TOPIC: Low water pressure with two different Merc 500's?

On my 65 Merc 500, the max water preesure that I get at WOT is around 3PSI. I installed a thermistor next to the number 1 spark plug, so I can monitor temps of the waterjacket cover. Temps are just fine. After replacing the water pump, there was no change. Exhaust cover is always ice cold and I get a stream out of the telltale. I just accepted the low pressure and have run the motor for 4 years with no issues related to cooling. (some pitting in drive shaft near water pump housing could be culprit)

Yes, I have checked the gauge to see if it reads correct pressures.

Well, I recently put together a 68 Merc 500 and after cleaning out the water passageways (removed exhaust cover, baffle plate, etc.) and replacing water pump, I figured I would see the proper water pressure. Low and behold, I see less than 3PSI at WOT. The motor runs great with a cold exhaust cover, water stream and low temps with the thermistor. The drive shaft has no pitting. Did these older Merc 500's just not genrate much water pressure?

The water pressure pickup is on top of both motors.

Do you have the pressure tube installed separate from the tell-tail tube? It needs to be separate.
Are you using a 15 psi gauge or a 30 psi gauge?
The 30 psi gauge won't be very accurate on these open ended systems.
Thom

15 PSI Teleflex gauge. The pickup is a separate fitting on the top of both motors.

I confirmed the gauge is accurate, as I have a small hand pump. I set it to 5 PSI and thats what the gauge reports.

The old Merc manual states that water pressure should never go below 5PSI. I guess if both run cool, the water pressure is irrelevant. I just use the gauge to monitor possible pump failure. So watching the needle move to 3PSI is all I need.

The is no leaking water from gaskets anywhere on either motor, as I replaced most of them. It's a mystery as why two different Merc's have so low of pressure.