4kq overheat problem

[Bernard Littau]

Hi Ben,

Nothing you mentioned rules out the radiator being massively plugged up.
Flow through a couple of passages would make the "hose" test look good, but
a couple of passages are not enough to shed the heat.  You seem to have flow
through the radiator, but not enough heat shedding.  Granted, it seems that
two radiators should not exhibit the same plugged up behavior.

Another possibility is that the return from the radiator is plugged
(severely constricted).  Just a little bit of flow occurring through the
radiator, and the radiator is shedding the small amount of heat easily,
could yield the temperature gradient you describe.  Check the hoses and the
ports they attach to.

Finally, perhaps everything is fine and you have a massive air bubble in the
system that makes your water pump ineffective.

What is the heater in the car doing during all of this?  If you are getting
massive amounts of heat from the in-car heater, then I would think the pump
and flow are OK (think blockage).  If the in car heater is putting out cold
air, then no flow.

By the way: behavior of cooling systems with the thermostat removed is quite
variable across different systems.  Some flow better to the radiator, thus
do not overheat or postpone overheating.  Others flow worse, and overheat
faster.

Generally, two port systems like the 4kq would overheat slower as they get
better flow to the radiator as no thermostat is less restrictive than a
fully open thermostat.  However, I've seen situations where the restriction
is necessary to cause enough flow through the engine.  If too much flow goes
through the radiator and not enough through the engine, the engine can
overheat while the radiator stays cool.

Three port systems, where the thermostat opens one passage while closing
another, generally overheat if the thermostat is removed.  Many of the older
BMW I6's use three port systems.

Best,

Bernard Littau
Woodinville, WA
'88 5ktq
'86 4kq

> Patient is '85 4000 quattro experiencing strange overheating problem.
>
> I noticed earlier around January that the radiator seemed to stay cool
> at the bottom and never kick on the fan even when head temps approach
> overheating.
>
> I have replaced - Radiator fan switch, thermostat, radiator, and now
> water pump which was good afterall.  After spending way more time on
> this than I ought, I'm  confused and frustrated at what is going on.
>
> It seems that the coolant is not flowing at all even though it should
> be being pumped by the water pump.  I even removed the thermostat and
> the problem took longer to happen, but still results in overheating
> with the fan not kicking on - radiator stays cool at the bottom, but at
> top boiling hot and developing high pressure.  The hose coming off the
> thermostat housing gets hot coming out of the block, but is cooler down
> toward the radiator intake, and radiator nice and cool at the bottom.
>
> I ran water from hose in both directios through the different
> radiators - they don't seem blocked.  I ran hose water through both
> directions in the engine, there seems to be no blockage.  The radiators
> supposedly are good used radiators and they seem to have good flow
> through with a hose and no noticable scaling inside.
>