Been out with the multimeter and i can confirm the idle control valve works (even took it out and saw the needle moving), throttle potentiometer works although the reaadings i was getting were twice what the manual suggests (could this be the problem?).

I now have no idea where else to look. The only other thing i can think of is whether, as mark chandler suggested, it need a power resistor (there are no wires for it and the guy i brought the engine from said that was the whole loom and all the boxes), but if this was the case, why is it running fine other than idle?

Any suggestions are more than welcome.

what type of sytem is it ? possibly hotwire ?

regards

Agriv8

Don't know much about the modern Jag engines, but your problem "could" be as simple as a leaking vac tube somewhere. Worth a look see. HTH Ray

Can you remove the throttle potentiometer from the throttle link and turn it manually to see if you can improve the idle, I’m thinking if its reading twice what it the books says, then the idle valve won’t be activating at the correct time as the engine thinks the throttle is open more than it really is.

I'm also thinking this might be whats wrong with my missys car...

Actually I’m reading that they need cleaned inside as they build up carbon from wear, a job for tonight on herself’s car I think

[Edited on 7/10/09 by Mr Whippy]

Agiv8 - Yes its hotwire

Chippy - Have inspected all hoses and they all look fine

Whippy - not sure what you mean by turning it manually

Just been out to check and although wiki says the engine has a hot wire system, there is no wire in it.

The unit says it is a mass air flow, does this mean it doesnt need a hot wire or it should and its missing?

if you have a unit called mass air flow sounds like a hotwire unit to me ( infact I think I have a jag one in the garage somwhere )

Check 1 leaks between flow meter and engine
Check 2 hotwire can get confused by turbulance are you running air filter ( and if room pipe between filter and meter )
Check 3 is the flow meter getting a clean 12v supply
Check 4 airflow meter attached correct way round ( here speaks the voice of experience )

A wiring diagrame may help

Regards

Agriv8

quote:

---

Originally posted by Agriv8
if you have a unit called mass air flow sounds like a hotwire unit to me ( infact I think I have a jag one in the garage somwhere )

Check 1 leaks between flow meter and engine
Check 2 hotwire can get confused by turbulance are you running air filter ( and if room pipe between filter and meter )
Check 3 is the flow meter getting a clean 12v supply
Check 4 airflow meter attached correct way round ( here speaks the voice of experience )

A wiring diagrame may help

Regards

Agriv8

---

Well, it seems the plonker who did the wiring (me ) may have forgotton to connect one wire - the one that supplies power to the MAF sensor

I have yet to try it as it is chucking it down here atm but will push her out as soon as it stops and report back.

Cheers

[Edited on 7-10-09 by speedyxjs]

does your MAF sensor look a lttle like this ?




Rover V8 ver of the Jag one IIRC

Regards

Agriv8

My VX Redtop engine will Idle fine and you can even drive it without the MAF/Hotwire thingy me bobby unit connected altho its like having an On/Off switch without it.

It may have no bearing on this system but it may help you from totally focusing your attentions on this.

Agriv8 - Yes it is.

MKindy7 - point noted. Not sure what else it could be though

Power resistor missing will screw it up at all speeds, early 3.6's are renowned for poor idle
linky

Hardly a week passes without at least one person phoning us about an elusive problem with an early (1983-87) 3.6 XJS running on the Lucas 8CU Digital EFI system. Light throttle driveability and idle quality have deteriorated, sometimes to the point where the car cannot pass an MOT test because of abnormally high CO levels in the exhaust gas. Often there will be a complaint of shunting in a trailing throttle over-run condition. Nothing seems to be wrong with the engine which performs as well as ever at wider throttle openings. Almost invariably the caller is convinced that the ECU is faulty - so please can we fix it?

It was back in 1988 when we received the first of these calls and after checking out the first few ECUs and finding no fault we began to wonder what was going wrong on these cars. In fact, it has nothing whatever to do with any failing of the EFI system - it is doing exactly what it is supposed to do -THE CAUSE IS THE ENGINE! It seems that after a certain age - and mileage is not necessarily a factor - the engine's ability to draw a vacuum at light or closed throttle deteriorates. The ECU, totally unaware that it is no longer receiving an accurate signal, simply does what it is designed to do - it supplies fuel according to the defective vacuum signal.

As Mr Whippy suggests, if the voltage from the throttle position sensor is too high at idle then the idle control function will not work.

quote:

---

Originally posted by mark chandler
Power resistor missing will screw it up at all speeds, early 3.6's are renowned for poor idle
linky

Hardly a week passes without at least one person phoning us about an elusive problem with an early (1983-87) 3.6 XJS running on the Lucas 8CU Digital EFI system. Light throttle driveability and idle quality have deteriorated, sometimes to the point where the car cannot pass an MOT test because of abnormally high CO levels in the exhaust gas. Often there will be a complaint of shunting in a trailing throttle over-run condition. Nothing seems to be wrong with the engine which performs as well as ever at wider throttle openings. Almost invariably the caller is convinced that the ECU is faulty - so please can we fix it?

It was back in 1988 when we received the first of these calls and after checking out the first few ECUs and finding no fault we began to wonder what was going wrong on these cars. In fact, it has nothing whatever to do with any failing of the EFI system - it is doing exactly what it is supposed to do -THE CAUSE IS THE ENGINE! It seems that after a certain age - and mileage is not necessarily a factor - the engine's ability to draw a vacuum at light or closed throttle deteriorates. The ECU, totally unaware that it is no longer receiving an accurate signal, simply does what it is designed to do - it supplies fuel according to the defective vacuum signal.

---

For the rover V8 there is a fault finding manual that takes you trough 20 tests that you can carry out with a multi meter.

dont know if there is somthing similar for the jag system

regards

Agriv8