Under the Hood: Erratic temperature gauge could indicate thermostat problem

Engine overheating is a situation that is fairly simple to avoid.
Engine overheating is a situation that is fairly simple to avoid. –Dreamstime

Q: I’ve noticed an unusual behavior in my pickup when driving it first thing in the morning. The temperature gauge keeps climbing above the usual place (a little less than 200) to about 220, and then it returns to the normal place for the rest of the day. Should I be concerned? It didn’t do this before.

Paul W.

A: This is unusual. I’d start by verifying the coolant level when cold (the only safe time to remove the radiator cap). If low, refill and investigate the cause of leakage. If full, it sounds like the engine thermostat is a bit sticky to open. This situation could worsen and lead to a serious overheating condition if one day the thermostat fails to open correctly. Engines employ a thermostat to promote rapid warm-up. It blocks coolant flow to the radiator when closed and to maintain minimum engine temperature, which in most cases is about 195 degrees F. It’s an inexpensive part and is typically fairly simple to renew.


This might also be a good time to consider replacing all cooling system hoses and coolant if they’re more than five or six years old, and perhaps the accessory drive belt as well. Your goal is to take steps to minimize engine, radiator, and heater core corrosion and to insure there are zero cooling system failures to come. Hoses deteriorate from the inside out and typically look good until they decide to let go. Feeling/squeezing hoses and checking for bulging, excessive softness, or a ribbed feel (occurs inside) is a way to find out if trouble is looming. Engine overheating is a tragic situation that is fairly simple to avoid.


Q: I just bought a car that has full gauges. I’m not familiar with a volt meter. It reads 14 when I’m driving, and this is in the preferred range. What am I concerned with if it doesn’t?

Suzanne T.

A: Your voltmeter indicates charging system happiness. A reading of 14-14.5 volts while driving indicates the alternator (a belt driven charging device) is meeting the needs of the vehicle’s electrical consumption and is charging the battery. A typical car battery, fully charged, provides 12.6 volts to supply power to vehicle accessories when the engine isn’t running and to start the engine. (Voltage momentarily drops to 11.5-ish while cranking.)


Think of it like a checking account, it needs refilling. The alternator meets the needs of all vehicle systems while driving and pushes against the battery (14 beats 12) to replenish it as needed.

Your most likely concern would be a low reading, such as 11 or 12 while driving, although this is rare. This indicates charging is not occurring and the battery alone is supplying all vehicle needs. In this situation one might be able to drive for one or two hours only, and an engine restart may not be possible. An overcharge situation (15+ volts) is even rarer, and this is caused by a charging system malfunction. This could be harmful to vehicle systems, and driving should be avoided. In a pinch, turning on every possible vehicle accessory may bring the voltage down some while the vehicle is driven to a better place.