Hi guys; I have a 1999 XJR. Recently had my import mechanic install forged aluminum pistons and am in the process of tuning my engine for better idle. It idles somewhat rough as is.

car has about 86,000 miles and have ordered 2 new bosch sensors . It dongs on me the car has 4 correct??

if so a guy would replace all 4 if under suspicion of worn out sensors correct??.(and not just 2 ?)

Are all 4 the same type if 4 need changing??

these are located right around the catalytic converters correct? I have the magnaflows cat backs and x pipe on order as well...

Again hi to all; any help/info will be greatly appreciated...thanks in advance. This forum is of great help.

 

My understanding is the fuel mix is controlled by the primaries and the secondaries are there simply to warn if the catalytic converters are working correctly for emissions purposes. I had a P1647 code just a month ago and after reading all the posts on here, I changed both primaries just to be thorough - code resolved. I would not suspect bad sensors for a rough idle tho. I usually start sniffing around for vacuum leaks first. Since the engine has been apart recently, there's been lots of chance to introduce vacuum leaks, not just hoses, but gasket mating surfaces not sealing up properly. As a home mechanic, that's what I usually suspect and find is human error - indeed my own.

 

No doubt it could be a vacum leak of sorts. My mechanic is tired of working on this project and does not want to spend much more time searching pehaps...long story....but I aim on finding the remedy so new o2 sensors go in as the car has over 85,000 miles...

thanks for responses truly appreciated....

1. Which are the primariy o2 sensors??
2. Is there a known methodology for finding vacum leaks? as in help!!! lol please explain in kindergarden terms if possible...thanks...I tinker enough but not a professional mechanic...

btw:my mechanic told me the miss at idle occurs while the larger bottom pulley was put on (eurotoys); and that he put the stock pully and did not see the miss...funny thing is the larger pulley was already on prior to piston replacement and no miss was occuring....I doubt it is the larger pulley causing the intermittent miss myself.

Thanks in advance again I need help on this....

 

Listen for hisses around the engine bay, check the breathers are all clear, especially the part load breather (right cam cover when looking from front) and also the throttle body. The part load breather pipe is a prime suspect for a vacuum leak too as this get very brittle with age and being disturbed may have split it somewhere.

What are the spark plugs like?

Check the intake 'snorkel' carefully for potential leaks especially in the flexible parts, the plastic goes hard and brittle with age and you could have a very slight air leak somewhere along there too.