Water pump question.
 

I had my 94 Jeep Grand Cherokee 5.2 in the shop, and the mechanic said he tested the water pump. He said it was only pushing 3 PSI, and it's supposed to be pushing 7 PSI. He said that if I replaced the water pump, that with the increased PSI, that it could cause leaks in the cooling system. Including the radiator, leading to replacing the radiator. Has anyone else heard of this? It seems that the cooling system components are designed for the normal PSI of 7, including the radiator. He said it's because the system is now used to the lower pressure.

I never heard of this. The water pump makes flow, not pressure. The whole system is pressurized at whatever the rad cap is set at. How can he measure how much coolant flow the pump generates while it's on the engine? It makes no sense. The system pressure is a lot more than 7 psi.

It didn't make sense to me either.

So he's telling you it's not making the pressure it needs, but says you shouldn't change it? What a dick!

He said he's concerned that if the water pump was replaced, it could cause the radiator to start leaking, and the radiator would need to be replaced.

Water pump question.
 

What the mechanic said doesn't make sense... Water pumps pretty much just throw the water water around and cause the coolant to flow once your thermostat starts to open. And the system is pressurized because it's getting hot..

Exactly. A cooling system is in all practical purposes a closed hydraulic system whose pressure is created by heat expansion and controlled by the radiator cap. Being a hydraulic system, pressure is equal and reactive throughout the system. In other words, if the cap is set to hold 16 lbs, there will be 16 lbs EVERYWHERE in that system. EVEN air bubbles will be at 16 lbs above ambient (normal atmospheric pressure of 14 psi). The reason cooling systems are held pressurized is to raise the boiling point. A solution of 50/50 antifreeze and water at 16 psi boils at about 260*. All the water pump does is circulate the coolant to maintain temperature equally throughout the system. The cylinder head(s) are the hottest part of the engine and is where most of the heat is generated. It must be noted that some smaller high stressed engines and especially diesels run higher pressures to control this heat.

SO, having said that, as long as the radiator cap is doing it's job, the radiator is already containing system pressure and changing the water pump won't affect it at all.

Print this post and take it to your mechanic.

Thanks Dave 1123. That makes a lot more sense than what the mechanic told me.

The water pump is not a positive displacement pump, it doesn't make pressure. The mechanic is full of crap.

Thanks, Busted. I forgot to mention it's just a paddlewheel, for C****** sake!

This topic shows that a normal pump builds 7 PSI of pressure.

Ya. I'd find another mechanic. If the water pump is not functioning correctly and I am not sure thats possible. Mine have either worked, or leaked, but not partially moved water. You would get under cooling, creating more pressure.

That "topic" as you call it says NOTHING about the water pump, it just tells you about compression leakage into the water jacket. Compression leakage will only build up to the radiator cap's release pressure, then blows off into the reservoir. If the cap can't release it fast enough, it will probably pop the radiator, heater core, or hoses.

My WJ's rad cap is set for 18 psi.

There ARE cases where the plastic impeller has shattered, but that generally results in an instant overheat.

The mechanic said he replaced the 195 thermostat , with a 160 thermostat. So far, it doesn't seem to be getting as hot, but the outside temperature hasn't been as hot lately. I haven't had the problem with starting, or running rough when it starts though.

Whatever.