Hi, XJ-S HE 3.6 manual, 1987 here.

New to the forum, and trying to improve my troubleshooting skills. Rather than throwing parts at a problem I want to get better at diagnosis. This time, though, I have already thrown some parts and, big surprise, the problem is still here!😳

To get started searching, I need to know what to call the following problem. Is it misfiring? "Rough running"? Something else?...

Seemingly out of nowhere, during a long drive in heavy rain, I started experiencing a surging/jolting/jerking problem. Initially it started off very light and occasional on motorway driving. It felt a bit like rapidly braking lightly for a split second. It got progressively worse, to the point that I limped the last mile home like I was riding a rodeo bull - that felt like rapidly slamming on the brakes pretty hard.

Assuming it was ignition related, possibly moisture-related, and in a rush to get moving again, I replaced the HV side of the ignition system - the ancient distributor leads, distributor cap and rotor arm, and spark plugs. The problem is better, but still present, and the improvement could well be totally unrelated to the work I've done so far.

Debugging advice is also very welcome.

Best,
Jeff

 

Jeff,

A minor point, but if you have a 3.6, it's not an "HE". HE relates to the V12 engine.

Anyway, is the problem now there all the time? In other words, is it always jerking / stumbling when you are now driving along in the dry? Does the engine stumble when the car is sitting idling, and when the throttle is blipped?

Also, when was your fuel filter last changed?

Paul

 

Hi Paul,

Thanks for the reply, and good to know about the HE.

I went for a short test drive yesterday, and the problem was still present but much less severe. It shows up when there is little demand on the engine, or I take my foot off the accelerator. Under heavier load I don't feel any jerking/stumbling/misfire. When stationary, the idle is fine. When I rev up and back off abruptly there seems to be a kind of faint, low pitched rumbling sound, which could be the car doing what it does on the road, but it's hard to be certain. When moving, I don't really hear anything out of the ordinary, I just feel the car jolting.

Looking in the service manual, in the engine troubleshooting section 12-15 under "Rough running, normal engine speed (less than 6 cylinders)" which seems to be the closest symptom to what I have (though I don't understand what the "less than 6 cylinders" is about), 6 of the 10 possible causes can be eliminated with a compression test.

I performed the test, and got 135 PSI +/- 5 PSI across the 6 cylinders, so that rules out anything to do with leaking valve seats / piston ring wear, etc.

So I guess it's fuel or ignition related.

By "blipped", do you mean abruptly pressing and releasing the throttle with the car stationary? It's hard to notice much when stationary - rather it's the joltiness that I notice while driving along.

Fuel filter was last changed about a year ago, I believe, when some work was done to repair a fuel leak in the boot. The fuel tank was replaced (unnecessarily, unfortunately, by a mechanic who just threw parts at the problem), along with all fuel hoses. I'd be surprised if he didn't replace the filter, but I suppose there is a slim chance. The fuel pump died a couple of months ago, which I guess could indicate crud in the fuel system...

How can I best distinguish between a fuel or ignition problem?

Cheers,
Jeff

 

Ok, new information. I finished my renewal work on the HV side of the ignition system and went for a test drive in the dry.

The car performed flawlessly for about 45 minutes under all conditions of speed, load and revs. Presumably temperature too, since I assume that will stabilise before 45min.

After that point, the car misfired/whatever-this-is-called twice in quick succession. It was barely noticeable, almost like going over a small bump. I was going at 60mph in 5th on the flat at zero acceleration.

Then slowly, over the next ~20min it got worse. The misfiring/whatever seemed worst around 1-2krpm when the car isn't doing much - e.g. just cruising through villages around 30mph. Under more load demand and higher revs, it tends to get better.

Once home, and stationary, i can now replicate the fault. It is worst when slowly increasing demand from 1 to 2krpm. It struggles as you creep up to 2k. Much above that it is fine. If you just slam the pedal down it doesn't really struggle.

I wonder what will happen if I let it cool down and try again. I wonder also what critically important tests/measurements I should be doing while the fault is present and the car isn't moving!

I've also made a rough video. The first 30 seconds are a bit crap. The audio isn't great, but it does show the fault pretty clearly (first 30 seconds the sound is a bit crap. It gets slightly better after that).

 

Quote:

The car performed flawlessly for about 45 minutes under all conditions of speed, load and revs. Presumably temperature too, since I assume that will stabilise before 45min.

The 'could be' list might get a bit long. It isn't unsual for ignition coils and igntion control modules to give trouble after absorbing engine heat for a period of time.

Coils are easy enough check with the caveat that the checking must be done while the problem is occuring. Not sure of the module specs/tests. Someone here might know. But the same caveat applies.

Cheers
DD

 

I agree with Doug, this is part of the ignition system failing when hot. I suggest the ignition amplifier (whatever the equivalent is on a 6 cylinder engine) is the most likely, but I would renew the coils, HT leads and amp, plus dizzy cap and rotor if your engine is a 3.6.

 

Thanks chaps. I've already renewed the HT leads, distributor cap and rotor, and the spark plugs (several days ago, before the long test drive).

I'm a big fan of the Scientific Method rather than throwing parts at the problem - my bigger goal, beyond a moving car, is making this a hobby - learning how to troubleshoot cars (at least old, analogue ones; my day job is electronics so I don't need any more of that!) - can you recommend any tests I can try? Are you confident it couldn't be a fuel problem? (with respect, I know zilch, I just want to know how you know the things you know!)

 

The Factory ignition module is expensive and is still available from Jaguar:
https://parts.jaguarlandroverclassic.../brand/jaguar/

However I am pretty sure that a generioc part will be available much cheaper. You signs are the classic ones of this item failing.

 

Thanks Greg. I actually bought a new ignition coil with the other bits, as one claimed to be compatible by Jayar was only £25. I haven't installed it yet, because I haven't reasoned my way into doing so, but as you suggest it I'll give it a go. My next move was going to be purchasing a series ignition spark tester, driving the car until it starts complaining, then parking up and testing the spark at each spark plug connector and at the output of the ignition coil. That would prove whether it's ignition or fuel related.

However you more experienced fellows have a more refined mental library of what different faults look like and what their causes are likely to be, so you can clearly shortcut past a lot of diagnostic tests and jump to a likely estimate of which part needs replacing.

I am still very much learning how to think in this space.

 

Just to be clear, the ignition module is a separate part from the coil.
In general the 3.6 is a very reliable engine; so do not be downhearted!

Another very important point, if you do not already know aboutn it: on the 3.6 there is a coolant hose hidden under the intake manifold. THis comes directly off the rear of the water pump. It fails catastrophically when old, because new owners do not know it is there. Renew it if you have not done so already.
https://parts.jaguarlandroverclassic.../brand/jaguar/

Part number 3 in this diagram.

 

Ah, thanks for clearing that up. That's the module that the service manual warns is full of highly toxic beryllium oxide .

Great, not downhearted at all. Just hungry to learn to troubleshoot.

Thanks, will do.

Also, a garage did a temporary repair on a perished hose in the vacuum advance system. I haven't got round to replacing it and I wouldn't be surprised if it were leaking. Could this be the cause? How could I eliminate it?

 

Not a cause of it cutting out, AFAIK. Anyway, replace any old vacuum hose with silicone vac hose. This is the best UK supplier ; very helpful guys.
https://www.viperperformance.co.uk/s...vacuum-tubing/

Check the internal diameter, usually it is 4mm ID on Jaguars of this era.

 

Wanting to troubleshoot versus throw parts is commendable. However.....

Some problems are easier to troubleshoot than others. It's generally easy to troubleshoot a circuit, or coil, or relay, or fuel pump, or fuel injector, or whatever, that is flat-out inoperative. What some refer to as a "hard failure".

More difficult is troubleshooting problems that occur intermittently, or only under xxx-circumstances.

Sometimes, too, diagnosis and testing undercovers xxx-widget that is right on the cusp of being within specification, or barely outside of specification....leaving you in a quandry as to whether the widget should truly be condemned and replaced.....or not. If it's a $20 widget you shrug your shoulders and replace it. It's a guess, but not a hateful one and it is based on some logic and testing.

If it's a $500 widget...well, this is where many would hope the problem actually becomes much worse so a more concrete diagosis can be made.

Sometimes a DIYer doesn't know the specifications or test procedure for xxx-widget. Your ignition module, for instance. The spirit is willing but there's no information readily at hand. So...you replace it or do more digging for the info you need. The Jaguar ROM probably has a test procedure.

A fuel injector problem can sometimes mimic an ignition problem, yes. An injector can be failing, or the wires to the injector failing, or the ECU (which controls the injectors) could be failing. Here again "hard failures" are easy to root out. Soft failures can be more challenging.

Sometimes xxx-symptom is the result of fault "stack up". That is, several minor faults contributing to the problem.

One thing for sure, though. Always, always check/clean/tighten all relevent electrical connections and grounds/earths before replacing parts.

Cheers
DD

 

IF nothing obvoius presents then Start at the plugs and work backwards. The plugs will tell a story. Google how to read a plug. Are they oily? Are they Carbonized? Are they white? (lean burn). Check the porcelain for cracks. May want to just replace them with a copper set, is still about 4$ each. Removing the HT leads will allow you to see if anything of them are broken. You can ohm them out. You dont need to klnow the correct value, than will all be close, the bad one will be doing it's own thing. Have a look in side the dizzy for arching, cracks or burned terminals.

If you can get it to misbehave at idle you can hold an un-lit propane torch near the intake duct and see if it smooths out. If it does then you've got a fuel problem.

You can run a long set of wires to a bulb or volt meter into the car, ground it on the dist and pickup the +12 from the coil's +12 supply and observer the bulb's or VM's behavior when the problem presents. That will inform you if you issue is a supply or ground issue at the ignition. A general check of all electrical connection is always a good idea. Also have a look at the Tach, if it dives when the problem presents then it a pickup or coil related issue.


Another technique is to use a spray bottle in the dark filled with tap water. Hard or mineral water works best. The technique is get to a very dark place, home garage with the door open on a dark night is good and open the hood, with all lights off and spray water on everything, if the engine stumbles you may see an arch in the dark or may have found a split vacuum hose. If the engine stalls you may have pushed a loose connection over the edge.

You may want to just replace the ignition module as a matter of course. If the old one is good put in in the arm rest or glove box as a spare.

 

Thanks Doug, that resonates with me from debugging other types of system, and makes a lot of sense. I certainly want to apply as much science as possible, but I will try not to be too disappointed when I resort to swapping out an affordable part without much evidence. Who knows, maybe I'm saving myself from a breakdown in the future

I swapped out the ignition coil and the fault remains the same. I've ordered a new ignition module for £50.

 

This is great, ics, some good debugging tests here - just the kind of thing I'm after.

I've renewed the plugs (which looked ok), HT leads, distributor cap and rotor.

The tach does dip slightly in sync with the misfiring, but I wouldn't say it dives. Good idea, though. The tach signal is sourced from the ignition amplifier in relation to the signals from the pickup / coil, so that does imply that the problem is to do with the amplifier/pickup and not the HV system.

 

jeffmakes,

So many owners (and workshops) seem to change parts when there's a problem without any definitive understanding of whether those parts are causing the issue. I'm a huge believer in diagnosing the problem before ever changing any parts. However, I agree with Doug that there are a few occasions where it just might make financial sense when the problem is difficult to pinpoint.

You also have one of those annoying problems that isn't about a no-start, or present at idle. The consistency of how the problem seems to manifest itself makes me also think it's electrical. And the fact that it doesn't seem to happen at WOT opening would make me think it's not fuel starvation or injectors. Having said that, if I didn't know the background to when the fuel filter was changed, I would do that anyway. I'd be minded to try 3 things:

- Remove one plug lead at a time and try the same rpm buildup. It won't get better, in theory it should get worse on every occasion, unless it's one cylinder causing the problem. If so, removing that lead might mean that the problem remains the same and you've pinned down the problem cylinder.

- Remove each injector and check one at a time for spray pattern in a jar, especially as you build the revs to the problem point.

- Warm the engine, remove all the plugs and clean them (noticing if any of them are a different colour to the others). Replace the plugs, run the engine, build the revs to the problem point and then immediately turn off the key whilst leaving the throttle position at the problem point. Now remove the plugs and check the colour. That can help you to clarify if it's a ignition stumble or a fuel stumble.

Good luck

Paul

 

You're a man after my own heart, Paul. I've pretty much given up on garages diagnosing problems - you just get an invoice saying "I fixed the problem by changing the right front wheel bearing, the glovebox closure sensor, the water pump gasket and left windscreen wiper. It'll be £300 in parts and 4 hours labour". Most recently I had a fuel leak in the boot and figured I'd get a "professional" to sort it out since it could be a safety issue. He replaced a bunch of stuff, including a perfectly fine petrol tank which he claimed was at fault. When I went to collect it I asked him to show me the problem. He pointed to some minor surface rust on the old tank and claimed it was "rusted through". I scratched it off with a fingernail but he still charged £600 for the new part plus more for the rest of the job.

Anyway, I digress.

Thanks for those three test ideas. Very valuable. Let me see if I understand:
the issue
- First one I understand
- Second - can you please elaborate a little on the spray pattern - I don't know anything about fuel injectors. Presumably I'm looking for a consistent pattern over the full RPM range?
- Third - when I remove the plugs and check the colour, I guess if the ignition is at fault, there will be unburnt (black) deposits, whereas if there's insufficient fuel it will look lean (whitish). Do I have that right?

 

jeffmakes,

Yes, spray pattern will be broadly the same irrespective of revs. Worth testing as I guess there's just a slight chance that one or more will dribble or stumble as revs build up if there's an injector or fuel supply problem.

As regards plugs, I think it's less likely that a single plug would stumble on revs opening but still be able to handle WOT rev build up. It seems more likely that it's a general ignition failure in which case they'll all be the same colour.

The bit that is slightly confusing is that it seems to stumble when you gradually build up revs but not if you floor it? We detest problems like that which are elusive to diagnose. Like others have said, I'm minded to think it's ignition not fuel, but I'd change the filter, also check the swirl pot for the usual sediment blockage first before doing any other tests.

Cheers

Paul

 

Since I had already ordered the new ignition amp before getting your advice on diagnosis I decided to fit it and go for a drive. I almost thought I'd fixed it, but towards the end of the drive I did notice a couple of little jolts. I parked up to get some food, and after about an hour cooling down I started the engine and the car had some new behaviour. When idling, it now cycles between approx 1500rpm and approx 500rpm, at a rate of about 1Hz, very repeatably.

Here's a video:

Given that I have now mindlessly replaced almost everything in the ignition system I will now move on to the fuel system, this time deploying more science.

I would welcome any more suggestions, of course!