1985 XJ-SC fuel delivery problem

[Fivethirty]

I have a 1985 XJ-SC with a 3.6 AJ6 engine (manual 'box). The car has been in the hands of an independent Jaguar mechanic for many months now, who can't seem to fix the problem of the engine cutting out after idling for anything between 15 - 30 minutes. Or, as happened to me twice, just cutting out while driving. This happens after the engine is running smoothly, pulling well, sounding sweet, without splutters or other noises. It just cuts out, like someone turning the ignition with off! And then it won't start!! My 'mechanic' has run up quite a bill for me (£2k+) replacing inlet manifolds, injectors, HT amplifier, plugs, leads, distributor and a host of other bits and pieces, including a reconditioned ECU. No luck!! His thoughts were that the continuing problem may be a voltage spike, damaging the ECU, possibly needing a new wiring loom or alternator (which is fairly new anyway).
Anyway, he decided (at his cost) to send both ECUs off to the fellow in the Isle of Wight for testing and they have been pronounced as 'perfect' by this chap and needing no repairs whatsoever.
As a last attempt, he thinks it may be a faulty water temperature sensor mounted in the region of the thermostat housing. (He called it a "Thermotime switch".) This 'switch' sends water/engine temperature information to the fuel injection system (presumably via the ECU) to allow rich fuel at start up progressing to a leaner delivery when the engine warms up. I've Googled "Thermotime Switch" without result. Has he got the correct name for this component? And can anyone throw some light on what the problem might actually be? Thanks...

[davidr]

Hi Fivethirty and welcome to the XJS Forum.
Have you isolated this to an ignition problem or could it be a fuel delivery issue?
Sorry if we're going back over ground you've already covered

[Fivethirty]

Hi davidr
Thanks for your welcome to the forum.
To answer your question, I don't know whether the problem is electrical or fuel. The rub is, neither does the "independent Jaguar expert" in whose hands it has been for several months! His latest attempt is to replace the (his description)...the "Thermotime Switch." I can't find reference to such an item on Google so wondering if he has the correct name. He described its function as enriching the fuel to the injectors when the engine is cold, whilst leaning out the fuel when running at normal temperature. He says it is attached close to (or on to) the thermostat housing.He further says that they are "not available." I'm trying to find out the proper name for this component so that I can, myself, try to track one down. If anyone can tell me the proper name for this component I describe, it would be a great help. Thanks, again.

[steve_m]

I think that it is commonly referred to as the CTS, coolant temperature sensor.
I think that there are 2, one supplies the temperature gauge and the other supplies the ECU

The following links may be helpful.

http://aj6engineering.co.uk/

https://www.sngbarratt.com/English/#/uk ... Down=false

[Fivethirty]

Thank you Steve. I will relay this info to my "expert".
Cheers.

[ptjs1]

Fivethirty,

If it is a fault in the thermotime switch causing a fuelling problem, has your specialist confirmed that you're definitely getting sparks at the plugs when the car is refusing to start?

Paul

[Fivethirty]

hi Paul,
Yes, he has confirmed there is a spark at the plugs at all times. He did replace the 'spark amplifier module' because he commented that the sparks were not "blue" but were yellowish and, therefore, slightly weak. I thought a spark was a spark but he insisted this isn't the case.
Notably, the plugs do get very wet with petrol (not oil).

[davidr]

I wonder if your coolant temperature sensor has failed. If either it has gone open circuit or there is a break in the wiring then the ECU will see that infinite resistance as a really cold engine temperature and so put in a much richer mixture than normal. The cold engine may start on this rich mix and run for a while but as it warms the rich mixture will become too much for it and it will stall and not restart. It's like starting a cold engine on full choke and leaving it there. That may suit it at first but as it warms it will run badly and possibly stall.
The ideal mixture of air to fuel is 14.7:1 and is known as the stochiometric ratio. The ECU will try to maintain this ratio when running in closed loop mode, that is the where it is taking into account the input it receives from the lambda sensors so that emissions are optimised. This may be varied by the ECU for different reasons. A richer mixture will give more torque and a leaner one better economy for example.
I hope that helps

[steve_m]

Honestly David, you are assuming that everyone on here knows what a choke is
For the youngsters among us, it was a manually activated fuelling enrichment device, usually operated by a Bakelite pull device, fitted somewhere to the left of the steering wheel.
Also really useful for your mum to hang her handbag on.
Yet to see any fuel injection system that can replicate that useful app.

[steve_m]

Honestly David, you are assuming that everyone on here knows what a choke is
For the youngsters among us, it was a manually activated fuelling enrichment device, usually operated by a Bakelite pull device, fitted somewhere to the left of the steering wheel.
Also really useful for your mum to hang her handbag on.
Yet to see any fuel injection system that can replicate that useful app.

[davidr]

Steve - you're quite right to call me out for being out of touch with today's 'yoof'. Guilty as charged!

[Fivethirty]

David,
I think you've hit the nail on the head! That explains the symptoms and effects to a tee.
In addition, I have read AJ6 Engineering's website that the ECU needs modifying (by AJ6, naturally!) to allow for adjustment for the drop off of manifold vacuum in a high mileage engine (like mine!). What do you think?
...and to both Dave and Steve; Choke(n), I thought that's what my wife wanted to do to me for buying such a duff XJ-SC - Choke(v)

Fortunately, I am of the age that knows what a choke is...and the obligatory clothes peg!...along with radiator muffs and paraffin lamps placed under the engine at night in winter. Happy days.

[steve_m]

Paraffin lamps, blimey, I bet you remember black and white TV!
Both channels!

[davidr]

David,
I think you've hit the nail on the head! That explains the symptoms and effects to a tee.
In addition, I have read AJ6 Engineering's website that the ECU needs modifying (by AJ6, naturally!) to allow for adjustment for the drop off of manifold vacuum in a high mileage engine (like mine!). What do you think?

The reduced vacuum issue may be one to address but I don't think that will stop your engine running - just give a poor idle and lack of responsiveness at light throttle. I'd fix the prime issue first.
If the symptoms I've described appear to fit then get your man to get his multimeter out to check resistance across the temperature sensor If it has failed 'open circuit' that will be an issue. Also check for a 5volt feed to the sensor and continuity to ground from the earth side connector. If either fail that could also be the issue.

[Fivethirty]

Good morning David,
I much appreciate your help on this. Thank you!
I've passed along your comments and hope to get the problem resolved.
He tells me he is having difficulty finding a replacement part. Can you recommend a source other than SNG Barrett?
Is the part common with other models of Jaguar, or even Land Rover do you know?
Cheers.
Peter