Hi folks... Well, when it rains, it pours.

So, I am setting aside the potential fuel system issue I have recently been troublshooting to ask about the coolant system on my 1990 F-150XLT with 351W and E4OD w/ A/C and 4WD.

So... I am trying to troubleshoot what is going on with a temperature gauge that keeps fluctuating quite a bit and slowly increasing until almost to the point to where it leaves the "normal" range. In town driving is fine, almost anything below 40 mph seems to influence the gauge reading very little from the "r" in normal on the gauge. Originally, until about a month ago, the gauge was very consistent, riding about on the "a" in normal without any fluctuation, even while puling a 18 foot flatbed with my 8N tractor on it.

However, starting a little over a month ago, as I travel down the road at 55-60MPH, the gauge slowly builds to that point where it begins to approach the upper edge of normal. If I stomp on the gas for a few seconds, the needle will drop to at least the "l" in normal (sometimes lower than that). Then, back we go to the same routine with the needle gaining temperature again within a few minutes of driving. What is going on here?

Let me explain what I have done... First, the system did have perfect, beautiful coolant and no obvious signs of rust or other contaminants. I assumed that it could be a thermostat. So, I added a bottle of flush (just to cover my bases, though the coolant initially looked great), drove it around for about 25 miles or so, pulled the thermostat and drained the coolant. Replaced thermostat and added fresh coolant.

No change whatsoever to the gauge readings. It is very consistent now in its fluctuating manner.

Does any body have any suggestions? Are we looking at a new radiator?

Thanks again for the help folks,

Regards...

Jason
Bandon, Oregon

 

Disclaimer I'm not a mechanic but if it's not losing any coolant, it's not contaminated (oil or coolant) and your thermostat is new and good I would check that the water pump isn't bad (not sure exactly how to do that but shouldn't be hard) and then ask someone more advanced than me. Another thought tho is the guage reading correctly? If u logically think about it, why would flooring it for a few seconds cause it to cool off and in town driving be fine but 60 mph winds cause it to run hotter. Know what I'm saying?

 

Thanks for your thoughts. I guess it could be a gauge, I do not have another means of measuring temperature to know if that is really likely, I'll have to keep working on it.

 

HF sells HF sells lasers to you can use to test the temp with.
Search results for: 'laser temp guns'

Does the truck physically heat up? Are their symptom other than you're watching the gauge? Steaming, Pinging, coolant over flowing the coolant bottle? Do you just feel a lot of heat coming off the engine?

 

It might be time for a radiator. It is 25 years old, after all.

Try unhooking both hoses, and stick a garden hose in the upper hose neck. The water should flow through freely. If it doesn't, the radiator needs work or replacement.

Water pump could be an issue as well. Impeller could be worn down a bit, such that revving it up makes all the difference.

Also could be an air pocket in the system somewhere. Water pumps don't move air nearly as well as coolant.

 

I'm going water pump. The impeller on mine was completely rusted away when I removed it. Short drives in town weren't a problem, but driving for more than 30 minutes or so would send the needle to the red.