Blakejk8

Hello everyone, I'm new to the forum and could really use some help on an issue with my Jeep that continues to baffle me. I have a 2008 Jeep Wrangler X Unlimited Automatic. For months I have been trying to resolve a P2181 (Cooling system performance code). So far I have:
Replaced thermostat
Replaced ECT sensor "Tried Aftermarket and Mopar"
Replaced radiator cap
Replaced cooling fan "Was only running in high speed due to a failed resistor/voltage divider."
Checked continuity from ECT sensor plug to corresponding pins on PCM Harness which checked out.
Installed Aftermarket temperature gauge to monitor coolant temperatures.
Live data shows the ECT reading at 165-180 after jeep has warmed up. Aftermarket temp gauge is usually 20-40 degrees higher at 200-220. Ambient air and intake air values seem to have normal readings.
At this point I dont know what else to try other then replacing PCM or TIPM. Any su

ggestions or a point in the right direction would be greatly appreciated. Also check engine light and lighting bolt pop up during a cold start in the morning with the P2181 code. After clearing codes the jeep is fine until the next morning or next cold start then code reappears immediately.

Mr.T

You've done a lot already, obviously it's not the usual suspects, at least initially.

Here's what the manual has for potential causes:
LOW COOLANT LEVEL
(K2) ECT SIGNAL CIRCUIT SHORTED TO BATTERY VOLTAGE
(K2) ECT SIGNAL CIRCUIT OPEN
(K900) SENSOR GROUND CIRCUIT OPEN
(K2) ECT SIGNAL CIRCUIT SHORTED TO GROUND
(K2) ECT SIGNAL CIRCUIT SHORTED TO THE (K900) SENSOR GROUND CIRCUIT
THERMOSTAT
ECT SENSOR
PCM

Here's some thoughts... Does the live data ever fluctuate strangely? Does the live data read an accurate and steady ambient temperature with a cold engine? If I remember correctly the sensor has a negative temp coefficient (temp goes up and resistance goes down), so a high resistance connection at the sensor reads low -- which is your symptom.

I had a similar intermittent problem of ECT reading low, not running the fan, and nearly overheating while towing. The problem was the ECT sensor itself. As it got hot enough to run the fan on high it would wander low erroneously and shut off the fan. While diagnosing, I caught the temperature changing unrealisticly. FWIW the bad sensor was all plastic and I replaced with a mostly brass unit from NAPA. I read you already changed the ECT sensor twice, but it may be something intermittent with the connector or wiring.

The PCM is a possibility, but an expensive one. I tend to think it's sensor, connector, wiring related. The TIPM is unrelated for this problem.

One more thougtt: If you had the ohm value per temperature chart, you could read the sensors ohms and see how close the live data reads. It's probably a standard ntc thermistor -- might be worth searching.

Stay in touch, hope you figure it out quickly!

Blakejk8

Thanks for the reply! I have indeed performed an ohm test on the ECT sensor to make sure it was with-in spec to a chart I had found online, everything checked out there. For the live data I have noticed erratic behavior for the ECT value. It will jump around every couple seconds. Example: engine warmed up it shows about 175 but will randomly jump to 200 or 185 etc very rapidly as you are viewing the live values. I don't know if this is normal on the Jeep in regards to live data but I thought it was strange how it jumps so rapidly and then returns. The ambient air values seem to be normal and nothing erratic from what I've seen so far. I just received my Bluetooth obd adapter today to link to my headunit so I can watch the values closer while driving. Last night I had cleared my dtcs and then checked my codes this morning before cranking the jeep and there the code sat P2181. The PCM is generating the code while the Jeep is off? I made sure codes were completely cleared and nothing pending with my flashpaq programmer. I believe you are right that it's possibly in the wiring. I need to double back and do a more extensive diagnosis from the ECT sensor plug to the corresponding pins on PCM. I do know it has continuity and that it's not shorted to ground on the harness plug but that's as far as I have got. Just to note, the PCM was replaced in 2020 with a remanufactured unit. I have had the TPMS light come on randomly even though tire pressures are good which is why I started thinking PCM is going out. After replacing the cooling fan the issue was gone for about 3 days after the install and I thought I was in the clear but here I am 😑. Coolant level is spot on. I've even went as far as to adjust my ac pressures to the recommended psi to make sure the pressures were not causing the fan to run out of spec thus causing my P2181. My Aftermarket temp sensor is installed midway of the upper radiator hose, do you think the temp is showing colder on the live data because the ECT sensor is placed after the thermostat maybe? THANKS AGAIN for the info and I look forward to your reply!

Mr.T

Interesting symptoms. Your KEY OBSERVATION is the temperature randomly moving rapidly on the OEM sensor. That can't happen for real, it normally moves slowly up and down. It's an intermittent sensor, connection, wiring, or worst case within the PCM itself.

Hate to say try another ECT sensor, but what you're seeing is nearly identical to the bad one in mine, and they are cheap + easy. The connector at the ECT sounds like the next most likely suspect.

More thoughts on your questions: If I understand correctly, the aftermarket ECT sensor is in the upper rad hose (coolant leaving engine), which gets no flow until the thermostat opens. The OEM ECT sensor is in the stock location which is upstream of the thermostat, and reads ECT regardless of the thermostat opening. In this case, they'll both read the same with the thermostat open enough to get significant flow, but on cold startup the aftermarket should tend to read near ambient until the thermostat opens -- then it will increase very rapidly when hot coolant reaches the sensor in the upper rad hose.

It won't set this code key-off, but all it takes is a momentary quick and implausible ECT reading with the key-on to set the code. It could even happen right after clearing and checking the codes but before key-off.

On a side note, OBD2 scanners scan much slower than the PCM scans sensors. So the PCM may detect a momentary implausible reading and set the code, but not show the bad reading on live data.

Mr.T

A few thoughts on the random TPMS light: If it transitions from out to blinking for about 90 seconds, then to solid on, it is malfunction detected. Usually it is missing communication with a TPMS sensor (C151C DTC in the TIPM, but it takes an enhanced scan tool to view these).

If it's low tire pressure the light immediately goes solid on. Blinking for about 90 seconds followed by solid on means a fault. This is buried in the owners manual and isn't very intuitive.

Most owners, myself included, get the fault blinking indication every so often. Sometimes it's RF interference from something like a charger or some electronic device like a programmer or scanner. Sometimes it seems to be from brake dust all over the wheels and valve stems, and cleaning appeared to resolve it. It could be from anything that interferes with the radio signal.

Blakejk8

I'm going to take your advice and pick up another sensor this evening from autozone, $75 at stealership for mopar! I see that the TPMS may be unrelated from what you're saying so I'm going to exclude that from my possible PCM verdict. I just drove it 50 miles today with no code since clearing this morning. It has to be something related to a cold engine that's triggering the code. I watched the live data again and the ECT is still flashing another value at me rather then gradually increasing or decreasing. The value it flashes seems like it would be the correct operating temperature but it's the lower value that shows the most. I will get the new ECT sensor installed in the A.M and keep my fingers 🤞. I'll keep you posted!

Blakejk8

Got the new ECT sensor installed and no luck 😪. Still flashing a higher value on live data rather then a slow steady increase or decrease in temperature. Getting steady voltage at ECT connector also. Do you think it's time to start looking into a new PCM or do you think it's likely it could still be in the wire harness somewhere? Thanks!

Mr.T

Note how pins 1 & 2 from the ECT sensor go through pins 18 and 4 of Connector 102 (C102). Splice 131 (S131) is buried somewhere in between, so first I'd try opening and re-seating C102 -- Which is a 20 pin connector right above the #3 plug wire leaving the ignition coil. Hopefully the schematic helps... The wire colors are abbreviated (for example, VT/OR is violet + orange stripe).

Blakejk8

Thank you for that! I will put that schematic to work in the morning. Hopefully just reseating the connector like you said will do the trick. If not I will further diagnose from that schematic. I feel like you've got me on the right track and I'm closer to solving this puzzle with your help. I'll keep you posted!

Blakejk8

I haven't been able to check out the connector yet but since installing the new ECT sensor I haven't had a P2181 code. Usually it will appear during a cold start in the morning and so far so good! Also, watching the live data now it is moving 1 digit at a time with no erratic behavior like previously mentioned. It is incredible to think I went through 3 ECT sensors before 1 worked. The new one I installed came from Advance but I can't believe the Mopar part didn't suffice. I'll post back if anything changes. Thank you so much for sharing your knowledge and time with me to get this issue sorted out Mr.T it's greatly appreciated!! 👍