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A few months ago, I was fortunate enough to be given the opportunity to own my Granddad's Jaguar, a big part of my childhood, that had been sold after he passed in 2004. I have vivid memories of riding in the car as a little guy, and it was an instrumental part in what become a life long passion and interest in cars, especially classic. The guy who bought the Jag after it sat in my family's garage for several years, revived it and drove it as his daily driver for many years, and did plenty of maintenance to it during that time. A year and a half ago he suspected the head gasket was blown, and parked the car. Then, in September of 2022, he sent me a message on Facebook and asked if I would like to have the car, for free! Who could say no to that?! I had the car towed home, and then eventually to a local British car shop where we began going through the vitals. We drained the bad old gas, threw in some new spark plugs, and it fired up like a dream. After further inspection, it seems that the head gasket is in fact fine, but the transmission will either need to be replaced or rebuilt, as it slips in second gear. Two very unrelated issues so not sure what's up there, but I'm pleased to not have to replace the head gasket all the same. Though I think I would've rather done that, than rebuild the transmission
Upon beginning to drive the car around a bit, it will run perfectly, but then suddenly die with no warning, as if someone reached over and turned the key off. Usually it will restart immediately, and other times take quite a while of twiddling thumbs on the side of the road (and optimistically jiggling wires) before it eventually starts up again. These stalling fits may happen once or twice during a drive, or not at all for a week or more. It also seems to have a difficult time cold starting, acting as if it's missing or flooded once it gets running, but if I shut the key off in these situations, and on it again, it will start normally and run fine.
Any ideas where I should start tracking down what could be causing this issue? A wiring problem in the ignition circuit, either right at the key, or related to the electronic ignition module? Or an ECU issue?
Any insight would be greatly appreciated. I'm looking forward to getting such an important time capsule back to being a reliable runner!
I would suspect the ignition switch because I had it happen in my 1984 XJ-6. ("suddenly die with no warning, as if someone reached over and turned the key off").
Replace the switch to see if it continues displaying this symptom. .
What a very cool story!
We have so many Gurus here, who are Fluent in Jag engines, as soon as they catch sight of your post here you'll have all the help you need to make this Lovely example road worthy again.
And, when you get a minute, you might stop by New Members section and post an introduction as Forum Rules require.
Until then, Welcome to our little corner of the Forums from ElinorB.
(';')
I would suspect the ignition switch because I had it happen in my 1984 XJ-6. ("suddenly die with no warning, as if someone reached over and turned the key off").
Replace the switch to see if it continues displaying this symptom. .
Very likely the answer. If you check where the ignition 12v goes to on the coil, and when it dies, ignition on, see if the 12v is missing, this would confirm.
I would suspect the ignition switch because I had it happen in my 1984 XJ-6. ("suddenly die with no warning, as if someone reached over and turned the key off").
Replace the switch to see if it continues displaying this symptom. .
Excellent Jose thank you. I think the car even came with an extra ignition switch, I will check and then pop it in if so!
Very likely the answer. If you check where the ignition 12v goes to on the coil, and when it dies, ignition on, see if the 12v is missing, this would confirm.
Fantastic, thank you, that is a great idea. The next time it quits on me I’ll be sure to have a multimeter on hand. Took it for a good drive today, but it didn’t stall out once!
What a very cool story!
We have so many Gurus here, who are Fluent in Jag engines, as soon as they catch sight of your post here you'll have all the help you need to make this Lovely example road worthy again.
And, when you get a minute, you might stop by New Members section and post an introduction as Forum Rules require.
Until then, Welcome to our little corner of the Forums from ElinorB.
(';')
Thank you, will do! This seems like a great community already with a wealth of information.
Mort: there are a number of possibilities but one that has not been mentioned (except by you) is the ignition amplifier, or more specifically the GM-made module inside it. This module is a regular culprit for these issues on the V12 version, but occasionally also on the 6. It is found on the front of the engine near the distributor.
Another possibility: the fuel pump module and the “main” relay that powers it. These are found inconveniently on the firewall.
Another, but less likely: the coolant temperature sensor on the water rail. This can cause starting issues as well as running issues.
Have you considered joining the Jaguar Car Club of Victoria? And, would you mind sending me a PM? Interested in the mechanic you are using.
Mort: there are a number of possibilities but one that has not been mentioned (except by you) is the ignition amplifier, or more specifically the GM-made module inside it. This module is a regular culprit for these issues on the V12 version, but occasionally also on the 6. It is found on the front of the engine near the distributor.
Another possibility: the fuel pump module and the “main” relay that powers it. These are found inconveniently on the firewall.
Another, but less likely: the coolant temperature sensor on the water rail. This can cause starting issues as well as running issues.
Have you considered joining the Jaguar Car Club of Victoria? And, would you mind sending me a PM? Interested in the mechanic you are using.
Thank you for the excellent tips. I was lucky enough to track down a test procedure for the ignition amplifier which I will perform this afternoon, and I’ll see if I can test the temp sensor as well. I’ve been noticing that, according to the dash gauge, it’s running a little cold but wasn’t sure how much to trust that.
I have not yet joined the Victoria Jag Club but certainly should. So far I am only in the Old English Car Club, which my Granddad as well as this Jag were members of in the ‘90s. I will send you a PM directly. Cheers!
if this is a Series III with fuel injection, one possibility for such random failures is in the ECU. My car is a V12 but I think the ECU is very similar to the XJ6. (Code 5CU for the 6 and 6CU for the 12 ?)
My ECU, caps marked My ECU cap marked
The electrolytic capacitors dry out with age (~40 years). It is very easy to open the ECU, identify the electrolytic capacitors (hint: they are metallic blue) and get someone with some electronics knowledge to replace them with new ones of the same value. Those ECUs are super simple inside, more like a digital alarm clock, with no processor or memory, just simple digital circuitry. Nothing to be scared of. The capacitors I got to replace mine were about half the size of the old ones, but that's ok as long as the values in Microfarads (uF) and Volts are the same. The good thing is that they are fairly cheap from electronics suppliers. There are about 4 or 5 and I had change from $20. About ten years ago this solved my problem, hasn't failed since then.
If it's not the ignition switch, it could be the contacts in the MAF that supply volts to the fuel pump relay. These contacts are tiny and closed once the engine starts. They are bypassed on engine cranking. Over time the springiness in the mounts plus corrosion of the contacts can mean they start to break contact unexpectedly. Like I had going up a steep hill on the outside overtaking everybody only for the engine to die on me. I laid a pile on the seat ! I had my family with me at the time, so it was quite upsetting. Why Jaguar had this stupid system I don't know.
Reviving my Granddad's XJ6
