Temperature Gauge Fluctuation

I have a 2000 Ranger 2.5L, and I have been experiencing some temperature gauge fluctuation problems. The temp gauge just started fluctuating between low and the mid-point on the temp gauge. It seems to only do this while driving the vehicle. When sitting at idle the temp stay at normal point. I went ahead and replaced the thermostat just to make sure that it wouldn't overheat and to see if that would resolve the problem. It still fluctuates. My coolant level is good and I grabed ahold of the bottom hose and had the truck started to make that the tube swelled up after starting to check the circulation of coolant. This tells me that the water pump seems ok. I also tested a new napa oem gauge sensor out and it did not fix the problem as well. I don't know if I need to go with a straight ford sensor or if there is something we are missing with the electrical system or something else.

Any advice would be appreciated?

That's some good information. Is there any air in the system?

-Drew

It's possible that both the first and second thermostats are bad.

No, I popped the cap of while it was running to make sure that there were no air bubbles. Unless, there is better way of removing air bubbles?

I didn't think that computer sensor would actually effect the gauge. Is this true?

Im trying not to overthink this to much. I was even thinking that it could be possible that the wire bridged itself to another wire, but its snowing here now so I have the desire to work on the truck?

I had a similar problem with an irratic temp gauge on my 92 Explorer bought 4 months ago from a private party.

The diagnosis-
The needle on the temp gauge, with engine at "normal" op temp, never got much above the "C". The upper and lower radiator hoses felt warm but not hot. A laser non-contact temp probe showed a max temp of 160 degrees on the hoses and 175 at the thermostat water outlet tube. This a problem as the thermostat is supposed to open at 195 and with the engine never reaching normal operating temp, it would affect my mileage and possibly kill my catylitic converter.

The Fix-
There was a TSB(1) on a replacement temperature sensor so I got one along with a standard and heavy duty Stant(2) thermostat and gasket. First I flushed and cleaned the system several times (filled with rust). I installed a Prestone in-line MPT "T" fitting in the heater line to back flush the cooling system with a garden hose to make sure I got out all the rust...no improvement. Then I replaced the temp sensor...no improvement. Then I removed the thermostat and examined it. It too was covered with rust which partially obsructed the flow. I cleaned it thoroughly and then tested it in a pan of boiling water to make sure that it opened at 195 degrees...it did. When I got it all back together the temp gauge needle was reading in the normal range.

(1) Replace Temp Sensor TSB: http://autorepair.about.com/library/faqs/bl511f.htm
There was a TSB on this problem. Replace the Temperature Sender with Ford part number F1SZ-10884-A and that should fix it.

(2) You want to make sure that when you replace the OEM thermostat that you just don't replace it with an "similar" one but an "exact" equal. If you don't want to spend the big bucks for the OEM, Stant makes 2 thermostats for the Explorer...one is the standard one (ST13959) with an air bleed off vent and the other is a heavy duty (ST45369), but without the air bleed off vent. If your OEM works, don't replace it simply to replace it...verify that it and any new one you put in actually functions by doing the boil test. If you do not use one that matches the OEM, expect air in your system. If you shop around, they can be purchased for about $7.00 and $10.00 respectively.

Good luck - Jay

Ranger00...there is a TSB on 97-2000 Rangers, auto or manual transmissions...it's called a coolant bypass kit , Ford # F8PZ-8522-AA that addresses temperature gauge fluctuation sometimes accompanied by a thumping or knocking noise on the passenger side. I don't have the exact TSB number, but I remember it saying that the problem was due to a buildup of steam in the system, and this bypass kit either blows it off or elimintes it altogether. Oh...it didn't state a certain type of engine, just 97-00 Rangers.

Fixed

Sorry guys, I ended up fixing the problem shortly after my first post. It was a bad temp gauge sensor. It turns out that the replacement sensor I bought was crap as well. I had gone ahead and replaced it again and it has been working great ever since.