Intermittent power loss problem 85 XJ6

XJ
1

Hi Everyone - it’s been a while… so long I can’t even recognize the site :slight_smile:

I’ve got an intermittent problem that I’m having trouble diagnosing because I can’t cause it to happen. Let me preface this by saying that all filters and other consumables have been replaced as well as wires, plugs, rotor, cap (both lucas oem stuff). Transmission was drained and new filter installed, etc, etc…

Drove the car on a long roadtrip (1600miles roundtrip). Car was fine going, but gave me trouble coming home. About and hour into the return trip I was going about 75 mph on the highway when it began to stumble or drop-out - sort of a lack of power. A tiny bit at first… it would do it and then be okay for a minute or two until eventually it lost power to the point that I had to pull over. The car never died and seemed to idle and rev okay while I was pulled over. The only thing I noticed was a whine from the original fuel pump which wasn’t that unusual because I’ve heard it before with no issue.

After idling about 10 minutes I couldn’t find any issues after wiggling every harness in the engine compartment so I figured I’d see if I could make it to the next rest stop (20 miles). It seemed to drive normally (although I was babying it) at about 65mph the whole way. I promptly changed the fuel pump with a new one in the parking lot because I always carry a spare.

After that I drove the car another 400 miles, babying it all the way and going about 65. Also stopped every 40 or 50 miles or so to make sure all was well. The issue didn’t happen again except I did notice a lack of power once while merging - not a stumble, just a lack of power.

About 200 miles away from home I got a little brave and didn’t stop for quite a while. The car started stumbling and losing power. First every once in a while and then it started to really get bad. My memory is kinda hazy about what I did because I was really tired at this point but I believe I put the car in neutral while traveling at 55 or 60 and revved it - I think it reacted normally but when I put it into gear the power loss got really bad so I started to exit. By the time I got all the way into the ramp the car would barely move. I nursed it super slowly into a truck stop and waited for the AAA man to bring us home. During all of this it never died - it just had no power. AAA tow-truck came about an hour later at which point I drove the car normally up to the back of the bed - no evidence of power loss.

I became convinced the issue was heat related and since the AAA truck was a broken-down nightmare that I didn’t think would get up our mountain, I had the guy offload the car at the bottom and after a brief drive to confirm that it was running okay I drove it up the mountain normally.

Since then I’ve driven the car around a good amount - never any road trips just regular driving. I also completely flushed the transmission, removed both catalytic converters and had the injectors cleaned and replaced all hoses. The problem didn’t surface again until a few days ago when I was coming up the mountain fast. Got a little stumble or drop-out and promplty continued home. I’ve driven it since and it’s normal.

So really the only thing these events seem to have in common are that they happened when I was going at highway speeds. I’ve jiggled, moved and molested every harness to make it happen again to no avail. I know this is reaching but could it possibly be a transmission issue? Like the torque converter giving up? I’ve also considered possibly that the fuel pressure regulator (original) may be failing but I when I’ve put a gauge on it in the garage it’s right where it should be. Of course I have no way to check while the problem is happening. I’m almost ready to start throwing parts at it - switching out the AFM and the ECM - but I hate to do that without a good reason.

Any thoughts?

~Mike
72 XJ6 (X2)
73 XJ6
85 XJ6

2

Welcome back, Mike! I can think of a couple of things, long shots. You didn’t mention fuel filter, but I assume you’ve dealt with that. Also, you might try opening the tank lids after a drive and see if you get the whooshing sound–clogged evap control system that causes a vacuum in the tanks against which the pump doesn’t like to pull.

Another heat related common cause is the ignition amplifier, but I notice you mentioned changing points. That’s odd for a US spec S3, but if no amplifier, then no problem there. Coil is another famous heat related failure point. Let us know what you find out.

3

Hi Bob! Good to be back and able to concentrate on the important things again (Jags)

I did change the fuel filter before the trip and I pierced the troublesome GM one way valve on the evap control system before it as well so no whoosh (it did whoosh when I first got it). If I mentioned the points it’s because I got series 1 on the brain (doing a manual tranny swap on one of the 72’s!) but the car has a normal Series 3 ignition.

Would a failing ignition amplifier or coil still allow the car to run and idle? I’ve never encountered it - usually when they’re bad the car won’t run… but is it possible? I should also mention that I rewired the coil wiring harness that runs next to the head and is usually a crispy mess (mine was)

This has really got me stumped - I feel like it might be something in the AFM or it’s harness because it never stops running it just has no power. Like maybe the ECM isn’t seeing the meter’s position correctly… I really hate that I can’t make it happen so it’s been impossible to diagnose. I’m thinking about rigging up a fuel pressure gauge so that if it does happen again while I’m driving I can at least confirm pressure (or not).

I do remember something in the receipts I have from the car that mention a yellow wire in the fuel injection harness having some kind of issue gotta dig that out and take a look at the S57 wiring manual.

Thanks!
~Mike

4

Mike, this a little out of my league…would be a good time for DD or other gurus to comment. I think that both the coil and the amp can fail when hot and then come back when cooled off. But I think you’re right that when hot and failed the engine won’t run at all. Not sure though.

Hope your Series 1s are doing well.

5

When this happens does the tach remain fairly steady, or does is drop way down? As a rule of thumb a fuel starvation issue will cause loss of power but the tach reading will remain the same, at least for a few moments. An ignition fault will usually cause the tach to drop down right away.

As mentioned both the coil and module are known for failing when hot. Sometimes with bucking and misbehavior, sometimes not. Be prepared to check your spark next time this happens. Bright blue/white is good, puny and pale yellow/orange…not so good. Or maybe you’ll get lucky and have no spark at all…in which case the possibilities are narrowed way down.

Just for giggles run a jumper from the battery to the coil “+” post and see if anything changes. If it does, suspect a faulty ignition switch or a wiring fault. Or, remove the under-dash trim panel and, with the engine running, give the wires at the ignition switch a few jiggles. Does the engine falter?

For even more giggles jumper the fuel pump relay and see what happens. Run your “+” jumper to the white/green wire(s) at the fuel pump relay. If the problem goes away then you have a problem with the fuel pump circuit…which is easy to trace.

Years and years ago I had a very similar problem with my Series III. Long story short it was a poor connection at the fuel pump relay (or was it the main relay?). I had already checked these connections but not carefully enough. It was just slightly loose. Simply re-tensioning (that is, bending) the terminal inside the relay plug solved the problem. Lesson learned.

Cheers
DD

6

Hi Doug,

In answer to your question about the tach… honestly I don’t remember. Too annoyed/tired.

It only happened to the point where I had to pull over twice and both times it was idling just fine when I did get to the side of the road. The third time was just a quick stumble and then I got it home within 2 minutes so it didn’t get a chance to continue… In retrospect I should’ve kept driving to see what it did.

I do remember that the first and second times I tried wiggling the key because I knew about the ignition switch issue but I didn’t remove the trim panel and really work it over from the connection.

If/when it happens again I’ll have the presence of mind to test it as you’ve described.

I can tell this is gonna be a challenge to figure out… if I could only force it to happen somehow. Hate this kind of thing.

Thanks!
~Mike

7

Two things to do whenever the engine misbehaves, Mike

  1. Change over to the other tank…

…one explanation of your symptoms is periodic clogging of in-tank filters by tank debris - which will drop away with less fuel demand, and certainly if the engine is stopped. With fuel restriction the engine will lose power, but may or may not idle and rev normally in neutral - as this requires but little fuel…

Changing to the other tank is in any case a vital diagnostic tool - if the engine instantly recovers; there is a fuel restriction to one tank. Saves looking for other causes. In the ‘other’ tank, previously unused, the in-tank filter will be clear - and the previous tank will clear will not selected. Adding that increased pump noise may related to the pump struggling to get fuel to pump…:slight_smile:

Driving such long distances invariably depletes tanks, and filter clogging is more likely with low fuel levels. It would also be helpful to notice if the problem appears only with a specific tank selected - to apply remedial action, filter and sump clearing there. As such clogging is unpredictable unless tank in use and fuel levels are deliberately supervised - it is utterly mystifying. And no action apart from cleaning tanks/filters will be in vain…

Adding that partial clogging of fuel lines and/or changeover valve may give the same symptoms, but usually respond to the same actions. And the pump may noise as it chews u debris…

  1. Supervise tacho, as Doug says…

…tacho anomalies implies an ignition problem - again a diagnostic tool…

The two actions must be automatic and in real time as the engine stumbles - usually one is too busy with traffic etc for conscious thinking about them… or indeed remembering ex post facto…:slight_smile:

Frank
xj6 85 Sov Europe (UK/NZ)

8

Hi Frank!

Debris being intermittently caught by the in-tank filter is a good thought… When it happens it ‘feels’ like a fuel issue. Since I’ve had the car I use one tank because the other one’s fuel level sender isn’t working - I don’t think I had the presence of mind to switch over while it was happening even though the other one has good fuel in it.

I’ve replaced the fuel filter prior to all this as well replacing one of the changeover valves because it was dead. Every time I opened the lines they were completely clean. Even when I replaced the pump on the road there was no visible evidence of debris. An in-tank filter intermittently clogged by floating debris would certainly be an explanation. The car did sit for some time.

After it happened the first time I stopped ALOT and refilled the tank pretty much every time so the level never got below 1/2 a tank. As you may recall from my first email that when I got brave and closer to home I drove it longer… and then it happened again. I’m about 90% positive the level was closer to 1/4 tank or less. Hmmmm…

Well I guess there’s nothing for it but driving around alot until it happens again and then keep my wits about me enough to perform the tests Doug and yourself have mentioned.

Thanks!
~Mike

9

RustfreeMike
December 7

Hi Frank!

Debris being intermittently caught by the in-tank filter is a good
thought… When it happens it ‘feels’ like a fuel issue. Since I’ve had the
car I use one tank because the other one’s fuel level sender isn’t working -
I don’t think I had the presence of mind to switch over while it was
happening even though the other one has good fuel in it.

I’ve replaced the fuel filter prior to all this as well replacing one of the
changeover valves because it was dead. Every time I opened the lines they
were completely clean. Even when I replaced the pump on the road there was
no visible evidence of debris. An in-tank filter intermittently clogged by
floating debris would certainly be an explanation. The car did sit for some
time.

After it happened the first time I stopped ALOT and refilled the tank pretty
much every time so the level never got below 1/2 a tank. As you may recall
longer… and then it happened again. I’m about 90% positive the level was
closer to 1/4 tank or less. Hmmmm…

Well I guess there’s nothing for it but driving around alot until it happens
again and then keep my wits about me enough to perform the tests Doug and
yourself have mentioned.

As said, Mike - an intermittent fault can only be diagnosed when it is
present…:slight_smile:

If there is in-tank debris causing the (likely) fuel starvation; the debris
collects in the sludge sump designed for the collection purpose - at the
tank drain. As there are no baffles in the tank, the shape of the tank
increases slushing about with the tank low - stirring up eventual debris.
Which is sucked up to the in-tank filter partially blocking it, and stays
there until suction ceases - then falls away. Point is that the in-tank
filters is to stop debris entering the fuel lines - keeping them clean…

I don’t say that this is necessarily the cause of your symptoms, but if
changing tanks restores engine to full power - it’s virtually certain.
Unless the other tank is empty, absent or also blocked - or your changeover
valve has failed…:slight_smile:

While driving around to provoke the fault and testing changeover effect; an
alternative is to remove the drain plugs and clean out the sludge sumps -
sort of preemptive strike. Drawback is of course that you may wait in vain
for the problem to reappear…:slight_smile:

A crude check of fuel starvation due to lack of supply is to open the filler
lids and listen to fuel returning to the tanks. Inadequate supply is likely
to hold enough pressure to the fuel rail to open the fuel pressure valve -
no fuel return…

Frank
xj6 85 Sov Europe (UK/NZ)

10

Welcome back!!!

Next time, don’t wait til things go astray. report the good with the bad!!!

  1. If you drain the tanks, you might be surprised at the detritus
    that falls out!!!

  2. HT wires react to heat. Increased resistance when hot. New HT’ s resolve that . Ohm hot vs cold. Big difference???

  3. Does "popping the bonett while underway in limp mode make it run better. Onl;y to the catch, of course, and preferably at modest speeds.

  4. Less likely. Dirty air filter. Cold engine tolerates rich mixture, even enjoys it. Hot engine eschews it!! Visible view of filter. Try sans filter run. Or merely, swap in a clean one.

Carl

11

Thanks Carl - good to be back!

I should probably drain the tank that it was running on when all of this happened but I’m also thinking it might be best to let it happen again and switch tanks as Frank suggested to confirm or debunk the ‘debris in the tank’ idea. Cause if I don’t figure out exactly what this is I’m never gonna be able to trust the car to another road trip which is unacceptable :slight_smile:

Plug wires are new, premium NGK’s as are the plugs and all filters are new as well. Something mysterious is definitely going on with this one - all the easy stuff has already been gone through.

~Mike

12

Hello, new to the forum world, and jags. Recently bought my mother a 1985 Jaguar xj6 and immediately had problems with it running out of power after driving anywhere from a few miles down the road when we first started it, to about a hundred miles where it blew a plastic coolant connector. Research on my own so far and input from the mechanic that sold it to us (also the previous owner) has indicated that it may be ethanol - I drive a Cooper S, so no stranger to higher octane. My problems aside, the vehicle continues to have this problem. After reading through this article at all the good input and great ideas, I’m curious if either I missed something about timing, or if anyone has considered whether the timing itself is actually being affected by a bad sensor in some in some or another…

I’m going to start working on my mother’s vehicle in about 2 weeks and I’m hoping that y’all true diehards can answer this question before I actually have to try it myself :rofl:. Thanks for any help, and all the help you can spare, your time is appreciated.

13

Hi Cam - you should probably start a new thread with your issue because this one is OLD :slight_smile:
I will tell you that a Series 3 XJ6 has a very basic distributor with no electronic control, so although you might be experiencing an ignition problem, it is probably not a sensor causing a timing issue.

One thing you should definitely look at however is a thin, wiring harness that runs along the intake side of the engine pretty much where the cam cover meets the head. It is almost always a crispy mess by now and that might be causing you some problems. You will find it connected to the coil. If you pull off the covering (if there is anything left) you might find some very nude copper in there whose insulation is all but crumbled to dust.

Ethanol is hygroscopic, attracting water so if you’ve got really old, stale fuel in the tanks draining 'em might be a good idea.

~Mike
72 XJ6(x2)
73 XJ6
85 XJ6
84 XJS

14

**
In short, Cam, the engine loses power after various driving distances - with what symptoms, misfiring or no/indifferent throttle response…?

Does it run OK when cold and during warm up? Does it run hot, as indicated by the temp gauge - does it misfire at any stage?

The items you mention should have a permanent effect - not coming on after some driving - so some more info is required…

Any loss of coolant - and where is the connector that blew?

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

15

Thank you much sir, I will indeed check out those wires and report back.

16

Frank, thanks for your response!

The first failure occurred when climbing a hill, the engine started to sputter and cough as if it was running out of fuel, died, and then wouldn’t start. We backed down the hill and after cranking it over several times I switched to the other gas tank and it started again. Attempting the hill again however, resulted in the same failure.

The right side tank was extremely low on gas and left side was a half a tank full, so we got some fresh 91 and filled the right side tank; ran fine after that. This is where we got the assumption that ethanol was involved.

I have less information about the second failure as it occurred after my father and I traveled an hour and a half to go pick up a different vehicle, (a Buick that had a failed alternator), and on the return trip it started having the same hiccuping and loss of power and then blew the coolant line. My father was driving the Jag and I was driving the Buick, I didn’t know about the failure until he text me an hour after I got home… luckily he has AAA.

I very much appreciate the responses, mostly wanted to throw the timing idea at Mike’s original issue. I can/will start a new thread on this issue to keep from crowding this one.

17

Cam, have you drained the (empty) tanks with sthe screw that you can see from below? After removing the large plug there will be a in-tank-filter. Then there‘s the fuel filter where the spare tyre is.
However if it occurs on both tanks it could also be a loose wire to the fuel pump in addition to the mentioned above.

Mine runs on E10 occasionally and without issue. I wouldn’t blame it, but maybe water… maybe.

18

No, not much work myself yet, my father is letting it sit a bit, n I’m unavailable for a couple of weeks. But I will look into all suggestions!

19

**
The first step when the engine starts misbehaving is instantly change to the other tank, Cam…

The point here is that with debris in the tank the in-tank filter or the line from the tank to the pump may clog up as pump is sucking petrol. Particularly when a tank is low, as the debris and petrol slosh about. Changing to the other tank the engine will then instantly recover - that the car started on the other tank sort of confirms the suspicion. If so removing the drain plug of the afflicted tank to drain of debris, and eventual water, from the tank’s sludge sump is a pertinent remedy - as David also imply.

It’s a recommended first step - but excessive debris may imply that a more thorough procedure is required…

The coolant is pressurized to around 15 psi - as coolant expands with heating up the filler cap on the expansion tank is fitted with a drain hose. The cap opens at 15 psi to vent out excessive coolant. If this hose/vent is clogged; the pressure will increase until a weak point ‘gives’, and vents coolant out. The force is otherwise on atomic level; like freezing to ice water will exert irresistible pressures…

When cold, the coolant contracts, and the proper level in the expansion tank is about half way up the sloping ridge in the tank…

The second episode may be the same as before, and the blown coolant line may just be coincidental. Indeed if the line broke first; leaking coolant may spray on the ignition system and disable ignition. In which case; replace the line, wipe off the dist and replenish coolant…:slight_smile:

However, question the driver; an overheating engine will lose power - so specifically ask about engine temp reading…

But the situation should be stabilized before innocent mothers are subject to inconvenience…:slight_smile:

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

20

Same problem on my series 1, opened tank filler and whoosh, in went the air and out went the fault. Turns out the air inlet in the filler was blocked and so I drove with the tank filler open a bit till by removing the washer. Now looking for a new cap.

Ian