Get a trouble light code indicating that the thermostat is stuck in the open mode. Engine temp gauge indicates normal heat range, not running hot or cold. Is there a temp sensor that goes bad or is this a common issue. Truck sits quite a lot and only has 10k miles. Any guidance would be appreciated.

Mike

 

What is the exact Code number?

There isn't a "stuck open t-stat" code.

All fuel injected engines have at least 2 coolant temp sensing devices, 3 if they also have an electric cooling fan.

There is a 1 wire Coolant Temp Sender, this is only used by the dash board temp gauge
Then the 2 wire ECT(Engine Coolant Temp) sensor, this is used only by the computer.
These two devices are usually located near each other, and look similar, so remember the 1 wire or 2 wire to ID which is which.

ECT sensor may be your issue, but without the exact code it would be hard to say.
If computer doesn't read that the engine is warmed up fully(195degF) after 10-15minutes of running, it will turn on the CEL(check engine light) and set a code about Open Loop or engine temp.

 

Not with the SOHC 4.0L.
There can be one or there can be two. Regardless of quantity, they are "two wire" as they sit in the top of the plastic thermostat housing.
Also, depending on the year, they are the same sensor & can be interchanged.

More than likely the 2011 only has one, as the gauge cluster is controlled by the ECU.

 

Good info, thermostat housings do show 2 "holes" for temp sensor and sender, '97-'02 4.0l SOHC, others show 1 "hole", lists 2001-2011 4.0l SOHC

If OP does indeed have 1 ECT sensor used for both computer and dash board gauge then the Code number would be even more important to get since the data coming from the ECT sensor would seem to indicate normal temp.

 

Hi RonD and Tee rev,

Thanks for your replies. I will recheck for the actual code #, however the Autuel code reader did indicate that the "thermostat was stuck in the open mode" when I hit the read codes indicator. I will also check the wires and contacts for corrosion. I will get back to you after it gets a little warmer this week. it was -2 Deg F this am.

Thanks,

mike

 

Gentlemen,
T
Thank you again for your responses. I ran the codes today and rec'd P0128 which indicated that the coolant temp was below the thermostat regulating temp. The upper hose was full indicating to me that the thermostat had opened and the gauge indicated normal while I had the engine running. I cleared all codes and did a Key On Engine On diagnostic run and rec'd several odd codes relating to ABS and ECM, also the diagnostics indicated that they could not be completed. Further code reading indicated that the battery was low. We have had extremely severe cold spells during the last few months and i think this is related to my issues. I did check the battery and was able to remove the sealed cell tops and behold all were very low. I have topped them up with distilled water and currently charging the battery. I think the combination of events caused most of these issues. Once i get the battery up and see if it holds I will let you know if anything returns.

Thank you again for your help,

Mike

 

P0128 means that the ECT sensor is not reporting at least 190degF(thermostat temp rating) with in a certain amount of time, usually 10-15min. after start up.
There are several reasons for that code

Upper hose should always have coolant in it, it just shouldn't be warm when engine is cold.
Next time you have the time start up engine cold and feel the upper hose, will be cold of course.
Let engine idle for 2 or 3 minutes and feel it again, should be just as cold, if it is warming up with the engine then thermostat is indeed stuck open.

If hose is still cold then shut off the engine and manually spin the fan, fan clutch should be loose and easy to spin, not tight and hard to spin.
If it is tight then fan clutch is not releasing and fan is cooling the motor too much.
4.0l is not a warm engine, in very cold weather they hardly get above the 190degF thermostat rating, so a fan blowing cold air over them could cause you issue.
A fan clutch works by radiator heat not engine heat, as radiator warms up fan clutch will start to engage and cause fan to spin closer to water pump RPMs, if rad is not warm then fan is not needed so it spins free.

 

Excellent info RonD, thanks. I will follow your advice check temp of hose during start up along with fan.

Mike

 

Just a follow up on all the excellent advice. I was able to get the battery to charge and checked the upper hose during warm up. No more codes and temp gauge is working with no issue. I believe it may have been several factors with the extreme cold temps and a discharged battery. My thanks to all.

Mike

 

Thanks for the update