1991 4.0 overheating issue

[Greg D.]

This summer I replaced my cooling system minus the water pump. Ran all summer in the Georgia heat with no problems. Now that it’s cold and I’m using the heater it keeps fluctuating from normal temp to overheating on the guage. Anyone have an idea as to what could cause this?

 

[franklin2]

Electrical problem with the gauge most likely. I do not like or trust the factory gauges. Does yours have real numbers or just L NORMAL and H?

 

[Greg D.]

It just has the L & H. I replaced both sensors when I did the overhaul of the system. The Guage worked fine. It started acting up when I started using the heat.

 

[franklin2]

Make sure you have the ground strap running from the firewall to the back of the engine. Not sure on your engine but most of the time they bolt the strap to one of the bellhousing bolts. This strap is often forgotten when the engine or trans is pulled for some reason.

If it's really overheating, you should have other signs such as pinging, a oily smell from the engine getting hot, and it may be overflowing excessively into the overflow bottle. Also engines rarely overheat instantly like you describe. If it's going to overheat, it will take a few minutes to climb, not climb instantly with the movement of a switch.

Till you get it figured out, you could buy one of these and check the actual temp of various components under the hood when you think it's overheating.

Amazon.com: Helect (NOT for Human) Infrared Thermometer, Non-Contact Digital Laser Temperature Gun -58°F to 1022°F (-50°C to 550°C) with LCD Display: Industrial & Scientific

Amazon.com: Helect (NOT for Human) Infrared Thermometer, Non-Contact Digital Laser Temperature Gun -58°F to 1022°F (-50°C to 550°C) with LCD Display: Industrial & Scientific

www.amazon.com