85 XJ6 Running Poorly/Rough

So I just got my xj6 working again and now I wanna tackle the rough running issue because it’s really bothering me now especially after I get out of my 94 xj12.

Some back story is that I got this car and the guy (a good previous owner) had replaced the distributor (Pertronix) and coil ( flamethrower) but supposedly couldn’t get it tuned properly and didnt have time so he sold it.

It still ran so I drove it home 20 miles. When I started looking around I found 2 plug wires mixed up… the car ran much better after that. I still thought there’s a misfire or something of that sort so I replaced the plugs and wires too but that made no difference. The timing I adjusted myself just till it seemed to run the best so I know that needs to be done right. But turning the distributor both ways does not make the rough running go away. I also had replaced the coolant temp sensor but it made no difference and looked new anyways

Now I’m not experienced with engine timing and distributors so I don’t know where to start on getting this thing running better. I do have the old distributor and coil that I could put if if I had to

With ‘rough running’, Jabraan - what are the actual symptoms…?

Like; what does the tacho read while idling - hot and cold. Is it steady or varying. And how does it sound at the exhaust end - even burbling or puffing? Does it smoke - black, grey or white…? Does the engine run smoothly at higher rpms - and does it pull OK and respond to the gas pedal on the road…?

There is no reason to suspect that reverting to the old EI and coil will improve matters. However; the Pertrinix advises specific coils - not arbitrarily put in a ‘high’ performance coil - or whatever. And, importantly; what is the actual ignition timing now?

Some further tests; connect a vacuum gauge to the fuel pressure vacuum line. Vacuum should be steady and somewhere around 18" Hg - if not; the readings may provide clues…

I suggest a compression test; it will clarify engine state. If the compression test is out of spec, possibly causing the problem; no amount of external replacements can rectify that…

xj6 85 Sov Europe (UK/NZ)

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First off I would do a compression test to see if there is something wrong there, actually just remove a spark plug lead in sequence and see if there is any noticeable change in the idle, that might lead you to a cylinder that is under-performing before the comp test.

I will get a video up of it running today. Tach, exhaust, and engine so you guys can see how it runs. Now the car is driveable on the road and highway, I’ve had it up to like 110 once. So it does get up there with how it’s performance is rn. But acceleration is a bit slow id say but this is the only xk and 3 speed car I’ve driven or been in. There kinda like a beat to it when accelerating in gear. In park it revs pretty free and smooth. @Frank_Andersen the ignition timing I did by hand I don’t know what it’s at I’ll check today. I just replaced the crank pulley because it was split so I couldn’t tell before @Robin_O_Connor I think i have taken off wires one at a time before and all of them just made it worse. I’ll try again tho. Edit. I just remembered I have a new o2 sensor for it so I’ll put that in too

@Robin_O_Connor @Frank_Andersen

So I drove the car around the block today after the crank pulley change ( sat about 6 months). It definitely accelerates poorly. Here’s the links for the videos one is a cold start / idle vid which it was a weak start even for this car but it had low battery. Second one is about 5 mins after the cold start and after I took it around the block.




after six months of sitting I wouldn’t expect any car to perform like a race car. Any reason not to take it out for some serious driving? Give it a chance to even out - if not, there is still a chance to address remaining issues.

Your starting point is poor - comparing a six cylinder car with three speed box with the 94 12 cylinder car is vain: no way the old Jag will stand the test.

When looked at by itself - ideally locked on a stick shift box - the XK engine is quite competent though.

From your videos the only malfunction I’d find right away is the pulsation of idle . My car runs at 750 in N without any ups or downs. That kind of saw teeth your car shows should have to do with FI and its control. Others know a lot more on these.

Good luck with your cars


75 XJ6L 4.2 auto (UK spec)

Startup a little weird with the backfire but not that bad. Idle speed not bad. They are not the fastest, especially not the 170hp, 2.88 rear end us models. But everything above idle should be smooth (the slight rough running is okay). If it’s a periodic miss when accelerating you are running of five. I’d say have a look at the spark plugs first. Maybe the injectors have been suffering. Oh, and look at timing, however if you changed nothing else it can’t be too retarded else idle speed would drop.
If you couldn’t find an underperforming cylinder via pulling leads (or injector connectors), Jochen might be about right and you should drive it until hot and a little, they like that.
My Europe-spec was able to do 0-60 in 10.5s, albeit with bad cam timing, so it had the power but the idle and so on were terrible.

@Jochen_Glockner. I haven’t taken the car out for a long drive yet but the car ran like this before too so that’s not gonna fix the way it runs. Your car runs at 750 in park? If so that’s more like a normal car on my xj6 if the idle was any lower it’d die in gear.

@davidsxj6 when in park revved up it’s smooth over idle but when under load / in gear I can feel the pulsating when accelerating. I’ll try to find an underperforming cylinder tomorrow and check timing with a light

Have you ever done a compression test on the engine, Jabraan?

If you find an underperforming cylinder in other ways - you still have to do a compression test on all cylinders, to find if compression is the cause.

In any case; there is no doubt the engine is misbehaving - and needs attention…

The difference in cold and hot idle is a bit too little; the AAV function should be checked - though it is an unrelated problem. Also, hot idle is set too high - does the engine respond to adjustments of the idle screw? If not; you may have an air leak - which may be part of the problem…

As the vibration damper has been changed; the timing scale/pointer alignment should be checked with the #6 piston verified at TDC. To ensure that the ign timing readings are trustworthy…

Verify compression - to ensure that other work is worth while…

xj6 85 Sov Europe (UK/NZ)

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I haven’t done a compression test. But I will check timing first then I’ll do the compression. I have the idle set there because any lower and the car will die In gear. I actually need to raise it a tad more so it doesn’t shut of turning in parking lots etc. the idle adjustment does work since I have it set at that. What should the timing mark be at with cyl 1 at tdc?

You need to set it at about 6 deg before TDC I don’t have the specs for your engine but that will get it in the ball park.
Don’t forget that No 1 cylinder is next to the firewall.

…which shouldn’t matter for static timing. 6 sounds about right.
The engine isn’t hot yet per the temperature sender. European spec is 750 warm, 1200 cold in neutral and ~600 or so in gear. I think the federal models are higher, 900 hot possibly. Mine happily went down to 150 before I did the head gasket, but on a bad day it would die @400.

Yep just pointing out in case the OP starts moving wires around.

Your car should have a decal on the underside of the hood (bonnet) showing the correct timing.

The attached picture shows the decal that was on my 1986 XJ6 parts car, a USA market Federal car. The timing on this car is set to 17 degrees Before Top Dead Center (B.T.D.C) at 800 RPMs and with the hose to the vacuum advance capsule on the distributor removed and plugged. Check the decal on your car for the correct timing for your car as different country markets had different timing specifications to meet local requirements.


I wanted to time the engine first but my local store doesn’t have a timing light so I’m about to do the compression test right now and I’ll post the results

Big clue!!! Pulsating under load!! I translate that to missing fire on one or more cylinders. Ie, ignition issue.

As mentioned, coil to distributor mismatch ?
Or, poor spark plug and coil wires. Run the car at night.
Open the bonnet. A light show of leaking electrons!!!

Get down and under. Find the braided strap on he right side. At the junction r of the engine and transmission. It must be clean and sound. don’t ask how? Looking OK not good enough, remove and clean or replace with a better one…

Twisting the distributor til the car runs best works. power timed… done it a time or two or actually a lot more…

110 is pretty durn good… As mentioned 170 and a high ratio… Not a light weight car, either.

As mentioned. Vacum and compression tests t for basic engine health…

What part of my home state, Texas??


When I replaced my points electronic ignition I turned the engine
manually to the proper timing mark I wanted it to fire.
Then adjusted the electronic the portion to the same point to fire.

Never had any problem since.

Same, but he‘ll need a static timing point for his car, for 0 rpm.

I’m new with the timing and engine stuff so I’m still learning. But I’m about to do the compression test on it The coils and wires on the car were good but I replaced them anyways but no change. @Cadjag. I’m in Dallas what about you

He‘s in California but I have never set timing before and did just fine. You need a static timing number, then you set the engine to that and turn the distributor so it just sparks from the main lead to ground so it’s not the hardest thing in the world; after you got the fan on this is just as annoying. Find uphill areas and if you can‘t get it to knock, advance it a little until it starts to. You will get a feel for it quickly. Or get a cheap timing light on ebay.

If you enter „static timing #xj“ into the search function you should find something quickly.