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- Common Lucas 4CU ECU failure symptoms

The Lucas 4CU fuel injection ECU fitted on 3.5 vehicles up to late 1989 - click here to see photo - has proved to be extremely unreliable and virtually all engine idling/running problems are likely to be due to faults with this ECU. This is unfortunate because the system design is very good and a 3.5 Range Rover with a rebuilt ECU has excellent performance with economy of about 23 mpg in normal use.

The airflow meter is the most reliable part of the system and should NEVER be forced open and cleaned or lubricated internally - the carbon track is meant to look scored through! Please click here if you think your airflow meter has been tampered with. Please click here if your vehicle is converted to run on LPG gas.


Car Electronic Services specialises in the performance rebuilding of these ECUs during which the ECU electronics undergo comprehensive re-engineering to restore the ECU to its intended power specification. Many owners have written in saying how delighted they are with the added power and economy of their Range Rovers following performance rebuilds and have reported 22 to 25 mpg in normal use with crisp throttle response and very high top speeds. Our performance rebuilds are guaranteed for 24 months. Surprisingly, about 50% of our performance rebuild customers already have LPG gas conversions. Please click here for full address and performance rebuild quotation request form.

Vehicles may exhibit more than one of the following symptoms. PLEASE NOTE - These are not the only failure symptoms - just the symptoms most likely to baffle!

Symptom 1

POOR OR ROUGH IDLE - vehicle can be OK for short while following service or tune up (vehicle does not retain CO setting - CO goes high/car runs very rich or can go to the other extreme - very weak). Diagnostic checklist

Symptom 2

ENGINE IDLE SPEED HUNTS (sometimes stalls) DURING WARM UP OR WHEN HOT - vehicle can actually rock. CO can be very low and unadjustable. Diagnostic checklist

Symptom 3

VEHICLE HESITATES THEN SURGES FORWARD WHEN PULLING AWAY FROM STOP. Sometimes feels as if vehicle not responding correctly to accelerator. Diagnostic checklist

Symptom 4

VEHICLE BREAKS DOWN AT IRREGULAR INTERVALS WHILST DRIVING BUT RESTARTS OK. Engine cuts out whilst driving - period of cut out can be 1 to 20 seconds, sometimes much longer. Diagnostic checklist

Symptom 5

ENGINE STARTS AND IDLES OK BUT SOMETIMES DIES THE MOMENT THE ACCELERATOR IS PRESSED. Restarting can be accompanied by clouds of black smoke. Diagnostic checklist

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