I have a 91 f250 that I bought about 3 months ago. It has always run very cool, like just under the N of the word NORMAL in the gauge. Right after I bought the the truck, I checked the SCA levels and they were high. 2 days ago on the way to work, within 10 mins of running the gauge was almost above the L, and the check engine light came on. It snowed the day before and I thought that maybe I didn't clean the grill off enough and it was blocking air flow. I pulled over and cleaned the grill and the radiator fins that I could reach, and the temp came back down to about the middle of the gauge. The temp that day got up into the middle thirty's, so I figured that if there was anything else that was in there blocking air flow it would melt out. On the way home that night the temp gauge peaked again. I stopped and added some water to the overflow tank that was empty, let it sit for a min, and took off again. The temp came back down again and was fluctuating between the R, and the very top of the gauge with the check enging light coming on when the gauge would near the top. Finally it just stayed up high so I shut the truck down, and checked the overflow tank again. It hadn't used anything out of it. I've never seen a gauge move as much, or as quickly as this one did. I kinda don't think that it's the gauge or sending unit though, since the check engine light would come also, and I thought it used a separate sensor. Would a bad thermostat act like this? I also thought about the fan clutch, but the truck was showing hot while running down the highway with no load and 25 degrees outside.

 

sounds like a bad tstat to me. I suspect the PO put in a cheapo tstat at some point, or the original tstat is going bad. always go with motorcraft tstats for these old engines.

 

I 2nd that on the t-stat

 

Only motorcraft tstat, if you want to belive the gauge my van stays on the o or the r...depending on load..

 

Okay, guess I'll tackle changing the thermostat this weekend. Thanks guys!

 

Okay, so I'm working on changing the thermostat today and I noticed some green drips on the dust cover on the bottom of the bell housing. The engine is covered in oil and it's making it difficult to see where the coolant is coming from. Is there any place that far back on the engine that could be leaking?

 

Finally got the truck back together and it's running fine. The thermostat was definitely the overheating problem.