outboard water pressure gauge questions

A water pressure gauge does NOT warn against overheating. It warns against a water pump impeller going bad that may LEAD to an overheat. Overheats are not always due to lack of pressure. Stuck thermostats, engine timing, lean carburetion are other reasons. Outboards typically have an overheat buzzer that warns of overheat. If overheat is what you are interested in then a TEMPERATURE gauge is what you are after. My boats have always had both gauges because they monitor two very different situations. If you are still stuck on a pressure gauge, some engines can have them plumbed into the tell-tale and will work fine. Others do not. The idea is to install a new impeller when you install the gauge. You then note the idle, mid-range, and wide open throttle pressure readings. If those number change significantly you know it is time to service the impeller again.

 

[MakoNY]

Re: Outboard water pressure gauge questions

Not to hijack your thread over60pirate, but I have a similar question on pressure range. I always thought that a water pressure gauge should read up somewhere around 15-20psi at cruising RPM. I tapped into the tell-tale on my old Johnson 115 and got around 20psi at cruising RPM, around 4200.

I recently had one installed in my '99 Merc 250 EFI and at idle it shows almost no pressure and only goes up to about 7psi at cruising RPM, around 4600 RPM. I asked the guy who installed it and he said that it's fine and that once the poppet valve opens up it dumps all of the water out of the head so the pressure is low. I use the water pressure gauge to monitor pressure as there are a lot of weeds by me and every now and then you look down and see the pressure drop and you can slow down and clear them off before the engine heats up.

He told me the exact opposite holds true and that if I saw a pressure increase it meant something was blocking the intakes and that a lower pressure was normal. Is this just a Merc thing or just due to the different type of system using a poppet in addition to the T-stats. I don't believe my old OMC used a poppet. I still don't think reading 7psi at higher RPM is normal.

Again I don't mean to hijack the thread but maybe other people have the same question.


Thanks.

 

[Silvertip]

Re: Outboard water pressure gauge questions

Here is an insallation on a 1998 75 HP Merc three cylinder. It reads in the 15 - 17 range at most any speed except idle. If the engine does not have the plastic fitting at the end of the tell-tall then pressure readings may fall a bit. Some engines work fine plumbed into the tell tale and some do not. Contrary to what your mechanic said, if the water inlet to the pump is blocked by weeds, pressure drops -- it does not increase. He obviously flunked high school physics. The cooling system on an outboard is an open system. Plug the inlet to the pump and with no water flow, the pump cavitates and pressure goes to zero.