Hey All,
I replaced my AJ16 downpipe and Cat last week with a MagnaFlow 'OE' replacement. I treated myself to 4 (now 5) new Bosch O2 sensors as well. When I started her up, the exhaust leak (the reason I replaced it in the first place) was gone, but the engine would hunt for idle. She would sit steady around 650, but then dip every 10 seconds or so down to 500 and vibrate for a few seconds, then return to normal. I bought another O2 sensor, thinking one of them must be bad and began the process of flipping them out one by one. No luck... still dips idle. Then I pulled the downpipe and checked the donuts, thinking that maybe a small exhaust leak was messing with the 'outside' reading of the upstream O2. The connections are tight as a drum and no leaks at all. So finally, I unplugged all the O2 sensors, and now she is purring like a kitten. Idle is steady as can be. Does anybody have any ideas on why she should be hunting for idle when all four O2s are properly plugged in and functional? Also, would I cause any damage just running without any of them plugged in? I've been doing that for the past two days, and she's been running perfectly. Thanks a bunch in advance for any advice....

 

O2 sensor re-orientation needed? I dunno. Just a thought. AFAIK this would normally be needed only if you got the wiring mixed-up; that is, got upstream/downstream wires mixed-up. Others will chime in on that.

Did you get correct oxy sensors? AJ16 sensors are somewhat unique although, being on my first two sips of coffee, I can't remember the name of the material that makes them so. Zirconium versus titanium, or something like that?

I'm not aware running with disconnected sensors doing any actual harm. But the engine is running in open loop, on a preset fueling 'map'...as opposed to the constant fueling corrections that occur in closed loop. Not efficient.

Cheers
DD

 

Thanks Doug!
I marked those O2 plugs before pulling them out, so I'm 97% sure that I got them plugged back in correctly. That being said, its an easy thing to start flipping them around and see what happens, so I'll give that a go. I'm reasonably sure I bought the right O2 sensors (Bosch 13789). They certainly look exactly the same as the originals... I'll keep playing around... thanks again!

 

Those are the correct sensors.

I'm gonna put my $5 on your intake manifold gasket. I had a similar issue with a dip every 32 seconds at idle. I didn't have a hunting idle though, but it did take the intake seal 32 seconds, whereas yours is failing every 10....

Took three mechanics to finally identify the leak. Many parts were thrown at the car in the meantime...including new exhaust manifolds and gaskets and donuts and 02 sensors....and coils and on and on...

While you're there, change out the ******* hose. It'll never be easier.

 

Doug - Changed around the plugs. I think the way I have them now is right. It certainly gets worse with some of the combinations. When you say 'orientation' are you saying that it matters how far around I spin them in their respective holes? Like, I should try backing them off, or torquing them down more to reorient the sensor?

Vee - I've got a bunch of throttle/intake stuff on my list, so I'll add the intake gasket onto it. I appreciate it!
D

 

If you have pre and post cat 02 probes (wouldn't have expected that but ...) then they only fit in specific places (at least on the XF) they may be O2 sensors but pre cat and post cat are very different.

O2 sensors work with the ecu by constantly varying the fuel delivery and checking for a response, by measuring this oscillation they confirm that they work, the pre cat is compared to the post cat and this identifies if the cat is working. This oscilation is generally undetectable - if you can detect it something is seriously wrong.

There reaches a point where the pulse of the injector is too long (high positive long term fuel trim), even though the lambda is correct this indicates a mixture too lean condition - basically this is an air leak or fuel flow or pressure issue. A short pulse (-ve long term fuel trim) can indicate excessive fuel pressure or leaky injectors.

I don't know whether the XJS operates a similar process but if you have 4 probes I'm betting that it does, if so the sensor types should be different pre vs post cat but I'm struggling to find any 02 sensors in my parts book let alone CAT assembly showing 4.

 

Getting the O2 sensors correctly connected does make a difference. Mine were cross installed for the front cylinders when I had a shop install new cats. I don't remember the idle fluctuation, but the car did not run well and I got a check engine light. I can't remember the code but it indicated the O2 sensors. Each of the onboard connectors has a different colored wire as follows: Front cylinders upstream is green and downstream is blue, rear cylinders is brown upstream and red downstream. That is an easy check for the O2 sensors. The other wires on the connectors are the same for all 4 connectors.

 

Orientation means upstream of the cat vs downstream. If the wires are switched, there is a procedure with dealer software to reverse the ECU to change which O2 sensor is up and downstream. It's not a usual proecdure.

 

There are 4 O2 sensors on the later XJS with the AJ16 engine, and V12's with OBDII. The sensors are all the same.

The AJ16 is very unusual in that the sensor post cat is the one that controls the air fuel ratio, the upstream one is the check. Every other engine I'm aware of is the reverse.

 

Since you have 4 oxygen sensors, all identical, an oxygen sensor reorientation makes sure that they plugs are all plugged into to the right sensor. I guess Jaguar figured it would be easy for a shop to screw up oxygen sensor replacements, so they made it possible for the car to figure out which sensor gets plugged inot where.

Make sense?

 

Thanks RickE. I labeled them (Bank 1/Bank 2) before I pulled them off, but wasn't 100% sure I put them back in the exact right way. Now I've looked at the wires and can confirm that I did. That at least helps me mark one thing off the list.

 

I have 0 experience with the XJS 4.0, but I do have a lot of experience with fuel injection and emissions controls in general. I suspect when the O2 sensors are disconnected it is forcing the engine into open loop and masking your real problem. It may be unrelated to the O2 sensors. If one of your old sensors had failed, you may have been open loop for a long time. It is unlikely to cause damage, but would fail an emissions test.