My 2002 xkr idles poorly within a few minutes after start-up. The DTC's are P0300, P1316, PO1649. My scan tool indicates the car is running in OL mode. Changing the coolant temp sensor didn't help. However, when I switch the coolant temp sensor connector from the coolant temp sensor mounted on the engine to a another loose coolant temp sensor not mounted to the engine, the problems go away. The engine idles steadily at 900 rpm. There are no DTC codes, the engine runs in CL mode, and the fuel trim is in the +5% to +7% range, although the engine speed is limited to 3,000 rpm. I have checked for vacuum leaks, the spark plugs are new, and I cleaned the MAP sensor and the part time breather. I have also disconnected the battery to reset the computer. Is there something wrong with the engine ECM which causes it to overadjust fuel or ignition management using the signal from the warm coolant temp sensor?
Thanks

 

Reset your codes and see which ones return.

We should see if P1649 actually comes back. This could be a serious ECM or network wiring fault if it is bona-fide. Sometomes weird codes could be set during previous service, so this should be verified before you start replacing expensive parts or doing complex debugging.

900 RPM idle is too high. By changing the coolant sensor to one out of the engine you are probably just compensating for some other, yet to be determined problem.

 

Thanks WhiteXKR. Reset codes. Ran again. All reappeared. P1649 popped up first.

 

What is the history of this car? Do you know of any service to the electronic systems previously or the wiring?

Have a look first at the OBDII connector with a bright light and a magnifier and see if you see any pins displaced and possibly shorting to another pin or to the chassis, or if there are any abraded wires in the area.

 

If you know the fuel trims, then you must have a real time scanner. Have a look at all the sensors to be sure there are no skewed values. Sounds to me like a lazy O2 sensor. Since the car has no Manifold Absolute Pressure sensor, I assume you mean the MAF. Look at the running values for that as well since it sets the mixture and is a major player in rough running.

 

I'll try your suggestions. However, I noticed the fuel trim indicates a lean condition when the engine warms. Both cylinder banks start out in the +5% range (with the coolant sensor properly connected) when cold, smooth idle, and then increases to 12% to 15% both banks when warm and rough idle.