Fuel Starvation? Engine Cuts Out

[jccesprit]

Hello, this is my first post. I wonder if anyone can help.
I have a 1977 XJ6 4.2 Coupe (Series 2).
I only do about 300 miles a year in the car, generally taking it out for a 20 mile run once a month.
Recently (last 3 weeks) I have had a problem whereby the engine starts first time every time, but only runs for 20-30 seconds before cutting out. I can start it again immediately (after say 5 seconds), but it only runs again for 20-30 seconds.
I have checked and rechecked the ignition settings and timings, and I have checked the floats and needles in the twin carbs.
I have also checked the AED unit as best I can - float and needle and diaphragm operation seems fine.
I have the AED unit, which I replaced with a brand new unit 3 years ago (1000 miles ago). I have a ballast resistor fitted to a brand new Lucas Sport 12v coil, and I fitted a 123 electronic ignition distributor a year ago. All working fine until 3 weeks ago.
Both my fuel pumps work (immersed in tank type), and the switch-over seems to work fine too.
My latest suspicion is that the fuel pumps may not be pumping at sufficient pressure/flow rate.
I have measured the flow rate from each pump/side, at the carbs, and its approx. 1.2 litres per minute.
Can anybody suggest anything? Can anybody confirm what the fuel flow rate should be?
Thanks very much for any help and advice.
John Campbell. Hampshire.

[PPV]

This does sound like a fuel flow problem, possible with a blockage occuring after running for a while or low flow but I am not sure what that should be for your engine. Have you checked the fuel filters or ran the fuel pumps for a little while into a can or something and see if any debris comes out?
To eliminate the ignition system as far as possible when the issue occurs take out a plug and check for a spark, it may be that something like the capacitor has failed and when it warms up causes an issue if there is no spark.
Regards,
Paul

[jccesprit]

Hi PPV -- Thanks for the response.
I have run the petrol into a plastic bottle, and that's the way I measured the flow rate.
Its the same rate on both pumps - 1.2 litres per minute.
I will try the trick with the spark plugs on Wednesday. I have seen that mentioned in the workshop manual, but since my engine sounds fine when it runs (albeit only for 30 seconds), I have not tried it yet.
The 1.2 litres per minute equates to around 16 gallons per hour if my maths is right
(at 4.5 litres per gallon, 60 x 1.2 litres = 72 litres per hour = 16 gallons per hour, roughly).
I have seen that on the ACDelco website, the pumps they suggest for the Series 2 XJ6 deliver nearly twice that - i.e. 30 gallons per hour.
However, the pumps they list (ACDelco Part Number EP42S) don't look like the right pumps to me.
I will let you know the outcome of my trials.
Cheers.
JCC

[PaulGover]

AFAIR, the Series II has a hot-air intake across the top of the engine, with a pneumatically-operated valve to switch to cold when the engine's warmed up. Which means there's at least one vacuum pipe to check - similarly ones for cruise control, if you have it. From what I recall (it's about 30 years since I had a II) there's a load of opportunities for air leaks.

That said, I'd go with your thoughts on fuel starvation, though I think your 16 gal/hour is more than adequate. My XJS V12 can sink to 3mpg at full throttle with kickdown (sustainable for at most something like 10 seconds, at which time it would be doing well over the ton), which at say 70 mph is 23 gph. Another possibility is electrical stuff that times out after 20 seconds; my XJS has loads of emission control stuff along such lines, but I don't recall the Series II being similarly encumbered.

[jccesprit]

Hi Folks, thanks again for the suggestions.
I tried a few more checks, but no real discoveries.
However, I also decided to dismantle the in-line fuel filter which is found just before the fuel line feeds into the carbs.
I had done that previously, checking that the filter gauze was ok (which it is).
I noticed this time that the assembly has to be re-assembled carefully, because if not, it is easy to block one of the two exit holes which allow the fuel to pass through to the carbs, having been through the gauze filter.
You guessed it - I had blocked one of those holes.
I re-assembled carefully, and hey presto, I now get a flow rate of 2 litres per minute = 27 litres per hour roughly.
This has helped, in that the engine now runs for about 60 seconds, instead of 20-30 seconds.
But it still cuts out.
Its strange, it cuts out as though something gets switched off, rather than cutting out after a bit of choking/hunting. It cuts off quickly.
I have checked my oil pressure switch (in fact I renewed it a little while back). It seems fine. I can disconnect it and it has no effect on the engine cut-out. My oil pressure runs at around 45-50 psi depending on rev speed.
I don't know of any other cut-off switches.
Speaking of rev speed, I have disconnected my rev counter and sent it off for refurbishment - it packed-up about a year ago.
I had the cut-out problem before I did this, and I cant believe removing the rev counter would have this effect, but I'm getting a bit stir-crazy with this problem.
So - my aim now is to try adjusting the mix in the carbs, and playing with the ignition again to try and find a balance/combination which works.
I wish I was better at this kind of thing!
Cheers
John.

[piman]

Hello John,

when it stalls, switch off immediately and then you can check the float chamber level, they would need to be almost empty if it is a supply problem. If you are just running the car at idle, the fuel demand is very low, normally fuel starvation shows up on the road when you accelerate.
Before you start tuning, just heve a look at the plug colour, this may give some idea of what is happening? Colourless is lack of fuel, black is an excess and wet is more likely due to an ignition problem. I wouldn't bother with the timing for now, that won't cause a stall after a short period of running?

Alec

[PPV]

I agree with Alec. If it was running ok before it is unlikely things have 'just gone' out of adjustment. As I said check plug for spark and colour. Is the fuel filter full all the while whilst it is running (rev it a bit to see if it changes), the level should not really drop much at all.
Regards,

[jccesprit]

Hi folks
Once again, thanks for the suggestions.
I dismantled the carbs, found nothing that looked out of place.
I tried taking out each plug, one at a time, and checking the spark.
Good spark on every one, at a distance of about 5mm from the block. And each plug looks a healthy dry brown colour, so I reckon the fuel/air mixture is ok.
Incidentally, the engine still started each time with one plug removed.
One thing I noted is that the spark looks orange to me, rather than blue.
I have read in a couple of places that the spark should really be blue.
So now I wonder if there is a problem in the distributor - maybe the capacitor, as PPV suggested.
My distributor is a 123 electronic, and I cant replace the internal capacitor - have to replace the whole thing at a cost of around £400.
So, I have decided I am going to give up for the moment. This problem is beyond my abilities I think.
I shall wait until my favourite Jaguar specialist is back at work, and I will send the car over to him to sort it out.
Many thanks for all the help.
I will post again once the problem is resolved, and let you all know what the root cause turned out to be.
Cheers
John

[PPV]

As you have a 123 distributor then the capacitor would not be an issue. When checking plugs you should not hold them away from the engine but rest them on the engine and see if the spark jumps the gap across the electrode. The spark colour is not that important as long as it is stong, regular and jumps the gap across the electrode (it may still break down under load but if the plug looks in good condition it should be ok).
Was there fuel in each carb whenyou took the top off? Is there fuel in the filter bowl when the car is running and is it still there when the car stops?
Regards,

[jccesprit]

Hi PPV
I have the SU HIF7 carbs, which have the petrol/float chamber integral to the body of the carb.
So its not possible to check the level of petrol with the carbs on the car.
When I took the carbs off, and opened the bottom cover, both carbs had plenty of petrol in them, which duly spilled out into my capture tray.
There is fuel in the filter bowl before the engine starts, and while it is running, and when it cuts out. It looks to me like the fuel level is constant in the bowl (its always full so far as I can see).
Whatever is root cause of my problem, it is not easy to diagnose!
Cheers
John

[PPV]

If that is the case it sounds more electrical with something happening as it heats up. When it has shut down you will need to check the spark at the plugs as if you have enough fuel into the carbs and they are not blocked (unlikely with SU to totally block) then it must be electrical and if nothing moved regards timing then it is down to there being no spark.
These engines can run in appalling state so it must be something straight forward to shut it down. I don't have knowledge of the XJ6 arrangement so cannot help much more.
Best of luck,

[PaulGover]

John,

I was checking my old Series II manuals, and see there were quite a few variants by country. Could you confirm that yours doesn't have any of the emission control stuff (air pumps and the like)? They're a potential source of the problems you see.

If you have a spare spark plug, as the engine will start with one plug missing you could remove one plug lead and connect it to the spare plug. Then start the car, and watch to see if it's still sparking when the engine cuts out. That should eliminate the ignition system from the case.

(Use a spare plug, otherwise you'll have an engine pumping petrol-laden fumes out through the open plughole near a naked spark. Recipe for excitement.)

[piman]

Hello John,
do you have an ignition strobe gun? It can be used to see whether the ignition stops coinciding with the engine stalling, this gives a clear indication of ignition failure. If it is fuel then it will continue sparking as the engine runs down.

I don't like electronic ignition, as most of the time a failure is a complete stop, not a deterioration as with the old fashioned points. However as this is not the case here it's probably serviceable, but there could be a poor connection somewhere in the supply, anywhere from the ignition switch to the distributor itself? A simple check is to run a wire from a good source to the distributor and see what happens?

Alec

[jccesprit]

Hi folks,
Thanks again for further suggestions.
My car does not have any of the later emission control additions.
It does have the air pipe from the back of the exhaust manifold round to the intake on the AED unit. So far as I can tell, that pipe is clear.
I tried putting a spare spark plug on each plug lead, and checking the spark. The spark seems good on each plug, and the spark seems to be present and regular up to the point the engine cuts out.
I haven't tried the check with the strobe light yet -- will do that today, and will report back. Suspect it will be the same outcome.
I have noticed that my plugs are getting a little bit 'sooty' with all this starting and stopping. I suspect its because the engine is only running in a 'start-up' situation at present. The plugs are not wet with fuel - just getting a bit black with sooty carbon.
Incidentally, a week or so back I was so puzzled by this problem that I took the camshaft/rocker covers off on each side and checked that the cams were pushing the valves successfully - I worried that maybe I had a stuck valve or something. But they seemed to be Ok too.
Don't you just love old cars!
John

[PaulGover]

It sounds like the electronics are OK. If you have, or know someone with, a Gunson's Colortune spark plug, you should be able to watch the colour of the flame inside a cylinder. If it's a fuel problem, as seems likely, it will probably progressively go pale and then out when the engine dies. Alternatively, bluer if the engine's flooding. Not sure what more steps you could take in either case, but it would be another source of information.

Do you have the air duct across the top of the engine to the top of the exhaust, as well as the hot air pipe to the back of the exhaust? That should feed via a flexible pipe to the temperature control flap in the front of the air cleaner. The hot air pipe should end at a tatty wire-mesh-in-a-can air filter hidden behind the exhaust. I guess if that's clogged your AED might get either run too cold, and/or not have enough air, and end up over-enriching the mixture. The filter's supposed to be removable for cleaning - loosen a jubilee clip, from what I remember it's likely to be held together by rust rather than jubilance.