S3 XJ6 - it ran until I tried to make it run better

(Beau LeBlanc) #1

Hi all, I am a newly registered forum user, but I’ve spent some time researching prior forum topics in search of answers on the issues plaguing my recently acquired 1986 XJ6. Any insight would be appreciated, or even direction to other forum topics that would address these problems. I’ll start with where I am with the car now, and the backtrack to how I got here.

Cold start takes about 3-4 attempts. It will start but immediately die the first few times. Once it gets to the point at which it stays running, it runs smoothly, but idle is high - hovering around 1500-1600 rpm. Giving it the slightest bit of gas kills it. Putting it in gear, whether drive or reverse, kills the engine immediately.

I bought it about 2 months ago, and it was running at the time, but with a known misfire at cylinder 6. The car had sat unused for a number of years until the PO bought it in non-running condition. He had the left fuel tank drained, replaced the starter relay and fuel pump, and refilled the left tank. It then ran, but with a misfire at cylinder 6 (per the Jag/Bentley/Aston mechanic who did the work) and the fuel gauge would not work. The shop recommended running injector cleaner through it because they suspected a gummed-up injector. PO would start it every week but drove it less than 15 miles in his year-and-a-half of owning it.

When it arrived at my house, I noticed that idle was high, at about 1100 rpm in park. It ran very rough, presumably from the misfire, but it started, drove, and stopped fine otherwise. In an attempt to get it running better, I ran a can of seafoam through it at the throttle body. Things did not go well thereafter. I tried to drive it around the block, but idle was then stuck at 1500-1600 rpm. As I would give it some gas, it would stall and die out as soon as it hit 2000 rpm. I made it home after it died four times. Sitting in the driveway in park, idle stayed at 1500-1600 rpm, and it would stall if revved past 2000 rpm. It would not stop, however - the tach would drop, and it would climb back to high idle. A local mechanic with prior Jaguar experience agreed to look at it.

When I started it to bring it to the mechanic, the misfire seemed to have been cured: the engine ran amazingly smooth. Idle still hovered at 1600 rpm, but now it would smoothly rev (in park) over 2000 rpm without a problem. The drive to the local mechanic was problematic though: in gear, it would stall and die with the slightest touch of the throttle. Further, the previously well-functioning brakes did little to stop the car despite depressing the brake pedal to the floor. The mechanic’s shop was accessible via a little-used side road, so I essentially coasted, foot off the accelerator, the entire drive. Apparently the engine got enough fuel via the high idle to continuously run at about 5 mph. I think I recall a noticeably louder-than-normal whine from the fuel pump, but I was distracted by the worry of ineffective brakes (not that I really needed brakes, because all I had to do to stop the car was to press the accelerator and it would die).

The local mechanic believed the AFM to be the culprit. Fortunately, the PO included a bin of spare parts with the car, and I had a new-in-box AFM. We installed it, but it still dies with the slightest bit of throttle. He believes that bad fuel is the problem, and he thinks the tank needs to be dropped and cleaned, but he no longer wants to work on it. It’s back at my house now, leaving its signature puddles on the concrete (three different leaks, which I’ll address once it’s running).

I don’t think that bad fuel is the issue, but I could easily be wrong. My first thought is that I somehow affected the throttle when I did the seafoam treatment: I did not have someone to hold the throttle open via the accelerator, so I manipulated the throttle lever manually while spraying the seafoam. Perhaps this affected some component of the throttle assembly? It looks fine upon visual inspection, but I admit that I’m not sure what to look for at this point. And it may be that the brakes are an entirely separate issue, but I can’t tell until it runs long enough to try to use them.

Any help would be appreciated. Thanks.

(Robin O'Connor) #2

I would check to see if there are any vacuum hoses that are loose, is the brake pedal very hard with little retardation? This could be due to a lack of vacuum from a dislodged hose.

(Frank Andersen) #3

A tale of some woe, Beau…

Firstly; engine rpms is dictated by the amount of air entering the engine. So your engine gets too much air and enough fuel to keep it running. As throttle is applied the mixture leans out too much - and the engine quits…

In principle; the AFM should increase the fuelling as throttle opens - but with an air leak bypassing the AFM it cannot do that. In short; a faulty AFM cannot in itself cause high rpms…

So you have a major air leak - and the prime suspect is the duct between the AFM and throttle body. First step is to remove it and carefully inspect all parts of the duct, especially the rubber parts for leaks - twisting and stretching. Then reconnect duct ensuring tight connections by the hose clops…

While the duct is out; also check and clean throttle body - and verify that the throttle gap is correctly set to 0,002"…

This is the first step in locating an air leak; if ineffective - further work involves checking the vacuum hose at the underside of the AAV. The AAV is mounted to the intake manifold, about midway; identified by a sort of tower leaning outwards, an electric connector - and a vacuum hose. Inspecting the ‘underneath’ hose is a bit awkward, but necessary. If it has fallen off the AAV it represents a major air leak…

As Robin says; inspecting all vacuum hose connections is necessary, but they will usually cause only a minor rise in rpms…

The brakes are a indeed an entirely separate issue; that the pedal bottoms means either an internal leak in the master cylinder - or an external leak in calipers or hoses. First step is to check fluid level in the brake reservoir - with both chambers full; the problem is likely the master cylinder.

Needless to say; a brake problem requires urgent attention, need to have - a properly running engine is just nice to have…:slight_smile:

However the brake hose connection should still be checked - detached, it is an air leak…

Crucial points; fuel supply and injector delivery problems cannot increase rpms - nor can a faulty AFM, or indeed an ECU or ignition fault. Engine can only rev up if air is supplied…

It is advisable to run a compression test - to verify engine state. It’s pure routine - it is pointless to search for other causes of misfire unless the engine itself is eliminated as a cause…

As various causes are checked and eliminated as factors - we shall dig deeper…:slight_smile:

xj6 85 Sov Europe (UK/NZ)

(Carl Hutchins, Jr. ) #4

Well covered. All I can add is to support the compression test. If even one cylinder is weak enough, the car will never run well.

Aye, as to a major leak. A plethora of hoses that can leak. but, not big leaks unless many do, then it adds up. The fast idle is major clue as to leaking vacuum lines.

Plus a possible big leak in the line or the booster can itself for the power brakes. A hard pedal says no boost!!

Seafoam. Chemical mechanic??? I suspect it merely cleaned out some crud from leaks making them even larger

Plus. essential to verify healthy spark, each and every time., sharp crackling blue. Wimpy yellow will not due.

(Beau LeBlanc) #5

Thank you all. I will investigate for air leaks this weekend.

On the last drive, the brake pedal was not difficult to depress. There was a “whooshing” sound when I applied the brakes, though, a few times before I noticed that the pedal became less effective. I will check the fluid reservoir and inspect for any visible leaks.

Carl - yes, seafoam is a “mechanic-in-a-bottle”-type product. The difference between how the car runs now and how it ran before is enormous. Now, if only I can get it to stay running.

Again, thanks to you all, and I will report with what I find this weekend.


(Paul M. Novak) #6

I agree with the others about the compression check. It should be your first step when tackling your engine issues. As a second step I recommend that you connect a fuel pressure gauge, in the usually recommended location, between the fuel rail and the cold start injector. The fuel pressure should be about 33-36 PSI at the fuel rail, a bit higher with the fuel rail primed but the engine not running. Keep an eye on the fuel pressure gauge as you start the engine, accelerase out of idle, and observe the reafings as the engine starts to stumble. If the fuel pressure continus to read 33-36 PSI, then you don’t have a fuel delivery issue. If the fuel pressure drops while accelerating the engine then you have crud in the tanks blocking the in-tank filter screens, or blockage in the fuel filter (located in the right front corner of the spare tire well).

I had a similar engine problem with my 1984 XJ6 Vanden Plas several years ago after it sat for avwhile undriven. It would start and idle fine but stumbled and died when accelerated or shifted into gear. I really suspected an ignition problem, but decided to check the fuel pressure reading first anyway. I put a fuel pressure gauge on and the dropping fuel pressure readings as I accelerated the engine were proof positive that I had a fuel delivery issue. I drained the tanks, found a lot of crud, but also a torn in tank filter screen I one tank. I also found that the fuel filter was partially blocked and full of crud from the tanks. The engine started easily and ran very nicely after I drained both tanks, installed two new in-tank fuel strainers, installed a new fuel filter, and added a few gallons of fresh fuel to both tanks. Since then I added two new aftermarket in line fuel filters, between the fuel tanks and the fuel changeover valve in each of my three Series III saloons to protect the fuel changeover valve and fuel pump.

You have had a “lot of cooks in the kitchen” on your car. But by proceeding in a logical step by step fashion you should be able to get you XJ6 running properly again. Confirming the compression is good and pretty much even plus or minus about 10% will be a good first step before spending additional time and money on this car.


(JR) #7

My brake booster went bad and no matter how much pressure applied to the brakes the car would barely stop. Replaced the booster with some direction form a lister and all is good now. Hope this helps!


(David Jauch) #8

As to the misfire on cylinder 6 (6 is the frontmost cylinder) I‘d pull the spark plugs. If it is the rearmost then there is a vacuum connection at the underside of the rearmost intake runner. Buy a can of throttle body cleaner and spray it around a little, small doses. If idle goes up then it has a leak. Can also be the injector seals. The high idle masks some of it very well. Most of us have a slightly uneven idle.
I further suspect that there is (another?) major vaccuum leak. This may or may not have something to do with the brakes, but does not explain your braking issues, separate issue.

I also like the idea about the fuel pressure but before attaching a gauge you can look into the selected tank. Open the filler door and push open the flap, there should be a fair amount of return flow with the engine running. If not you definitely lack fuel pressure somewhere. Furthermore I hate seafoam, and retrace every step you took to get it in via the throttle body.
No shame though, I had a few cases including no-starts after repairs, like forgetting to attach the brake booster hose to the manifold, leaving the afm disconnected, forgetting the fuel pump, most famously hydrolocking the engine because somehow the injectors shorted out and filled the cylinders. Everything turned out fine though.

The engine has cold start enrichment. If there is too much air available it will have a hard time starting, then run lean until the enrichment phase is over, at which time it eventually dies. More throttle input will make the engine die because there is much more air available but the pressure differential the AFM sees is not as high as it should be. Also remove the ducting before the AFM and see whether pushing in the sprung flap (this tells the computer to add more fuel) makes the RPM go up. Report back with what happens; also, this will give basic information about the AFMs functioning.
And pursue the brakes before driving further.
Good luck!

(Frank Andersen) #9

If the pedal bottoms, Beau - either air in the system or a leak. If it goes half way down but then holds firm - one brake circuit is out. Leaking internally in the master cylinder - or externally…

As you apply brakes; air is let into the brake booster through a passage co-axial with the pedal rod - causing the ‘whoosh’. Usually not very audible unless listening carefully. The ‘whoosh’ stops as the booster reaches the position indicated by the pedal. Constant ‘whoosh’ while the pedal is held down, indicates the booster is leaking…

Clarify; ‘…pedal became less effective’. There is a difference between loss of braking effect (requiring more pedal pressure) - and pedal bottoming (with little/no brake effect)…so…?

xj6 85 Sov Europe (UK/NZ)

(David Jauch) #10

So, check brake level and flush brakes first or move on right away?

(Con Saris) #11

Hi Beau, your problems are almost exactly the same as I had on my 85 4,2 Sovereign. I too suspected ignition and spent quite a bit of time checking and replacing parts I eventually solved it by the same method as Paul. I even had the loose filter screen floating around in there. Must have been a bad batch of filters out there worldwide. I even blew out the outlet pipes from the boot back into the tanks while I was draining it.They were both clogged with crud. I found that the fuel pressure was fine while idling but as soon as I tried accellerating the car died. Very frustrating when trying to merge into traffic!
So sounds like a couple of problems, get those sorted becfore any on road driving. I can’t help you with the engine miss, I am lucky at this stage to have not that problem.
The giys here are very knowledgeable and will help you get it going again
Good luck
1985 Jaguar S3 4.2l Sovereign.

(David Jauch) #12

Con, did you ever check your return flow back then on your issue?
As I said the vacuum line would be interesting and the brake cleaner. Then the spark plug and then the compression test. Easiest to most difficult as always.

(Con Saris) #13

To be honest David I didn’t get to it. I was just glad to get the Beasty running. I was able to have a trouble free drive to the petrol station and back in a long round about drive that was further than I had driven in the car for about 6 months. I a so busy at the moment I haven’t had a chance to drive it again and may not now until after Xmas when I have a couple of weeks off. Beau’s talk of brake problems has got me wondering if mine are working properly but I do stop just not as quickly as I would think but it is quite a bit of weight so maybe ok.

(David Jauch) #14

Does it brake straight and can you eventually lock the brakes up? Doesn’t hurt to find a free spot of road and check. I don’t know if locking tyres are an indicator for anything but it will tell yot that the brakes are strong. As long as there is nothing really strange going on you should be fine, but you eventually want fresh brake fluid if you never done that. I lost my brakes eventually and changed the fluid on my own. I don’t believe that this had ever been done before since its import from UK by the least.
When you come around to driving you‘ll find it runs better every day. As soon as you feel relatively safe do a longer cruise, at the least half a tank or so, and you suddenly don’t want to get out again.

(Aristides Balanos) #15

That’s a sign of a broken booster diaphragm Beau.

(Frank Andersen) #16

It’s not, Con - something is wrong…

Two different scenarios; the booster does not boost - the only force acting is the drivers foot on the pedal. Which requires quite a Herculean effort for effective braking - and is not safe.

The master cylinder is mechanically connected, through the booster, to the pedal rod - allowing unassisted braking if the booster fails. Typically, the pedal is then quite firm…

The second is a failed circuit, front or rear axle. Pedal then goes halfway down before it bites - and as there is braking only on one axle; half the braking effect is lost, requiring twice force for the same braking effect. However, with the booster working; it’s a wee bit safer…until the second circuit also fails…:slight_smile:

As said before; you are out of the woods regarding the other problems - but you need to sort out eventual braking problems…

xj6 95 Sov Europe (UK/NZ)

(Con Saris) #17

Sorry Beau, I didn’t mean to hijack your thread. I think you may be right, again, Frank and I did wonder if it was taking too much effort to stop. I thought maybe the pads were a little glazed from little use but the boost not working is a definite possiblity. The pedal doesn’t seem to drop too far before the pedal gets hard, Another problem is that I compare to the more modern cars that I drive every day. David it does brake straight but I haven’t tried slamming them on to lock them up …yet, but will soon. As always time is short this time of the year. Looking forward to the Xmas break!

(Carl Hutchins, Jr. ) #18

Lock up is a feat of the past. Once four wheel skids were considered as good braking and a fool proof means of speed detection. Flawed, for sure…

  1. My Jeep with ABS brakes much differently than my Jaguar. the pedal far “touchier”.

  2. I consider the brakes on my jaguar in good order. No leaks and boost at the pedal is good. But, the pedal is not as “touchy”. I’ve made an emergency stop or two. No four wheel locked skid marks laid down. It just "squatted and came to a controlled stop. Good tread gets a bite.


1.The best braking effort is just short of lock up.
2. Locked front wheels do not respond to steering input.


(Beau LeBlanc) #19

Thanks to all for the continuing advices. I only had a short time on Saturday to work on it. I checked the air flow ducting, but it had no obvious cracks or loose connections. The accordion-type hose attached to the AFM had a sort of fabric exterior over a rubber inner layer, and the exterior was separating from the inner layer at the edges. I had an extra, new one, so I replaced it. I had a chance to look at the throttle body, and it appears clean - no visible buildup anywhere. I checked the hose under the AAV: it’s not detached, and appears fine.

I started it up, and it started a bit easier. Idle initially sat at 1600 rpm. I let it warm up and then tried revving it, and it revved fine - I got it up to almost 3000 rpm with no problem. In “drive,” it sputtered and the tach dropped low but then recovered and sat at about 900 rpm. Back in “park,” and idle hovered at 1250 rpm. I put it in “drive,” and it dropped to 900. Back in “park,” up to 1250. In “reverse,” down to 900. Back in “park,” up to 1250. In gear, the brakes held, but there was a bit of a lurch forward when I’d put it in “drive.”

On Sunday, however, it started very reluctantly and would not stay running for more than a minute or so. It would start, run for a moment, and then die out. I had intended to try the brakes in the driveway, but it wouldn’t stay running long enough to put it in gear. I checked the brake fluid reservoir, and it appears slightly low. I didn’t have time to check anything else.

Thinking back to the time that I most recently drove it, I don’t recall much about brake pedal feel; I simply recall having the pedal to the floor, and the car kept creeping forward for a bit.

I’ll update as I have opportunity to check other things.


(Frank Andersen) #20

All this is perfectly normal for an initieal cold engine but with the idle set too high…

When cold, a functioning AAV will increase the hot set idle by some 200 - 400 rpms. From 1250 hot, the idle will drop when in gear, as the engine has to work against the converter. As converter resistance increase with rpms; idle will nominally be around 750 rpms in gear within a rather wide range of idle settings - but some 900 rpms from 1250 is sort of normal. And a jerk is perfectly normal when gear is engaged from a high idle…

That it again misbehaved on Sunday does imply an intermittent fault of some kind - which makes the way forward somewhat bumpy…

An intermittent fault can only be effectively tackled when present.

But when/if the engine again behaves as a good engine should; try adjusting the idle screw to bring the idle down, engine hot, from 1250 to some 800 - 900 rpms. If the engine does not react to idle adjust - it is a sign of an air leak. The AAV should be routinely checked; cold (half open) - and warm (fully closed).

Returning to the throttle body; make sure that the throttle gap is set correctly to 0,002". Operate the throttle repeatedly to ensure it returns to the set gap - this is important…

One possible intermittent fault is a sticking throttle pedal/cable - leaving excessive throttle opening. Which will cause difficult starting; the xk engine is sensitive to throttle position while cranking - throttle must(!) be closed, ‘feet off’. And the engine may not catch and run with too much throttle - easily checked…

Another easy test; have you tried switching to the other tank when the engine misbehaves - a routine check. One tank may be intermittently clogging between the tank and the pump. This will simulate an ‘air leak’ by leaning out the mixture - and the engine will stall with pedal input.

A fuel pressure test, as mentioned by others, will then be conclusive. A subsidiary test is to open the filler lid, and listen for fuel return - with too low pressure there will not be any return…

Also; remove air filter and push the AFM flap to full open and slowly release, ensuring that the flap moves with even resistance - a hanging AFM flap will interfere with fuel mixture…

Neither of the above fully explains both high idle and then dying with pedal input. But you may have two separate faults acting independently, and an engine otherwise in good order - so keep looking…

xj6 85 Sov Europe (UK/NZ)