Rough idling and dieseling

Good morning Jaguar enthusiasts!

I am a proud owner of a 1976 XJ6 Series II.

However, I have had the problem for a long time that the engine does not really run smoothly when idling. The engine starts well, runs quite well while driving. When parking, the engine continues to run for ~ 0.5 sec.

I have already checked the following things:

Intake bridge checked for leaks (with smoke leak detector)

Ignition cable new

Ignition coil new

Ignition distributor 123 new

Ignition distributor cap including distributor rotor new

new and checked spark plugs (not oily and not sooty)

AED completely overhauled and adjusted (distance from the upper edge of the square screw to the sealing edge of the housing approx. 8.5 mm)

both carburetors (HIF7) were cleaned and got a service kit CSK 82 (Gaskets and Seals, main jet and needle valve and seat float bowl), then carburettor adjusted according to manual, damper oil checked (15w40), idling 830 rpm

Compression checked:

Cylinder - bar
1 - 12
2 - 12
3 - 11.6
4 - 12.1
5 - 11.3
6 - 11.7

Ignition (measured on the 6th cylinder, without vacuum): 11 degrees before TDC

Valve clearance measured (the WHB says 0.012-0.0014 for intake and exhaust):

Cylinder - Intake - Exhaust


The values ​​for intake (1 and 6) and exhaust (2 and 5) differ slightly. Is that still within the tolerance range?

Does anyone have another idea? Meanwhile I am desperate :frowning:


P.S.: Kindly excuse my bad English:-)

Good evening André,

great to have the figures at hand. Compression looks great, clearances at least totally undisturbing.

What is the static timing of ignition though? It should be 8° BTDC. What is your CO concentration in the exhaust? Anything around 3 - 4 % at idle should be good. What do your spark plugs look like?

A dangerous cause of dieseling may be carbon residues burning off - but that is rather a horror story it seems. Most often dieseling comes from too high idle (750 RPM in N are o.k.), too hot spark splugs (specs are Champion N11Y), too lean mixture or too late ignition. No need to be desperate - just take one step at a time.

Good luck


75 XJ6L 4.2 auto (UK spec)

I think the idle is a bit high as well, that would be the first thing I would try, as Jochen suggests 750 rpm would be my target, (not that I can get mine down to that unless it is in D or R :confused: ) because mine idles high I always shut it down in D.

Hi Andre. Everything the guys say. For rough idle when warm check the carb air balance with a length of hose or a UniSyn. Not familiar with HIF7 carbs yet but I think the tuning process is similar to my HS8’s. As above check the spark plugs - should be brown on the insulators. It took me a long while to get my 4.2 engine to idle well - part of the problem was the faulty AED. I know you have repaired your AED but it might be worth isolating it when the car is warm and idling by isolating the fuel and blocking of the warm air intake. Paul.

Andre if timing is too advanced it can cause dieseling and running on.I find the best way to set timing with a warm engine,providing carbies are set well is remove dist vacuum pipe loosen dissy,bring engine up to 3000 rpm then rotate dissy for optimum revs.tighten dissy.Easy to do and nothing to loose JOHN


In the meantime, I have adjusted timing (8° BTDC), idle (750) and the valve clearance to spec. Unfortunately, it did not help…

After a little research and the purchase of an endoscope, I finally found out that oil is getting into the cylinder via the inlet valve. The assumption is that the valve stem seal is no longer sealing properly and causing the engine to idle erratically and dieseling after shut down. Furthermore, there is positive pressure in the crankcase (at the oil filler cap and dipstick).

Can this be the reason for my problem? Does it make sense to replace the intake valve stem seals?


This could very well be your problem.
Check your PCV valve/system.

Yes it does but it means taking the head out.

It doesn’t mean taking the head out.
But how do valve stem seals cause dieseling? They will cause higher oil consumption and a bit of blue smoke after engine braking….
A bad valve stem seal will not cause higher crankcase pressure either. The actual valve seals the combustion pressure and the intake is under negative pressure so if anything it will lower the crankcase pressure.


you might exclude the oil issue by temporarily pulling the crankcase ventilation hose from the intake, plug the intake and take a serious ride. Then check the looks again.

In my case dieseling went away once a vacuum leak was identified, cured and the engine tuned correctly. Your intake looks just sooty to me. What do the spark plugs look like? I had five dark black and one white - result of tuning the engine to 4 % CO with one cylinder running lean …

Good luck


75 XJ6L 4.2 auto (UK spec)

Given that you have a camera - what do the piston crowns look like? You might have some sooty hot spots igniting the fuel/air mixture? The incoming oil will make this worse.

1 Like

The ‘run-on’ is caused by the carbs delivering fuel as long as the engine is turning, Andre - being ignited by glowing carbon deposit. The best bet is an anti run-on valve - that either malfunctions or is not fitted on your car. The anti run-on valve is operated by a specific connection in the ign key - which is powered with ign ‘off’ - and an oil pressure switch. Thus operated; the valve applies manifold vacuum to the carbs’ jets - which prevents the carbs from delivering fuel to the engine if oil pressure is present as engine is turning with ign ‘off’.

First check if you have this set-up; the valve itself is in the ‘left’ front wheel well, with electric and vacuum connections - and is connected to the carbs. there is and extra oil pressure switch at the right side of the engine block - and an extra wire connection at the ign key…

The set-up was fitted to late carbed cars to cure the run-on. However, the real problem is that carbs deliver petrol as long as engine is turning - and petrol being ignited by carbon ‘afterglow’ after ign is turned off. Failed valve seals will certainly allow oil into the cylinders causing carbon build-up. But failed seals do not in itself cause uneven idling or engine misbehavior - they just prevents oil being sucked into the cylinders from the head. It is the valves, opening and closing correctly, that runs the engine…

Changing the valve stem seals is of course desirable, but will not cure the run-on - decarbonizing the head is also necessary, but carbon will eventually build up again. There is a method to replace the valve seals without removing the head - involving a method to hold one valve at a time closed while removing the valve spring and tappet. It’s described in the Archives, but is lot of work - changing seals it’s better done when head is removed for other purposes…

In short; your best bet is to check and verify that the anti run-on system is fitted and working…

xj6 85 Sov Europe (UK/NZ)

Good morning !

Thanks all for the fast response / support!

@Aristides : I have already checked the PCV system. No problems here.

@Frank_Andersen : good hint, but the car has no run-on valve.

@Jochen_Glockner : I have already blocked the crankcase ventilation hose from the intake a few months ago, Dieseling is getting a little better. But the rough idling remains unchanged…

In fact there is positive pressure at the oli filler cap / dipstick, when the engine is idling.

My guess: due to the defective intake valve stem seals, there is positive pressure in the crankcase (oil vapors and may be fuel vapors). This and the carbon afterglow causes the dieseling when the engine is switched off (as the crankshaft housing is directly connected to the intake, even if the throttle valve is closed). Could this also be the cause of the rough idle?

What exactly do I have to do to replace the seals without removing the cylinder head? The work should be comparable to adjusting the valves, which I have already done. Correct?

  • Set crankshaft/camshaft to TDC
  • Lock crankshaft (at the flywheel or balancer?)
  • Remove both camshafts (intake and exhaust)
  • Apply compressed air to the cylinder through the ignition port (to secure the valve)
  • Replace intake valve stem seals
  • Install camshafts (using the special camshaft tool)

@Frank_Andersen : can you please send me the link to the archives, where the method (replace valve seals without removing the head) is explained ?

Any help or advice is welcome !

Thanks André

Not quite as easy, you need to be able to pressurise each cylinder to hold the valves closed as you remove the bucket and valve retainers.

Crankcase pressure is normal and natural, it has to be vented (to the intake for emissions reasons and because it makes sense). Connect everything as intended and make sure the line is not blocked.

You are definitely on the wrong track if you think that old intake valve stem seals cause anything else than a little blue smoke at the tailpipe.

Rough idle can be lots of things.

I see what you’re thinking but I doubt it… the engine on carbed cars could idle forever because the mixture is supplied as long as the engine turns, that’s why the „ignition“ is switched off and not the fuel or air…


ahm … I don’t get the line of your problem analysis: if you have pulled off the crankcase ventilation from the intake manifold there is absolutely no source for oil to be pulled in and cause dieseling. - Flat engines have a tendency that during extended periods of sitting oil tends to accumulate and run along the lower valve stem, around the worn shaft seal and drip out. You have separated the intake and the combustion chambers from any supply of oil.

In my view you simply have an engine that has been operated under less than ideal circumstances, maybe overly rich, too much short-distance use, no revs, and thus built up a lot of carbon deposits. Just stop trying to tune the engine and simply drive the car over longer distances and with increasing load. Then repeat the picture and see whether it has changed.

Good luck


75 XJ6L 4.2 auto (UK spec)

Oh yes, there is. Down the valve guide, which is why the rubber seal was introduced. And it’s intake side only because the intake side pulls oil in while the exhaust side tends to be pressurized.

Yes, put the crankcase ventilation etc. back together and let it run properly for a few dozen miles. And then run it a little harder and see if you get any dieseling (all that as long as the engine feels normal and not terribly uneven or lean or very rich).

Defective/missing intake valve seals cannot increase crankcase pressure, Andre - it can only cause oil to be sucked into the cylinders from the valve covers. Which increases carbon build-up, which of course is ‘not good’. Crankcase pressure may force oil from the crankcase past the piston and rings, but only if there is considerable cylinder wear - which in itself induces carbon build-up, and this blow-by will in itself increase crankcase pressure…:slight_smile: Cylinder wear is best tested by a ‘dry’ and ‘wet’ compression test…

Proper ‘dieseling’ is the ignition of petrol by compression pressure - while the likely cause of your ‘run-on’ is petrol being ignited by carbon ‘afterglow’, more akin to glow plug ignition. Compression ignition may of course happen if excessive carbon build-up increases compression enough - and a combination of both may cause the run-on. However, petrol is still required, the compression is very unlikely to reach the level of igniting oil - hence the use of the anti run-on valve. Without this, only decarbonizing is likely to effect a cure - just changing the inlet valve seals is unlikely to cure the run-on…or the uneven idle

As others have said, uneven idle may have variety of causes, not necessarily tied to the run-on. But residual carbon may ignite the mixture randomly rather than sparking by ign timing…so…

As Jochen says; disconnecting the connection from the crankcase ventilation housing will vent the crankcase to open air. But also; have you checked and cleaned the filter in the housing? If it is clogged, the system will not vent properly. And as he also says; dawdling driving will in itself cause carbon build-up…

Correct, and you procedure outline is to the point. But very high, and continuous, pressure is require to ensure that a valve does not fall into the cylinder - which definitely require a ‘head off’. Can’t give you an appropriate link to the Archives - it’s as difficult for me to find it as for you…:slight_smile:

An alternative method, safer but more cumbersome - done at two cylinders at a time. Set the engine to TDC mark; #1 and #6 is now at their TDCs - and verify the the dist rotor is pointing to #6 (frontmost) plug led. Remove both cams and spark plugs the cylinders at their TDCs - this will then keep the valves safely closed while the seals are replaced. Then the engine is turned 120 degrees the next two cylinders to their TDCs to repeat the process. Again turn the engine to bring the last two cylinders to their TDCs TDCs to finish the seal changes. Then turn the engine to the TDC that the rotor points to #6 and refit the cams using the cam setting tool.

xj6 85 Sov Europe (UK/NZ)