Cooling System Troubleshooting
Revision B – 12/10/2007
The Miata
cooling system is described on pages E-2 through E-14 of the 1990 Miata Factory
Workshop Manual, and much of the material below, including illustrations,
is taken from that manual. Model years beyond 1993 have a number of differences,
but the basic troubleshooting procedures should be similar. Hopefully this
article will be useful to many of those having cooling issues. However, there
is really no substitute for the FWM if detailed information is needed.
A knowledge
of what conditions should exist in the cooling system is invaluable in troubleshooting.
So for the benefit of those who do not have a complete set of manuals for
their car, this article starts with a somewhat comprehensive set of desired
cooling system parameters.
Basics
Coolant
Capacity and Mixture
The cooling
system capacity is 6.3 U.S. quarts. Mazda’s recommended anti-freeze solutions
for the Miata are as follows:
|
Minimum Temperature
|
Volume Percentage (%)
|
Specific Gravity @ 68°F
|
|
Water
|
Glycol
|
|
-16°C (3°F)
|
65
|
35
|
1.054
|
|
-26°C (-15°F)
|
55
|
45
|
1.066
|
|
-40°C (-40°F)
|
45
|
55
|
1.078
|
Table 1. Coolant Mixture Recommendations
(Mazda)
Remember
that the primary task of liquid cooling is to transfer heat energy from the
metal parts of the engine to the radiator and then to the air. Water has a
specific heat of 1.00, but the specific heat of glycol is only 0.571, meaning
that a given quantity of glycol will carry away only about 57% of the heat
that the same volume of water will transfer.
A more complete
table of freezing points vs. % ethylene glycol is given in the Handbook of
Chemistry and Physics:
|
Freezing
Temperature
|
Volume Percentage (%)
|
Specific Gravity
@ 60°F
|
|
Water
|
Glycol
|
|
-3.9°C (25°F)
|
87.5
|
12.5
|
1.019
|
|
-6.7°C (20°F)
|
83
|
17
|
1.026
|
|
-12.2°C (10°F)
|
75
|
25
|
1.038
|
|
-17.8°C (0°F)
|
67.5
|
32.5
|
1.048
|
|
-23.3°C (-10°F)
|
61.5
|
38.5
|
1.056
|
|
-28.9°C (-20°F)
|
56
|
44
|
1.063
|
|
-34.4°C (-30°F)
|
51
|
49
|
1.069
|
|
-40°C (-40°F)
|
47.5
|
52.5
|
1.073
|
Table II. Freezing Point vs. % Ethylene
Glycol
The boiling
point of water at 14.7 psia is 212°F, and if the pressure is increased, it
rises. The pressure in the Miata cooling system should be between 11 and 15
psi (25.7 to 29.7 psia) maximum, which would place the boiling point of a
50/50 coolant mixture at around 265°F (at 15 psi) according to the Prestone
labels.
Unfortunately
I could not find information correlating the “H” reading on the Miata temperature
gauge with coolant temperature. The 1990 FWM is written as though the Water
Thermosensor is the temperature gauge sensor, which it is not. There is no data on the actual gauge
sensor, although the manual spec on the gauge itself states that the gauge
should point to “H” for a resistance to chassis ground of 20 ohms (18 ohms
for Canadian cars).
Coolant
Flow
The coolant
flow is shown in Figure 1. The arrows clearly show the flow, but the diameters
of the heater hoses are actually considerably smaller than those for the radiator
hoses, as you can tell from looking at your engine bay. If the thermostat
is removed, flow will be unbalanced, since the water pump and the thermostat
housing are both at the front of the head. As a result, the rear cylinders
will run hotter than the front. The lesson here is to keep the thermostat
installed; if you have cooling problems, removing the thermostat is not a
good long-term solution.
Note that
the diagram shows an “IG” relay. That is actually the Cooling Fan Relay, as
you can see on the cover of the Main Relay and Fuse Box.
Figure 1. Coolant Flow
Note that
there is no coolant shutoff valve at the heater hose connections. Coolant
always flows through the heater core regardless of the position of the heater
controls in the passenger compartment. It is very important that the heater core path from the rear of the head
to the water pump inlet not be blocked, since otherwise hot coolant in the
rear of the cylinder head will stagnate, making the rear of the head overheat.
Note the
small hose at the bottom of the thermostat housing that carries coolant output
from the cylinder head through the input side of the thermostat housing and
down to the water pump input. This assures that hot coolant from the head
flows through the thermostat housing when the thermostat is closed, exposing
the thermostat valve to the coolant. If this path is blocked by metal shavings
in the thermostat housing outlet nipple, or
anywhere in the small hose [Rev
A], the coolant will not flow through, and the thermostat will not open
early enough. A symptom of a blocked path here would be temperature rising
above normal on first warm-up, then suddenly dropping to normal sometime later.
This connection can be seen in the lower right portion of Figure 2.
Figure 2. Thermostat Housing
Some coolant
flows from the cylinder head into the Idle Speed Control valve below the throttle
body (called the Idle Air Control valve on the 1.8L engine), then out of the
ISC valve and through the Air Valve mounted on the engine side of the intake
manifold, returning to a “tee” into the down tube from the thermostat to the
water pump input. (1.8L engines do not have an Air Valve.) The Air Valve on
the 1.6L engine is open when the coolant is cold, raising the idle speed,
and closes gradually as the coolant warms up to normal operating temperature.
Thermostat
and Water Thermoswitch
The OEM
thermostat opens in two stages. There is a “sub-valve” that opens at 182 –
188°F, and a main valve that opens at 188 – 193°F. As shown in Figure 2, the
main valve is oriented to the front of the thermostat housing. On the cover
of the thermostat housing is mounted a “Water Thermoswitch.” This switch is
open when coolant temperature is below approximately 207°F and closed when
the temperature is higher. The closed switch activates the Radiator Cooling
fan. If A/C is installed, but not turned on, the A/C Condenser Fan (on the
right side of the radiator looking forward) will not
activate when the switch is closed.
Fan(s)
and Fan Relay
The base
Miata has one fan, mounted on the left (looking forward) side of the radiator,
called the “Radiator Cooling Fan.” The Radiator Cooling Fan motor should use
5.3 – 6.5 Amps. If A/C is installed, there is a second fan on the right side
of the radiator called by Mazda the “A/C Condenser Fan.” The A/C fan motor
is smaller and the fan shroud fits on differently spaced mounting points,
so the two fans are not interchangeable.
As seen
in Figure 3, when the Water Thermoswitch is closed, the Cooling Fan Relay
contacts close and power the Radiator Cooling Fan. The connection to the Diagnosis
Connector goes to its “TFA” terminal. If the “TFA” terminal is jumpered to
the “GND” terminal, the effect is the same as closing the Water Thermoswitch,
i.e. the fan should activate if the ignition switch is on. This is a good
test to determine that the relay and fan motor are working. (This test will
not activate the A/C Condenser Fan.)
If the test
above fails to activate the radiator cooling fan, then the 30 amp Cooling
Fan fuse in the Main Relay and Fuse Box is blown, or the Cooling Fan Relay
or Main Relay has failed open, or the fan motor is bad.
Rev B
Figure 3. Cooling Fan Wiring Diagram
A/C Condenser
Fan Circuit
The A/C
Condenser Fan is activated by the A/C on/off switch on the heater/air conditioning
control panel. When the A/C switch is on, both the A/C Condenser Fan and the
Radiator Cooling Fan are activated by the ECU, through the A/C relay. Figure
4 shows the wiring for this circuit.
Figure 4. Air Conditioning Condenser
Fan Circuit
Radiator
Cap
The OEM
cap’s positive pressure valve should hold closed until at least 11 psi and
not more than 15 psi pressure. The negative pressure valve should be easily
opened with two fingers and should seal closed when released.
Coolant
Reservoir
This tank
connects to the radiator at the cap, just above the positive pressure valve.
The tank is not under radiator pressure. It should be sufficiently full to
allow coolant to be sucked back into the radiator as it cools off. If air
is sucked in instead of coolant, it will expand in the cooling system as the
engine warms up, and will result in air and coolant being expelled back into
the reservoir if the pressure is over 15 psi.
The small
tube from the reservoir input to the bottom of the tank sometimes gets clogged
with rust or other deposits from the cooling system. It should be cleaned
for good operation of the system.
The other
tube from the reservoir is an overflow to vent excess air and coolant. If the hoses are removed from the reservoir
cap, be sure to reassemble with the cap’s yellow tab pointing toward the rear
of the engine; the long tube connects to the radiator hose. [Rev
A]
Troubleshooting
Leakage
The OEM
radiator is aluminum with plastic end caps. With age, the black plastic will
turn a brownish-green color and develop the appearance of hairline cracks.
At this time leakage may begin to occur at the plastic/aluminum interface.
Replacement of the radiator is recommended if this happens.
The water
pump may also begin to leak, as evidenced by coolant on the lower front of
the oil pan and/or engine block. Again this usually means the pump is due
for replacement, although the pump gasket might be the culprit.
Any of the
hoses shown in Figure 1 can deteriorate and begin leaking. If a heater hose
splits, be very careful removing it from the heater interface tube, which
is thin copper and will deform easily. If the hose has to be replaced anyway,
it’s best to slit the end of the hose along its length where it slides over
the tube, instead of trying to pull it off or pry it off. If the tube is deformed
it may be prone to leaking after hose replacement.
There is
a rubber cap over the end of an unused coolant tap at the rear of the engine
block shown as “cursed water plug” in Figure 5 (Thanks for the picture, Harry
Sue.)
This cap
has been known to leak and is difficult to trace, since it cannot be seen
from the top or bottom of the engine. To get at it with the engine in the
bay, I think the CAS cover has to be removed. However, I have never had to
do this and I may be wrong about that.
A hose leak
at the rear coolant housing generally requires the coil assembly to be removed
before trying to release the hose clamp.
Figure 5. Coolant Cap at Rear of Engine
Overheating
Mazda states
in the 1990 FWM the following causes and remedies for overheating:
|
Possible
Cause
|
Remedy
|
|
Coolant level insufficient
Coolant leakage
Radiator fins clogged
Radiator cap malfunction
Cooling fan malfunction
Thermostat malfunction
Water passage clogged
Water pump malfunction
|
Add
Repair
Clean
Replace
Replace
Replace
Clean
Replace
|
Maybe we
can be more specific…
Overheating
at Idle
At idle,
if the temperature gauge indicates above normal, the Radiator Cooling Fan
should be running. If not, test with the “TFA” to “GND” jumper in the Diagnostic
Connector and see whether the fan turns on when the ignition is on. If it
does, the Water Thermoswitch needs to be replaced, its connector needs to
be cleaned, or its wiring needs repair. If not, and there are no other problems
with the engine that are electrical in nature, either the Cooling Fan Relay
(the “IG” relay in Figure 1) or the fan motor is bad. Of course there is always
the possibility of an unplugged connector somewhere, or a broken wire. In
a few cases the fan blades had come loose from the motor shaft and were not
turning, although the motor was OK.
If the fan
is running but the engine still is overheating at idle, feel the top radiator
hose. It should be HOT! If not, the thermostat may be stuck closed. If it
is hot, feel the bottom radiator hose. It should be warm, but perhaps 20°F
cooler than the top hose. If it is cool, there is blockage in the radiator,
or possibly in the lower radiator hoses, preventing flow into the water pump.
Sometimes old bottom hoses collapse under a vacuum created in them by the
water pump straining to pull in water at its input.
Finally,
check to make sure there is no blockage in the heater hoses, which would stagnate
the water at the rear of the head where the temp gauge sensor is located,
and cause the temperature there to be much higher than at the front of the
engine.
Overheating
at Speed
This problem,
assuming there is adequate uncontaminated coolant in the system, is either
poor water flow through the Figure 1 path, or poor airflow through the radiator.
Poor water
flow can be caused by a stuck thermostat (only partially open), collapsed
hose, clogged radiator, or a bad water pump.
Poor airflow
through the radiator can be caused by air blockage due to a license plate
configuration, bugs etc. stuck in the fins, bent fins due to mishandling,
and in rare cases, the Radiator Cooling Fan running backwards!
If all of
the above items have been checked and the engine still overheats, the cooling
system should be flushed to remove any scaling or corrosion inside the water
passages.
Draining
and refilling of the coolant with a lower percentage of glycol in the mixture
(see Tables I and II) would help if the glycol percentage is too high (over
50%).
Also keep
in mind that temperature sensors and gauges do go bad occasionally. If the
gauge says the engine is too hot, but there are no other apparent symptoms
like lower-than-normal oil pressure, steam or bubbles escaping into the coolant
reservoir, or a very hot under hood
temperature, maybe the gauge is giving a false reading.
Figure 6
is a page from the FWM showing radiator removal, and Figure 7 shows water
pump removal. Figure 8 is a simplified block diagram of the electric cooling
fan system.
Figure 9
is a troubleshooting tree for reference.
Figure 6. Radiator Removal
Figure 7. Water Pump Removal
Figure 8. Simplified Block Diagram
– Cooling Fan Wiring
