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Xj81 Running Rough on idle

Posted: Mon Jun 24, 2019 8:46 pm
by panarabica
My 94 Xj81 just got motd after sitting nearly a year. The car was in good running order when parked but has developed over time a habit of stalling when idle, and often only running at 300-400 rpm when cold. Once warm it’s all good, and the car idles at 620 rpm. No VCM code. Now the check engine light has come on, and I have done a bit of research.
A culprit seems to be the o2 sensors.i cannot find any ,material on how to replace those on a xj81 let alone how many there are. 2 or 4? How complicated is it to change those?
After having spent 110€ on trying to gas up my ac system, and having a leak on the compressor I am not in the mood to experiment to much with spare parts.
I know there can be plenty of other causes to the problem, but I think the o2 sensors are a good guess.

Any thoughts?

Robert

Re: Xj81 Running Rough on idle

Posted: Tue Jun 25, 2019 9:51 am
by Walker
My understanding of the o2 sensor is that they only come into effect when the exhaust temp is within a certain range. My V12 knowledge is next to Nil, however despite they're complexity there are a few members on here who know them well, hang tight for their attention!

Re: Xj81 Running Rough on idle

Posted: Tue Jun 25, 2019 11:23 am
by naki_k
Clearly the revs should be much higher when cold. 1100-1200rpm. I would think the Extra Air Valve is not working/ stuck?
New parts for this are expensive. Used are v scarce/ expensive. I did a little post on them. With contributions from other members showing a refurbishing route, for a lower cost.

If there’s no VCM code showing it’s less likely to be Oxygen sensors. You can manually measure their resistance values (search for the figures online to compare them to).

Re: Xj81 Running Rough on idle

Posted: Wed Jun 26, 2019 8:50 am
by newyankee
620 rpm is too low for a warm engine. Spec is 750 in Park/Neutral and 650 in D.

I would definitely check the AAV first, adjust idle screw to above values. If this doesn't work, clean AAV. Only if the AAV then doesn't work to get an acceptable idle on a cold engine, I would proceed further. On well used AAVs idle is usually higher when cold, but that's due to the degrading wax bulb.

As an additional measure, I would check both throttles for clearance and clean them.

Re: Xj81 Running Rough on idle

Posted: Wed Jun 26, 2019 1:54 pm
by panarabica
Thanks for the reply's on my idle issues. Having read through several posts, I understand that the AAV is a hot candidate for my problems. As i understand it can be cleaned, as per a procedure mentioned in a post by leo-denmark. Now I am a bit confused as I read that there is a wax/oil in the valve that operates at certain temperatures. There is no mention in the cleaning procedure of adding any oil.
Has anyone done the cleaning procedure and has a working AAV?

Re: Xj81 Running Rough on idle

Posted: Wed Jun 26, 2019 6:52 pm
by naki_k
Did you read through this already?

viewtopic.php?f=20&t=998942&hilit=Extra+air+valve

Re: Xj81 Running Rough on idle

Posted: Tue Jul 02, 2019 1:37 pm
by panarabica
Yes I did. But still not sure about the topic of the wax bulb. Maybe I am just misunderstanding this. Had the car running more or less normal, without throwing an error code. Then I let it idle for .5 min, and the error is back on.

Re: Xj81 Running Rough on idle

Posted: Tue Jul 02, 2019 2:00 pm
by newyankee
Low or high idle due to a poorly working AAV doesn't throw VCM error codes or a CEL. If the CEL comes on, there should be an error code. You will need to read the code.

If you have e.g. dripping injectors causing a rich mixture, failing O2 sensor(s) you will have error codes. Check for those first. Secondly, I would probably take a look at all spark plugs to see if any are wet, black, sooted. You might have ignition issues somewhere. A single cylinder not firing will also cause the idle to drop.

Re: Xj81 Running Rough on idle

Posted: Tue Jul 09, 2019 2:05 pm
by panarabica
Having read the posts, I will go forward with following measures:

- Check that all ignition leads are mounted correctly and are properly attached
- Remove all spark plugs and and throw some new ones in
- Double check that the dizzy cap is clean and dry (replaced it before it went into storage)

and will see how the beast runs then. Sometimes it's the basic stuff that solves the problems. The car has also decided to not throw the check engine light any more, but she is still not running a 100% smooth as I would expect on a V12.

Rob

Re: Xj81 Running Rough on idle

Posted: Wed Jul 17, 2019 9:13 am
by naki_k
Any luck with this then?