Hello,

I recently acquired a 1996 Ford Explorer XLT with a 4.0 OHV engine 62K mile. During vehicle evaluation I found a small coolant leak on coming from the timing cover gasket. I had the timing cover gasket and water pump replaced by a trusted mechanic.

Upon receiving the vehicle from the shop I immediately noticed wildly fluctuating engine temps, going above normal to nearly overheat and then suddenly dropping too cold. This would happen as I am driving at speed or at idle. I replaced the t-stat with a fail-safe type and find that it still does the temperature fluctuation. Called mechanic and he recommended burping the cooling system. Freed air in the system by topping off during cold start and adding coolant till full, and the condition seemed to improve but not fixed.

The fluctuation is much more minimal now, but the engine temp is always lower than normal on the gauge display. Before the initial service, temp gauge read dead center at operating temp always. Now it always runs colder, no matter what. The heater works too, and sometimes it seems to affect the temperature when I engage the heater during driving after warmup, the temp will start fluctuating.

I noticed that the upper radiator hose is much hotter than the lower radiator hose. In this case, after a drive the upper hose had surface temp of 200 F and lower hose had surface temp of 130 F. I have heard of backwards water pump manufacturing and I am beginning to think that is culprit. I can see the flow indicators on the upper hose showing coolant flowing from the radiator to t-stat, but hose surface temps testing suggest that it is operating backwards to me. I do not have the original water pump to comparison.

It became apparent that the fan clutch may be sticking some. When I cold start (30 F ambient) the fan runs at high rpm for several minutes before slowing. Even warm start fan will spin up with engine until second or third gear, then spin down. Is that normal operation?

I am in need of assistance with this matter. Can anyone offer insight from the description I have provided?

Thanks

 

If you don't replace your thermostat with a genuine Motorcraft one, you will have all sorts of problems, including wildly fluctuating temperatures and overcooling.

 

Thanks, I am considering another thermostat, but something to note is that the original thermostat was still in place when the fluctuating temperature started, immediately following water pump replacement. Replacing the thermostat seemed to have no effect. It was after I had bled the air from the cooling system that the engine temp stopped fluctuating greatly. Now it fluctuates slightly but below normal operating temp even after extended driving.
Going to test temp with USB OBDII scanner.

 

Go with a Stant thermostat... the Motorcraft are junk! I read it here and it's true. You will need to replace the motorcraft every couple of years.

 

OK, the temperature fluctuates when the heater is engaged but not when the heater is off. I saw a high temp of 240F while testing with USB OBDII device with the heater on. Without the heater the highest temp reached was 199.8 F. What does turning the heater on make the engine heat fluctuate? Backwards waterpump?

 

No such thing as a "backwards waterpump." Not sure where you picked that up.

The arrows on the hoses are alignment marks for assembly and have nothing to do with which way the coolant flows. Coolant flows out of the engine, through the thermostat, and to the radiator through the top hose.

If you are seeing 240F you need to stop driving the vehicle immediately. This is overheating and you are asking for severe engine damage.

Drastically fluctuating temp is due to air in the system. If your "trusted mechanic" cant purge the cooling system, then take it to someone that has the knowledge and equipment to do it properly. He obviously didn't even verify the repair before giving it back to you.

 

It is possible with incorrect belt routing to make the pump spin backwards. However, the 4.0L is a known application where some aftermarket pumps can cause severe cavitation. If it was a descent name brand, this shouldn't be an issue.

You need to have your system pressure tested, and it needs to be properly purged.

 

FireME - Great reply
Thanks! I got the idea of the improper waterpump manufacturing from a different forum where it indeed turned out to be the case.

When the temp rises ( when the heater is engaged) it is very short-lived and rapidly drops below normal within a few seconds. When the heater is off, it remains below normal on the gauge but around 200F when t-stat opens and 185 F when closes again. (According to OBDII USB data graphs)

It must be air in the system. Someone recommended lifting the front end of the vehicle to purge air effectively. I will get system purged.

Thanks