99 temperature gauge ECT sensor issue

[gba88]

Hi Guys,
I have a 99 4.0 153k on it. Installed new radiator 2 years ago. About a week ago I was waiting to pull onto the highway and temp gauge started bouncing around and heard a little hesitation and check engine light came on then off. Drove it for about a week then it happened again at a red light but this time the engine light stayed on with a P0117 code (low voltage to coolant circuit). I did some quick research and was familiar with replacing the temperature sensor to solve a similar issue on another 99. I replaced the sensor and everything was ok for about 3 days. Yesterday at a stop light the gauge started fluctuating again but not as wildly.... I turned on the heater to see if it had any effect but it didn't seem to. I turned onto the highway and it was fine the rest of the way home (no check engine light).

I had read about another guy with the same issue who found that a wire from the sensor in the harness was shorting out on the rear of the engine. I had not disconnected the battery when I replaced the sensor so I decided I would disconnect the battery, clean the terminals and test everything. Everything was good with the battery and alternator...left it disconnected for maybe 15 minutes. I saw that the main harness that goes to the PCM had a spot where it had rubbed through, but the affected wires did not appear to be the ones coming from the ECT. I zip tied it up off the contact point though.

Drove to my dad's and home last night (only about 7 min each way) and work this morning with no issues so far. I'm not confident since the only changes were 1. disconnected the battery and 2. lifting the harness up off of the contact point it was laying on. If this continues, any idea what I should be looking at next? Coolant was changed in 19 when radiator was replaced, reserve tank stays full, no obvious leaks anywhere and good heat with heater on.

 

[RCP Phx]

I think you answered your own question! You've got some wire failure probably due to heat around the rear of the head. Only thorough inspection and testing will find the damage.

 

[gba88]

I think you answered your own question! You've got some wire failure probably due to heat around the rear of the head. Only thorough inspection and testing will find the damage.

Thanks for the reply! I guess I was wondering if it could also be the thermostat or waterpump that would cause this fluctuation? Since heater works well didn't think thermostat but really didn't know. The wires from the sensor seem to go right back through the fuel rail then across the firewall and into the PCM up front by the airbox. I was also wondering if not having disconnected the battery to "reset the computer" could have caused it. The outer covering on the wires is really brittle and breaking apart in areas too so i guess it may take a thorough inspection as you said.

 

[Tim_MN]

If the Check Engine Light was ON, or is ON, your Jeep is trying to tell you what is wrong.

 

[gba88]

If the Check Engine Light was ON, or is ON, your Jeep is trying to tell you what is wrong.

Yes, it was on with the P0117 code. Most info online I found pointed to the ECT sensor. As I already said I replaced this sensor on a different 99 in the past and that was the issue. I replaced the sensor and reset the code on my current 99. It is NOT on now, but the behavior appeared to be coming back after having replaced the sensor.

I also found info pointing to the coolant condition as a possibility for the code as well as the water pump or the thermostat. My point of the post was to see if anyone else had the same situation that was fixed by something OTHER than replacing the ECT sensor or repairing a short/faulty wiring.

 

[Tim_MN]

Wiring issues or a cheap crappy Chinese sensor are the primary suspects. Coincidentally they are also the easiest and the cheapest fixes.


P0117 JEEP Engine Coolant Temperature Sensor Circuit Low

Possible causes:

• Engine Coolant Temperature (ECT) sensor harness is open circuit or short circuit
• Engine Coolant Temperature (ECT) sensor circuit poor electrical connection
• Faulty Engine Coolant Temperature (ECT) sensor
• Faulty engine coolant thermostat
• Low engine coolant level

 

[gba88]

Wiring issues or a cheap crappy Chinese sensor are the primary suspects. Coincidentally they are also the easiest and the cheapest fixes.


P0117 JEEP Engine Coolant Temperature Sensor Circuit Low

Possible causes:

• Engine Coolant Temperature (ECT) sensor harness is open circuit or short circuit
• Engine Coolant Temperature (ECT) sensor circuit poor electrical connection
• Faulty Engine Coolant Temperature (ECT) sensor
• Faulty engine coolant thermostat
• Low engine coolant level

Nice... Ok, so the sensor I bought was from oreilly, so it probably was a cheap chinese one. Here it is:

https://www.oreillyauto.com/detail/...ive-suv-1999-jeep-cherokee?q=ect+sensor&pos=0

do you have a suggestion or a link to a non-cheap chinese one? I have no problem spending more or buying OEM but it was sunday afternoon and needed a local option to get the thing ready to go to work monday morning. Coolant is definitely not low so the last item I see is the thermostat. I wonder if it could be sticking or just starting to fail and causing the issue. In any case, I DID think that possibly the "new" sensor was junk out of the box. Thanks for the reply!