0044 (0)1625 573556 roger@aj6engineering.co.uk

A Common Problem with the Early 3.6 XJS (1983-87)

Excessive Richness at Idle & Light Throttle.


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.

Routine engine tests at this stage tend to be inconclusive, the only clue being that at idle the manifold vacuum is rather lower than might be expected, but in all other respects the engine seems to be in sound condition. We think that a number of quite minor failings compound each other to cause the problem: slight leakage past the piston rings and the valve seats (remember on a four valve engine there is much greater seat area over which to leak) added to some stretch in the timing chains causing the cam timing to be a little retarded, and maybe the construction of the cylinder head allows the valve clearances to close up slightly. None seems that important but added together their effects become serious yet the idea of an expensive engine rebuild is hard to justify, although any excessive timing chain wear must be rectified.

Fortunately there are a couple of alternatives. A simple method which helps if the problem is not too bad, is to relocate the vacuum signal source for the ignition vacuum advance system by taking it to a manifold tapping (at the rear of the manifold plenum) instead of the original throttle edge tapping. The idle speed and mixture will now need to be readjusted but as there may be a tendency to stall under steering pump and air conditioning loads the idle speed may need to be raised slightly. A slightly rich exhaust CO level of around 2-3% will probably be found to give the best idle quality.

Eventually the problem will still get worse, indeed all these cars will suffer in this way sooner or later. We offer a complete solution to the problem by providing the ECU with adjustment so that conditions other than at full load can be quickly optimised to suit the individual car via a simple trimmer screw. This has the great advantage over simple recalibration that if the engine is later overhauled or replaced, the ECU needs only to be readjusted. The cost of this quick turn-round conversion is £165 plus £7 postage (UK) plus VAT.


Update – January 2019.


The above was originally written in 2003. It is quite surprising to find now that I seem to be doing more of these ECU conversions than ever. Of course the price has risen since the figure of £165 was quoted back in 2003 and is currently (2019) in the full website price list at £300 plus £11 postage (UK) plus VAT.

A lot of these ECUs sent for modification also need repairing. 30 year old electronic devices are always likely to drift out of calibration and develop various faults ranging from dry solder joints to actual component failures. These are generally repairable unless the main custom chip fails because these have not been available for many years. Fortunately such failures are not very common.

I rectify simple calibration errors for no extra charge but more significant repair work will incur charges although less than just for a repair on its own without the modification added.

A repair includes completely de-soldering the main circuit board and renewing all the solder joints to clear any latent dry joint problems. An ECU should then have many years of useful life.

A repair alone is likely to cost £200 – £225 but if combined with an upgrade or modification the repair cost can usually be reduced by £100.