From: Wally Plumley [wplumley@avana.net]
Sent: Friday, March 03, 2000 7:22 AM
To: 928
Subject: [928] Overheating

I recently received the following request for info on our tech line, and
since those of us in the Northern hemisphere are eagerly awaiting
Spring, I though that the reply might be of general interest.



>>> I recently purchased an 86.5 928 auto.  I have other Porsches
(944, 944T)  and the 928 seems to run much hotter than the 944...  The
temp gauge regularly is in the upper 1/3 rd of the gauge- versus the
944's which runs in the lower 1/3 and maybe @ 1/2 range at the
hottest.   Is this normal?  I can take a look a radiator, perhaps
install a lower temp T-stat or perhaps a lower wtr. temp fan switch.
Any other thoughts, recommendations? <<<

Normal operating temp for the 928 should be between the two center lines
on the gauge under most conditions. Porsche says that the gauge may show
higher temps under high speed operation, but should not go above the
upper mark, and should go back to normal when speeds are decreased.

The engine generates heat, with the amount generated depending upon the
power being generated - more power = more heat.

The cooling system transfers heat from the cylinder heads to the water,
and from the water to the air. During normal operating conditions, the
amount of heat removed is controlled by the amount of coolant flowing
thru the engine (engine speed and water pump), the amount of coolant
flowing thru the radiator (engine speed and thermostat) and the amount
and temperature of the air flowing thru the radiator (car speed and
fans).

There are many other things that can play a part in this balancing act,
but most of these will affect one of the major factors mentioned above.
(As examples, leaves may collect between the A/C condenser and the
radiator, blocking air flow, or coolant level may be low, limiting
coolant flow.)

Troubleshooting starts by defining what the problem is, and when it
occurs. If the gauge shows a high temperature, but there are no physical
signs of high engine temperature, there may be a gauge problem. If the
engine overheats only at low speeds, or only with the A/C operating, the
problem is often air flow. If the engine overheats at highway speeds,
the problem is often coolant flow. Low speed/high load operation
(climbing a mountain in fifth gear) can easily cause overheating, as can
stop and go traffic in high ambient temps with full A/C.

It is possible for a radiator to be partially plugged, usually by
"stop-leak" additives, but this is pretty rare in a 928. The most common
radiator failure is leakage at the joint between the plastic side tanks
and the aluminum core. A good radiator shop can replace the gasket or
the plastic tank.

The engine needs a properly functioning thermostat, and a properly
functioning system will not overheat with the correct temp thermostat. A
lower temp thermostat may mask the problems, but this is not a cure. A
lower temp thermostat will decrease engine life and economy, and
increase emissions.

Thermostats may fail closed, open, or anywhere in between. Signs of a
failed thermostat that is closed include overheating under normal
driving conditions. Signs of a failed thermostat that is partially open
are very slow warm-up and overheating under highway speed/high load
conditions, but not at low speed/low load conditions. Signs of a failed
thermostat that is open include no warm-up and poor economy.

A properly functioning 928 will not overheat with the normal temp fan
switch. A lower temp fan switch will often prevent problems in a
marginal system by increasing air flow before the engine and coolant
gets too hot. A lower temp fan switch will also often improve A/C
performance slightly.

It is possible, but unusual, for a water pump failure to cause slight or
intermittent overheating. The most common causes of that type of failure
would be slippage of the impeller on the shaft or erosion of the
impeller. The most common water pump problem is failure of the bearings,
often caused by failure of the shaft seal allowing coolant into the
bearing. Any water pump leakage on a 928 is a critical problem, and
should be dealt with immediately. If the water pump fails, it will often
destroy the engine, either by causing timing belt failure, with
subsequent valve/piston crashing, or by eating the block with the
impeller.

A bad radiator pressure cap can cause overheating thru loss of coolant,
and will allow coolant to boil at a lower temperature. A broken cap cost
one of our list members an S4 engine thru sudden and catastropic
overheating.

Some other thoughts:
The viscous coupling on the earlier belt-driven fans often slip, causing
low air flow. If there are no mechanical problems with the coupling, its
performance can sometimes be improved by refilling with silicone oil.
The flaps and variable speed fans on the late cars must function
properly to prevent overheating.
Pure anti-freeze/coolant is a poor conductor of heat, and both boils and
freezes more quickly than the proper mixture of coolant and water. The
best coolant for a 928 is a 50/50 mix of clean or distilled water (not
chemically pure water, which is highly corrosive) and a phosphate-free
coolant. A good choice is any GM-approved "Dex-Cool" coolant.
Some list members recommend "Water Wetter" as a coolant additive, but
most car manufacturers don't recommend any additives.
Check for leaves and crud between the A/C condenser and radiator.
A lean fuel air mixture (such as might be caused by the common 928
vacuum leaks) can cause overheating.

There are literally hundreds of other potential causes of overheating,
but this should give you some ideas on what to do.

Wally Plumley
928 Specialists





From: Walt Konecny [mailto:wkonecny@burgoyne.com]
Sent: Monday, July 15, 2002 1:41 PM
To: John Pirtle
Subject: Re: [928] cooling fan speeds


John,

If the temp is going high while driving at highway speeds it could be a few
things, some easy, some not.

In order of severity:

Low coolant level.

Problem with t-stat.

Debris between the A/C condenser and the radiator, or "stuff" plugging the
cooling fins in the condenser and/or radiator, enough so that sufficient air
can't be pushed/pulled through to reduce the coolant temp.

Inside of radiator partially plugged with "stuff", not allowing proper
circulation of coolant.

Waterpump going - - yes, I know it's fairly new, but "stuff" happens.

Now the bad one.....blown headgasket, cracked head, cracked block.

wk