Engine Water Pressure Drops with Engine Speed Increase

[dvanaken]

Q1: is a reduction in cooling water pressure with increase in engine speed normal for 90-HP two-stroke-power-cycle outboard engine used on a 2002 170 MONTAUK?

[Moderator’s note: the author needs to explicitly mention the manufacturer and model and year of the engine.In a follow-up post seven days later, the engine was identified as a c.2002 Mercury 90-HP two-stroke-power-engine.]

BACKSTORY
After being in winter storage since Fall 2022, my 90-HP engine water pressure gauge is now showing 10-PSI at idle, but drops to 3-PSI at 3,000-RPM and stays at 5-PSI or lower at 4,500-RPM. When engine speed returns to idle, water pressure again rises to 10-PSI.

A few years ago I replaced the [original] tubing [(presumably the hose connecting the gauge to the engine)] with a larger 1/4-inch poly tube. That has been working fine.

Since the water pump had never been replaced, my first action was to drop the lower unit and replace the impeller and related parts of the water pump. But that did not remedy the odd water pressure indications on the gauge.

Next, I pulled off both ends of the tubing [connecting the water pressure gauge to the engine]. I used compressed air to blow out the tubing. A nice stream of water came shooting out. I was sure this would resolve the unusual readings on the water pressure gauge. But the behavior was unchanged.

The engine overboard cooling water confidence stream is noticeable at all speeds, but [volume of water flow in the confidence stream] looks less vigorous when the [indicated water] pressure drops [on the gauge].

I'm confused as to the predictable drop in pressure when engine speeds up.

Q2: is there a pressure relief function [built into the engine cooling system] that might be [the cause of the apparent drop in cooling water pressure with increase in engine speed]?

I'm out of ideas.

[goldstem]

While not familiar with this specific engine, most large outboard engine cooling systems have a poppet valve, and as cooling water pressure increases with engine speed the valve opens, bypassing the thermostat and increasing the water flow to keep the engine cool. The water pressure would then read lower from this point on.

[jimh]

The narrative suggests that the unusual behavior was not seen previously and now is occurring for the first time.

The behavior described for the engine cooling water pressure to decrease with increase in engine speed is very unusual, and is ABNORMAL.

The suggestion that the water pressure indicator SHOULD show less pressure with increased engine speed due to the action of a pressure relief valve (mentioned above with the generic term poppet valve) contradicts my experience with classic two-stroke-power-cycle engines.

The observed correlation between (what must be inferred as) actual decrease in volume of water flow via the cooling water diversion to the confidence stream outlet and the reduced pressure indicated on the cooling water pressure gauge is normal. This tends to confirm that the pressure indicated on the gauge reflects the actual pressure in the cooling system at the measurement point.

[jimh]

Many engines have a pressure relief valve that works to increase the flow volume of cooling water at higher engine speeds. The opening of the valve requires it to be subjected to a particular pressure of cooling water acting on the valve. The notion that once the valve opens that this action would result in a decrease in water pressure acting on the valve would also mean the valve would then close. This clearly is not how it works. The valve stays open because it is being forced open (against spring tension) by the higher water pressure. The valve closes as soon as the water pressure reduces below the valve’s opening pressure threshold.

When the pressure relief valve is closed, there is no water flow through the valve, so if the measurement point for water pressures were in the downstream path from the valve, there would not be any water there to create the pressure to be sent to the gauge.

[jimh]

A loss of pressure with increasing engine speed sounds like the pressure is being lost because the vanes of the impeller are losing contact with the pump housing. Make sure you installed the water pump service kit parts in the correct orientation.

[dvanaken]

Thank you all for comments. I'm not inclined to blame the impeller. When I pulled off the old impeller, it was in very good shape. The blades snug up against the housing. I installed a new Mercury OEM part and confirmed correct rotation and operation of the pump components.

The symptoms now are identical to those prior to the new pump. This causes me to consider looking for another reason for the symptoms.

[jimh]

Looking at this situation with the assumption that the water pump is operating flawlessly and exactly as intended and that the cause of a significant loss of cooling water pressure with increased engine speed must be due to something else, my thinking is as follows:

There must be a significant leaking of water out of the cooling system via some path that requires a certain minimum pressure to open. Whatever this path might be, once it opens how can it stay open if it causes the big drop in pressure? The system seems like it could only work if there a large control action hysteresis in whatever opens, allowing it remain open even when the pressure acting on it decreases below the threshold needed to open the device.

At this point, I would contact a Mercury dealer and inquire with them if the behavior described is NORMAL for this particular Mercury 90-HP of unknown year and unknown model.

[jimh]

The manufacturer of the 90-HP engine can now be inferred to be Mercury from the oblique mention of use of a Mercury OEM part.

The year and model designation remain unstated.

Again, the author should EXPLICITLY mention the manufacturer, the model designation, and the year for this 90-HP engine.

[dvanaken]

@jimh - thank you for your comments. It is in fact a Mercury 90HP 2-stroke as delivered on the Montauk 170 in 2002. It has had very little use, and has under 200 hours on the clock. I think I may temporarily install my shop pressure gauge directly to confirm that the hysteresis you suggest is not occurring at the gauge. Any reason to think the poppet valve may be the culprit?

[jimh]

The most logical reasoning for why the replacement of the water pump impeller did not provide a remedy is to assume the original water pump impeller was reinstalled by mistake. Alternative, the new replacement impeller was installed, but was defective.

The water pump impeller has the greatest influence on the force produced on the water by the water pump.

[Phil T]

Q3: have you checked that the gauge is operating correctly?

[dvanaken]

Thank you all - I will confirm operation of the gauge. I suppose there are other components of the water pump system (such as the tubing and O-rings) that could be failing, but the impeller installation was correct, and, as I stated, the impeller I pulled out looked to be in very good shape. That supports the idea that this [unusual decrease in engine water pressure with increased engine speed] is not [being caused by] the water pump, as replacing a working impeller with another working impeller made no difference in symptoms.

Before I can get back with my manual pressure gauge about two weeks will pass, but I will reply to this post once that determination has been made. Again, many thanks.

[dvanaken]

[The engine that is under discussion in this thread is a ] 2002 Mercury 90-HP two-stroke=power-cycle model.

I have been able to connect a bench pressure gauge directly to the tee fitting where the dash mounted gauge is fed. The behavior is exactly the same - Idle and low RPM I get 5-10 PSI, on accelerating to 3000rpm, pressure drops over a 3 second period to 0psi. The telltale looks equally strong throughout.

Again, the water pump impeller and gaskets have been replaced earlier this summer. I did not replace the o-rings where the water pump output connects to the "up pipe", but given the strong telltale, should I suspect poor water flow? If it's not reducing water flow, it must be something opening up to release pressure downstream. Any ideas where I can look for that? I would think that I'd see water coming out somewhere in that case.

This is baffling and I'm reluctant to run it more than a few minutes at a time. I really need some ideas from those of you more experienced.

Thanks.

[Phil T]

Make sure the impeller was installed correctly. There is an orientation to them.

[MarkCz]

I am assuming your running the engine in the water and not on a set of clamp on water muffs when you are running at 3000 rpms. I suggest removing the engine cowl and take the boat for a ride and either use an infrared temperature gauge or even your hand to see how hot the engine is getting.

[dvanaken]

Thank you all for your comments.

I ran the engine in a lake for all tests. I tested each cylinder with an infrared temp gun. After 25 [minutes] of cruising, the temperature I was measuring never exceeded 125-degrees-F.

[NEW TOPIC has been moved to its own thread for discussion in the SMALL BOAT ELECTRICAL forum..]

[jimh]

I ran the engine in a lake for all tests. I tested each cylinder with an infrared temp gun. After 25 [minutes] of cruising, the temperature I was measuring never exceeded 125-degrees-F.

I don't think engine temperature was a concern, as this thread was always discussing engine cooling water pressure.

Q4: have you solved the unusual situation where the water pressure in the engine cooling water system decreases with increasing engine speed, and in some cases the water pressure decreases to zero (0-PSI)?

After some further thought, perhaps the fitting to which the water pressure hose to the gauge has been connected is not installed in the correct place,