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XJ81 - V12 stumble and running lean

newyankee
Posts: 61
Joined: Mon Oct 06, 2014 7:32 am
Location: Hamburg, Germany

XJ81 - V12 stumble and running lean

Postby newyankee » Fri Jul 31, 2015 8:59 pm

I have had this slight stumble which is mostly noticable at idle since the beginning. In fact, I have seen or driven four other cars that had the same problem (two of which I own, one from a forum member, one X305), all above 80000 kms on the clock.

I have since carried out quite a bit of work on the car, checked fuel pressur, replaced fuel filter, replaced distributor cap, rotor, spark plugs, ignition coils, cleaned and re-sealed all injectors, fixed any vacuum leaks (there was one split hose), cleaned and adjusted throttles, adjusted TPS, checked temp sensors, and replaced both oxygen sensors.
Effect: ZERO.

I did check with a CO-Meter today, and CO at idle was 0,3% after cold start which is a little low, when warmed up, it was less than the unit could measure. I suspect that the engine is running lean. As this appears to be a systematic problem: What could be the cause?
--
1994 XJ12
1964 MkII 4.0 project
1968 TR5
2016 RRS SDV8 4.4
2013 Touareg 4.2 TDI totaled

ChrisW
Posts: 1611
Joined: Sat Apr 14, 2012 6:05 pm
Location: Whatlington, East Sussex

Re: XJ81 - V12 stumble and running lean

Postby ChrisW » Fri Jul 31, 2015 11:02 pm

Check that the small rubber caps are still in place under the inlet manifolds. They blank off unused vacuum take offs and have a habit of falling off, resulting in too much air and weak running. Ive lost two during my ownership.

Where item 3 is on this diagram

Image
http://www.springballchange.co.uk
'93 XJR
'93 XJ12
'73 MGB Roadster
97 Range Rover Autobiography

MagicDoormouse
Posts: 20177
Joined: Sun Apr 08, 2012 10:37 am

Re: XJ81 - V12 stumble and running lean

Postby MagicDoormouse » Sat Aug 01, 2015 12:33 pm

That number 3 thing is a solid cap Chris. You are correct in saying there are blanking caps under the manifolds and they do indeed fall off. I used tyre valve caps and filled them with black RTV, result? perfect even running.............
1990 Sovereign (Brian)
1994 Sovereign Silver
1993 Daimler (Parkhouse)
1990 XJR Solent
1993 Daimler Double 6
1993 XJR Flamenco
1991 XJS V12
1976 Capri 3.0 V6

ChrisW
Posts: 1611
Joined: Sat Apr 14, 2012 6:05 pm
Location: Whatlington, East Sussex

Re: XJ81 - V12 stumble and running lean

Postby ChrisW » Sat Aug 01, 2015 12:49 pm

Yes, I ordered 4 thinking they were the rubber caps but got these solid brass things instead.
I use anything rubber with the right dimensions. Those rubber caps on top of the bolts in each corner of the engine bay work well, for a while. I'll try adding some rtv and see if that helps.
http://www.springballchange.co.uk
'93 XJR
'93 XJ12
'73 MGB Roadster
97 Range Rover Autobiography

newyankee
Posts: 61
Joined: Mon Oct 06, 2014 7:32 am
Location: Hamburg, Germany

Re: XJ81 - V12 stumble and running lean

Postby newyankee » Mon Aug 03, 2015 9:58 pm

Thanks so far. All those blanking caps are present, I had previously checked for their presence and secured them with cable ties. They are still in place as of this afternoon.

Reading the V12 service manual, I have come across the list of potential stumble/rough idle/rough running issues. Checking and adjusting valve clearance might be something to consider. Has anyone ever done this before?
--
1994 XJ12
1964 MkII 4.0 project
1968 TR5
2016 RRS SDV8 4.4
2013 Touareg 4.2 TDI totaled