Fixing a stranger's 97 del Sol S 5spd. She originally was driving the car and said the temp gauge moved from about 1/3 to 1/2 then shot straight up to H. Car overheated and cracked rad. I replaced the radiator and thermostat, filled it with coolant, let it bleed. I let it idle for 15min and drove it 10+ miles only to have it show C on the temp gauge. Upper rad hose is hot to touch, lower is cold to touch, car didn't really seem to overheat on the test drive, seemed to drive fine. I'm assuming its the coolant temperature gauge sending unit. I just want to be sure, I'm also considering a stuck open thermostat even though its new. Thanks.

 

You likely need to bleed the system longer. The bypass hose busted on my Del Sol last week while on the highway.... no busted radiator of course.

 

Turns out the upper hose was warm because the engine was warm but lower hose was cold because it just wasn't up to full operating temp. The coolant temperature sending unit WAS faulty, I think it got fried when it was exposed to the very high temperatures from the initial overheating. Put in the new sending unit ($18) and the gauge instantly started showing movement when warming the car up, it then reached almost exactly 1/2 way between hot and cold and stayed there the whole test drive. Problem solved!

Just for reference, if you've got a gauge that is not reading any temp or is sparatic, check the following:

• Air in the cooling system (There can be pockets of air trapped in the system after service, the coolant temp sending unit is designed to read coolant temperature, not air temperature so if its not completely exposed to the coolant it will read low)
• Gauge in the cluster (The gauge in the cluster typically lasts the life of the car but it can sometimes become faulty, it can be tested by grounding the wire the leads to the temperature sending unit)
• Coolant temperature sending unit (These sending units are pretty simple in design, basically just indicates a varying electrical resistance to the gauge in the cluster, they should have a resistance of about 36-40 Ohms when the engine is COLD & about 140 Ohms when up to full operating temp (195-220 degrees)