Intermittent power loss

XJ
1

Good evening everybody,

currently, I’m getting a bit more of that old Jag wrenching than I really care for:

Some weeks ago I had an unhappy experience when the Jag engine - warm and running without choke - all of a sudden lost power. The engine would sputter, didn’t rev, but could be kept running. I was able to limp it back home, but had to cancel an appointment in Zurich as I didn’t dare to go on the motorway with a car in such condition. Some time later the engine resumed to proper functioning. As that hasn’t occurred again I thought it might have been the result of some water in the tank.

Unfortunately, it happened again yesterday and today. The malfunction is intermittent. Sometimes it occurs in a mild form, kind of strangling the engine but letting me travel on at like 40 mph, sometimes it is violent and then the car will barely creep on at like 10 - 15 mph with strong sputtering. I switched tanks with no immediate reaction. It seems to occur mostly upon marked changes of positions (once following a left turn, today when going through a trough), but sometimes just anytime. It’ll go on for some time until regular operation kicks in.

There is no visible smoke behind the car (at least not visible from the driver’s seat), there is no strong smell either. Twice I stopped and opened the hood to notice nothing impressive, again no smoke, no smell today. Last time I saw some fuel underneath the rear carb.

My initial thought (and feel) was fuel: the engine behaved as if it were flooded, but there was no black smoke nor any overwhelming fuel smell or puddles of fuel.

The symptoms might also occur if the engine didn’t net enough fuel - or one carb was not getting enough fuel. Typical rusty tank debris in carb scenario - but I have the standard fuel filter in the trunk and two additional fuel filters before the fuel pumps. Could it be an intermittent power out at the fuel pumps? I switched tanks to no avail and the pumps have individual grounds, so it could only be the power supply of the pumps upstream.

Finally, spark: a 1-2-3 ignition is installed that has performed flawlessly over the last 12 years. Is that a typical failure mode?

Any idea welcome!

Thanks as always

Jochen

75 XJ6L 4.2 auto (UK spec)

2

Jochen, I’m no expert but I’ve had a similar intermittent situation twice.

Once it was failure of the fuel tank venting system (both sides). a partial vacuum in the tanks overcame the fuel pumps’ ability to create a head of pressure. Opening both tank caps solved the problem once I diagnosed it. Unlikely I suspect.

The second time was finally traced to a sticking float valve on the rear carb. The valve stuck in the closed position, not the open one as is more common.

Viel Glück!

3

My Series 2 did the same with a failing coil. Does the coil feel hot? Tape one with the same connections in place and swap when it happens. Also check the king lead for night time arcing.

Of course I cheated. I knew it was ignition, not fuel, because the symptoms were the same whether I switched from gasoline to LPG or vice versa. I did like that old Daimler.

4

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Robert’s and Peter’s suggest valid possibilities, Jochen - another is crossfiring…

Check dist lid for cracks and dirt; in one instance a leaking/loose radiator hose dripped coolant on to the lid - with symptoms as described. Checking for arcing in the dark may also reveal something…

And checking the rotor is advisable - but a plain electronic fault in the 1-2-3 is not too improbable…

The real problem is that it is intermittent - you can only find a fault when it is present…

Frank
xj6 85 Sov Europe (UK/NZ)
**

5

I’m no detective I would start here, at the float bowl. Good luck. Paul

6

Thanks Robert and Paul!

Fuel ventilation in the tanks should not be a problem in the 75 UK car as the fuel tanks are not sealed against the atmosphere. They are so loosely closed that there shouldn’t be any vacuum building up

The stuck float valve could be an idea, as you suggest, especially as I once spotted some wetness around the rear carb float chamber. Will go after this!

Vielen Dank und bis bald

Jochen

75 XJ6L 4.2 auto (UK spec)

7

Good to hear that spark must not be ruled out, Pete and Frank. Your coil theory, Pete, is supported by the fact that the misconduct always occurred with a hot engine and - the first time - on a very hot day. Night time checking for arcing or crossfiring is a good idea and an easy check as well. In fact, Frank, 123ignition recommend a new cap every 30000 kms - and I could be around that mileage.

Thanks again

Jochen

75 XJ6L 4.2 auto (UK spec)

8

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It’s a bit odd, Jochen - the cap is not really a wear part…

However, electronic ignition in general operates with much higher ignition voltages - which may increase risk of component failure. And the 1-2-3 voltage/energy may be higher than usual…

Frank
xj6 85 Sov Europe (UK/NZ)
**

9

I had a similar problem with my series 1, it was indeed the cap on the fuel tank not venting so when the cap was opened the tank expanded and there was a whoosh of air going in. Cleaning the venting system fixed that. I also had the same problem with my series 2 E Type which had a 123 disributor and the problem was indeed the cap. The central contatc jammed and didn’t drop to connect properly when it got hot. As well as intermittent rough running the tachometer bounced about all over the place as well.

Good luck

Ian

10

Three points:

  1. Open tank vent system. Does the cap seal and another opening is provided? If so the other means to vent could be occluded…

  2. Ditto, on the tach behavior at the time of distress. The tach is triggered by the ignition!!!

  3. As Frank mentioned, odd that cap replacement is a regular maintenance item?? Why? Intriguing…

Carl

11

Thank you all, gentlemen,

next time it happens I’ll pop up the fuel tank lids.

As for the ignition, Carl - don’t ask me why 123 want new caps and rotors every 30000 kms, but they do https://www.123ignitionshop.com/manuals/123JAG6.pdf (last page). Frank may have a point as regards higher voltages; I’d suspect they simply want to make sure they don’t get any bad publicity or unsatisfied customers for reasons unrelated to their product.

The symptom you mention, Ian, may rather speak against an ignition issue: the tach is absolutely calm and evenly mirrors the low revs during malfunction.

Have a nice week end

Jochen

75 XJ6L 4.2 auto (UK spec)

12

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With the CE electronic ignition, the tacho uses the amplifier ign amp to rune the speedo, Jochen - but doesn’t really care if the sparks does not reach the plugs. Ie, a faulty rotor/dist lid, or a coil fault, will show normal ignition on the tacho…

Your 123 may work the same way - or, like some EI systems, drive the speedo directly.

Basically; the tacho shows only whether trigger signal is working - and if not; ignition will certainly fail…:slight_smile:

Frank
xj6 85 Sov Europe (UK/NZ)
**

13

Good you mention that, Frank: a friend who plans to put a 123 ignition into his TR 250 told me he’s looking for a Jaguar SII tacho as they work directly with the 123 ignition. He intends to put the innards of the Jag tach into the housing of his TR tacho.

But that means that at least my signal is good.

Thanks again

Jochen

75 XJ6L 4.2 auto (UK spec)

14

Sorry to revive this thread, but …

in 2019 I swapped the coil and had the impression everything was good again.

Now, i.e. in the course of the last weeks, I got the feeling the engine felt “restricted” every now and then, but still intermittent, i.e. freed itself up after some seconds. Today, for the first time it occurred violently, allowing barely to limp at 900 RPM, but still, intermittently, sometimes allowing to rev freely in N, sometimes allowing the car to go normally through the gears. All through these incidents voltage, water temp and oil pressure keep normal and constant, no smokes, no loud backfires.

My next plans are to

  • check for arcing in the dark
  • undo the rear fuel bowl to check for debris or any other sign of a sticky float
  • get a new rotor cap, as recommended by 1-2-3 ignition
  • get a second new coil - any recommendations? The last one looked nice and was labelled Lucas, but obviously not original.

Co-incidence or accidents:

Last week it seems the car spilled out a larger amount of oil shortly after - not upon - being started, but no clear indication where the oil came from nor repeat situation (maybe it even was another car - as a Jag driver you always feel guilty when there is a puddle of oil close to where your car was parked).

Last week I had a spirited drive, some 100 mls with 50 mls autobahn at + 80 mph, no ill effect except the usual gurgling and - not usual - some coolant being spilled out upon parking. Closer examination revealed the water filler lid on the expansion tank hat disintegrated: Out of 360° metal rim holding the rubber seal in place only about 20° were left, the rest “down the drain”. Could these metal parts in the coolant circuit somehow affect engine running?

Standing underneath the car with the engine on and the car in drive, you hear occasiona “tuc” knocking sounds from the gearbox - could be some kind of governor just shifting in or out, nothing exciting, but: does it belong there?

Any further idea is welcome.

Happy Easter holidays to whoever celebrates and/or is able to enjoy them

Jochen

75 XJ6L 4.2 auto (UK spec)

15

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By ‘violently’, Jochen; do you mean ‘very much enhanced’ or ‘shakingly’…?

It sounds like fuel starvation - which may indeed be water in a carb, or some obstruction.
Did you change to the other tank at the indication of misbehaving?

An ignition fault would normally lead to pronounced misfiring/irregular running - but signs may not always give a clearcut difference between fuel and ign…

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It will only affect coolant circulation - which in turn may affect engine temperature. In principle, the expansion tank is not involved in actual water circulation, so you may find the bits in the tank? Being metal they may drop further down, but as there is no fast coolant flow; they should stay there?

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The most likely reason is the gearbox, or whatever, is harmlessly ‘rattling’ against something - to be investigated. Actually, it sounds more like the noise that is best counterd by turning up the radio volume…:slight_smile:

But honestly, being under a car with the engine idling in gear is not my thing…

Frank
xj6 85 Sov Europe (UK/NZ)
**

16

Thanks Frank,

as I wrote there was nothing like backfires, let alone mechanical noises, just what you’d expect from a seriously restricted engine. It felt like with the choke on for too long, just without the black smoke. I used the term “violent” just to tell that this wasn’t just abound some top end power, but really serious forcing me to limp home at 900 RPM and 15 mph, hoping it would last … until the engine would free itself up and resume regular operation. I switched tanks to no avail and - luckily - there was no “water incidence” over the last weeks…

Thanks for your thoughts about that coolant lid - that’s what I was expecting as well. As once or twice the car would accept pedal input in first, but not in D, I wanted to make sure it isn’t the auto trans playing up. You’re right of course - standing underneath a car on a lift with running engine and in D is no comfortable scene, but it seemed safe enough for me, as the lift is reliable and I didn’t have any clue hinting at mechanical faults. Of course, the man in the driver’s seat had one hand on the key, the other on the gear lever …

Today I checked the float chambers, to no avail. Gas inside was clean and on equal level. The floats looked brand new and would move the valves easily. Nothing gummed up, nothing grimy or dirty.

The one thing I noticed though was that the elbow rubber tube coming from the breather pipe to the rear carb had split over almost one inch at the end. For the moment I fixed this with some plastic tape. It might well be that, depending on forces of gravity and load the split end might have moved and either admitted air to enter behind the rear carb thus leaning out the rear three cylinders or closed itself thus resuming operation. Is that just wishful thinking or a well-founded theory? Of course, I’ll put in new rubber to falsify!

Thanks again and all the best

Jochen

75 XJ6L 4.2 auto (UK spec)

17

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Falsifying is the right way forward, Jochen…

Loss of feed from one carb is plausible, of course - the reason for that is another matter. The problem with an intermittent fault is to catch it ‘on the hop’; the fault is not detectable when engine is running normally. It is so much easier when the fault can be provoked at will - like fiddling with the broken hose, or whatever…?

And as it is ‘not good’ to fall in love with a specific theory - keep an open mind. Like, when/if the fault appear; look at the tacho. It should faithfully show the actual engine rpms - if it is not; the 123 unit may be involved…

Frank
xj6 85 Sov Europe (UK/NZ)
**

18

That was what I thought of. The coolant system is not bothered by a few metal flakes, there is larger particles in there, wires from casting and general debris. Watch your coolant tank!

Pull off the rubber elbow and see if the symptoms are the same. Mine idles like that but would it run? Standing under your car was fun albeit always a bit scary when they come to life. I would watch the oil puddles closely; and the transmission will make some noise in action, keep an eye on that too; I have a free spare if something exploded, as long as you check the level in P while running, the shifts are decent and the oil looks good don’t worry.

19

Thanks, David and Frank,

you’re right - I should have immediately tried to run the engine and fiddle with the split elbow hose to find out about the effect.

Will do that and, in case it is not conclusive, follow the fuel track. As switching the tanks didn’t help and SII cars have two fuel pumps I will try to check the single in line fuel filter in the spare wheel well.

Before getting back to the ignition track just one question to the experts: During all the acting up of the engine the tach always mirrored flawlessly what was going on, no cutting out, no bouncing, no oscillating. Well, if it did I’d have strong evidence against the ignition system, but does correct operation help in the opposite direction?

Thanks again and stay safe

Jochen

75 XJ6L 4.2 auto (UK spec)

20

Hi Jochen,

Just out of curiosity: what type of fuel do you feed your cat?