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Re: Request for help with overheating issue on 1990 Mazda MX-6
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CarGuru > Technology > Re: Request for help with overheating issue on 1990 Mazda MX-6 29 May 2006 12:38:30
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Re: Request for help with overheating issue on 1990 Mazda MX-6

Kevin 29 May 2006 12:38:30
 
<HLS@nospam.nix> wrote in message
news:Tzeeg.34620$4L­1.16357@newssvr11.ne­ws.prodigy.com...>
<ZacInAus@gmail.com­> wrote in message> news:1148808761.050­825.30020@u72g2000cw­u.googlegroups.com..­.> > Hi folks,> >
Appreciate any suggestions the board might have to help keep my 1990> > Mazda MX-6 4cy NA on the road.> >
The issue is that in the hot Texas weather, the car will overheat but> > only in a strange set of circumstances:> > -- if i drive stop/go around town, under 55mph from the start, the> > car will run all day without overheating, even at lights, in 100+> > weather> > -- If i drive on the highway, it will go all day without overheating.> > -- ONLY when i drive on the highway, then exit and return to around> > town driving, does the car overheat. If i find a road where I can> > reach higher sustained speeds it will cool back down again.>
I believe you have a somewhat marginal airflow through the radiator (or
conditioning condensor and radiator), or perhaps a slightly restricted
water> flow through the radiator.>
(1) Restricted Airflow ?> Your around town driving indicates that heat generated at low speed is> pretty> well removed by the fan/radiator etc. Maybe not great, but adequate.>
At highway speed, the extra heat generated (by the higher rate of fuel> consumption)> is pretty much removed by the air passing through the heat exchangers> (radiator).> But, when you slow down suddenly from high speed driving, the extra heat
dissipated immediately and it shows up as an overheat until the fans,> radiator, etc> can cool it.>
Check to see if there is a little trash, grass, bugs, etc in your AC> condensor or> between the AC condensor and the radiator. Make sure the airflow is not> impeded> with trash, bent fins, etc. Also check to make sure any shrouding (the> manufacture> may have used to direct airflow from the fans through the radiator) is in> place and> functioning.>
(2) Restricted water flow ?> - Sometimes a collapsing radiator hose on the suction side of the cooling> system> will cause the symptoms you are seeing. A new rigid hose will correct
this> if it> is a factor.>
-Although you may have flushed your radiator, it is still possible that> there is something slightly blocking the water flow. This can be some> blocked tubes,> mineral scale, whatever. Silicate scale from antifreeze decomposition> products has> had this effect in some cases. If the radiator is in poor shape, you
have to> take it to a radiator shop for cleaning and repair, or may even have to> replace it if> it is 'rotten'.>

HLS offers the most plausible explanations for your symptoms. I would
mention one other possibility that I have experienced on your particular
model. The ground connection for the cooling fan may develop resistance due
to corrosion and as it gets hotter it finally reaches enough resistance to
lower the cooling fan circuit current. This causes the fan to run at a much
lower speed than it should and does present cooling problems at slow speeds.
It is very difficult to visually tell that the fan is running slower, so the
best way to diagnose it is to monitor the cooling fan current flow with an
ammeter. If the current is less than specs (About 10 amps if I remember
correctly) then you need to find the unwanted resistance in the circuit.
Another way to check for excessive resistance is to feel the ground wire and
it's connections. Any resistance will act as a heater and cause the location
to be physically warmer than the rest of the circuit.

Sometimes the resistance can get high enough to keep the fan from running at
all when it is hot, so you might consider pulling over when the temperature
gage is indicating overheating, and take a look at the fan to see if it is
indeed running or not.

BTW, you did not say if you had a manual or automatic transmission. On the
automatic transmission equipped vehicles there are two fan relays to used to
make the fan a two speed fan. The slow speed relay is controlled directly by
the temperature switch on the front of the engine and uses a dropping
resistor in the fan motor circuit to provide slow speed for normal
operation. The other relay is used to provide high speed when the A/C is
turned on or during overheating conditions. That relay is controlled by the
ECM as well as a second temperature switch on the engine. You said you
replaced the relay, but did you check both of them? If either low or high
speed control circuits are not operating correctly then you might experience
the type of symptoms you described.
--
Kevin Mouton
Automotive Technology Instructor
"If women don't find you handsome, they should at least find you handy"
Red Green





Add comment
Kevin 29 May 2006 12:38:30 [ permanent link ]
 
<ZacInAus@gmail.com­> wrote in message
news:1148889600.616­207.321740@i40g2000c­wc.googlegroups.com.­..> Wow, thank you both so much - your help is very much appreciated!!>
I am going to spend some time working on it tomorrow and tracing down> both of your suggestions. I have a few short questions first if I can> bother you more...>
1. It's a 5 speed car. Are there still two relays/two speeds?

Not according to the repair manual from Mitchell. The manual transmission
equipped vehicles only show one relay and a single speed fan, but I never
worked on one with a manual transmission, only the automatic with two
relays. It may be that there are two relays and a two speed fan even with a
manual transmission, if it also has air conditioning.
Originally, the fan was not coming on at all, and this was fixed by> replacing the relay that is under the battery tray. I know the fan is> spinning, but I guess I need to check the resistance to as you> suggested, Kevin. Do you know where the ground (will cleaning it> help?)

There is a connector (front of left front fender). Try following the ground
wire from the fan motor. The ground connector is one that can be unplugged
and that is where the high resistance would most likely be. It might feel
hot to the touch after the fan has run for a while if it has a poor
connection.

and the second relay are-- the haynes manual for this car is> worthless in this respect as there are so many minor differences> between mx-6, 626, manul, auto, na, turbo, etc, i'm sorry to have to> ask.... I wonder, I don't use the a/c becuase it needs a charge> (leaking dryer), but if iI turn on the system i get a check engine. I> need to pull the code tomorrow, but could that be related to an issue> with the same relay?

Possibly, the ECM is involved in turning on the High speed relay when the
A/C is turned on. The ECM grounds the relay control circuit in response to a
signal from the A/C switch.
2. Which side of the cooling system is suction? i can feel water flow> if i pinch off the top hose into the thermostat, though it's not as> much pressure as I expected. Is there a specific hose i can test?

Lower hose sucks into engine, upper hose flows out to the radiator. Squeaze
the upper hose to test for flow and pressure when reving the engine. Observe
the lower hose to make sure it is not collapsing.
Again, thank you so much for your amazing help and time spent. I'll be> working on the car tomorrow and will be sure to give you a full report.> Wish me luck~!>

good luck,

--
Kevin Mouton
Automotive Technology Instructor
"If women don't find you handsome, they should at least find you handy"
Red Green


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CarGuru > Technology > Re: Request for help with overheating issue on 1990 Mazda MX-6 29 May 2006 12:38:30
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