Jaguar Mk2 not starting when hot

Hi all,
Ive purchased a 1968 Jaguar mk2 240 and am encountering a problem when engine is cold it fires up instantly without any problem whatsoever and runs perfectly, but when switching off it simply doesnt start until it has cooled. Its dead like a flat out battrey, Leave it to cool down and back up like it never had a problem. Any help from where to start debugging this problem please?


We need to know if it will not crank over when hot or if it cranks over OK but does not start and run.

One problem I had with one of my cars was similar…It could be your condenser or coil, both of which can cause problems when they get hot.

Like a flat out Battery ?
do you mean it cranks over slow ?
Check your live and earth strap , could be a starter motor or solenoid issue .

It will be vaporisation in the carburettors. Possibly due to a low setting of the float or a non-seating (faulty) needle and seat. Check the float levels and replace the needle and seat.

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I just finished chasing this exact problem in my 64 3.8. I initially thought vapor lock as Timothy stated. But after a complete overhaul of the carbs, the symptoms remained. I then systematically changed all the usual suspects; points, condenser, coil, wires, rotor, and cap. Flushed the gas tank, new fuel filter, and pressure checked the pump. No difference. Finally, I put a new set of plugs in and used a Colortune to refine the mixture. Bingo. Problem solved.

Cant understand how that tipped the scales, just one more thing that make these cars so endearing…

I had a similar problem due to the original oil filled lucas coil failing once it got hot. From memory I can’t remember if there’s a ballast resistor. Maybe that could fail when hot too.

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It has a ver low sound trying to start but nothing more, like a completely flat out battrey type of sound or one that is about to go completely flat

Sounds like a Battery , starter , Solenoid , or Battery wire connection ,
Battery could be on it’s way out , maybe charging system is not working , bad earth .
Don’t think the oil , or fuel , coil , vaporisation would make engine turn over slow :thinking:

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But when left to cool down it will start without any problem what so ever. Like it never had a problem

Hi and had a similar issue with my 69 e type… it was the ignition… a rotor with a pinhole in it. Opted to move to a Pertronix system, with the accompanying Pertronix coil, so all would play nicely. Tired of bad quality control on rotors, condensers and points.

Yes, Ian you sound like you are on the right track. Vaporisation won’t cause the engine to turn over slowly as you said. If all the earthing points are checked and cleaned then it could well be the battery is breaking down when hot.

Sorry , but if the Rotor arm was removed , Engine should still crank fast .
I would go for long run ,after checking volts in Battery , then back home , and turn engine off , then try and start it .
If engine cranks slow and won’t start , check the volts of the Battery again ,
No point in spending any cash , till you know the fault /

I have had the same problem on my last 3 Jags.
It was caused by Vaporisation in the carbs due to the engine running too hot. The jag engine requires a special bellows-type thermostat, which opens and closes other water channels in the head, allowing better cooling. Most owners fall into the trap of purchasing a new Jag thermostat from their local Motor Factors shop, but although labelled suitable for a Jag, it will not allow adequate water flow, causing a overheated head and hence vaporisation. Expect to pay about £20 for the correct item and purchase from a classic parts supplier and your problem will be solved.
Hope this helps

Yes, that’s right, naturally. My old MK2 used to be difficult to start on a hot day and that was vaporisation but it certainly didn’t spin over any slower. Ignition issues can be a real pain! I bought two “good quality” coils which both boiled on me when the car was hot. :hot_face: That only stopped the car from restarting and it would misfire when hot. The best coil I have found, eventually, is one that is branded Lucas but is made in India. It is fitted to almost all Indian taxi’s so that’s a very test group. They are available through The Distributor Doctor in the UK. All his stuff is first rate.

I’ll still go with a battery or charging issue as you have said, Ian. It could also be the Voltage regulator malfunctioning when hot - just a thought?

Not wanting to state the glaringly obvious but does the red Ignition light come on when the car is going?


Am I missing something , how can Vaporisation make a Engine turn over like it has a flat Battery :rofl:
Don’t think a new Thermostat will make it spin faster lol

Thank you for pointing out such an obvious oversight.
Without people like you, the forum would be full of misinformation, so full marks to you! However, sometimes you have to look further than the end of your nose! The advice regarding the non starting when hot, may not be applicable in this instance, but might be a help to someone suffering such familiar symptoms. I have run jags for 30 years and presently run a 3.8 Mk2, an Xk140 and an XK150, so I have plenty of experience with this marque. Out of curiosity what Jag do you own???!!

I have made a few mistakes in the past , got things wrong , just needed pointing out , ignition and fuel would not make a engine turn over slow ,
Had my 240 for close to 40 years , much modified , all by me , only thing I did not do is put the screen in !

You should take into consideration that the slow turning-over, could be a ‘red-herring’ and a by-product of constantly spinning the engine over, thus weakening an older battery. Believe me, when your engine has vaporization, no amount of engine-spinning, will make the engine fire-up … But hey, what do I know!

True John , I did ask :thinking:

has a ver low sound trying to start but nothing more, like a completely flat out battrey type of sound or one that is about to go completely flat