Temp Gauge goes down when Heater is on - Ranger-Forums - The Ultimate Ford Ranger Resource


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Old 01-12-2016
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Temp Gauge goes down when Heater is on

All,

I'm new to these forums, and I would like to ask a question about my 97 Ranger 4.0 OHV. As far as I can tell all looks good with the Coolant system. The truck does not overheat at all. The Temp gauge never goes pass the half mark. It pretty much stays constant below the mid-point mark until I use the heater. When the heater is turned on and while driving the Temp gauge goes down to just above the C and then it goes back up to the mid-point mark. It does this time and time again while the heater is on.! Can someone tell me if this is normal for this truck or is there something wrong with the Coolant system that I should look into. Thermostat, Rad Cap, upper/lower Hoses are new and radiator has been flushed. Also the heater works great. Nice and toasty in the cab.

Thanks
Ben_97
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Old 01-12-2016
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sounds like you still have air trapped in the cooling system

find a place to park with the nose up , remove radiator cap , start engine and let it run to bleed out any remaining air bubbles
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Old 01-12-2016
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Thanks cheese_man for the reply. I'll definitely try that and will reply back with the outcome..
Thanks again.!


Quote:
Originally Posted by cheese_man View Post
sounds like you still have air trapped in the cooling system

find a place to park with the nose up , remove radiator cap , start engine and let it run to bleed out any remaining air bubbles
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Old 01-12-2016
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On a 1997 do you have a heat control valve on the heater hoses?
My '94 doesn't

If so this is probably failing, it will have 4 hoses connected, the 2 hoses coming from the engine are the by-pass for the water pump, so they need to be connected ALL the time.
So the coolant flow can either go from one hose to the other, "by-passing" the heater core, or the coolant can flow thru the heater core maintaining the flow from one hose to the other.

If this flow thru these two hoses is erratic then temp gauge will randomly go up and down, never overheating but needle/temp will go up and down, because flow thru the hoses isn't consistent.

Because it only does it when heat is selected it could also be the heater core is starting to get plugged up.
When checking this by-pass valve, Back Flush the heater core.

Next time you are driving and engine is warmed up have the heater on, engine running and open the hood.
Feel each heater hose at the firewall, one will be warmer than the other, the warmer on is the IN line, mark it or remember it.

Back flushing means you reverse the flow on the heater core to flush out any obstruction the way the came in.
So you put garden hose on the OUT line and let water flow Backwards thru the core and out the IN line, hopefully flushing out any blockages.
You will be able to tell if flow in core is restricted by doing this.
If it is you can try cleaning it with CLR and warm water mix, then wait 20min and flush again, repeat as you see fit.

Last edited by RonD; 01-13-2016 at 09:17 AM.
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Old 01-12-2016
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I think this could also be caused by a slow reacting thermostat. The heater core works like an additional radiator when used to heat the cab. The cooling capacity of the cooling system is increased and the thermostat has to work harder to keep the coolant at the normal operating temperature. If the thermostat is slow to respond the result could be this fluctuation in coolant temperature.

Does the needle change quickly from the up or down position? Or does the transition take place at a slower rate?
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Old 01-12-2016
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both very good points

i had that type of problem years ago, could not get any excess air to bleed out

turns out that the little air bleed nipple on the thermostat was blocked by calcium build up
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Old 01-13-2016
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Thanks guys for your replies and suggestions. I will definitely try them out this weekend. Will let you know what happens.

Again, Thanks for your input it is much appreciated.
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Old 01-19-2016
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FYI,

I worked on Burping of the system this past weekend with no positive results. The Temp still goes down when the heater is turned on.

Procedure done this weekend was Cheese_man's suggestion:
1. Raised front end of truck.
2. Ran it with cap off. The system did throw out some air bobbles. So there was air in the system.

Next step is to try RonD's suggestions. Even dough I did flush the Heater core system before it started getting cold up here in NY. Before the Core flush I was not getting any heat at all. After the flush back then, the cab is nice and toasty. I had tried different Thermostats, I had replaced the heat control valve when I did the core flush as well.

But at this moment I'll try anything again. I am hearing the thumping noise, at times, when the heater is on.....
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Old 01-19-2016
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did you pour CLR into the heater core and let it sit for several hours

your year of ranger is when the big 3 were still using the green prestone coolant loaded with calcium

i suspect that the temperature sender is coated with caked calcium buildup

and will only send temperature data when the cooling system pressure is around 12 psi or higher

replace the temperature sender

i personally would take the ranger to a specialized radiator shop and have them power flush out the heater core ( basically very hot water under pressure from compressed air )
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Old 01-20-2016
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No I did not leave the CLR for several hours though. I wasn't sure if it will do more harm then good leaving it in the Core for a long time. At most If I remember, was for about half an hour.
The Sensor scenario makes some sense. I could just try replacing it. The truck never does get Hot. It never passes the mid-point, so I think it's just something as simple as cleaning the Core better and replacing the sensor as you suggested.
Will try it.!
Thanks

Last edited by Ben_97; 01-20-2016 at 10:39 AM.
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Old 01-20-2016
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the engine probably reaches normal operating temperature

but i suspect that the temperature sender is so encrusted with calcium deposits , i only sends signals when the coolant temperature reaches a certain pressure

i had to replace a sender on my old jeep straight 6 engine , there was so much calcium crud on the internal part , i was surprised it was even working
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Old 01-30-2016
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Hey Cheese_man,

Looks like it was the sending unit sensor. I finally had a chance to replace it this weekend and so far so good. Temperature is maintaining steady right below the mid point with heater on and driving around. So I guess that's what it was after all.

Again, thanks to you and to the others for the help and support with this matter.
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Old 01-30-2016
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you are welcome
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