So I've been driving my 1992 FD with no issues. Been driving it to work, some late night parking lot slides, and even a 3 hour drive up in the mountains last week. At the time, the car only had an aftermarket boost gauge, but the stock temp gauge was reading in the middle and staying there during all these drives. The coolant was changed not that long ago and is full. Anyway, today I installed a Defi water temp gauge. Used the piece that fits into the radiator hose to hook up the temp sensor. Started up the car and the gauge seemed to be working fine. However, after about 15 minutes of easy driving, the warning buzzer started going off on the gauge and it was reading around 115 degrees Celcius. The stock temp gauge was still reading ok, however, and today was a pretty cool, overcast day. So I'm really confused because how can my car overheat after 15 minutes of easy driving, yet I took it on a hard 3 hour drive just 4 days ago and never had a single issue. Surely if it was overheating that easily, the engine would've gave out, not lasted 3 hours of speeding through winding mountain roads. So could the gauge possibly be bad? Could the car somehow be overheating suddenly? What could happen that would cause the car to overheat so quickly? Like I said, it's full on fresh coolant and the fans seem to be operating fine. Car is mostly stock, only engine modifications are a pod filter, aluminum AST, and exhaust.

 

At 115c, I'd have expected your stock coolant gauge to be heading towards the top.

 

Try testing your sensor in a pot of hot water to see if it is accurate.

 

Sorry, went back and looked at my gauge today and realized that it didn't hit 115, it was more at 108 or 109. Still too hot though. Thought maybe my thermostat was bad, so I pulled it out, filled it back up with coolant and went for a drive, only to have to start overheating again after about 10-15 minutes. The temp gauge steadily rose to 80 as I was driving, slowly made it to 90, then after 90 it moved really quickly to 100+. The belts are fine and the water pump seems to be ok (don't see any fluid dripping out of it, which is apparently the sign that it's bad).

 

Haven't tested the gauge yet. As soon as the engine cools down so I can remove the temp sensor, I'll test it.

 

Since the coolant was recently changed are you sure all of the air is out of the system? I do not have much of the stock cooling system left but getting all the air out is really time consuming. It is at least 3 to 4 full heat cycles and a nice Lisle funnel.

 

Cannot emphasise how much easier these make bleeding the FD.

 

Yep and it's still a b***h to fully burp it.

 

Removing the thermostat can make these cars run hotter because a large portion of hot coolant will bypass the radiator. Don't do it. Get a new 180 degree thermostat an gasket. Make sure it has the jiggle pin and you install it in the 12 o'clock position.

Use a lisle funnel to properly bleed the system. If you have throttle body coolant popping off the rear hose helps when filling the system.

Get a Power FC or other standalone computer to tune the car and set the fans to come on earlier. I like 88-90 degrees C. Or get a FC fan switch which will also turn the fans on sooner.

Use only tranditional green coolant mixed 50/50 with distilled water. In hotter climates you can go to a 60/40 mix. More water will cool better.

 

^This.
And the stock thermoswitch triggers fans at 108 C. (226 F.) so seeing that on the gauge may not be all that unusual depending on circumstances. Many replace it with the S5 FC switch which triggers fans earlier, at 96 C. (204 F.).

DO NOT pay attention to the stock gauge. Not linear. That means if it moves higher than just below midway, it's likely too late.

 

If it's at 108C then it's behaving as a stock car would be expected to do. Look into the FC thermoswitch as mentioned.

 

I can confirm, 108C is not unexpected out of a stock car. Mine is mostly stock and my aftermarket temp gauge sits around 90 to 96C for the majority of cruising. If I push the car hard, I'll see 105C.

I saw 110C only once but that was a blistering heatwave summer day (record breaker) where old people were dying.

I don't have a PFC but I do have manual control of my fans from inside the car- if I see the temp get to 90+ celcius, I'll turn the fans on and that will drop it back to about 80/85C within a few minutes no problems.