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  #1  
Old 11-26-2012, 08:02 PM
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Default Fluctuating Temp Gauge Issue...

Apologies if this is covered in an existing thread. I found this issue in a few threads, but I've tried the solutions listed with no luck... The symptom is a fluctuating reading on the temp gauge of my 1999 Ranger 4.0 liter. It gets to the middle (normal) and then dips down almost to the cold and works its way back to the middle over and over again. This cycle runs about every 2 minutes or so and I recently had a couple codes come up (PO171 and PO174) which indicate lean conditions. Truck seems to run just fine but not sure how to fix the gauge. I’ve replaced the thermostat (180 degree) and also replaced both the Temp Sensor and Temp Sending unit in the hopes that it would resolve the problem. I cleared the codes and so far they haven't recurred but the gauge is still oscillating… The coolant is clean (changed this year) and as far as I can tell there is no air pocket in the coolant. I have NOT checked the gauge but wonder if a failed gauge would fail in this way. Once the truck warms up I have heat from the heater, etc and it doesn't seem to lose any heat that I can feel. I'm the original owner and prior to this, the truck would only fluctuate once when the thermostat opened and the "cool" coolant from the radiator hit the sensor after which the temp would stabilize at the half-way mark. That being said, has anyone had this problem and what was the fix? Thanks for any info/help!
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  #2  
Old 11-27-2012, 06:48 AM
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Default Re: Fluctuating Temp Gauge Issue...

I have a mechanical gauge installed on the pillar. The factory gauge will read "normal" anywhere from 140* to 210* according to the mechanical.

This I think would rule out a possible overcooling issue, if the clutch fan was seized up. I would guess its the actual temp gauge in the cluster if you have changed all sensors.

One thing I noticed with my mechanical gauge and my flex fan is that the flex fan cools faster than the truck heats up. So what happens is the thermostat closes, the truck gets hot, about 195* and then T-stat opens and it cools right back down to about 130-140. Its an instant drop on the gauge.

Im assuming that the stock fan wont do this, as I had the mechanical gauge prior to my flex fan and it never did that.

I guess what Im getting at is, even if you fan clutch was locked up (allowing no slip) at most it would cool as good as a flex fan, but probably not. The flex fan cools the engine down to 140* and the stock gauge stays at normal. So your gauge reading indicates the engine is dropping to say, 90* or so. Which eleminates a fan issue. Must be the gauge.
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  #3  
Old 12-11-2012, 06:54 PM
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Default Re: Fluctuating Temp Gauge Issue...

Thanks for the help. I did some more troubleshooting and with regard to the fluctuating temp, I should have specified that the thermostat I installed wasn't Motorcraft... :-( Replaced it with the EOM motorcraft thermostat and now the temp looks normal again. (After full warmup it stays in the middle). It still seems to be triggering the PO171/174 codes tho'. Lean condition on banks 1 and 2. I'm going to see what I find here for that. Thanks again -I always wondered what the high and low readings on the temp gauge corresponded to in degrees F.
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Old 12-11-2012, 07:01 PM
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Default Re: Fluctuating Temp Gauge Issue...

Trust me, your about the 5,000th person on here to have a brand new non-OEM thermostat fail out of the box. They SUCK. It makes troubleshooting a real pain. Also, the "Normal" scale is deceiving. The bottom is about 150 and the top is about 250. But that's a bunch of bullshit because actual normal operating temperature range is 190 to 220. If your less than 190, it won't run right. And if you're climbing over 220, you've got a cooling problem. 250, which is the upper end of the "normal" range, is nearing critical and very very bad. A 50/50 mix at normal operating pressure will start venting at 260ish and start boiling at 270.

Those codes are almost always a vacuum leak.
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Last edited by FireRanger; 12-11-2012 at 07:07 PM.
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  #5  
Old 12-29-2012, 04:39 PM
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Default Re: Fluctuating Temp Gauge Issue...

Thanks for the info on the codes. Do you by chance know what the intake manifold vacuum pressure should be at idle for this OHV 4.0 engine? I'm reading 16 in Hg Vac (400 mm Hg Vac) at idle now. I'm almost certain there is a vacuum leak and it's the intake manifold but not sure what the normal vacuum pressure should be. Thanks again,

-Sam
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