Poor starting on 4.2 ser III xj6

1985 Ser III xj6 4.2 engine .
If i leave my car for a few days before starting its difficult to get it too start. When the engine is cranking it does not want to start, but on releasing the key it tries to start .Sometimes it will start after a few tries and be fine for the rest of the day,sometimes the battery gets a bit slow and i jump with power pack and it starts very easy . Can anyone point me in the direction to look ,also could it be low battery voltage as it seems to start very easy with power pack .
Thanks Ian

How old is your battery?.. It be a prime suspect…
Second option, there is a fuel check valve in the fuel pump that keeps the pressure in the system.
After a few days it looses pressure. Try to prime the system before you start by tuning the ignition on for a couple of seconds two or three times letting the pump build up pressure and see if it gets better.

Keep an eye on battery voltage, Ian…

You may have an external (or internal) current leak. One test is to disconnect one battery clamp and measure current (with an ammeter) connected between the battery post and the disconnected clamp. Current should be (with ign/lights ‘off’ and doors/boot closed) 35 mA - radio and driving computer stand-by power.

Indeed, a bad battery will lose charge internally - as Aristides points up. You don’t say whether starting difficulties relates to slow cranking - a typical symptom of battery loss of power…?

xj6 85 Sov Europe (UK/NZ)

Yup good advice. I had a similar case were a trunk light was staying on and it kept running my battery down to the point it killed a 1 year old Bosch battery for good. thank god for the their 3 year no questions asked warranty!

My thought here is that the ignition switch is failing.

But, first things first. If there’s doubt about the battery, address that issue and then move on.


I would…run a hot line direct from the battery to the coil and crank the engine…if it starts then I would suspect an ignition switch
I would…spray some starter fluid into the air intake and crank…if it starts then I would test the cold start injector…

I like that approach !

The original post has a significant clue. Not enough voltage to crank and fire. Release the key and it gets volts for an instant and tries to fire. The added volts by jump box support that as it seems to crank and run then…

Poor battery and poor connections, one or both?

  • that ground strap down and under. Ills in my car fixed by an added ground. Wing wall ground post to engine. Wow, big difference…


Hi all
Thanks for all the suggestions. Just found today that if I don’t touch throttle before starting it seems to start better, I was flooring the throttle before switching the ignition on ( as in the old carbs cars where you had to put on the choke before starting), cant see how it makes a difference but will see. Also getting the battery tested tomorrow not sure how long the battery has been on,as when it went for its nct test it was an advisory that the tyres are 6 years old , yet they looked new .
Regards Ian

Never floor it to start. Don’t even touch the throttle as it‘ll upset everything. If the hot idle is per specification and the cold somewhat higher (~1200) it is all good. Maybe the cold start injector if it takes very long in freezing temperatures, and the ignition if it only starts on releasing the key (there should be two circuits, one for crank and one for run? Not sure).

The xk is a strictly ‘feet off’ starter, Ian -a David emphasises…

However, any pedal input before turning the key is completely immaterial - it has absolutely no effect.

The amount of air during starting is regulated by the AAV, the throttle gap and the idle screw setting. Touching the gas pedal for more air while(!) cranking will disorient the ECU - and the engine will baulk an may fail to start.

With starting difficulties; the idle should be adjusted to specs with the the engine fully hot. The throttle gap should be verified (0,002") - and the AAV function should be checked as a first step. After verifying the battery state of course…:slight_smile:

xj6 85 Sov Europe (UK/NZ)

As I “see” it, indeed two circuits to operate. One to crank and fire, and another to run.

Done in the ignition switch. Brown wire, always hot. Full right on the switch connects the brown to the white ignition and the yellow white crank at the starter relay.

Release to run and the brown s connected only to the white ignition to the coil +.

Way back when, I had a 39 Plymouth pickup or was it a Dodge. One and the same, badge difference only. A bad starter that took all the volts to crank. Trick, get it spinning pretty good. Switch to run quickly and it just might catch!!!

Racers used two switches. One to crank only and the other to run. a way to avoid starter kick out because of advanced timing…


Interesting: how would that work?
(Nevermind: starter kick back, not engine kick back!)

I’m lazy and didn’t read the entire thread; and I’m quite sure the battery topic was covered.
I know Frank is pretty much on top of this one. He’s helped me with my '86 - ignition issues.

I just finished a video about a “no start” on my '85 Series III. Put the spark back in your Jaguar to get right to it, jump to 2:28sec. I stopped at the ignition module because was the issue… I will be following up on a tune up with my next video. ANYWAY back to your hard starting.
You’re gonna need a multi-tester.
1.Battery should read somewhere between 12 & 13 volts with the ignition off. test the battery from pole to pole.
2. with the ignition on, check the voltage to the ‘+’ side of the coil (negative side to the negative on the battery), should be not less than 1 volt than the result from above. This checks the condition of the ignition circuit. If you Jag is fitted with a ballast resister, then check the resistance from one end to the other. The resistance should be between .08 & 1.0 ohms. If no ballast resister, then the voltage should read the same as step 1 above.
3. Check the condition of the primary windings on the coil. disconnect the amplifier connection from the coil, and put the test lead on the positive tester lead on the negative pole of the coil and the negative lead on the negative pole on the battary. The voltage should read the same as number 2 above. If the measured reading is 2 volts less than measured from number 2 above. then replace the coil.
4. plug the amplifier back onto the coil, using a lighted test lead. Ground 1 lead at the battery and the other lead on the negative side of the coil. Crank the engine over, the light should flicker. If not, then it’s time to replace the amplifier.
5. Ignition pick up resistance should be no less be between 2.2 and 4.8ohms. Test with the ignition off and the two amplifier leads disconnected from the coil, or disconnect the pick leads from the distributor to the the amplifier. this is your best bet to isolate the magnetic pick up. Testing from the coil is really not conclusive because it could be wire related to the ignition amplifier, the amplfier itself…
6. check the gap between the pickup should be .008-.014 in
7. The angry pixies shooting out the top of the coil should measure at no less than 25k volts and at the plug should be no less than 10k volts.

Now, the one thing I didn’t cover in my video and is glossed over here in this post is the ballast resister. What does it do? It servers 2 purposes

  1. regulate the voltage to the coil during starting. the biggest drain on a battery is starting. the ballast ensure consistent voltage to the ignition system to TO ADDRESS A HARD START CONDITION (I’m not yelling just emphasizing to draw your attention to this point, in case you don’t want to read the whole post.). if the voltage to the ignition drops below 10v then chances are the Jag will be a little bitch and be hard starting or leave you stranded.
  2. The ballast also regulates ignition system from too many volts during higher revs. Essentially keeps the ignition system safe from too many angry pixies running through it which generates heat; and we all know what heat does it cooks stuff; if it’s not edible, then it’s all bad.

Hope this helps. I also hope I didn’t just waste nearly an hour of my time because you already know it or it was already covered.


I just do not see the need for the ballast on this version.

tis another name for a resistor. converts volts to heat. Unwanted either way!!!

Lots of pixles needed at high revs.

Leaned something yesterday. Modern cars have smart alternators. The PCM governs alternator output, beyond that done in the alternator by the bride.
That plus an over running clutch at the drive!! The example was a middle aged Toyota. Object, more miles per gallon…

Carl …

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In a cold, really cold environment, the ballast does help with hard starting. I deleted it from my '86 and when the temp drops below 28f then the Jag won’t fire off at the first bump, like it does normally.
I understand that alternators have diods. & regulators to prevent. over charging. These do fail (such the case on my '86 XJ. Once I got past the no start, I discovered the voltage shooting up to 18v Pop went the radio, & I then discovered bits of metal from the mag pick up blobbed onto the cam on the distributor. Mag pick up not a problem. The cam is a NSS part. A new distributor? well that just pissed me off. ended up popping the distributor out, and with a mini belt sander, sanded the blobs off the pickup rotor.

I’m not a fan of ballast resisters either, but in certain circumstances they do serve a legit purpose. Especially if it’s consistently cold enough to make thing shrivel.

A ballast resistor does two things, and two things only, Mark - it restricts current flow and, in the process, lowers voltage…

That a resistor can improve cold starting is quirky…?

xj6 85 Sov Europe (UK/NZ)

You’re right Frank. as usual.
however, I think we are saying the same thing; I was just putting it in a real world context.
Also the Jaguar Dealer training material I read on the Series III states the ballast resister “added” to the ignition system because of hard starts in cold climates. The '85 did not have a ballast, where as the 86 did. maybe it’s the other way around.

Anyway, I removed it, then discovered when it was really cold, the car doesn’t start right away, and if the battery charge is remotely low, it won’t start at all. my new ballast resisters just arrived today. I’ll install them and let you know. It’s supposed to snow here next week. Ugh!

If it’s a 6V coil with resistor: yes and more spart to start
If it’s no resistor it will be a 12V coil…
Anyways, let’s wait for what the battery tester says. I reckon that isn’t the problem.

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a simple multimeter test can be revealing, use an online chart that represents charge percentage

anything that wont hold above 12.4V for at least 24hrs is no good for sure

6yrs is old for a battery

With really cold weather start problems - the external coil resistor is the last thing I would suspect, Mark…:slight_smile:

However, as the saying goes; ‘never trust an electron’ - though I’m at a loss to see the mechanism behind the adding of a resistor to aid cold weather stating. And I though I knew every trick in the book to get an engine going at below -40C…:slight_smile:

xj6 85 Sov Europe (UK/NZ)