My 86 F150 with the 4.9 6 cyl. has been running borderline hot. The gauge is at 'L' of normal range.

Recent parts include;
newer radiator
180 thermostat
water pump

its been flushed several times, 3 days ago again.

I once ran the truck without a thermostat and it ran cool.

I am wondering if there is a wrong way to install the thermostat on this engine or I have heard of some thermostats having a bypass hole in it.

Not sure what to do next.

 

How do you know it's running hot besides the factory gauge? I would put a real gauge with numbers on it under the dash and see what temp it's really running at.

 

Those factory sending units for the gauge fail a lot. I had one go from normal to hot on me in 10 seconds going down the road once. The top radiator hose wasn't even hot yet.

 

Yep, these guys are right. The factory gauges are notorious for being wrong. I've overheated one engine because the gauge said all was well. And I troubleshot one cooling system until I was at my wit's end, only to find it was the gauge that was wrong.

 

While the factory guages can be inaccurate, they usually tend to be predictable about their inaccuracy. Meaning if 1 is way off, they all tend to be off in the same way. (aside from the alt gauge, which by now, most never move at all)
Is the engine actually running as hot as the gauge indicates? Is the oil pressure gauge reading much higher than than it used to?
Yes, it is very possible to install a t-stat backwards, especially on these older trucks (and cars). Fords are known to over power the spring enough to not meltdown when this has been done, but they do tend to run rather warm.
The spring should go into the head, not the t-stat housing.

 

thank you for the input guys I appreciate it! I will check the engine temp with another temp gauge.

The oil pressure and everything else is normal.

And I did install the t-stat with the spring side into the head.

Dan

 

Find someone who has an infrared [hand held] temperature gun and point it at the thermostat housing and then the radiator with the engine at operating temperature. You will quickly get a ballpark temperature reading. Most mechanics have one in their toolbox.

Since you replaced the water pump, are you sure you did not get one for a later model truck? Serpentine belt driven pumps run opposite to v-belt driven pumps. Some pumps have directional vanes and will not operate correctly if running "backwards".