Old Merc 150 pisser question

[seabob4]

IMO a water pressure gauge > temp gauge

X10. 

[Fletch170]

Cool, Novice question.

Is there a factory sender on my motor that senses water pressure, what would relay water pressure to said gauge?

[Georgie]

IMO a water pressure gauge > temp gauge. At Idle you should get 3 - 4 Ibs of pressure, if you have that the water pump is suppling water to the block.

Please forgive my merc ignorance, but I don't think the pressure gauge could ever tell you if your Tstats/poppets were clogged or failing to open.  You'd be running hot with a ton of water pressure b/c the impeller is working, but nothing is passing through the engine and out the exhaust, and you'd be none the wiser when in the water, right?  :017:

Cool, Novice question.

Is there a factory sender on my motor that senses water pressure, what would relay water pressure to said gauge?

If it's anything like OMC, you'd just run a hose from a T-connector spliced into your pisser forward through your wiring trough and connecting it to the back of the pressure gauge.

[Capt. Bob]

Well, we have been down this road before and both gauges have their respective base (like trim tab types).

While I don't currently have a dedicated water/head temp gauge on my Yamaha, I have toyed with the thought for one since I got it in 2009. I agree that a water pressure gauge cannot tell if the t-stats are functioning correctly and it tells nothing of the operating temps at speed. Still, as stated it can indicate water pump servicing at idle but to my way of thinking, not a lot more than the visible telltale does. With modern gauges/alarms/motor controls, the water/pressure gauge is more like a belt with suspenders and since regular service/replacement of these parts (impeller/tstats/poppet valve) is highly indorsed by Forum members here, the question is ,"are they still required or have they gone the way of points/condenser ignition"?

[seabob4]

Keep in mind CB, Yamahas, the port for the WP fitting is on top of each head...the water feeding your gauge has to pass through the entire head cooling jacket before it hits that fitting.  So it's not like you're just seeing WP off the water pump.  Same with Mercs.  On OMC V6s, the fitting is installed at the base of the starboard head, but I would assume the principle is the same. 

It would be of no use if engine manufacturers designed the motor such that the WP fitting was upstream of the head cooling passages, as that would give absolutely 0 info as to the health of the water stream through the motor...

[Fletch170]

Bob, I'm going to go see if my motor has anything like that at the top of the head.....I dont think it does, but then agian I'm wrong.........alot.

[Capt. Bob]

Keep in mind CB, Yamahas, the port for the WP fitting is on top of each head...the water feeding your gauge has to pass through the entire head cooling jacket before it hits that fitting.  So it's not like you're just seeing WP off the water pump.  Same with Mercs.  On OMC V6s, the fitting is installed at the base of the starboard head, but I would assume the principle is the same. 

OK, while I am aware of that, it isn't the point I'm trying to make. While each gauge has merits, the pressure seems a less useful tool to my way of thinking. At high speed operation, it shows water flow but not temperature change. Same at low speed operation. If you have a cylinder running lean and causing an elevated temperature it can't indicate that.

Both sides can make a point for their gauge but at the end of the day several more things can cause overheating besides poor water pressure and none of those can be detected by the pressure gauge. Lastly, because some tend to enjoy the state of boating rather than being focused on actual operation, no matter what gauge one chooses, it only works if you tend to look at it from time to time. Modern engines move that responsibility to the manufacturer and it's probably in the the best interest for all concerned. Pay more attention to where you're heading and less to the function.

Ok then, why do I still consider a temperature gauge? I'm old in my ways and it was an important indicator on engine condition before all the fancy computer/sensors/save your butt systems we have today (remember my outboard was built in 1991 but has these things). I also have a GPS that I like a lot but still use my compass every time I'm out. Old habits are very hard to break.

[Fletch170]

Just read through my Shop manual, the Offshore carb does not have any water pressure sensor. Bummer.

The only reson I'd like that is here in Jersey, we have a big tide swing, and some really muddy bottoms.

[seabob4]

Ryan, don't know the year of your motor offhand, but read this thread from iboats...http://forums.iboats.com/forum/engine-repair-and-maintenance/mercury-mariner-outboards/657582-1996-mercury-150-water-pressure-test-point

[seabob4]

There are no "sensors" for WP.  You have to install a fitting that feeds a pitot tube running forward to the WP gauge...

[Blue Agave]

This sure has become an interesting thread.  Like debating Chevy vs Ford,  Blondes vs brunettes. 

Please forgive my merc ignorance, but I don't think the pressure gauge could ever tell you if your Tstats/poppets were clogged or failing to open.  You'd be running hot with a ton of water pressure b/c the impeller is working, but nothing is passing through the engine and out the exhaust, and you'd be none the wiser when in the water, right?  :017:

This is why outboards come with an overheat alarm.

[GoneFission]

The telltale (pisser) should have water coming out all the time.  The line for the telltale comes off the cooling lines before the thermostats.  A water pressure gauges connects to a port that is on the water jacket for one head.  Some folks find the water pressure gauge is more effective than a temperature gauge because it will tell you of a problem with water pressure BEFORE it impacts engine temperature.  By the time you see a high engine temp, the problem is likely too far gone and you are not likely to fix it on the water.  Mercs will run at "normal" temperatures with only 5 PSI or less of water pressure.  A water pressure gauge helps you better see the condition of the system - if it normally is running 10-15 PSI at speed and you notice water pressure is down to 5-6 - you know you probably need to change an impeller, clear a clog , or check the water pickup.     

[Aquamaniac]

Actually, as OP,  this thread has been interesting and helpful to follow. 
Particularly if you are trying to learn your way sorting out an old motor!
For example:
   here is a pic of a fitting I was wondering about.....look familiar?

The hard plastic line is just bent and closed off. Very professional....go figure...... ::)

Anyway, thought I would point out that while a thread might get hum-ho to folks very knowledgeable in the subject, there are those of us still crawling up the learning curve and discussion is a great source of info.
Thanks!

[seabob4]

At least that hard plastic line didn't crack where the PO bent it.  Would've had salt water spraying all over that powerhead...

[Capt. Bob]

OK then,
If the tell tale should be pissin' at start, you may just have a blockage at the exit. Might want to try a thin wire inserted into the nozzle and see if it clears the line.

My Yamaha is set up that way with the WP line on the left bank and the temp on the right, similar to your photo.

Good luck.