I put a new thermostat in my '95 F150 4.9L back in March but can't get the engine up to a decent operating temp. It's in the 90s outside but the gauge shows it barely into the normal range-I mean just barely over the bottom line. Kills mpg. I suppose I could have gotten another defective factory part from NAPA (Mexico has no better quality control than China) but a thermostat isn't exactly rocket science. I've bought dozens of thermostats over the years and never had one bad from the factory. Short of blocking half the radiator off with cardboard or replacing the thermostat again anybody got any ideas as to why?

 

If you're relying on the 23-year-old factory gauge, that's your first problem. It's NOT a thermometer - it's just a gauge. There's a reason it DOESN'T have numbers. Click this, read the caption, and buy a non-contact IR thermometer:


(phone app link)

But you didn't say what brand or PN the current t'stat is, or at what temperature it's intended to open.

 

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