Alright folks, see if you can figure this one out. I have a 1996 Ford Ranger 4.0, 5 speed 2wd. I just did a complete head job on the engine (due to a head gasket leaking). The temp gauge now fluctuates drastically from cool on up to the hot side of the gauge. My water level is staying constant. No oil is in the water and vise versa. Water level stays constant. The engine never did this before it blew the head gasket. I have changed the thermostat several times and finally put in a factory Ford thermostat. The fluctuation is not quite so drastic now and the temp gauge goes about 3/4 of the way to the hot side. It stays on the hot side for only a matter of seconds and then drops back to the cool side and then repeats the cycle all over again. However, it is still fluctuating and makes me uncomfortable when driving it. I have checked all hoses and have replaced the radiator, radiator cap and water pump also. Heads were slightly shaved and magna fluxed, valves resurfaced and new valve seats. Any ideas as to what is going on? There is no knocking or thumping in the radiator. Thanks for any reply and/or suggestions.

 

sounds like the air has not been properly purged, or the water pump is having issues. Head gaskets blown because the engine had been overheated, not the other way around, so suspect there is a problem with the effectiveness of one or more key components. You have fixed the thermostat, but a bad water pump can sometimes be difficult to diagnose. A collapsing hose can also cause temperature surging. You said you replaced the pump, not sure what years, but Ford had a TSB on water pumps, they came out with a new updated pump design, but most aftermarket designs would likely copy the original faulty design. The design would cause cavitation and surging. Not sure if that applies to your year.

When the system was being purged, was the heater of full? You need circulation through the heater to get proper purging, otherwise air trapped in the system can plague you.

 

Khan, heater core was full and works just fine...... After reading your reply, I am leaning toward a faulty aftermarket water pump design. Thanks for your input.

 

replace the temperature sensor !

 

While it is unlikely that a temp sender would produce a wildly fluctuating reading, it is a cheap part, and that would help. Since you obviously want to bite the bullet on this, I would also replace the upper and lower radiator hoses. The lower one is spring reinforced, but it could still collapse internally and cause problems, hoses are relatively cheap, and it wouldn't hurt.

 

I just put in a temp sensor but still have the same problem. I failed to mention that I had replaced the hoses when putting the heads back on. The parts man at NAPA tells me, to fix the problem, to take the blue oval off of the grill and put a Bowtie on it!! My reply was " I'd rather drive it than push it".

 

Temp sender is one thing ECT is another. How is the truck running? If coolant is doing it's job, the truck should run fine. The ECT, Engine Coolant Temp is a sensor that reports to the ECM. The temp sender is a varistor that reports a voltage drop which tells the gauge to move. I'd check the connector to the sender first. Make sure it is secured to the sender.