Ok, my '85 GSL 12A is exibiting some weird warm-up behavior. Weird in that it only does this from time to time, coolant levels are normal, and outside air temps don't seem to play a part.
What happens is that *sometimes* when the car reaches operating temperature and the choke **** retracts (ie: choke opens) the temp guage continues to climb from it's normal range of about 1/3rd of the guage to a point where it is 1/2 of the way up the guage. It then stays there for a few minutes, then slowly returns back down to its normal setting of 1/3 of the guage.
I'm thinking thermostat, but before I start replacing stuff has anybody else encountered this problem? I've owned four 12A 1st gens including this one and have never seen this happen intermittently.

 

Thermostat..........



J-

 

Ditto....thermostat

jerij

 

That sounds like the culprit. I'll replace the t-stat on Monday. Thanks for the help.
BTW, are you saying you've personally experienced these same symptoms?

 

Mine does the same, wont after the t-sat change.

 

Everybody knows 'what', but do you want to know 'why'???

The thermostat is getting stuck in it's closed configuration for longer than it should when the engine finally comes up to temperature, reducing the amount of coolant that can flow through the block to bring the engine temp to the normal range (1/3). When the engine gets hot enough (1/2), the thermostat suddenly 'pops' open, allowing for full coolant to pass, and brings the engine temp down to where it belongs.

The thermostat is just a bi-metal spring which opens a poppet valve at a specific temperature. When that spring gets old, heat cycled, or rusty, it can open at the incorrect temperature, or get stuck in the closed position, leading to higher than normal temps during start up.

Nobody else mentioned this, but you'll do well to get a Factory Mazda thermostat with the jiggle pin to replace your unit. The jiggle pin is installed to the top of the housing, and allows air bubbles to pass through instead of being captured in the block, reducing cavitation of coolant as it flows. HTH,

 

Thanks LongDuck,
I knew how thermostats work, but I wasn't aware that they could do this only sometimes, while working perfectly fine at other times. Guess I'll be paying Mazda a visit tomorrow.

 

With me it only happened on the "first" warmup. Once the sucker popped open it would open up "on time" the next cycle. However, cool down, restart, it would do it again. I got afraid the next time I started the car, it just wouldn't pop open anymore. Ergo, overheat city.

jerij

 

Had to wait an extra day for the new rad hoses to arrive from Toronto (decided to change those while replacing t-stat), but managed to get the job done this afternoon.
Temps are now back to normal (thanks again for confirming my suspicions), but while changing the coolant I noticed that there was an undue amount of sediment in the rad and water pump housing--- not good, since one of the things I had done 10 months ago when the brake hoses and ball joints were being replaced was a cooling system flush. There is NO WAY that there should be as much crud in the cooling system as I found after only ten months--- unless it was done wrong (ie: not back-flushed) or not at all.
I think I'll be paying that mechanic a visit tomorrow...