Keith Park

This is a stumper! Posted on it earlier this year but noticed a few new stumping issues



93, bone stock, 35Kmi, new radiator, on its 3rd Mazda thermostat, flushed the system every 5 years. Temp gauge is on the filler neck

Below the thermostat in the front.



Car will sometimes try to overheat, get up to 240 on the gauge, moreso if the AC is on but it WONT ALWAYS DO THIS, sometimes its fine… and this can happen even if I force all the fan’s on so its not a fan issue. But here’s the stumper… turn off the AC, it wont cool, turn off the interior fan, It cools immediately. This morning it was 37 out there, had the heat on full and it headed for 220, turn off the interior fan, comes down quickly, turn it back on and up it goes again… conventional wisdom would say if your running hot turn on the heat to shed some of that heat load, but any time the interior fan is on, AC or Heat, it CAN (doesn’t always) try to overheat or at least run significantly hotter than Normal.



Ive sat there, cool summer morning, heat blasting, all the fans on, idling and temps would not come down from 235 until I turned that inside fan off.



Stock gauge comes up and paints its needle on the scale as it always has since the car was new, I never let it get hot enough for that to move but with the digital gauge I can see whats happening and I know what used to happen before all this began.



Any ideas? Its all tied to the interior vent fan, does that fan turn something else on? Or, what else does it control?



Im really stumped with this one! Could I have an air bubble? I can open the top to the filler or the expansion tank and they are always full, am I missing something?

Thanks



Keith

DaleClark

iTrader: (55)

What brand gauge do you have? I'm almost wondering if you have a gauge you can't trust.

Not a big fan of thermostat housing temp sender placement but for the purpose of this problem it's fine.

Do you see the high temps just idling in the driveway or driving around? Airflow makes a huge difference. If it's 37 deg F outside there's no way in hell I could get my car to overheat.

What kind of radiator do you have?

Also I think the "turn the interior fan off" is a red herring, I almost wonder if it's a funky electrical noise issue where that fan is messing with gauge readings. Running the heater full blast makes very little difference to engine water temp, the heater core is a fraction of the size of the radiator and the airflow from the interior fan is far less than the radiator fans.

If the car is bone stock, these cars run HOT from the factory. Stock fan switch is 107 deg. C which is about 225 deg F. That's why the FC fan switch is such a good upgrade.

Do you have the precat or a downpipe?

Dale

DaveW

That was my 1st thought seeing your latest symptoms - the voltage drop or electrical noise from the heater fan is affecting your aftermarket gauge reading. Otherwise it makes no sense.

Keith Park

Its a cyberdyne gauge circa 2005 when I got the car, Im used to what it does and what it tells me so this is different from what it was before a couple years ago anyway,
this has been in issue for a few years but getting worse. Its why I put the new radiator in, its a Mazda replacement, I was told it was for a 99 with the extra row of cooling in it but to be honest, when I got it, it looked identical to the original.

It will do this sitting idling OR going down the highway, thats another wierd part... its like there was a fan issue but I can force them all on and still, will do it. yep! Makes no sense, why im here! :-) it wont overheat on a 37 degree day, but it will hit 220 - 225 and it never used to do that.

It has had the fan switch upgrade.
Running a racingbeat catback, rest is original stock exhaust.

Another quirk that the temp guage has had since day 1 is that it will how WAY hot 260+ if you shut it off and start it again soon, comes right back down once coolant start flowing, always thought this was normal although that seems quite hot for the filler neck.

Where does the stock gauge measure it? Ive been hesitant to let it get hot enough to move that stock needle as Ive been told by the time it moves its too late, and even years ago when all this worked properly I could get it to 240 on a hillclimb (100mph up a hill) and the stock gauge would never move.

So there is no "valve" anywhere in the coolant system that could be acting up other than the thermostat? anyone have an impeller come loose on a water pump?

Keith

DaveW

I still think it's a gauge or sender voltage sensitivity issue. More likely to be the one of the 2 that is wired into the same circuit that supplies the heater power, since heater fan seems to trigger the issue. So I'd look for a bad (power or ground) connection in either the gauge wiring itself, or the heater fan circuit that could cause a voltage change across the gauge terminals.

Of course, this is just an educated guess, so measurement is the key to what it really is.

Keith Park

Well what your saying makes sense, the gauge is only as good as its voltage supply... thing is, nothing has changed, well... that Ive had anything to do with.
What i will try though is putting a thermocouple on that same spot and see if they track one another, and if the readings im seeing on the gauge correspond,
probably time to verify that gauge!

Thanks

Keith

Keith Park

took it into work today, cold morning, 215 when I arrived, manually switched all the radiator fans on and it jumped to 250! Yea, Now i dont believe that,
ive had it run hotter with the fans on too in the past, drove it home 215 all the way, should have been 185 to 190, next day... no problem!

YEa, I think something fishy is up with the gauge but next spring when I drive it again Ill put a thermocouple on that to verify things, then look to see where
I grabbed the B+ for that gauge.

Keith

scotty305

iTrader: (2)

Coolant temperature won't change more than 1-2 degrees per second, unless you have air pockets or steam in the system. If you're not sure you can trust your coolant gauge, try an infrared temperature gun. Pointing the temp gun at the thermostat housing (near the upper radiator hose) usually gets pretty close to the coolant gauge temperature. You should see coolant temperature goes down when the fans turn on, even if it's not immediate. Ground problems might cause a temperature gauge to do weird things when electrical loads (like the coolant fans) turn on, especially if it uses a sensor with just one wire connection (using the engine/chassis as the sensor's ground reference).