Overheating mystery

[Foolish]

Driving the P5 home from work the other night I sat for a while at an intersection, and when I got the green, I shot across the road, looked down at the gauges, and noticed the temp gauge was all the way to HOT!

I backed off the throttle, and drove slowly the 1/2 mile home. I turned the heat on, and the temp fell back to a normal range. Since then, I've driven the 3rd car to work.

Today I read a bunch of threads on here about diagnosing overheating, then took the P5 out around the neighborhood to try to figure out what's going on.

It stayed cool for quite a while, but eventually it got hot. Once the needle moved off of the normal range, it climbed VERY rapidly to full-hot.

At one point when it went full-hot, I pulled over, jumped out, went to the front of the car. I could not hear the fan running.

Another time, the cooling fan came on when idling in the driveway, and kept the gauge low, so I know the fan/switch/relay isn't completely shot. Both fans come on with the A/C.

The radiator hoses are both hot, making me think the problem isn't the thermostat.

Coolant is at the proper level, and isn't boiling, overflowing or super hot in the bottle.

So: What the hell?

Could the temperature sensor be failing and not telling the fan to come on, but only sometimes?

Could the thermostat be failing and not sending coolant through the radiator, but only sometimes?

Could the Fan switch be failing and not turning on the fan, but again, only sometimes?

More importantly, how can I figure which one it may be, without replacing all of it?

Thanks for any suggestions. I'm a bit baffled.

 

[StealthWyvern]

your thermostat could be sticking and cause intermittant issues.... replace it... change out your coolant or you could have air pockets in your cooling system.... Either way change out the coolant and T-stat.. worse case its maintance that is due sooner or later.

 

[lolwut]

your thermostat could be sticking and cause intermittant issues.... replace it... change out your coolant or you could have air pockets in your cooling system.... Either way change out the coolant and T-stat.. worse case its maintance that is due sooner or later.

+1

Replace thermostat, and coolant temp sensor since its only like, 25 bucks. Use a multimeter to test relays and electrical components, then bleed the cooling system.

 

[tweety2300]

+1 on thermostat it could just be stuck half way open which is what's causing it to get warm so slowly. Btw don't drive it hot unless you want to be replacing the head gasket

 

[Foolish]

So, I've been trying to figure this out, and at this point, I think the fan just isn't coming on when it's supposed to. I ran the car around today, and stopped every so often to check temps with an infra-red pyrometer. I checked the upper and lower radiator hoses as well as the radiator itself. From normal operating temp (middle of the gauge) to HOT (top of gauge) there's about a 30* temperature difference, showing a high of around 214* on the top hose, 208* on the bottom. The fan never came on, and as soon as I opened the hood, the temperatures started dropping. Also, the temp seems to climb most rapidly when moving along in traffic behind another car. When I'm alone on the road, it stays cool, making me think the car really isn't getting air flow across the radiator because the fan isn't coming on.

So, what controls the fan? It will come on with the A/C but not on its own.

 

[Foolish]

Put in a new temperature sensor, took it for a drive. Stayed cool, fan came on when running and parked, thought I solved it. Ran an errand, super exited that the car was fixed, then on the way home, saw the gauge climb back to the top, pulled off the road to find that the fan was again not running. Popped the hood, waited a few minutes (running, but no fan) temperature dropped, drove home, car stayed mostly cool.

Now what?

 

[03protolx]

How many miles on your P5? I had a similar problem when I had my 97 proto 1.5...intermittent overheating and cooling fan never coming on.

I wound up replacing the temp sensor along with water pump....no avail. Then replaced the cooling fan relay....no avail. Went ahead and hit the junkyard for some junk fans and voila! the fans worked.

I would check your water pump as well....

 

[Foolish]

It's at about 90K miles.

 

[Foolish]

Talked to a parts guy at a Mazda dealership today. I was going to go get a fan switch since neither the coolant temp sensor or the fan relay solved the problem. He told the the Coolant Temp Sensor IS the fan switch. At this point, it seems I must have a wiring problem somewhere that's keeping the cooling fan from coming on independent of the A/C fan.

Joy.

 

[Jeezus]

Same thing happened to me. Radiator cap was bad, failed to keep pressure in the cooling system causing it to boil over at much lower temperature.

I had the exact same problem as you, even remembered jumping a wire from the battery to the fan to get it cooled down on the side of the road. Pulled the radiator cap and the spring fell apart in my hand.

Easy $10 fix

 

[Foolish]

Ah, forgot to update this!

Turns out it's just the gauge. I tried everything I could think of then finally took the car to my mechanic. $95 later, they'd diagnosed that the car wasn't actually getting too hot, which is why the fan wasn't coming on when the gauge showed hot.

Bought a gauge from a friend who has parted a few of these cars out, and once my new LEDs come in, I'll swap out the gauge and the lights behind the cluster.

I confirmed the diagnosis by driving the car with an OBD2 cable hooked to my laptop. Even when the temp gauge showed OMG! HOT! the temps were holding in the normal range, typically below even the 207* threshold I've read for the fan to come on.

I tested this confirmation by laying down 13 autocross runs in the car last Sunday. No problems at all! :-D

 

[StealthWyvern]

Good to hear that it wasn't anything to major!