Fluctuating Temp Gauge with Heater On

[coreweave]

I just recently picked up a '96 Explorer Eddie Bauer (4.0L) and replaced the head gaskets. It is running pretty good so for besides an issue with the temp gauge only when the heater is on. The heater is blowing out hot air.

When the heater is on, the temp gauge will climb normally to the middle. When the thermostat opens up, the needle will fall down to the "C" notch of the normal operating temp range. This will happen whenever the heater is on.

With the heater off, the needle stays in the same spot (middle). No fluctuating of the needle.

I have purged the cooling system about 4 to 5 times, with no success. With the engine cold, I rev the engine to about 2.5k rpms, remove the radiator cap, and add when necessary. The symptoms that I'm having leads me to believe that there is air in the system or being sucked in when the heater is on.

The previous owner informed me that he replaced the "heater control valve", which connects to the heater core. I am starting to wonder if that is the culprit. Could someone please post a picture of how the valve should be mounted?

Mine was mounted with the diaphragm pointed down (vacuum nipple on bottom). I went to the junkyard and could only find one explorer and it was mounted the opposite direction (vacuum nipple pointing up).

Thanks in advance.

 

[alohamonte]

heater control valve is inline with the heater hoses. it take 10 mins to replace it. it will normally cause drip/leak when it fails.

pressure test the radiator cap for leaks.

 

[96eb96]

I just recently picked up a '96 Explorer Eddie Bauer (4.0L) and replaced the head gaskets. It is running pretty good so for besides an issue with the temp gauge only when the heater is on. The heater is blowing out hot air.

When the heater is on, the temp gauge will climb normally to the middle. When the thermostat opens up, the needle will fall down to the "C" notch of the normal operating temp range. This will happen whenever the heater is on.

With the heater off, the needle stays in the same spot (middle). No fluctuating of the needle.

I have purged the cooling system about 4 to 5 times, with no success. With the engine cold, I rev the engine to about 2.5k rpms, remove the radiator cap, and add when necessary. The symptoms that I'm having leads me to believe that there is air in the system or being sucked in when the heater is on.

The previous owner informed me that he replaced the "heater control valve", which connects to the heater core. I am starting to wonder if that is the culprit. Could someone please post a picture of how the valve should be mounted?

Mine was mounted with the diaphragm pointed down (vacuum nipple on bottom). I went to the junkyard and could only find one explorer and it was mounted the opposite direction (vacuum nipple pointing up).

Thanks in advance.

Welcome to the forum!

Change the thermostat to a motorcraft and change the 1 wire sending unit near the thermostat. I have the same truck, the temp moves around a bit when the heater is on. Before those changes it would move into the C region like yours. Vac nipple should be on top.

 

[coreweave]

Thanks for the replies...I really appreciate it.

The heater control valve is brand new. It does not leak.

I have replaced the thermostat, radiator cap, and even the replaced the coolant return hose. I will get the heater control valve mounted with the nipple pointing up. As for the thermo-sensor on the thermostat housing, does anyone happen to know the acceptable resistance range?

BTW, I am very impressed with this Explorer. I would have never thought about buying one but I got a great deal on it with a blown head gasket ($1k).

 

[410Fortune]

How about you are low on water?
If your system is not completely burped or full the temp sensor can read air temp above the water, and fluctuate.

If you still have the issue the sender itself may be suspect, followed by the OEM gage (pssst stock gages SUCK, if you want to know your actual water temp, get an aftermarket gage)

 

[96eb96]

Thanks for the replies...I really appreciate it.

The heater control valve is brand new. It does not leak.

I have replaced the thermostat, radiator cap, and even the replaced the coolant return hose. I will get the heater control valve mounted with the nipple pointing up. As for the thermo-sensor on the thermostat housing, does anyone happen to know the acceptable resistance range?

BTW, I am very impressed with this Explorer. I would have never thought about buying one but I got a great deal on it with a blown head gasket ($1k).

They are pretty good rigs!

Are you purging air by letting it warm up with the cap on 1/2 way and heater on high?

I don't know the temp range but they are known for going bad on these years. Maybe someone else has a chart.

Also, I suggest you replace the idler pulley and the tensioner pulley even if they look good now. They sieze up randomly, and fail when you don't want them to.

 

[coreweave]

How about you are low on water?
If your system is not completely burped or full the temp sensor can read air temp above the water, and fluctuate.

I believe there is air trapped in the system, but I am at my wits end trying to get it out. At idle with the heater on, the temp gauge reads steady. Once I start moving and put load on the engine, the need slowly climbs up to the middle, and takes a nose dive down to the "C" notch. This is when I can hear the "gurgling" coming from the heater core. This all repeats until I come to a complete stop or shut the heater off.

They are pretty good rigs!

Are you purging air by letting it warm up with the cap on 1/2 way and heater on high?

I don't know the temp range but they are known for going bad on these years. Maybe someone else has a chart.

Also, I suggest you replace the idler pulley and the tensioner pulley even if they look good now. They sieze up randomly, and fail when you don't want them to.

What's the benefit of having the cap on 1/2 way? On all the cars that I've worked on, I've removed the radiator cap entirely. The problem with this Explorer is that with the heater on, the needle barely goes past the "C" notch and nothing more even after I hold the rpms at 3k. I don't think the engine is getting warm enough to open the thermostat at idle because the coolant slowly creeps back up the funnel that I have in the radiator filler hole and eventually overflows out.

I also failed to mention that I have a rough idle problem. This makes it difficult to purge the system unless I hold the rpms steady above 1k. Also, I have been reading about using thermostats without the jiggle pin. I bought mine from AutoZone which has the jiggle pin.

Also, thanks for the heads up on the idler and tensioner pulley. I will look into replacing those after I figure out this problem.

 

[robertoa1a]

He has a 4.0. rember he has the 2 sending units. 1 for the computer and 1 for the guage. Did you get the right one?
deg k ohm
50 58.75
104 16.15
140 7.7
194 2.8
212 2.07

 

[96eb96]

I believe there is air trapped in the system, but I am at my wits end trying to get it out. At idle with the heater on, the temp gauge reads steady. Once I start moving and put load on the engine, the need slowly climbs up to the middle, and takes a nose dive down to the "C" notch. This is when I can hear the "gurgling" coming from the heater core. This all repeats until I come to a complete stop or shut the heater off.



What's the benefit of having the cap on 1/2 way? On all the cars that I've worked on, I've removed the radiator cap entirely. The problem with this Explorer is that with the heater on, the needle barely goes past the "C" notch and nothing more even after I hold the rpms at 3k. I don't think the engine is getting warm enough to open the thermostat at idle because the coolant slowly creeps back up the funnel that I have in the radiator filler hole and eventually overflows out.

I also failed to mention that I have a rough idle problem. This makes it difficult to purge the system unless I hold the rpms steady above 1k. Also, I have been reading about using thermostats without the jiggle pin. I bought mine from AutoZone which has the jiggle pin.

Also, thanks for the heads up on the idler and tensioner pulley. I will look into replacing those after I figure out this problem.

No problem, there are a few other things to look at too, like corrosion on the hard brake lines (they leak) and your internal power steering tubes. If they use salt in your area keep an eye on these.

Just telling you the service manual procedure for bleeding coolant, they say leave the cap 1/2way. Maybe to build some pressure? This engine has an internal bypass. They also say its normal to lose a cup of coolant over 7500 miles, quoting from the owners manual. The heater must be on high so the heater control valve opens.

There is a TSB for a similar issue for the 4.0 engine, dealing with noise, steam formation and gauge irregularity. Another TSB mentions changing the water pump. I never got the gauge to work correctly without changing that one wire sender though.

ISSUE:
A temperature gauge fluctuation and/or a "knocking/thumping" noise may be audible in the passenger compartment on some vehicles. This noise is most pronounced with the engine at normal operating temperature. The noise is caused by steam formation in the cooling system. The noise is then transmitted through the heater hoses and heater core.

ACTION:
Install a Coolant By-Pass Kit. This increases coolant flow which reduces steam formation. Refer to the following Diagnostic and Service Procedure for details.


DIAGNOSTIC AND SERVICE PROCEDURE

Check for excessive exhaust system restrictions. A restricted exhaust could elevate cooling system temperatures.
If dash-mounted cooling temperature gauge fluctuates, check that gauge and sending unit are functioning properly.
Check for restrictions or kinks in the radiator or heater hoses.
Pressure check cooling system for leaks.
Confirm proper Engine Coolant Temperature (ECT) sensor signal to EEC-V processor.
Check for correct 50/50 coolant mix. Verify that freezing point is -37°C (±-15°C) or -35°F (±5°F).
If concern still exists, install service Coolant By-Pass Kit and verify that concern has been resolved. Refer to the Application Chart in this article.

 

[coreweave]

He has a 4.0. rember he has the 2 sending units. 1 for the computer and 1 for the guage. Did you get the right one?
deg k ohm
50 58.75
104 16.15
140 7.7
194 2.8
212 2.07

That's a big help! Thank you!

Are those values the same for both thermo-sensors (sending units)?

 

[coreweave]

There is a TSB for a similar issue for the 4.0 engine, dealing with noise, steam formation and gauge irregularity. Another TSB mentions changing the water pump. I never got the gauge to work correctly without changing that one wire sender though.

ISSUE:
A temperature gauge fluctuation and/or a "knocking/thumping" noise may be audible in the passenger compartment on some vehicles. This noise is most pronounced with the engine at normal operating temperature. The noise is caused by steam formation in the cooling system. The noise is then transmitted through the heater hoses and heater core.

I believe the temp gauge is working properly. When the heater is off, the temp gauge reads steady. With the heater on, it fluctuates which I believe is from the air bubbles passing through the thermostat housing. The gurgling of the heater core is a good indication that there is a large air pocket in the heater core.

I will be trying to jack the front end as high as possible and try to purge the system that way. If that fails, I will try the boiling water down the upper radiator hose trick.

 

[robertoa1a]

I think my old 4.0l had a simular problem for a long time there was a funky un-burned fuel smell near the exhaust. It was running rich and the obd1 system kept telling me to replace the ect sensor with no other codes. Everything checked out good though it drove me nuts for a while. I felt like I was getting close to solving the problem when someone bought the truck from me.

The cel was not iluminated, It had a stinky exhaust, there was nothing wrong with the sensors and my guage fluctuated. I thought it was a fluid Issue too. It didn't get cold enough here to know if the heater worked but I was mostly conserned with it's high fuel consumption.

I'm going to get on the wagon with those who belive it's a coolant problem. The thermoster(ect) has to be wet to work.

Does does the exhaust smell, and Is your mpg lower than it should be?

 

[slimsol]

I think my old 4.0l had a simular problem for a long time there was a funky un-burned fuel smell near the exhaust. It was running rich and the obd1 system kept telling me to replace the ect sensor with no other codes. Everything checked out good though it drove me nuts for a while. I felt like I was getting close to solving the problem when someone bought the truck from me.

The cel was not iluminated, It had a stinky exhaust, there was nothing wrong with the sensors and my guage fluctuated. I thought it was a fluid Issue too. It didn't get cold enough here to know if the heater worked but I was mostly conserned with it's high fuel consumption.

I'm going to get on the wagon with those who belive it's a coolant problem. The thermoster(ect) has to be wet to work.

Does does the exhaust smell, and Is your mpg lower than it should be?

Did you find out what the problem was? I have similar problem. How did you fix it?