'86 XJ6 VDP extremely rough running and cutting out

Hello Jaguar Lovers,

This site has been a huge help to me throughout my endeavours, and I think its time for me to post my own story and questions.

I bought a 1986 XJ6 Vanden Plas in November of 2022. The owner informed me of some issues, the headlights didn’t work, the right gas tank either, although it was brand new along with the fuel changeover valve. Whatever, I’d fix these problems when I got it to my garage. Despite these issues he maintained it was drivable, and I did indeed drive it the 240 miles home with no issues (save for some scary moments with the failing brake booster/master cylinder). I and my more-mechanically experienced father set about working on the car. We replaced the:

-Valve Cover Gaskets (Old ones were nonexistent and replaced with RTV sealant. Terrifying, but at least the valves were in tremendous condition)

-Brake Booster

-Brake Master Cylinder

-Repaired/replaced headlight and highbeam wiring/relays

-Repaired/Replaced front left indicator bulb/wiring

-Right fuel tank filler and gasket (Was more RTV sealant)

-Power Steering Rack

-Custom Exhaust Headers (Had a bad experience with a slow fabricator (car sat for 4 months) after being unable to find any fair condition manifolds for sale. There’s still a medium exhaust leak where it connects with the stock system after the catalytic converter delete. There is also the appropriate bung for the O2 sensor.)

Along with other cosmetic changes like

-Wooden veneer replacement for the center console, (previous was cracked from 37 years of sun damage)

-Replaced Aftermarket stereo with newer aftermarket stereo connected to new dashcam/backup camera

-New speedometer/odometer unit (Old one was stuck at 120,000 miles)

After fitting the exhaust, we discovered an issue with the engine cutting out whenever it was shifted from park. We did some more work, taking apart and refitting the air intake system which we had been cleaning seemed to fix this problem for some reason. Then the engine would run poorly, then die.

We replaced:

-The ignition switch


-Ignition Amplifier switch (the GM Module inside the amplifier, not the amplifier itself)

-ballast resistor + repaired wiring

-new distributor cap/rotor (discovered burn marks)

-spark plugs

-spark plug wires (chewed on)

-coolant temperature sensor

At some point through this, fuel started coming out of the tanks while the engine was running. We

-performed a successful fuel pressure test

-new fuel injectors

-fuel pressure regulator

-Also tested for vacuum leaks with a smoke test, didn’t find any.

At this point, after all of that and losing spark, I decided to take it to the local Jaguar specialist. They did regain spark, after determining the air gap in the distributor to be incorrect, and replacing the right return valve and rewiring the changeover valve fixed the fueling issue. However, the car still ran extremely rough, and the engine cuts out about 40 seconds or less after its jump. I remember hearing the mechanic saying he advanced he timing to help it out. They offered to keep it and keep working to make it run properly. Their 150/hr labor rates and invoices as descriptive as a sentence per 5 hours, combined with lack of care to even put the trunk panels back together or include the air filter in the car when I take it back leads me to shy away from them. I’d like to have a day soon in which I lock myself in my garage and just methodically work through possible issues, but I don’t really have many ideas. I’ve looked into ground straps, and learned of their importance. Upon touching the ground strap between the motor and body on the passenger side underneath the car, cubic centimeters of dirt fell off. I know it can’t help, and its certainly something I’ll clean up, but I don’t think that’s my issue. If its dirty now it was just as dirty 9 months ago when I drove it 240 miles with no issues. Beyond that, I’m pretty lost on possibilities.

I can’t afford to let other people work on my car. I only went there for their expertise, but I believe this site has more collective knowledge to offer. I imagine I’m one of the younger members here, and my lack of background knowledge has been slowing me down the whole time. No one in my life understands why I dedicate so much time and effort to such an expensive car as a broke college student, but I’m certain people here do. I bought the car with the intention of persevering through any and all costs and efforts, and I intend to continue. I’ll post a video of the engine running/dying soon, any advice would be greatly appreciated.

Thank you


If the starter works fine it’s not going to be the ground strap. Maybe the ones at the rear of the coolant rail.
Does the engine restart well, does priming the fuel pump a few times (key to crank in gear) change anything?
Search forum for ‘bullet connector injectors’ and verify injectors just after it dies.
Check fuel pressure while dying, do post the video.
40s sounds too long for a no fuel issue, but maybe too little - check fuel flow by making sure there’s a healthy return into the selected tank as can be seen through the filler.
Do you have lambda/ catalytic converters?
Does it run better with some throttle input? Does idling improve or worsen when you spray some starting fluid into the intake as it stumbles?

Welcome on board, Graham!

Refitting the air intake system probably fixed an air leak there, Graham - which likely caused the stalling in gear…

We need some details on the idle/stall situation, and David’s remarks are very pertinent; does the engine run, and keep running, with throttle input or does it die within the same time frame? In which case; it is likely some restrictions causing fuel pressure drop - and supervising fuel pressure with the engine is important.

Connect a fuel pressure gauge to the cold start injector and watch fuel pressure. As David also says; open fuel filler cap and listen to fuel return - it is a crude test of fuel pressure. As long as pressure is adequate there will be a return to the selected tank - if return stops; it means pressure is lost, confirming there is a fuel problem which is a grand step forward…

If the engine does run at higher throttle settings; connect a vacuum gauge to the vacuum hose at the fuel pressure regulator and set the engine to run at initial 2500 rpms. Watch the vacuum gauge; if it gradually drops; it indicates clogging exhaust/converter - which will kil the engine. To confirm you may disconnect the exhaust at the downpipe - if the engine then behaves (it will be veeery noisy; pursue the matter…

This may be a heat problem; as the engine warms up, the mixture is gradually leaning out. It is normally countered by idle settings to prevent idle dropping to the stalling point. Hence, the idle must be set, fully hot, to 800+ rpms - a functioning AAV will the run the cold engine at some 200 - 400 rpms higher than hot set idle. If the idle is set hot with the AAV open the engine may be difficult to start and may stall as it warms up…

There is a seldom occurring fault with the non-return valve between the pump and the fuel filter. Bypass the valve by connecting the fuel pump directly to the fuel filter - it may rectify the problem…

I don’t know what the Jaguar shop actually did; the certainly should have checked compression and at least pump fuel pressure, but maybe not regulated pressure - ask them. To their excuse; unless they have a old mechanic used to the xk, they are handicapped, and tome must be used/wasted.

What I can say is that you now have a lot of working spare parts, if you have kept changed items. If there is no change to engine behaviour with a parts change; the part is not at fault - and is likely in working order. Example, failure of a brake master cylinder will cause the pedal to go halfway down, but if braking is retained with no excessive pedal force; the brake booster is working. The fault is only with the master cylinder…

Keep trying; the problem is likely minor - and once found ‘easy’ to fix…:slight_smile:

xj6 85 Sov Europe (UK/NZ)


welcome aboard … and don’t worry: there’s enough Jaguholics unanonymous around here!

It might help others help you though, if you gave more of the specifications of your car: year, market and state of originality are crucial … even though some issues are fairly universal.

If this is your first Jag it might be helpful to get yourself the official Repair Operations Manual and the S57 electric guide.

Then do exactly what you described: check air, fuel and spark methodically. Start with a compression test to exclude major issues. While you’re there, what do the plugs look like? Then continue with the HT wires (gotten mixed up?). Can you check a sound spark?

From there on it gets more SIII specific: fuel pressure, injectors operative, AAV, o2 sensor?

Good luck


75 XJ6L 4.2 auto (UK spec)

The ground strap at the end of the coolant rail does seem a little dirty, but nothing like the one underneath. I’ll still clean it, but it’s good to know the ground strap at the bottom isn’t my main issue.
The engine did restart after, though as poorly as the first time. I haven’t verified the injectors or fuel pressure during the death, but its at the top of my list of things to do on my next day off. I’ll post the video tomorrow morning and save my neighbors the exhaust leak noise now.
I’ll look through the filler and verify a healthy return too. I have a lambda sensor but no catalytic converter, I had the fabricator include a space in the custom headers for the stock O2 sensor that’s in place now.
It did run a bit better with some throttle input, and I’ll see how starting fluid affects the idle too.
I appreciate your help

Thank you

The air intake leak does make sense for that issue.

If I remember correctly the engine ran better on the throttle, but I let go of it before it died so I’m unsure if it would continue to do so with throttle input. Its certainly another thing I’ll test my day off on Monday.

Good to know about the lack of fuel return signaling loss of pressure, I’ll test with the vacuum gauge at the fuel pressure regulator just in case, but I don’t think the exhaust is clogged - there’s about a 2 centimeter exhaust leak where the end of the downpipe meets the rest of the exhaust system. I still need to fix that up, but I’ll do so when I’m able to drive the car to a local exhaust shop. My video tomorrow will be a bit noisy.

The heat problem is really interesting, I did the car to another mechanic prior to the Jaguar specialists and I know they adjusted something with the fuel mixture. That Mechanic, despite specialising in European cars had been more knowledgeable with German and Saab and didn’t have the desire to continue something he knew little about. I know the Jaguar specialists at least took a look at the mixture screw. It does coincide with the engine cutting out as it starts to warm up, if not the rough issue. As I understand it, it slowly closes as the engine warms up. I’ve read here to test it by comparing the opening when cold to directly after the engine had been warm, and it makes sense to me.

Are there any downsides to the non-return valve bypass? If not I won’t hesitate to make the change permanent.

The Jaguar shop failed to check compression, but I know they at least tested fuel pump operation. I tested fuel pressure back in June and it was fine, but I’ll definitely monitor it as the engine dies. Of their invoice and what they told me, all they did was: readjust the distributor air gap, check fuel pump operation, clean and remove spark plugs, repair wiring for the coil, amplifier box, fuel changeover valve, fuel injector pulse from the control module, and put a new return valve for the right tank.

I do have a lot of spare parts, I have kept almost all of the items. You’re right about replacing parts maybe not being the best first option, but with the condition of everything I’ve replaced there’s nothing I regret upgrading. The Master Cylinder for example had degraded to a point where it had leaked into the brake booster, and both needed replacing. The way I see it, if it lasted 37 years, the new ones will last me the next 37.
Thank you for all of the help and suggestions. I’m creating a list from all of these responses to methodically test. Considering the cars’ mechanical ability to drive me 240 miles almost flawlessly when I first purchased it only 9 months ago, it makes sense that its something small holding it back now. But all of the ideas and suggestions here have certainly provided a lot of options for me to check and report back.
Thank you for your help

Thank you
Certainly, my car is an Automatic US Spec 1986 XJ6 Vanden Plas - The mechanical parts are as far as I can tell completely original (before my own changes like the custom exhaust headers), but there have been some less-than-quality wiring…enhancements made long before my ownership, primarily on the intake side of the engine. Apart from that, the only nonstandard parts of the car had been the aftermarket stereo and the antique German fire extinguisher I found under the passenger seat.

I’ll invest in the Official Manual at some point, and after looking at the S57 Electric Guide I’m surprised I haven’t heard of it sooner. I have at least found myself a Hayne’s Manual and a less thorough colored wiring diagram coupled with the one that came with the car have been enough for now, but they omit important things like where the actual grounds are located on the car.

I’ll do a compression test just to rule it out, and take a look at the plugs. The wires should be in the correct order, as they were last put on by a reputable local Jaguar specialist shop, but I’ll double check both that and the spark strength while I’m there.
I’ve created a list of things to methodically do/test my day off Monday, including
-cleaning the water rail ground strap
-testing the engine restarting/throttle input/with fuel pump priming
-monitoring fuel pressure and injectors while dying
-check fuel flow into tank
-fuel pressure gauge cold start injector
-testing for clogged exhaust
-testing AAV
-compression test
-fuel pump pressure test
-fuel pump check valve bypass
-double check plugs, spark and HT wires
I’m going to slowly go through everything and report back in as much detail as I can.
Thank you for your help

Indeed, Graham - it is easily checked. Remove the air hose on top of the AAV and look at the slide. It should be about half open with the engine cold and closed with the engine hot. The slide is spring loaded and sometimes sticks - you can ‘exercise’ the slide with a small screwdriver to loosen it.

The gist is that your cold idle should be 1000 - 1200+ rpms, and the idle should then gradually drop to hot set idle as the engine warms up. I’m not sure if your problems have allowed the engine to fully warm up? The bimetallic spring is also heated by a coil, which will heat up and close the slide shortening the closing. The heating coil should read either 22 or 33 ohms confirming it is intact ; check.

However, another hose on the underside of the AAV connects it to the throttle body. Check that this hose is connected - either by looking from underneath or by removing the AAV to verify. If this is disconnected; it is a massive air leak, and if the hot idle is set with the hose disconnected - the engine will likely stop as it heats up…

The valve, a ‘brass’ hexagonal contraption with a piston inside. When pump pressure is applied; fuel is delivered through the valve to the filter and fuel rail. Without pressure, pump stopped, the valve closes, and the fuel pressure is retained in the rail - to have fuel pressure at first crank, before the pump builds up pressure.

However the delay is immaterial in practice, but also; as the piston in the valve moves to the ‘no pressure’ position it opens a vent line to the fuel return hose - to vent air from the pump. As a permanent bypass of the may cause leaking into the boot - some care must be taken. Otherwise the bypass has little negative effects.

We have, over the years, had a couple of instances where the piston has ‘tumbled’, partially blocking the fuel flow. Leading to starvation symptoms that may be the cause of your problems. bypass the valve as a test, but if no change - keep the valve ‘on line’.

The ‘to do’ list you presented to Jochen is certainly spot on in addition to the above. Both to compensate for things not done by the Jaguar shop, with limited knowledge of the xk engine - and particularly the ignition sequence. It is very easy to ‘cross’ the plug wires - and considerable care must be taken to get it right…

xj6 85 Sov Europe (UK/NZ)

The video is uploading to youtube as I write this, I had some technical difficulty uploading it directly
The Engine ran longer than I expected, about 2 minutes. The exhaust leak certainly occupies some noise, but it seemed to me that the engine ran better with some throttle input, but I’m not sure how much the extra noise just masked anything. After holding it at 2000 RPM for a bit, I then pushed it up to 3000 for a second before letting go of the throttle, that’s when the engine cut out. So far the AAV seems like the most plausible option to me, As the engine does idle at about 800 RPM cold, and it’s incredibly possible for it to have just been set without taking engine temperature into account. It seems to me like the higher revs just warmed the engine up more, causing the stop as you theorized.

Thank you for the information on the fuel valve, I’ll take care to put it back if nothing changes.
The to-do-list still remains for my day off, and I’ll update and provide pictures and I’m sure many other questions
Thank you for your help


It’s not the aav. The engine seems to run fine, but not on all cylinders. Pull one injector (plug!) at a time until you identify the missing cylinder.
I think the ignition is working well because the tach is steady, so it gets all its pulses. A broken wire or plug or swapped leads are still possible. The distriutor, amplifier are okay. One injector may not get its pulse.
Then go from there. Might be a simple fix. You can also compare spark plugs - they will be sooty, one might look different, that’d be the one to investigate.
It only smooths out at rpm because of inertia.
Once that’s fixed you can look into the sudden death issue of the end of the video

It is still possible that your idle set-up and AAV is perfect, Graham, and the problem is related to any of the factors mentioned earlier - either singly or in any combination of two or more…

Idle setting involves setting the throttle gap to 0,002", then, with the engine hot, verifying that the AAV is closed - then adjust idle with the idle screw to the desired level. The verification of the AAV function is then the elevated cold idle - dropping gradually to set idle as the engine warms up. The AAV can of course be tested, out of the car, by heating it up and watching the slide positions…

However, as the throttle is opened; both the idle setting and the AAV, engine hot or cold, are ‘redundant’ - overridden by the increased air flow through the throttle…

Specifically; above 1000 rpms the xk engine should be perfectly smooth - even with some rather large faults. Ie, at higher rpms the engine should not ‘seem to run better’ it should behave perfectly - if idle/AAV is the sole problem.

Of course; as throttle is released, the engine falls back on idle settings - and if it cannot idle, for whatever reason, the engine stops. So idle is still the prime criterium for locating system faults.

If the engine runs at high rpms without stopping; lack of fuel volume/pressure is not a likely cause for stalling. Fuel pump, while running, is always at full capacity - higher revs just requires more fuel. Ie, mixture lean out may cause lack of power enough to idle, but whether by lack of fuel, leaning out due to an air leak - or some other factor is responsible for engine misbehaviour still requires going through the ‘shopping list’…:slight_smile:

As an aside; the brake booster is not affected by master cylinder leaks/failure. Both have their separate, independent and distinct failure symptoms…

xj6 85 Sov Europe (UK/NZ)

To be precise, it would be counterproductive to mess with any of it at this point.

Indeed David - I agree…

However, there is no harm done in verifying the throttle gap and AAV function - as part of overall testing. And indeed verify that the hose under the AAV is properly connected - lean fuelling, vacuum leak, is a possibility, and that hose may have been overlooked in the smoke test…?

xj6 85 Sov Europe (UK/NZ)

The engine seems to start and idle really well, missing aside, verification of settings if you insist, but I would not change anything.

I really just meant to remind him on not focus on one thing and neglect other factors, David…:slight_smile:

Something seems amiss with my audio, so I did not benefit from his video?

xj6 85 Sov Europe (UK/NZ)

Yesterday I found some time, did a little work (crude tests, ground strap cleaning), and the cutting-out problem seemed to go away. Until this morning, but I think that’s related to me improperly re-attaching the AAV air hose, I removed the air hose and looked at the slide, here is hot

And here is cold

I exercised it with a screwdriver, hopefully loosening the spring a little. I ran out of time to check the heating coil for the AAV, but I did see the hose underneath was too kinked for my liking, possibly reducing airflow. I’ve added it onto my list.

Additionally, the idle was too low. 800 cold, 500 warmed up, dropping to 200-300 in gear. Still, I’d prefer to get all of my cylinders firing properly first before adjusting it.
I haven’t performed compression, check valve, or non-crude fuel pressure test yet, but I did experiment with the spark plug wires, taking them off and putting them on the plugs to test for any audible change in the engine. I believe it to be cylinder 1 (closest to the firewall) that is misbehaving, as its the only one in which I didn’t hear a change in the engine whether or not the spark plug wire was connected or not. I’ve tested for spark at each plug wire, and all of them get their required spark. I thought maybe the fuel injector would be malfunctioning, but I was able to hear all of the pulses with a stethoscope for all 6 injectors. Still, I suppose just because I can hear the pulse doesn’t mean it isn’t malfunctioning, and I’ll pull it out and test if it actually injects fuel soon. I’m waiting for a borrowed compression testkit just to rule it out, as even though I doubt the cylinder has lost compression from just 8 months of sitting, I’d rather definitively rule everything out.

With the same stethoscope I also heard the fuel return to both fuel tanks, crudely indicating proper fuel pressure as you mentioned before.
The engine did restart faster with priming the fuel pump, and it is capable of restarting immediately after death.

Tomorrow I’ll try to do as much as I can, here’s my updated to-do list:
-Unkink AAV air hose
-Investigate spark plug condition cylinder 1 vs others
-Compression test
-Cylinder 1 fuel injector test
-Actual fuel pressure test
-fuel pump check valve bypass
-test AAV heating coil

Thank you all for your help, I’ll continue to update and learn and test until it’s solved

AGAIN. Don’t adjust ANYTHING before all cylinders work! Get it to run FIRST!

Start with the #1 spark plug, then check the injector. 12v is too much voltage for testing it, better, safer to go with less.
For now, save yourself the pressure, compression, AAV stuff for later - if you ever need it. I don’t think so. Fuel pressure, air supply will be okay. Compression could be an issue, compressed air would be enough for checking for compression leakage.

Exercising the slide frees the slide’s tracks, Graham - over time they get dirty, impeding slide movement. The bimetallic spring bends according to temp, moving the slide to a temp relevant position.

Methinks the slide, cold, is less open than it should be, possibly due to track still ‘dirty’ (more exercise and possibly some CRC for further cleaning) - otherwise the AAV is responding correctly.

The purpose of the heating coils is to close the AAV early - it is not vital for function, and a simple ohm check, verifying coil continuity, is enough.

‘Unkinking’ the AAV hose is relevant, it may indeed interfering with air flow, causing low cold idle - but the low cold idle may also be caused by insufficient slide opening…

‘Priming the fuel pump’ means running the pump to build up fuel rail pressure before cranking - it may shave less than 2 seconds off cranking time. The pump only runs with ign in ‘crank’ - but also, in ‘crank’ the CSI is briefly active.

Fair enough, but the ‘disconnect’ of plug wires is sometimes deceptive; focussing on #1 is fair enough - but run the compression test on all cylinders. Fairly even compression is the benchmark - testing one cylinder may be inconclusive…

xj6 85 Sov Europe (UK/NZ)

And again, get the engine to run on all six first before messing about. Besides that it‘d run great 800 cold 500 hot, it’s just not to spec.

I just did a compression test which went well, here are the figures, in order with 1 closest to the firewall
1: 138psi
2: 135psi
3: 141psi
4: 135psi
5: 137psi
All of the plugs were pretty black

They all look like this, indicating its running too rich? I’ve yet to test the cylinder 1 injector
I’m mostly surprised at the condition of plug #1 matching the rest, given the running condition of the others.