Hey all,

First I'd like to thank everyone for the help with some of my other Jag related issues, without the input from the team here I don't know where I'd be

So when I brought this old Daim the throttle micro switch was disconnected.
The switch is the type that detects a closed throttle and I guess activates overrun fuel cut off. (Non Emission type Jag)

Easy answer: Maybe the ECU has been replaced with one that doe not need the throttle switch. Connecting it confuses the ECU and the engine stalls.
Just a guess.

This is what I've figured:
(This could be wrong, this is just hope I think they work from the research I have done)

I believe there are two sensors in this system, the closed throttle sensor on the throttle body and the vacuum switch next to the throttle body.

I believe that the two switches combined sense four different states:

1. Full load enrichment. The Throttle is NOT closed and the vacuum switch senses high vacuum, the ECU supplies more fuel.

2. Overrun Cut off. The throttle IS closed and the vacuum switch senses high vacuum, the ECU cuts fuel to the engine and uses no fuel during overrun.

3. Normal fuelling conditions. The throttle IS open and the vacuum switch is not activated by high vacuum.

4. Idle. The throttle is closed and the vacuum switch is not activated by high levels of vacuum. ECU continues to supply fuel and air is supplied as per normal idle.

Both 3 and 4 are not really to do with the system but its good to keep in mind seeing as the switches could be effecting these states.

With these states in mind I have come up with the following:

1.The switch kills the engine at idle, so it must be activating fuel cut off regardless of if the vacuum switch is activated by high vacuum.
So maybe the vacuum switch is stuck open?

If the vacuum switch is stuck open the ECU would be supplying Full Load Enrichment at all times because the closed throttle switch is currently disconnected.

2. A different symptom could be caused by the Vacuum switch stuck closed (not sensing the higher vacuum when under load) and with the throttle open, this would cause the ECU to never supply Full load enrichment.

The interesting thing is, when I drive up hills I find that the engine seems to sound a bit like a tractor if I'm not careful with the throttle. Maybe because its struggling with a mixture of fuel and air that is not rich enough when under load. Revs stay steady but it makes that sound when you change up a gear instead of down a gear when your driving your daily manual, say your manual Toyota.

3. Another idea:
- Maybe a vacuum leak is causing the vacuum switch to not operate and therefore causing no full load enrichment.


Where I read about stuff:
Fuel injection and the Jaguar XJ6 4.2 Series 3 / AJ6 Engineering

Fuel System Maintenance
The OVERRUN VALVES section. This is about the XJS but the principle is the same.

Fuel injection - control unit, lambda sensor and throttle body - Carburettor, fuel injection and air filter - Jaguar XJ6 Series I-III and Daimler Sovereign Series I-III (1968-1987) - Jaguar - British Cars - SC Parts Group Ltd
Numbers 83 and 21 are the switches I'm talking about.

 

Well said Harry, in sharing your sentiments you speak for many of us!

 

The struggling up a hill could be your ignition timing too far advanced. Hows the power?

 

I remember this tiny little switch on my Series 3 I once had. As I understand it, it was a device to improve petrol consumption, so when the driver closed the throttle whilst driving, this little switch cut off the fuel injection until the engine revs (or maybe the speed of the car), dropped to a certain value. I can't remember how this was done, but fuel supply was restored once this revs/speed value was achieved, and thus the engine could tick over OK as the revs/speed value was above tickover.

Edit
I just looked in Nigel Thorley's book on the XJ (Jaguar XJ - The Complete Companion) of 1991. This mentions the fuel cutoff switch on the overrun. Fuel is cutoff until 1200 revs are reached.

 

Thanks for the reply! I have been looking into this, I checked with a timing light last night and got 6 BTDC, at idle with both the vacuum hose off and on.

Now comes the hard part, where is it meant to be set? Ive read the workshop manual by cant figure out what version on\f emissions she is, so therefore stuck as to where to set it.
Theres no CAT, no lambda and no evaporative emission control.
The only emission control is the positive crankcase ventilation, which I have read (haynes manual pg.229) is only on UK spec vehicles, which is what she is, 1980 BTW.

So what version of emissions is she then? I've attached the list from the service manual with the timing settings. I feel like she might come under "rest of the world"

I believe you are completely correct, I managed to get the switch connected with the car running and no trouble since. I think that the little metal arm was bent in a way that meant it didn't always click on when the throttle was closed, a few careful bends and its as good as new. Which would explain why the engine died when it was connected. NO FUEL haha.

 

So is all sorted then Harry?

One way you can check is to get the VIN off the car, this should tell you what market your car is for, certain letters mean certain levels of trim, intended market etc. A quick look on the net and you should be able to decode it.