Lucas Petrol Injection

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PI engine


The Lucas mechanical petrol injection system was orignally designed for military applications, and was later adapted for use in cars. It was extensively used within racing, including Formula 1. It was also used in Maserati and Triumph road cars.

The original design was done by Harry Bottoms, and Jim Littlehales did the development for use in road cars.

The design is basically sound, with a very high performance potential. The system does, however, need a certain degree of preventive maintenance to ensure a good long term reliabilty.

There are mainly two good sources of documentation available for these systems. The first is the Petrol Injection Mk II manual from Lucas is the original description from Lucas. This is a very thorough description of theory and service, but does in some cases refer to special equipment. A reprint of it was made a few years ago, and it may still be available.

In containing the "collected wisdom" of many people and many years of PI experience, the Triumph 2000/2500/2.5 Service Notes is probably more comprehensive and indeed more practical for the average PI owner. Section 5 covers the PI system.

General description

PI overview

Fuel flows by gravity from the fuel tank to the filter. A swirl chamber, a seperate fitment on Mk 1 models, integrated in the tank on Mk 2 models, maintains the fuel supply in curves.

From the filter, fuel is drawn into the spur-type gear pump and pressurised. A relief valve maintains a line pressure of 100 PSI, and returns excess fuel back to the tank. This constant circulation of excess fuel is an important means of cooling the fuel pump, which otherwise would be very prone to fuel vaporization. Pressurised fuel is fed to the metering distributor located in the engine compartement.

This unit is metering fuel in amounts proportional to the engine vacuum by means of a tiny piston (shuttle) located within the distributor body. The movement of this shuttle is clearly audible as small metallic clicks when the engine is running at idle. Fuel is distributed to the respective cylinders where it is injected into the inlet tracts. There is a throttle butterfly per inlet tract.

Fuel that leaks past the metering shuttle is fed back to the fuel tank via a return line.

Fault finding guide

Select what the main indication of the problem at hand:

Engine stops in hot weather

This is usually caused by vapour locks in the fuel pump due to insufficient cooling, and is one of the more common problems with the Lucas PI system. Is is diagnosed by a higher pitched whine from the fuel pump than usual, a sure sign that it is free running in a cloud of fuel vapour instead of pumping liquid fuel as it should.

It is important to realise that the primary source of pump cooling is the petrol being pumped around the system. The amount of petrol consumed by the engine will not be sufficient - the majority of the flow through the pump will be petrol being returned to the tank via the shunt pressure regulator.

A quick check to find out if the circulation is reasonable is to listen for petrol being pumped back to the tank with the ignition on (pump running) and the engine off.

For a system that is beginning to show signs of this problem, it helps to avoid running with a low level of petrol in the tank on hot days.

Hot running problems can usually be due to a number of reasons, in some sort of priority order:

Excessive fuel consumption

Really excessive fuel consumption will be indicated by black smoke from the exhaust, and is usually easy to diagnose. Check the following:

Engine running unevenly

Check the following:

Fuel leaks

Due to the high pressures involved, the system can be quite prone to fuel leaks. Plastic and rubber tubing used in the system can become brittle over the years, and should be inspected regularily.

Leaks from the petrol pump is designed to be drained out via a tell-tale pipe located in the middle of the lefthand rear luggage compartement well. It is worth making a habit of checking for fuel dripping out of this pipe while the pump is running at regular intervals. If the outlet is clogged, petrol will flow into the luggage compartement well.

Most leaks have obvious causes and remedies, but some items warrant special mention:

Engine not up to power

Check the following:

Starting problems

Check the following:

Engine stops

Check the following:

Engine does not run after a rebuild

It does take a while to prime the fuel system when dismantled. A good approach is to run the engine on the starter, with the plugs removed, untill the system is primed. This will also help oil circulation, by the way. Remove the injector with the longest pipe, and run untill a fuel spray is visible.

Further detail

Vacuum at idle

The PI system uses engine vacuum for fuel metering, and correct vacuum is thus very important for proper operation.

Check the engine vacuum at idle. It should be approx 12" (305mm) Hg for a standard PI cam, 7" (180mm) Hg for a TR5-cam. If it is too low, this may be caused by wrong tappet clearance, leaking valves or maladjusted throttle butterflies. A leak in any of the vacuum hoses may also be suspected.

Fuel pump

The pump should be able to deliver at least 1.2 litres per minute at the rated pressure of 100 PSI and a voltage of 13.5V. A quick check can be made by disconnecting the regulator tank return line, and measure the amount of petrol coming out of it when the pump is running. It should be able to fill a litre measure in 50 seconds.

Be sure to check the voltage at the fuel pump terminals. Any noteworthy difference to the voltage measured at the battery terminals will cause degradation of the pump performance.

The pump should not consume more than 5.5A at 13.5V when operating at the specified pressure. At 8 volts, the current should not exceed 6.3A. The light running current should be 1.4A at 2200 RPM. The armature resistance between adjacent commutator bars should be within 0.16 and 0.24 ohms.


The most likely cause of problems here is debris collecting on the valve seat, especially on cylinders #2 and #5. It will usually indicate itself by the engine running on 5 cylinders at reduced throttle openings, and occasionally run properly at full power. The debris is usually caused by the remains of the rubber O-rings on the metering unit banjos becoming frayed. Clean by blowing compressed air down the injector. In an emergency, open the injector nossle by hand, and try to blow the dirt away. In a pinch, you might even try to pierce with a tiny piece of very soft wood (i.e. a toothpick).

As injectors get old, the small O-ring within the injector body may become tired, and start to leak. This will usually manifest itself by the inner injector tube being loose with respect to the outer tube.

The injectors should start spraying at a pressure of 45 to 55 PSI. It should produce an evenly distributed cone of fuel, with no dripping. At 40 PSI, the allowable leakage is specified such that the time for a droplet to appear at the injector tip should be not less than 60 seconds.

Fuel pressure regulator

The regulator adjusts the fuel pressure by allowing excess amount of petrol return to the tank. The fuel pressure should be 100-110 PSI, and can be set by adjusting an internal adjustment screw (clockwise to increase pressure).

The most common fault with the regulator is that it becomes clogged. In this case, the fuel pressure will be too high. This will usually cause the pump to run too hot, causing petrol vapour lock in the pump.

Valve judder may occur if the rubber hose from the pump to the regulator is very short.

Fuel supply curve

Contrary to popular belief, it is fairly simple to measure the fuel curve. Note, however, that the fuel supply curve itself does rarely go out of adjustment, and problems with the PI usually have other causes. So don't alter the fuel supply curve settings unless you are quite sure that you know what you are doing. It makes good sense to verify that the supply curve is correct, however, especially if major surgery has been performed on the system.

The tools required are a set of feeler blades, and a reasonably accurate vacuum meter. Unless you have the possibility to calibrate them, the cheapest of vacuum gauges are probably a bit too inaccurate. The vacuum gauge must be connected via a T-piece, so that you will get a continuous reading.

You also need a source of vacuum. An arrangement with a vacuum pump and an adjustable bleed valve is sufficient, but you might have problems setting up a stable vacuum. The simplest and probably best approach is to suck yourself, closing the hose with the tip of your tongue. This is also a very good leakage test, by the way.

PI adjustment

The fuel curves can then be checked according to the tables. The quickest test is to check the gap between the cam follower face and the rollers on the datum track. The maximum fuel screw (overrun stop) should be set according to the maximum vacuum reading specified.

An alternative method of calibration is to measure the quantity of fuel delivered at the injector nozzle. The quantity of fuel per 1000 injections is specified in the tables.

Saloon PI cam:

Vacuum (inches Hg) Gap (mm) Min fuel (cm³/1000) Max fuel (cm³/1000)
0.0 1.44 41.0 42.0
0.7 41.0 42.0
1.0 35.0 38.0
1.4 25.0 28.0
3.0 0.76 21.5 22.8
6.0 0.60 17.1 18.3
9.0 13.6 14.7
12.0 0.33 10.0 11.0
24.0 0.07 1.5 3.0

TR6 PI cam:

Vacuum (inches Hg) Gap (mm) Min fuel (cm³/1000) Max fuel (cm³/1000)
0.0 1.44 41.0 42.0
0.7 41.0 42.0
1.0 35.5 38.5
1.3 27.9 30.9
3.0 0.78 21.5 23.1
6.0 0.58 15.7 16.8
9.0 11.3 12.1
10.5 0.33 9.4 10.0
20.0 0.05 1.5 3.0

TR5 cam:

Vacuum (inches Hg) Gap (mm) Min fuel (cm³/1000) Max fuel (cm³/1000)
0.0 1.47 41.5 42.5
0.9 41.5 42.5
1.1 38.0 40.0
1.4 28.0 30.0
3.0 0.63 19.9 21.5
6.0 0.48
9.0 0.35 11.7 12.3
17.0 0.30 8.0 9.0

Note the the TR5 fuel curve requires an other set of springs in the control unit than does the regular fuel curve. Ensure that you have got the right springs for your application.

The fuel curve is also available in graphical form.

A more thorough description may be found in the Lucas PI manual.

Preventive maintenance

Change the fuel filter at regular intervals....

Drain off any collected water by opening the valve at the bottom of the fuel filter from time to time.

Check for fuel leaks, and check the condition of the fuel pipes.


The fuel mist from the injectors is very easy to ignite. Perform all tests on operating injectors out-of-doors. Have fire extinguishing equipment ready at all times.

Keep the fuel spray well away from the eyes.


The following is a list of ideas for modifications of the PI system. Several of these will make the system more reliable, but they cannot replace proper maintenance and repair.

PI overview

This page is being maintained by Egil Kvaleberg. Please report errors, updates, suggestions and comments to

Latest update: July 12th 2000