1996 Explorer 4.0L Fluctuating Thermostat

[pmonahan]

I've read other threads here about the fluctuating temperature gauge and have followed all of the troubleshooting steps, especially burping the cooling system.

I am aware of the TSB #97-18-9 describing this problem and the recommendation to replace the waterpump with a F7PZ-8501-AA waterpump and gasket.

My waterpump was replaced in 2016 with an aftermarket and I am assuming that it would be a suitable replacement for the F7PZ-8501-AA, not "old inventory" or the original design.

Can anyone tell me the fundamental difference between the original waterpump and the recommended F7PZ-8501-AA replacement? Is there a way to identify what's on the truck by appearance? I inlcuded a photo of the aftermarket replacement.

 

[lobo411]

No idea, but unless you're having the exact problem specified in the TSB--fluctuation AND a loud thump--I'd be looking more closely at the usual leak suspects in the cooling system. Check you hoses and especially the places where they connect (those clamps have a nasty habit of leaking when you're not looking...like 5 mins after you park, and never under any other conditions), check for signs that the thermostat housing RTV seal is leaking, consider whether the upper intake manifold gasket could be leaking (unexplained coolant loss), replace the radiator cap, etc...

 

[pmonahan]

As I stated, I've followed all of the recommended steps.

The problem is exactly as described in the TSB.

There is no coolant loss. The cooling system passes extended pressure tests. The problem began after replacing the waterpump and radiator; never existed prior to that.

As I understand it, the problem is cavitation; I am trying to find out if the impeller is different on the F7PZ-8501-AA compared to the aftermarket water pumps.

Thanks

 

[pmonahan]

Update:

I did find that the impeller is correct and my water pump is working.

The problem persists and I've noticed that my cooling system has not been pressurizing when the engine runs. I added UV dye to the radiator and decided to perform another pressure test. I performed the test cold. As before, the cooling system passed an overnight pressure test, no leaks. Then I decided to run the engine with the pressure gauge connected and the system operates at 16 PSI and holds pressure; no knocking, no fluctuating pressure gauge. The Upper radiator hose is clearly pressurized and I have normal operation. I had never run this test with engine running before when Iperformed the prior pressure test.

When the pressure gauge is connected, the system holds pressure and operates normally.

Suspecting the radiator cap previously, I had replaced it a couple of times. With any of 3 radiator caps in place, the temperature gauge fluctuates and the truck overheats for short periods, followed by rapid cooling. The upper hose collapses easily as there is no pressure.

I'm at the point where I believe that the radiator cap specified for the radiator is an incorrect fit and the lower seal does not make contact when the cap is connected, because it does not sit deep enough in the radiator neck. I will try a Motorcraft radiator cap, if it seems as though it will sit deeper in the radiator neck. The upper seal is adequate to prevent a leak since the is no pressure, but since the lower seal is not making contact, there is a leak into the overflow reservoir.

I have a CarQuest #4324212 radiator and the cap that is specified is CarQuest #10233. An alternate cap is the CarQuest #10333

Update II:

The radiator neck was defective, preventing the cap from making a proper seal. Replaced today and the cooling system now builds pressure for the first time since the raditor was first replaced.