1996 Explorer 4.0L Fluctuating Temperature Gauge, Steam Thumping - Fix

[pmonahan]

Let me start off by stating that I am the 2nd owner of my '96 Explorer and it's my "everyday driver". I take really good care of this truck. It has 330,000 miles on the original motor.

I've been dealing with a variety of issues over the past two years that started with replacing a fan blade and fan clutch. After that, I just kept going, sometimes because I just like working on the truck.

In a moment I will detail everything that's been replaced for the cooling system, but the last remaining issue that I've been dealing with, is the fluctuating temperature gauge, accumulation of steam in the cooling system and air being trapped. Before people start screaming "head gasket", I will explain that this had been ruled out with a variety of tests, because it concerned me too.

It's not your imagination. There's a Ford Technical Service Bulletin on this issue and it's been posted many times here, so I won't duplicate; I'll just state that the recommended Ford "heater bypass kit" that was available at one time, has not been available for years. There is no factory fix.

Some Observations:

• The truck normally runs rather cool in all seasons but summer. Then it's spot on temperature in the summer. Overheating has never been a problem.
• Heat works great.
• The problem *never* happened when the heater bypass valve had vacuum (OFF and Heater Core Bypassed).
• Even when the temperature control called for AC, the heater bypass valve had no vacuum and water flowed through the bypass and heater core. (makes me wonder why the bypass is needed if it's almost always in use for heat or cool)
• The truck did not lose coolant, but frequently had air trapped in system.
• Cooling system pressure check has always tested OK
• Even after cooling overnight, the cooling system maintained some vacuum allowing recovery tank to function during cool-down.
• The cooling system, including heater core seemed to have really good flow while being flushed.

What Was Replaced:

• Heater core (Motorcraft OE)
• Radiator
• Thermostat
• Water pump
• Radiator Cap
• All Hoses
• Heater Bypass Valve
• Both Temperature Sensors
• Fan
• Fan Clutch
• Pulleys
• Belts

After eliminating all of the aforementioned items by replacing, I decided to try and eliminate the heater bypass valve with "straight through" barbed fittings. This did not solve the problem, so the bypass valve was not the culprit. The common denominator was the heater core, but it flow tested well.

I noticed that even though the cooling system and heater core had good flow, there wasn't always enough coolant getting back to the water pump. I don't think there were any restrictions, I just don't think the heater core and hoses are *capable* of delivering enough flow through heater core for the waterpump. Then suddenly, enough coolant would flow.

I created a bypass using 2 barbed tees, hose clamps and a 3" piece of heater hose. After installing this (as in photo), this allowed coolant to be delivered to the water pump if enough coolant does not flow through the heater core. I was then able to remove air from the cooling system and the steam/air buildup has ceased. The temperature gauge stopped fluctuating in extremes and the water pump is satisfied when pulling coolant. There is no pulsing in the heater lines and the "thumping" condition has been eliminated. I have good heat (140° air temperature in duct). One other cool thing is that because I used nylon fittings, they are translucent and I am actually able to observe the coolant flow and there is no air visible.

I'm going to fabricate a more elegant metal solution with less separation between the lines, that will allow me to re-install the heater bypass valve and I'll post an update, but for now, the problem is solved on my Explorer.

One other thing that might work is to install a "stop screw" on the Ford Heater Bypass Valve, so that when vacuum is lost, the valve will never fully open or close. I don't that will work however since it won't increase the volume of coolant made available to the water pump.

I hope this helps someone else.

Peter

 

[abikerider]

Same problem and solution

I had the same problem and came to the same conclusion after a lot of parts replacement. I finally noticed that when the heater selector was on no heat or max air that the problem went away. Then I realized these were the only two positions where the heater core was bypassed. I got some reducer tees and some heater hose and made a bypass and, what do you know, the problem went away. I still get plenty of heat too.

 

[abikerider]

Same problem and solution

I had the same problem and came to the same conclusion after a lot of parts replacement. I finally noticed that when the heater selector was on no heat or max air that the problem went away. Then I realized these were the only two positions where the heater core was bypassed. I got some reducer tees and some heater hose and made a bypass and, what do you know, the problem went away. I still get plenty of heat too.