I have a 2000 Ranger with a 2.5L. I just purchased it from the original owner 3 months ago. Super clean, all receipts for service and repairs and 77,000mi. Recently I noticed when coasting down a steep hill the temp gauge dropped to below cold and then came back up. A few days later the temp gauge would only rise when the RPM's were higher and then drop immediately to cold at an idle. So far I have:

1. Replaced the thermostat - worked for about a week, then went back to cold and never moved.
2. Flushed, changed the coolant and burped the radiator - no change
3. Replaced the temp sensor - no change
4. Replaced the temp sender - now comes up to temp sometimes and then drops back down.

I was at a loss and took it to a local mechanic. They checked all the systems and said thermostat is opening and closing, engine is coming to temp, electrically everything is fine. They could not figure anything out and did not want to replace unnecessary parts as they were stumped too. They recommended I take it to the dealer. The dealer said it would probably be a $500 diagnostic before they would service the truck. I'm not ready to shell out that much for a mystery issue, but the gauge moving up and down is driving me crazy! Any ideas?

 

I had similar issues with my 98 Ranger with a 2.5 engine.

I replaced the thermostat and temperature sender and then everything worked properly. I tried after market parts that worked for a few days and then got the same issues back again.

I finally broke down and bought parts from Ford. They were expensive, but it has been working fine for over a year now.

The two after market thermostats that I tried had a smaller rubber seat at the end compared to the Ford unit. I don't think it ever seated properly in the engine.

 

Thanks for the input! I put a motorcraft thermostat, but the sender is aftermarket. I'll try a new sender and see how it goes.