Jag 4.2 S3 won't run properly, europeen version

Hi all, first time I post something here, and it’s because of problems with my jag.
I am not very familiar with the use of a forum like this but i am a bit desperate.
Situation is this: car won’t start normaly, this happened from one moment at the other, before this i had a good running car.
I am pretty sure it’s a mixture problem. cranking is good, battery good, fuel relais good, petrol good, pressure 2.2 bar when running, new pressure regulater last year, new coolantsensor also. Cleared all grounds from cables. Checked for vacuum leak but it seems this good.
So the car won’t start normally but when i remove the connector from the coolant sensor she starts and runs quite normally, bit tumbling or stuttering, don’t now how you call it, only when i press accelerator a bit she stalls.
I ve exchanged the airflow meter with my spare one and get the same results. I’ve made a loop for the wire to the sensor and same result. I don’t know if the red selenoid pack could be responsible for this or what else. When the car is running and i connect the coolant temp sensor she immediatly stalls, What else could cause this?? Thanks ahead to all who make time for my problem

Welcome on board, Herman…

The ECU uses the resistance in the coolant temp sensor to adjust mixture according to temp. The higher the resistance the fatter the fueling - disconnected, the ECU reads infinite resistance, a very cold engine…

Measure resistance between the prong and ground on the CTS. It is temp variable; at 0C (32F) the resistance should be 5 Kohm, at 20C (68F) it’s 2,5 Kohm. At -10C (14F) the resistance is 9,2 Kohm…

If the resistance is very low/shorted (CTS faulty) the ECU fuels as if the engine is very hot - and a cold engine will not start. If the engine is reasonably cold, the engine will likely start and run, but will likely misfire a bit due to excessive fueling. It will run normally as it warms up - but if still cold engine will stop if the CTS is plugged in. Have you tried using the old CTS?

The diode pack has nothing to do with this. There may be other causes to pursue - but start with the CTS…

xj6 85 Sov Europe (UK/NZ)

What’s your history with this car?

Are you saying you had a good running car and then suddenly it started giving you problems? Of course the question would be did you do anything right before the car started running rough?

First thing I would do is a compression check. These are probably the easiest engines in the world to get to the spark plugs, so there’s no excuse not to do it. If there are no major differences in the compression of each cylinder then … and only then, would I move on with my troubleshooting. Remember you’re dealing with an engine that’s over 3 decades old and until you eliminate any internal mechanical problems (stuck or burnt valves, broken piston rings, etc) you are just chasing your tail.

Hey, that’s why the first thing a doctor does is listen to your heart and take your blood pressure !

Always a good idea to check compression, but I 'd be quite confident that it will only be for peace of mind and exclusion of other causes.

Go for the CTS onwards.

Good luck

75 XJ6L 4.2 auto (UK spec)

Hi all, I want to thank everyone who took time to help solve the defect on my Jag, i am glad i can say she runs again. She had a broken vacuum hose, the one from the inletmanifold to the aav, In dutch we call it false air, dont know the english word for. Anyway minor issue and I am very glad she runs again!!

In German “Falschluft”, in English “vacuum leak”, Herman.

Yes, old hoses splitting and letting liquids out or air in is one of the usual suspects on old good running cars and sudden deaths …

Keep the spirit


75 XJ6L 4.2 auto (UK spec)