2000 XJR
Car starts but runs too rich and air temp sensor reads -40 degrees. We have replaced air temp sensor, replaced wiring for the air temp sensor, had ECU rebuilt, replaced both fuel pumps, and verified fuel pressure. The ECU still thinks it is -40 degrees outside and is increasing the injector pulse which is dumping too much fuel into the engine. I am thinking the next step is to replace the ECU because I have read on the forum that rebuilds are not 100% foolproof.

Thoughts? Opinions?

 

Which air temp sensor did you replace, assuming you're addressing the IAT (intake air temp)? There are two; one in the MAFS and one on the rear of the bank 1, right side, charge air cooler. What's the fuel pressure reading? Any codes showing?

 

I would do more diagnostics before thinking the ECU is bad.
- I assume AJ27 ECU for 2000 XJR

per hispeed42, there are multiple temperature sensors, including:
- the Intake Air Temperature sensor (IAT) which is integrated into the Mass Airflow Sensor
- the Intake Air Temperature 2, which measures the temperature post supercharger
- the Ambient Air Temperature sensor, which connects to the air conditioning control module

If you disconnect the MAFS connector, what resistance do you measure at the MAFS (Black-Green wire and Orange wire, pins 4 and 5)
- I would guess something in the 600 ohm range



Do you get the same measurement when measuring at the ECU connectors, if you disconnect them to make the measurement (and reconnect the MAFS)?
- EM82-17 and EM83-13
- which would check that the wiring is good up to the ECU connector



If you are able to splice into the orange wire while everything is connected, do you measure a voltage (referenced to a suitable ground)



The resistance from the IAT sensor (EM-82-17) goes to an analog to digital converter pin (AN53) on CPU1, there is not much to go wrong with that as far as the ECU is concerned (unless everything is dead, and if the car starts, most things must be ok)
- The voltage is measured based on 5V voltage fed via 2k7 resistor, which feeds the IAT thermistor which is grounded on the other side, then via spike protection circuit into the CPU A/D converter.

 

Easy test to short/open the circuit and watch for changes, verify by backprobing at the PCM. Much more likely you have issues there than a fault in the PCM, if it's been repaired.

 

Change the Coolant Temperature Sensor.

 

+2: My default for most issues in starting and smooth running -- at change of seasons -- the coolant temperature sensor.