Water pressure gauge Johnson 70hp

[medic one]

Hi all. It's been ages since I posted, so I started fresh. I'm doing some work on our 15' Whaler.

I recently updated my 1994 70hp Johnson with the tell tale relocation to the top of the exhaust manifold.
As I now have an open port at the bottom of the manifold, it seemed like an opportune time to install a pressure gauge. I have run tubing from the lower outlet (old tell tale outlet) to the input of the pressure gauge. It seems to work just fine reading 24psi at cruising speed.

My question: Do I need to be concerned that there is water all the way up the tube, to within an inch of the gauge?

The gauge came with several adapters for tapping into the tell tale stream including a "T". I thought a direct connection to the exhaust manifold would provide the best pressure reading rather than the "blow by" pressure of the tell tale via a "T". It also means I can plug the old lower outlet in the exhaust manifold. But I suppose one benefit of using the "T" method is that the tube to the gauge might drain it's self after shut down via the suction of the draining water.

Thanks for the time.

 

[emdsapmgr]

You should be ok, but there is a word of caution. If the gauge ever leaks at the water hose connection at the back of the gauge, it is always possible that some water will travel through the hose and could enter the back of the gauge where the bladder is. Gauge will still function that way. Now, if the boat is stored outside and the water freezes inside the gauge over the winter, the bladder will expand and can damage/bend the needle link arm. Next spring, the gauge may work, but won't read even close to what it should. (This is the same problem with a leaky speedometer, too.) Will need to repl the gauge. Usually this is not a problem, just make sure the hose is clear- when installed (blow back thru it) and that it seals tightly to the gauge hose barb.

 

[medic one]

Good to know. Thanks.
I saw a diagram online somewhere, that used a plugged/capped "T" in line towards the gauge that acted as a drain when necessary so you wouldn't have to disconnect from the gauge each time you needed to clear water. Probably an good way to go.

 

[gm280]

I have to say I am surprised to see you are getting 24psi pressure. I didn't think typical water pressure would be that high. As for water in the tube, Yes if the tube comes off the gauge or there is ever a break in the tube, it will allow warm water to flow in without stopping. And the is merely an inconveniences unless the temperature is up from the engine cooling. So it could be quite hot. So it depends on where the break would occur and the temp of the water if anybody could get hurt. JMHO!

Oh and one last issue... :welcome: to iboats medic one. Glad to have you aboard... :thumb:

 

[medic one]

Thanks for the welcome and the info.

I too was surprised at the pressure. I was getting some leaking around the tube where it meets the barbed end of the gauge. I decided to try the alternate installation where the pressure is taken off the telltale via a "T". Disappointingly, I show only a few psi when at idle and maybe around 7psi at WOT with this set up. Not too much of a surprise, as the water flow in the telltale has a direct path overboard which doesn't create much back pressure for the gauge. In fact I suspect there is a little venturi effect happening watching the air bubbles going away from the gauge.

So my next evolution with be a modified set up direct from the exhaust manifold to the gauge, with an inline small diameter "T" which I hope will bleed a little pressure and allow for draining/expansion of the gauge line.

 

[interalian]

I have an original OMC pressure gauge on my '82 90hp and with a fresh impeller, I get about 25psi at WOT. Mine is teed-in a couple inches above the telltale exit. When the boat was newer, it used to peg the gauge at 30 - who knows how much it was really doing.

 

[Fed]

Tee or no Tee the pressure should be the same everywhere in the system and water moving up to the gauge means you have a leak.
https://en.wikipedia.org/wiki/Pascal's_law

 

[gm280]

Tee or no Tee the pressure should be the same everywhere in the system and water moving up to the gauge means you have a leak.
https://en.wikipedia.org/wiki/Pascal's_law

Not necessarily Fed. If the pressure is taped at an internal passage, it will read one pressure. But if you "T" in at the output of the pee tube, a lot of the pressure has bleed off as it exits and it will read a lot lower. For a good example, read the water pressure at the facet and then read it as it is flowing out the end of the hose. Those pressures will not equal. In a closed system, the pressure is totally equal at any point, where that is at the beginning or the very end. But once water is flowing, it changes along the way because of friction in the flow... JMHO!

 

[medic one]

No Title

With some creativity I was able to produce a satisfactory pressure at the gauge. The flow now runs from the old telltale outlet in the lower corner of the exhaust manifold through a "T" then to the gauge. The "T" directs some of the flow/pressure to the telltale hose. I think this will also improve the setup because each time the motor stops, some excess water in the telltale line dumps overboard which pulls some water out of the line to the gauge via venturi. Also, there won't be pressure in the gauge line at rest.

Now I'm getting around 10psi while at high idle and around 20 at cruise.

In the photo, the tube on the left reaching up to the left hand corner of the pic goes to the gauge. The horizontal tube runs to another "T" that intersects the telltale. The telltale tube descends from atop the exhaust manifold and then around to the starboard side to discharge.

 

[Chinewalker]

As with many of these gauges, it is less important that they be accurate, and more important that they give you consistent readings. Their only purpose is to notify you of an issue with your cooling system.

 

[phillnjack2]

No Title

you should have the tell tale on TOP of the engine block not on the exhaust cover at all.
this modification was a bulletin sent out in 1992 , so your one should be on top of the engine block.
for a pressure gauge it should use the old original tell tale orifice on the side of the exhaust cover.
the tell tale should NOT be used for the pressure gauge, it should go straight out the bottom of the cowling and also have its grommet either removed or enlarged.
again this was another service bulleting for the cooling of these 56 cu in engines.

Put the line to the gauge before you connect to the elbow fitting in the exhaust cover. Once fitted to the gauge then fit onto the exhaust cover elbow.
then you will NOT get any water up near the gauge as the gauge works on air pressure not water pressure.
from the gauge you need the pipe to go down lower than the fitting on the engine, this will stop any water travelling to the gauge.

so many people start the engine, wait for water to come through the pipe then fit to gauge, this is totally WRONG, and should not be done.

if you get water at or very near the gauge you have a air leak/water leak at the gauge fitting, these gauges do not have water inside them.
it is just a air pressure gauge, the water from the engine pushes the air in the pipe towards the gauge thus causing pressure that the gauge reads..


Water pressure is measured from the exhaust cover and should not be measured with a tell tale also fitted to this connection as it will not let the
water into the top of block under its correct pressure, this was the whole point of the engine service bulletin update.it was to stop the problem of the no1 cylinder being destroyed due to air pocket and not enough cooling,
by fitting a tell tale to this location you would infact being doing harm not good.
the tell tale fitting needs to be ON TOP OF THE BLOCK so it can act as a air release and then straight out the cowling with no restriction.

Like said in an above comment, you can never get the correct pressure if its a T fitting with a tell tale connected to the T.
this is why so many people think they have low water pressure, most is going straight out of the tell tale so no pressure build up in pipe to gauge.

this photot shows where the tell tale should be placed on the omc triples from 1987 onwards for correct cooling..



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[Fun Times]

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Thanks phillnjack2 for the additional info for helping members out so thoroughly.:tea:
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