Car doesn't warm up

Possible bad ground. Bad grounds will cause the gauges to be off unless you have mechanical gauges.

Grounds are important to any electrical system, and especially to computers.

1.) The main power ground is from engine block to battery: it is the power ground for the starter & alternator.

The secondary power ground is between the back of the intake manifold and the driver's side firewall. It is often missing or loose. It supplies ground for the alternator and other electrical accessories such as the gauges. Any car that has a 3G alternator needs a 4 gauge ground wire running from the block to the chassis ground where the battery pigtail ground connects.

3.) The computer has its own dedicated power ground that comes off the ground pigtail on the battery ground wire. It uses a barrel type quick disconnect connector inline with the battery ground pigtail. Due to it's proximity to the battery, it may become corroded by acid fumes from the battery.

4.) The engine mounted sensors have a common separate ground. This includes the TPS, ACT, EGR, & BAP/MAP. These sensors share a black/white ground wire that connects to computer pin 46.

5.) The O2 sensor heaters have their own ground (HEGO ground) coming from the computer. This is different and separate from the O2 sensor ground. It comes out of the fuel injector harness near the EGR valve.

6.) The TFI module has 2 grounds: one for the foil shield around the wires and another for the module itself.

7.) The computer takes the shield ground for the TFI module and runs it from pin 20 to the chassis near the computer.

8.) The computer's main power ground (the one that comes from the battery ground wire) uses pins 40 & 60 for all the things it controls internally.

If you have access to a digital voltmeter and would like help diagnosing grounds, make another post and I will try to help.

welcome to the boards. do you have a mechanical or electrical aftermarket gauge?

Wow, someone else had my same problem, my 1990 gt got to a point where the temp guage would barley move ,never warm up. I know it wasnt the thermostat because i changed it, and i know it wasnt the guage cause the heater would blow luke warm air when it was cold out. even sitting still ideling, it would never heat up. I home flushed the radiator, changed coolant, and thermo at same time thinkin i had fixed it, heater air was probably 10 degrees warmer. I never did get it figured out. I just recently changed heads, cam and intake and it runs nice and warm now with a 190 somethin thermostat in it. I burped the cooling system and made sure i had plenty of coolant so that wasnt it either, but i would love to know what it was incase it ever happens again. Good luck!

A 160* stat in the winter can allow the car to run so cool the heater has no decent sustained heat.