Bigbird

Hey guys,

Recently my 1983 RX-7 GLS 5SPD 93K (orig.) has been acting in a rather strange way.

A) The car seems to drive just fine.

B) While driving, the temp gauge stays either right below the half mark or at the half mark, like it is supposed to.

C) When I apply the brakes the gauge begins to climb, and will climb until it pretty much reaches the overheat point, however, since I do not let that happen it usually dips a quater over the half mark - which is still TOO DAMN HOT!

D) But here is where I am puzzled! If I let off the brakes and let the car idle normally, the water temp ducks down to normal or even below the half point. How does that make sense???

E) Also, when its beginning to overheat according to the temp gauge and I begin to floor it and drive agressively it will lower down to normal once again.

F) This is very strange, because it seems that braking and slowing down makes the engine overheat, but if I start moving and floor it - it goes back to normal OR if I just let it idle without applying the brakes it goes back to normal as well. (When I say "idle" I mean that I am parked, not moving).

What is the malfunction here??? I checked the coolant, the oil level, the piping, plumbing - it seems to be just dandy.

I know this is a 20 year old car, but is there any rhyme or reason behind this?

WackyRotary

Sounds like a intermedate open wire/shorting wire/corrosion that is effected by motion of the car. I had something simialer happen to my temp gauge one day. THen I disconected the wiring harness and spray'd WD-40 on all the connections and then reconnected. The temp gauge worked fine after this.

Bigbird

Well, it changes REALLY REALLY slowly, enough to let me know that most likely it is not an electrical problem, but rather the actual temp flactuating like that. Also, sometimes when I would be driving on the highway, it would suddenly beging to overheat (surge up), yet as soon as I just roll in neutral and restart it would go down a little and just stay there (slightly above normal).

So at this point, although I will check, I do not suspect an electrical gauge prob.

WackyRotary

Well thermostats are cheap, put a new one in. But I still think its a electrical problem from your discription. If it was a inner rotor housings seal failure, it could behaive in a simialer manner you are speaking of, HOWEVER, it would spew coolant out the overflow resivor. Is it doing that? You didn't mention that if you need to add coolant or not.

Bigbird

I checked the coolant, and it was just fine. I will check it again tommorow to see if it is low.

FD Racer

hows the fan? Usually as the temp rises, you can hear the fan clutch grab the fan more producing more fan noise.

Zulu

makes a nice roar when it does

Bigbird

You know what? I don't think my fan turns on! That is what I just realized. I never hear it kick in as it does on my other cars. Uh hoh! What to do? What to diagnose?! Crap

Manntis

Just because the needle moves slowly does not mean it isn't an electrical problem... the temp gauge is measuring liquid temperature - even with catastrophic failire of your coolant system it can still take full seconds to change temperature.

Couple of things:

-clean all electrical contacts

-remove and inspect the thermostat.

Is the sending unit loose? It might be swinging into an air pocket when the nose of the car squats under beaking and therefore reporting hot air temperatures instead of semicooled liquid temps.

Similarely, the electrical resistance might be changing due to a physical shift pinching a wire, which could report a false change in temperature

If your fan was in fact failing ou'd be heating up at low speed and idle, not highway speeds. A fan is there to pull in air when you're NOT driving fast and ramming air into the radiator.

Bigbird

Where can I find this "sending unit" that you speak of?

Manntis

The thermostat is located at the front of the engine, inside a housing just to the right of the alternator (there'll be a hose connecting this housing to the radiator). As your temperature fluctuations seem to be related to the car being in 'nose down' attitude (deceleration) it's unlikely there's a failure of the thermostat. Rather, a physical shifting of a connector, temperature sending unit pieces, or even coolant level is likely.

Are you sure your coolant is topped up and free of air pockets?

The coolant temperature sending unit is on the left side of the engine, by the oil filler neck.

Connect an Ohmmeter to the coupler connection coming from the sensor and check the ohm reading. With the engine cold, the reading should be between 105 and 233 ohms.

Warm up your engine to operating temperatures and take another reading. The reading should be between 20 and 50 ohms, give or take.