Jaguar XJ6 1979 EFI Rich fuel problems. Advice needed

after trying to start with low voltage, the engine misfired. after this, the engine has had too rich a mixture and cuts out after starting. If I disconnect the fuel pump while the engine is running, it runs clean until the system is empty. petrol pressure is 2.5 bar, and nozzles inspected. Has anyone experienced the same? Jaguar XJ6 Series 2 1979

Fuel pressure is way too high. Should be about 2.5 bar. Suspect a faulty regulator or restricted return line


Yes, i suspectet the same. The return Line i replaced with a bypass witch doesnt help. The pressure regulator have been changed. But the new is still 5 bar.
I read somewhere That this is the correct pressure. But perhaps not. .

I should have asked before but how/where are you taking your pressure reading?


Sorry. - i was incorrect. The fuel pressure is 2.5 bar, mesaured at the hose for on of the injectors.
Is it possible that fuel is lead to the intake system in any other way?
The cold start injector is blocked, to be sure it is not the problem.
Br. - and thanks, Ivan

Welcome aboard, Ivan

2,5 bar (36 psi) is the correct pressure and is not the cause of fat running. Are you sure it is running fat - stopping after initial start may have other causes…

There are nominally only two different ways of getting fuel into the manifold - the injectors and the CSI; and having the latter fuel hose clamped eliminates that source.

That a second start with fuel pump disconnected got the engine running; did you retry - or reconnect fuel pump once the engine was running?

As the engine starts, the AAV will cause the idle to rise to some 1200+ - did it? Point is that if the idle is not high enough, the engine may stall due to cold engine drag - a cold engine that cannot idle will not start/run.

The xk engine is very tolerant to ‘fat’ running, and is always fat when starting cold, but then require good sparks - and the low battery may not suffice initially. Check spark quality - and recharge battery…

But also, if mixture is indeed verified too fat; do check/measure the CTS - if failed the mixture may be very fat indeed…

xj6 85 Sov Europe (UK/NZ)

Wow, but that takes me back at least 10 years. 100’s of J/L postings on one topic, and weeks of group thinking trying to figure out why an XJ6 would travel around the block with the fuel pump relay out. Ended up the coolant temperature sensor was shot, and ECU was increasing injector pulse width…enough to make the vehicle travel. Check you CTS.

SD Faircloth


Hi Frank, an thank you for the advice.
The motor is hard to get started, and disconnecting the fuelpump by the switch in the flowmeter, it is running. Ok.
As son as i switch the pump back on, Black smoke appears and the motor stops. The CTS is tested as Well.
It all started by the backfire in the intet manifold. Could that be a cause of any trouble?

[quote=“IvanC, post:8, topic:448253, full:true”]
…It all started by the backfire in the intet manifold. Could that be a cause of any trouble?

Indeed it can, Ivan; an inlet backfire may damage the AAV (flow meter) resulting in incorrect data to the ECU - and incorrect fuelling with correct fuel pressure…

As minimum; remove air filter to access flap - verify the flap is closed (endstop). Push the flap slowly through its full range; there should be smooth, even resistance throughout the sweep - and the flap, when released, should smoothly return to closed. Absolutely no binding any way…

This does not absolutely verify AFM function, that is a more complicated procedure - but it must pass this test. The sweep, passing over the resistor board, wears the track of the latter - and is an occasional source of fuelling fault. It’s not necessarily easily visible - but you can look at it.

Since you are using the reed switch(?) in the AFM to stop the pump, I assume you have opened the AFM. You can then observe the movement of the sweep to assess the movement of the flap. But do verify the position of the sweep with the flap positively closed - it should be at the extreme end of board.

If inconclusive; the best is to try another AAV…

Another result of a backfire is to blow the air duct off one of the connections (and sometimes split it). Inspect carefully - though it invariably leads to very lean running…

xj6 85 Sov Europe(UK/NZ)

I would expect the AFM to be reporting a voltage back to the Ecu in the range of 1v at idle and close to 12v at full deflection.
I don’t have the Jaguar manual, or wiring diagram for this, so have just made some assumptions based on other use of Le-jetronic Efi. (which I assume this is)

Ahhh, but how ? Did you ohm test the CTS only ? or did you also test the connector ?, and did you test the wiring all the way back to the ECU ? As in disconnecting the wire harness from the ECU and ohming the two wires coming from the CTS while all the rest remains connected ? SD Faircloth

Indeed important, SD - it should read some 2Kohm. And must not vary when bending or twisting the connector. ‘Break’ means fat indeed - and a short means very lean…

xj6 85 Sov Europe (UK/NZ)

I think we can forget the Cts since the O/p stated.

Dear all, i thank you for the good advice
I have measures the flow meter and find the values as described in manual. I tried to change with another flow meter which didn’t help as well.

The temperature sensor is measured as described in this forum. Both ways. All values on spec.

I have not removed the injector’s yet, which could be the next thing. Still it seems unlikely that they would all break after the inlet backfire?

Hope to be running soon

Thank you frank. Did not help. Unfortunately

Unless the backfire was a symptom, not the cause.

For a backfire there must be fuel and a spark into the inlet manifold.
Check the ignition timing and if necessary, the valve timing and compression leak down.

That would be a world first, Ivan…:slight_smile:

Are the symptoms still the same as described in your first posts? The replacement AFM fitted may of course have the same fault as the original - an AFM fault would fit the bill…as would any of the things mentioned and tested! That all this happened after the backfire might be a coincidence - or the actual problem caused the backfire. The ‘US’ has a throttle switch that enriches fuel during full throttle - and the ‘European’ a full load switch operated by vacuum for the same purpose. If either fail it will enrich fuelling, but usually not to the extent of causing symptoms described. However during cold start they may lead to excesses? Which one do you have?

The sole purpose of the ECU is to ground the injectors for a prescribed time based on sensor inputs. If the sensors, all tested and found ‘good’; the ECU itself may the problem. Best tested by a swap, of course if you have, or can borrow a good one. Alternatively using an oscilloscope to measure the length of the grounding - it is in the mS range…

Releasing/removing the injectors is fair enough for checking spray patterns, leaking (causing excessive fuelling) - or being clogged. However it is less effective for testing if the ECU is delivering too much - the difference is usually not easily discernible…

xj6 85 Sov Europe (UK/NZ)

I agree with that…but have you tried pinching/clamping off the hose to the cold start
injector ? It could be open and continuously delivering fuel. Could its’ pintle have been blown open by the backfire ? Who knows, but pinch off the hose and see what happens. SD Faircloth

Second that, SD - indeed a prime step, I thought he had tried it. Though I cannot readily see the backfire can interfere with CSI or other injectors…?

xj6 85 Sov Europe (UK/NZ)

Hello Frank. I thank you for your help.
Mine It is an US model 1979. The car was running smoothly till it stopped by low voltage. ( faulty generator) by restart the battery was low in voltage, and the backfire appears. So in my mind is also some kind of electronic that did not like the low voltage.
I hope it is not the ecu :grimacing: