Mattstark889

I own a 1986 F150 4spd Inline six, Straight Piped.

Lately when the weather has been hot, the heat gauge spikes a tad. I let it idle today and the gauge kept going up and up until i started driving it to work, then it went to normal. (Just so everyone knows, it's not an engine issue.)

has done this every once in a while, and almost makes me afraid to drive into traffic for this reason. I know when i shut the engine off, the process of heat soaking begins, because the temp gauge will rise a bit. But it doesnt explain the random gauge spikes while idling. Haven't let it overheat yet.. or it hasnt yet. Any input greatly appreciated.

Also, after work, i let it idle about 10 - 15 minutes and the temp gauge was fine.

crankymod

i this issue with an 1984 300 6 4 speed after i installed an after market thermostat had to install a ford thermostat then i had no issues.

JCR 56

My truck was doing that and the impellors were completely eaten away on the back of the water pump. My fan clutch was in good shape.

JLTD

That year has the belt driven cooling fan with a viscous clutch IIRC. Tthat clutch could be worn, keeping the fan from turning fast enough to pull sufficient air through the radiator for cooling while you're not moving.

I'd think about replacing the fan clutch first before tearing into it, if it was the thermostat then I would think it would overheat on the road as well.

Mattstark889

If it was the Thermostat, I would also believe it would overheat on the road and it does not. I feel like this has something to do with "Heatsoaking"? the gauge will only spike up after I shut it off for maybe 5 minutes and then start it up again, I got off work and let it idle about 15 minutes last night and it never spiked. Remained absolutely normal, as well as this morning. So im unsure. :/

Mattstark889

After running to work today, i let it idle for a while before going into work. The truck did not over heat, but the temp stopped at about 60 to 65 % of the way. Or at the end of the M on the Normal Before the C or H.

Tonight driving home it idled perfectly vertical on the gauge. Probably because it was colder tonight. But im still unsure.

JLTD

Guess there will be more monitoring in warmer conditions.....

Mattstark889

I have the original Owners Manual, and it says this is normal?

JLTD

So the OM says that the temp moving up while at idle and not moving is normal? If it never actually gets to the level of an overheat, but stabilizes before it gets there, then no worries IMO.

JCR 56

Doesn't sound normal to me, just keep an eye on it.