Running on after engine switched off

[piman]

Hello Richard,

if it doesn't pink then it does not need to be retarded, it may even need a bit more advance? Don't forget that you have a vernier adjustment on the distributor?

Alec

[RichT]

Hi Alec

Doesn't look like there is venier on 123 has a non mechanical vaccum advanvance thingy

All has been fine once better fuel run thru so I am leaving well alone until salt stops play, when I will commence servicing and improving stuff (or messing up and breaking suff depending on which way it turns out

Cheers

Richard

[J44EAG]

The super fuel has better antiknock properties and allows a higher advance level to happen without detonation occurring. My modern Paramount tuned V8 is a different car on super and runs like a sewing machine on it. The antiknock sensors when pushing on obviously detect that detonation is not occurring so allow the engine to really open up safely at full throttle.

My gut feeling is that your car is a tad over advanced and as a consequence hot carbon coals within the engine glow when you switch off and the engine detonates and runs on. i would be taking a tad of advance off the engine and looking to try to remove internal coke up by some method. A decoke in past years used to be the standard and normal method. It was normal procedure.

Obviously you will want to avoid that strip down work and some of the wonder cures now available in a bottle might be worth a try. Going the full hog, I am thinking that perhaps a Terraclean session might be of benefit. No doubt you have seen the Wheeler Dealer XK8 session on TV or youtube and more recently a post on the XJS board where one of our members had the process carried out.

Hope that helps.

Mike K

[piman]

Hello Mike,

we have different views, I think, about the timing and I don't believe that it would contribute to internal hot spots.
One effective method to remove those soft deposits is to drive the car hard. It was quite a common procedure many years ago, but finding a quiet road is harder these modern times. Ideally a long uphill road where the throttle will be open wide but the speed not too excessive.

Rich, I was forgetting about the 123, did you say it has a vacuum advance as the carburation on our cars is set up for one, with advanced ignition at part throttle.

Alec

[RichT]

Hi Mike and Alec

I think the higher octane fuel will help as you say it will allow more advance without detonation I seem to remember reading somewhere about steadier slower flame fronts.

I would be inclined to agree with Alec about carbon deposits, because if carbon is there it will glow regardless of advance although incorrect timing can cause overheating and an over advanced engine would maybe be just that bit hotter I don't understand why this happens and need to read up more. I thing knocking a degree off may also be a good plan and I will use the other tips on this post to check the static timing marks

The use of better fuel also coincided with giving it some beans on the motorway so I think this must have helped also the premium fuels are supposed to have better additives. Modern fuels and oils largely mean decokes are a thing of the past, I have been reading some interesting stuff about the invention of the removable cylinder head being a revelation in making this process easier in earlier motoring history.

When I change the plugs I will stick the 8mm boroscope I got off ebay for£20 this is a great bit of kit for looking inside stuff to see if you have broken anything ( I use the same kit for looking down dogs throats at work)

This 123 distributer does have a vacuum advance it also does away with the mechanical advance removing spring problems which can lead to the distributor not retarding when the engine slows down (I also need to lower my idle speed slightly)

The model I have has various ignition curves but the thing has to be removed to get at the adjustment switch.
I have up to this point assumed the previous owner has set it to the correct profile which I think is b, the latest ones have a USB port so you can make changes with the thing in situ and even on the move.

My plan is to go to the xk engine seminar in November to hopefully learn so tips on how to break the engine properly before I get stuck into it

Cheers for now

Richard

[J44EAG]

It is worth remembering that these cars were designed and built to run on fuel laced with tetraethyl lead.

TEL has the effect of reducing fuel mixture to auto ignition when compressed. It also has the advantages of cooling an unburned charge within the inlet system and reducing temperatures within combustion chambers. Instances of uncontrolled detonation or auto ignition reduce as higher quantities of TEL are traced into the fuel. TEL has the advantage of reducing value and seat wear although plug fouling increases.

Highly compressed, high flying aircraft piston engines as used during WW2 suffered from destructive detonation issues. Fuel octane levels increased to 150 octane to counter the problem. These issues were drummed into me at Aviation College when I undertook my Licensed Engineers course over twenty years ago.

Detonation causes cylinder carbons to remain rather hotter that carbon which has been heated to only moderate and acceptable temperatures where carbon temperature is low enough to avoid the hot coals effect. Advanced timing as we know particularly will also tend to raise internal combustion chamber temperatures and lead to residual hot carbon temperatures. Use of unleaded fuel also exacerbates combustion temperature issues and can lead to burned valves and seats. Switching an engine off immediately after a hot run is likely to run on if hot spot carbon continues to glow due to residual heat remaining in the chambers. A short period of cool down at idle speeds before switch off will allow carbon to cool to acceptable levels and reduce the likely hood of auto detonation as power is cut from the ignition system.

One has to consider why an engine runs on after electrical energy has been removed from the spark plugs. Is this due to carbon glowing or fuel with an octane level low enough to auto ignite due to insufficient octane level for that particular engine directly linked to compression ratio, advanced timing or a high level of carbon build up within the combustion chambers?

I well remember running 2.5 PI Triumph engines. They were high compression units and ran quickly with slightly advanced timing. However wear within fuel injection components and high carbon levels on a second hand car could make those engines rattle and run on at the slightest excuse. Whenever my Father and I bought one of those cars, the first thing we did was rip off the head and do a thorough decoke. Detonation issues within those engines greatly reduced once we had cleaned carbon from combustion chamber components. Overhaul or replacement of the PI fueling unit also helped.

Periodic subsequent removal of the plugs at service intervals had us injecting raw Redex upper cylinder lubricant into the cylinders which we would leave overnight to soften piston crown carbon. We would then crank the engine over with plugs out next day to eject the Redex before fitting new plugs. That kept the issue under control.

More on TEL in these two links.

http://www.google.co.uk/url?sa=t&rct=j& ... GU&cad=rja
http://www.google.co.uk/url?sa=t&rct=j& ... GU&cad=rja

It may be worth trying some form of octane improver to see if matters can be improved. "Instead of Lead" is a product that comes to mind. If the run on subsides slightly or fully when such an additive is used, it might then point to an octane increase being needed to counter high compression and a lack of TEL when using unleaded fuel.

Mike K

[piman]

Hello Mike,

if the engine does not pink then it's not over advanced, given that the ignition curve is as it should be. Obviously it is worth while doing some adjustments either way to see if it does eliminate the problem and then perhaps review what figure it brings up. Also worth doing an advance reading above 3000 rpm to check the full advance (without vacuum)

As I remember aero engines are different to car engines as they cruise at constant speed which brings it's own set of problems.

Redex reminded me that it was common in the sixties to dribble about half a pint into the carburettor with the engine running then rev the engine, the smoke produced was quite dramatic, although I don't know if it was very effective?

Alec

[oldtimer]

My memories of taking the cylinder head off XK engines during the sixties was that the back of the inlet valves were heavily coated with black coke like debris which came off relatively easy when compared with back of the exhaust valves which seemed like reinforced concrete. The crown of the piston and combustion chambered were really not that bad unless some other defect was revealed.

So what am I trying to say. By twiddling all the external knobs and whistles you will never turn a pigs ear into a silk purse.

[piman]

Hello Oldtimer,

I rather feel that calling the XK engine a sow's ear to be rather unkind, it was, after all, designed in the 1940s and quite an exceptional design at that.
It's unfortunate with modern traffic that it can't be driven with as much verve as it deserves; I think that engines that are driven hard run better and sweeter than those that aren't.
There's a lot to be said for the 'Italian tune up'.

Alec

[oldtimer]

Alec,
You miss the point. When there are significant build up of contaminants on the engine internals you cannot get around the inevitable work required to remove them. Once you lift the head a bag of worms is usually uncovered. Even an XK engine is a "Pigs ear " if neglected for long enough. So, as I said fiddling with all the knobs and whistles on the outside of the engine will not cure the ills inside OK.

As for road condition, nothing much has changed. There were no motorways and plenty of congestion. One could drive like a" looney " and still only achieve an average of 30mph over a journey. ( Not that I would admit to driving like that) Of course there were no speed limits so driving in a spirited manner at umpteen miles per hour on single carriage roads could be a bit hairy.

I must have some regard for the Jaguar brand else I would not still be with it after over 50 years of it, despite all the tribulations

Oldtimer

PS Way back it used to cost £1 .50 for a complete head gasket set - how about that.

[oldtimer]

PS. When did you last drive up the Motorway ! I've been chased by a high top van at over, well. a lot of miles an hour, thinking why don't I just settle for a van.

Traffic don't hang about so there is plenty of scope to give a Mk 2 a good thrashing. Remember the gearing was set for single carriage roads where low speed acceleration was crucial. The 3.8 Mk 2 was a star performer --- I'll tell you about it one day. Oh and the heater was never much cop either.

Oldtimer

[piman]

Hello Oldtimer,

perhaps I didn't make my point very clear, build up you are referring to is reduced if the engine is driven hard.

Motorways are quite gentle on the car, 3,500 rpm in a Mk2 with overdrive is easy and likely to incur a ticket if regularly maintained. Driving on open A roads is more what I had in mind with burst of overtaking, hills etc where wide open throttle is used.

Alec

PS look below, owned for over thirty years.

[oldtimer]

Alec,
Just noticed your location. There's interesting. In the late sixties I was commuting at night between West London and Anglesey at the weekends, this lasted for about five months which was in my 66 S type. Picked up the A5 at Gaily corner, just off the M6 I think. The new coast road was not there then.

It still makes me shudder. Arriving in Llangollen in the middle of the night, slowing for traffic lights which were hung across the street high up on a cable and with the outline of the mountains silhouetted in the background by the night sky. Never arrived to green lights always red. So creep up in third , drop it in to second when the lights went green and off like the start of a Grand Prix and head for the mountains. Sometimes snow was trying to lay especially when you got up beside Mount Snowden. Quite un-nerving, all on your own and not another car in sight.

Gosh, that brings back memories

Oldtimer

[wimgohres]

https://en.wikipedia.org/wiki/Dieseling

I have the same problem I found out it is called dieseling. I have 3.8 with 123 electronic ignition. My car has been restored in the past years, but the dieseling is a recent issue. I have the feeling it relates to the carburetors which need revising. Because they are weak the flow of petrol is not cut down completely once the gas pedal is released. If the engine has run for a long time, the condition becomes a bit worse. You can stop the dieseling by putting the gear in 3rd and release the clutch.

The wiki link summarizes the issue as follows:
Dieseling can occur for several reasons:

Built-up carbon in the ignition chamber can glow red after the engine is off, providing a mechanism for igniting unburnt fuel. Such a thing can happen when the engine runs very rich, depositing unspent fuel and particles on the pistons and valves. Similarly, rough metal regions within the piston chamber can cause this same problem, since they can glow red. It has also been suggested that an improperly rated spark plug can retain heat and cause the same problem.
A carburetor that does not completely close can contribute to running once the engine is off, since the extra fuel and oxygen mixture can combust easily in the warm piston chamber. Similarly, hot vaporized oil gases from the engine crankcase can provide ample fuel for dieseling.
Incorrect timing.
An engine that runs too hot or too lean may produce an environment conducive to allowing unspent fuel to burn.
An idle speed that is too fast can leave the engine with too much angular momentum upon shutdown, raising the chances that the engine can turn over and burn more fuel and lock itself into a cycle of continuous running.