I have some issues with this car, 84 XJS HE v12. It starts but it idles rough. But after some time, 15 mins. of starting and turning off, it catches itself and idles and drives normal. Crazy.
Now the OXY Sensor light came on so I dont know where it is on a V12 and I am wondering if its a easy job for me to do.
Also, my gauges are messing up on the speedometer and fuel, they are both off a little bit. Help!

 

Not sure how it goes, but I can tell you a story about my '92.

Bought the car, and it had the check engine light on and a FF code (forget which but it was for the right bank). Read up on it, bad o2 sensor right side. Was going to order on and replace it, but figured I'd look at it first.
On the exhaust on the right side, if you turn the wheel to the left, you should see the sensor screwed into the exhaust pipe. Mine had a new one, so at least I knew I was on the right track.
Opened the hood to check the plug, followed the wires up and it wasn't plugged in! Looked some more and found the old sensor laying in the engine compartment still plugged in to the harness! Unplug the old one and threw it away and plugged in the half installed new one and everything is AOK. Apparently they unscrewed the old one, stuffed it up the hole, installed the new one and stuffed the wires up there, intending to finish it later (I'm guessing the car was on a rack) and forgot about it.

Check the exhaust pipes for the sensors underneath, you should see them screwed into the exhaust system.

howlinowl

 

The O2 sensor light comes on at a predetermined mileage. it does not indicate a failure. Cars of your vintage, not just Jaguar, had a service interval for the O2 sensor. I want to say but I could be wrong that non heated cars the interval was 30,000 miles and heated ones were 60,000 miles. Many cars had a light to warn the driver it was time to replace the O2 sensor. Your car has the non-heated sensors unless someone has added a circuit to allow the later heated style to be used. This is a highly reccomended upgrade.

When you say 15 mins of starting and turning off. Can you be more specific? Is the engine stalling from a low idle on a cold start. or is this after shutting off hot? If you try to increase the RPMs does it backfire or does it rev up smoothly?

You very well could need O2 sensors but most likely it will not fix your symptoms. You may simply have a stuck AAV valve.

 

Greg, thanks for the tip(s)! On the O2 I will research. But your latter question on the idle. Yes, engine stalls on cold start, but after a few tries it idles at about 16 rpms, but, tap the gas peddle after a few mins. it stalls or bounces on the tac meter from 0 to 9 rpms and then stalls. ON the other question, it DOES backfire when rev up, then stalls, what with that?
LOVE V12 JAGS DONT WE?

 

These older fuel injections systems have very little ECU controlled adjustment. Pretty much everything has to be spot on. Let her sit overnight and measure the resistance of the coolant temp sensor at the ECU and at the sensor. Make sure the readings are the same. Then compare the readings to the chart in the manual. You will need to know ambient temp. Then if the resistance matches the temp. you know the coolant temp sensor is good. Then next is to check and adjust the TPS voltage. That is covered in Kirby's book in detail. Do a key on engine off sweep of the TPS(slowly open throttle) it should smooth go from about .32 volts up to 5 volts. if there is any jumping of voltage there is a bad spot and will need to be replaced. Have you set the throttle plates? If not time to do so. Then adjust the linkage. Again covered in Kirby's book and service manual. Once you have verified these very basic tests start the car and warm up. Then check and adjust timing per service manual. If after these things are checked and corrected then you need to feel the tailpipes to make sure the exhaust pulse is smooth on both sides. If not you have a misfire that needs correcting. Report back after you have done these basic tests and adjustments.

If you approach this with logic and a plan you will find your problem. Many times it is just a few things that are not right that add up. As you will see on these forums there are many who want the simple answer and do want want to perform all the tests and checks needed. Don't fall into that trap or you will just be wasting time and probably money.