Hi all,
Finally got ahold of my grandpa's 91 RS 5.0.

Having some issues with it, not really sure what to make of it. Figure this was a good place to ask some questions. Apologies if these have been asked before, but I didn't see them when I looked.

Here we go:
When I brought it home, I noticed the temp gauge never moved. Got it into the garage and the coolant reservoir was violently bubbling. Looked like it may have been boiling? I am not too sure. I read that I should check the radiator cap, as the seal might be bad and there may be some air in the lines. I let it cool down and replaced the cap. Haven't seen or heard any bubbling in the reservoir since then.

The temp gauge is still not working. I replaced the sensor, and that didn't really seem to do anything. Gauge stays at 100 degrees. Not sure what is going on here. Maybe a wiring issue from the sensor to the gauge?

This all brings me to my main question. I decided to drive the car to work today. No issues there, and seemingly no issues on the way back. I parked the car when I got home and came out to a pool of coolant in the driveway. Seems like it drained the reservoir. Couldn't see exactly where it was coming from, best I could tell was the bottom radiator hose. My question is: with a better seal from the new cap, would that have created a higher pressure in the coolant line causing a hose to crack or just a leak in general?

Is it possible that the thermostat is faulty and not cycling the coolant, keeping the hot coolant in the engine and causing the engine to overheat?

This is all kind of hard for me to tell as My gauge isn't working. I can't really tell is it is overheating. Is seems pretty hot when I pop the hood, but it's August in Southern California so everything seems hot!

 

I would not be surprised if the temp gage went bad but you would want to test your wiring and verify you changed the right Temp sensor.



Yes I would say it is common to fix something that was causing a pressure loss for awhile you will see leaks in some case a lot of them.



Test the thermostat is easy. For someone that has never done it before there are visual indicators inside the Cap and you can feel the temp difference, but I would suggest getting a infrared thermometer. You can buy them at Harbor freight for 20ish dollars. You will see a dramatic change in temps when the thermostat opens on the thermostat housing.

A pressure tester can be borrowed from most parts stores and check YouTube on “How to pressure test a coolant system”



A word of warning. Coolant systems need pressure. The pressure stops the boiling process. Without pressure the boiling releases gases that get trapped in the engine. They will rise to the top (the heads) and anywhere the gases are trapped does not get any coolant. IMO this is one of the biggest causes of head gaskets getting blown. Blown head gasket should show up on the pressure test.

 

As Gorn suggests, a tight coolant system raises boiling temperature from 212 degrees to 260 degrees if pressurized to 16 PSI.
Temp gum helps a lot, aka, when t-stat opens, radiator temp difference between inlet and outlet min 20 degrees.