Re: [xj-s] Dual drive water pump (was overheating)

["Richard F. Dixon" <rfdixon@chs.cummins.com>]

Have you guys ever noticed that the coolant comes in the bottom of the
engine and leaves from the top? Have you noticed it goes in the top of
the radiator and out of the bottom? Ever wonder why? It is so the
coolant will to continue to circulate after the engine is shut down due
to the thermosiphon effect. (hot water rises, cold water falls). This a
carryover from the very infancy of internal combustion engines. In the
very early engines this was the only way cooling water circulated. They
didn't have water pumps. Of course it doesn't circulate nearly as fast
as a pump would circulate it, but it does circulate. That is also why
the electric fan eventually shuts of in the XJ-S. The sensor is on the
engine right? When the fan runs, it cools the water in the radiator.
When the cooled water reaches the sensor it turns it off. My Ninja 900
motorcycle has an electric fan that is energized all the time. After I
ride it in hot weather and then shut it off, the cooling fan will cycle
on and off several times in the next half hour. The sensor is on the
radiator. When the fan cools the radiator down it turns off. But as the
coolant siphons through, the radiator gets hot and the fan will come
back on. I'll wager if you bypassed the bootstrap circuit on the XJ-S
you would see the same thing happen. The only reason I can figure the
Jag has this circuit, is a safety feature so the fan won't come on
unexpectedly with the ignition off. (like it will on some late model US
made cars).

As for water pumps to circulate hot engine coolant, forget the water
pumps at the RV store. Those are diaphragm pumps that wouldn't last more
than a few hours pumping hot coolant. What you need is a bronze
centrifugal pump so water will circulate with the pump not running.
Finding one suitable that operates off of 12 volts might be a bit of a
challenge. They are marine cooling water pumps that are 12 volts, but
they usually have rubber impellers and are designed to pump cool water.
If you did find one, just where in the world would you mount it? These
things are not small and there isn't much free real estate under the
hood of an XJ-S as we all know. IMHO, I think just concentrating on a
good electric fan with suitable circuitry to power it would do the job
of post shut off cooling.

As for the radiator surround foam, I left mine out of the sides. I
didn't think the small area left open would affect the cooling of the
radiator. After I had my radiator cleaned out at the radiator shop and
replaced the fan clutch, it runs nice and cool. Always below N with 180
deg. thermostats here in Charleston, SC with 95+ deg. weather. However,
I figured the engine compartment could use some cooler air. And these
openings are right in front of the air filter intakes.

Richard F. Dixon
82 XJ-S HE

---

Follow-Ups:

• Re: [xj-s] Dual drive water pump (was overheating)
• From: "Robert C. Warnicke" <Obstinate@worldnet.att.net>

---

• Prev by Date: Re: [xj-s] Time for emissions again, ok to use carb. cleaner?
• Next by Date: [xj-s] RE Questions of many areas
• Prev by thread: Re: [xj-s] Dual drive water pump (was overheating)
• Next by thread: Re: [xj-s] Dual drive water pump (was overheating)
• Index(es):
  • Main
  • Thread