Re: [xj-s] Water pump hose

["Kirbert" <palmk@mailer.gadcomm.net>]

> From:          Mike Morrin <mikem@southern.co.nz>
> 
> I am feeling argumentative, so could you explain how the pressure on the
> suction side of the water pump gets less than atmospheric, in a presurused
> system with bypass thermostats? Unless of course your radiator is plugged...

It all depends on the pressure capability of the water pump and the 
flow restriction around the circuit.  The pressure capability of an 
automotive water pump is a function of rpm, so the higher the rpm the 
more pressure on the outlet side and the more suction on the inlet 
side.  If the flow is unrestricted, the coolant will easily flow from 
the outlet side around to the inlet side, equalizing the pressure 
difference somewhat.

The pressure relief valve -- known as a radiator cap to most of us -- 
puts an upper limit to the pressure at one spot in the circuit.  From 
the pump outlet to that point will generally be HIGHER pressure than 
the cap rating, while from that point on around to the pump inlet 
will be lower pressure.

How low will it get on the suction side?  Frankly, I have my doubts 
that it would go from 17 psi or so at the header tank all the way 
down to sub-atmospheric unless, as you suggest, the radiator is 
plugged.  OTOH, when the engine is cold and hasn't built pressure 
yet, it's only too easy to suck a hose flat.  And once the hose is 
sucked flat, it may STAY sucked flat!  Even though the engine heats 
up and overheats, the pump is now sucking against a plugged hose and 
may KEEP it plugged.

My wife's Honda CRX had a related problem and it took the longest 
time to figure out what it was doing.  A hose somewhere would suck 
flat, and the engine temp would go up to overheating levels.  As soon 
as it started to overheat, apparently a lot of coolant would turn to 
steam and build enough pressure to open the hose.  The temp would 
rather suddenly drop to reasonable levels, and then eventually rise 
again.  All while driving constant speed on level ground.  Turned out 
to be a bad radiator cap, the little check valve that's supposed to 
let coolant back in when the engine cools off was stuck shut.  So, 
when cold, the cooling system actually had a considerable amount of 
vacuum on it, and the pump would just add to this and apparently suck 
hoses flat that weren't protected with coils because they normally 
don't see that much vacuum.

Trust me, you don't want a lower radiator hose that can be 
sucked flat.  If it hasn't got a coil inside it or some other means 
to prevent flattening, put a coil in it.

 -- Kirbert      |     If anything is to be accomplished,
                 |     some rules must be broken.
                 |          - Palm's Postulate

---

References:

• Re: [xj-s] Water pump hose
• From: Mike Morrin <mikem@southern.co.nz>

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