Jaguar Repair: Rough Idle, airflow meter, tail pipes

Question
Mr. Charles,

I have a 1985 xj6 which has a rough idle. From a could start it seems to run
fairly smooth. When the car warms up is seems to idle rougher.  I have
checked the timing and it is set at 17degrees. I have changed the cap and
rotor. I sprayed carb cleaner around the intake while the engine was running
and saw no difference in the idle. I did notice however that the tail pipes are
black and suety.  Could it be running to rich?

Kim

Answer
Hi Kim,
A rough idle can be caused by many things and mixture is just one of them. Not counting that the series III XJ-6 is noted for having a rough idle.

There are a few things you can test. Look at the 0-2 sensor to see if it is a one wire or three wire connector. Take a volt meter and put it on a low volt scale. Warm the engine up to operating temp and If it is a one wire sensor unplug the wire and with the volt meter on the sensor wire and the engine at idle, read the voltage that the sensor is producing. .5v is ideal and above .7v is rich and below .3 is too lean.
If you have the three wire system unplug the sensor and test the blue wire from the sensor for the same readings. The isle mixture adjustment is on the airflow meter.

Also note your mileage. If you have about 65 to 75 K miles and the head has not ever been off you may be due for a intake valve carbon removal. The "L" Jetronic injection system is noted for causing the intake valves to coke-up. If this is suspected you can remove several injectors and the plastic seats and take a tinny probe light and look at the intake valves to see if they are getting thick carbon covered on their under side.

Howard