Hi,

I have a 2004 XC90 2.5T.
The temperature gauge stopped working, and I began seeing codes p0116, p0118 and p0128.
I replaced the ECT, but the problem persists.
The temperature gauge will sometime kick in and work fine, then it will suddenly turn off.
The car is easy to start from cold, but if I stop for gas, it is often very hard to start, i have to crank for 5-10 seconds, then it runs rough, and throws a lot of extra gas.
I hooked up my scanner and watched live engine temperature data -- 2 things confuse me:
1 - There are 2 readouts for engine temperature - is there a 2nd sensor?
2 - The readout from the sensor I replaced is erratic. It will start at -40, even if the engine is full hot (running 20 minutes on a 90 degree day), then will jump to 3 or 4 degrees, then drop back to -40. I would suspect the sensor, but I have replaced it twice, and I get the same response. Also the same with the original sensor.
If I start from cold, both temperature displays in the scanner smoothly come up to an expected range (165 degrees), and the car runs normally, no codes.
If I stop the engine and immediately re-start, both displays start at -40, and do not come up to normal temperature -- 1st display is erratic, 2nd display stays around 20-30 degrees.

What is going on? should I replace the thermostat?

I have statically tested the ECT with my Fluke - it has about 1500 ohms when cold, and had about 267 ohms when hot (165 degrees). It does not fluctuate on my Fluke.
I also tested voltage to the ECT - it is steady at 5.04VDC.

Thanks for any help you can offer!

Judas

 

Since no-one else is jumping in, I'll answer my own question. Maybe it will be of use to someone in future (probably not.)

I replaced the thermostat, housing and ECT (again). No change in behavior.

I updated my scanner so it can read Volvo codes - I was then getting code 2000 - ECT out of range - low. Occasionally the temperature gauge would start to read, but most of the time it was off.
I tried all 3 ECTs (2 new, 1 original), and was getting the same code 2000 with all.

I started tracing the wiring harness back from the ECT, to see how hard it was going to be to find a broken or shorted wire... The harness runsa acroos the injector rail, down the right side of the engine, across the bottom front of the engine, then up the left side to the computer modules. As I traced the path, I wiggled the harness everywhere i could easily reach it.

After wiggling the harness, I turned the car back on -- temperature gauge is now working correctly, code 2000 is gone, car starts easily when hot. I ran it for about 45 minutes, with no issues.

I know I haven't solved the problem fully yet, but it is working well enough that it should be able to pass inspection. I will trace the wires another day.