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[Kevin Wood]

Fuel Trim Malfunction codes (0170 and 0173) and Cat Efficiency problems (0420 and 0430) have been mentioned a few times recently, particularly with reference to LPG conversions. I've also chatted with a few members via PM who have been interested in what's behind it and the following explanation evolved, which I thought probably deserved to be available in public.

Fuel trim codes can be a common problem where an LPG system hasn't been tuned accurately or where another malfunction of the engine or associated systems has had an effect on the tuning of the engine. So, first things first. What is fuel trim?

The petrol engine ECU has a number of inputs to tell it about the engine operating conditions and from these inputs it looks up in a calibrated table (called the "MAP") how much fuel to inject into the engine. This results in the petrol injectors being opened for a precisely controlled length of time during each engine cycle called the "injector duration". Since the injectors flow petrol at a constant rate when open, the volume of fuel is directly controlled by the injector duration. This typically ranges from 3 milliseconds (0.003 seconds) at idle to perhaps 15 milliseconds at full load.

The above describes an "open loop" fuel injection system, of the type that were common before catalytic converters were mandatory. The map was calibrated for a given combination of engine, injector flow rate, induction and exhaust system, cam profile, etc. Provided none of this changed significantly the engine remained in a reasonable state of tune, with only the odd mixture adjustment required at MOT time to keep the emissions at idle within specification. Crucially, the ECU is operating "blind" in an open loop system. It delivers fuel to the engine, but has no idea if the fuel mixture turns out to be correct.

With the advent of 3 way catalytic converters the fuel mixture burnt by an engine became more crucial. A catalytic converter will only clean up the exhaust emissions successfully when there is a chemically correct mixture of fuel and air being burnt in the engine. This means tighter control of the fuel mixture. Lambda sensors are the mechanism by which this control is achieved. They give the ECU feedback on the actual mixture burning in the engine.

The most common type of Lambda sensor, and the type used on all Omegas, is termed a "narrow band" sensor. This is because it measures the fuel mixture over a very narrow operating region centred around the chemically correct mixture. Such a sensor can be viewed as indicating whether the mixture is "lean" (too little fuel) or "rich" (too much fuel) with respect to the chemically correct mixture. Basically, the petrol ECU will use the output of the lambda sensors to adjust the quantity of fuel injected (the injector duration) so that the lambda sensors are always at the point of switching between "lean" and "rich", so the average mixture is correct.

It does this by maintaining a variable called a "fuel trim" for each bank of the engine (on a V6 - just the one on a 4 pot). If the lambda sensor reads "lean" it will increment the fuel trim periodically. If it reads "rich" it will decrement it. The fuel value from the map is multiplied by the fuel trim before calculating the injector duration so the fuel trim has a negative feedback effect, adjusting the injector duration read from the fuel map to keep the mixture correct.

The fuel trim is used to compensate for variations between engines, fuels, injectors, etc. to ensure the mixture is always right for the catalytic converters. However, it has a maximum range based on how much trim would normally be expected. If the fuel trim exceeds a threshold programmed into the ECU, a fault code is stored and the EML comes on.

Many ECUs take this a stage further and store a "long term fuel trim" value based on the long term average of the fuel trim that is in use. This variable is often stored in non volatile memory so it is available from start-up before the Lambda sesnors have warmed up. It is also often applied to areas of the map where the closed loop correction is not active, for example at full throttle where the mixture is too rich for the Lambda sensors to provide useful feedback.

Many ECUs also store long term fuel trims in a so-called "block learn table" which effectively stores separate fuel trim values for different areas of the fuel map, as the fuel trim required when idling is often different to that in use when motorway cruising, for example.

Add LPG to the equation and the LPG ECU is watching the injector duration in use by the main ECU and using it to calculate the LPG injector duration required. The tuning of the LPG ECU relates these two durations, and also applies corrections for the temperature and pressure of the vapour being delivered to the engine so if, for example, the petrol ECU uses a 3ms injector duration, the LPG injector duration might be 5ms for the same amount of fuel. If this is done correctly the fuel trim remains roughly the same as on petrol and the main ECU is happy.

Now, let's say for a 3ms duration the LPG ECU only injects 90% of the fuel required, due to a fault in the LPG system or to bad calibration of the LPG system. The fuel trim is increased by the main ECU until the mixture is right. This will require a fuel trim of +10% to correct the situation. So, it is straightforward to see that, if the LPG ECU is not correctly tuned, the main ECU adds fuel trim to make the mixture correct again, and if that trim is too large, the EML comes on.

Incidentally, you can watch this process happening on a Tech 2 or, on later engines, using an OBDII compatible reader. It allows you to view the variables inside the ECU in real time.

[Kevin Wood]

So, having established what is happening inside the ECU with regard to fuel trims, what can cause a fuel trim fault code? Well, essentially, anything that causes the ECU to have to make too large an adjustment to the fuel delivery. Starting at the beginning, the ECU has a number of inputs that determine the required fuel for the engine. Coolant and air temperature, mass air flow, throttle position, RPM and battery voltage all have an influence to a greater or lesser degree, so if one of these inputs to the ECU is in error, the wrong value from the fuel map will be selected, and a correction will be required to achieve a chemically correct fuel mixture. Rememeber also that an air leak can cause the MAF reading to be completely wrong, and that changing the sensor will not help in this instance!

Assuming the inputs to the ECU are correct, perhaps the injectors are not delivering the expected volume of fuel. This brings in the possibility of clogged injectors or poor spray patterns, or incorrect fuel pressure due to a faulty FPR or a clogged fuel filter...

The characteristics of the engine itself must obviously match the map in the ECU. Change the cams and the fuel trim must be able to take up the adjustment. If the cam timing is incorrect it can have a huge impact on the required fuel, and hence the fuel trim.

A clogged exhaust system or cat will restrict the engine's breathing and cause a large correction to the fuel trim.

Finally, do not discount the fact that the Lambda sensors could be misreading. A tired sensor should be identified by the ECU by its' slow response time, but not always. An air leak into the exhaust upstream of the Lambda sensor via a manifold gasket, EGR valve or SAI system fault will make it read incorrectly. This will cause an incorrect fuel trim to be applied and, potentially a fault code if it takes it outside the allowed range.

The clue to many of these potential problems, on a V6 at least, is whether the fault occurs on both banks or just one. Have a think about which of the above causes would affect both banks simultaneously, and which would affect one bank in isolation...

[Kevin Wood]

Another issue which is often grouped with fuel trim, although not necessarily related is the appearance of "cat efficiency" (0420 and 0430) codes on the newer Omegas with 2.2, 2.6 and 3.2 engines. Everything we have discussed above regarding fuel trim relates to the front "pre-cat" Lambda sensors. These engines have a second set of Lambda sensors after a small "pre-cat" in the exhaust elbow.

The only purpose of these "post-cat" Lambda sensors is to determine if the pre-cats are working effectively and to light the emissions light if there is a problem. They are not involved in fuel trim at all - for a very good reason. The fuel mixture is measured by the front sensors by detecting the proportion of unused oxygen in the exhaust gases following the chemical reaction that occurs in the cylinder. Put a rich mixture into the system - one with excess fuel - and all the oxygen will be used up burning this fuel. Put too little fuel into the cylinder and the fuel will all have burnt before the oxygen is used up. Once the exhaust gases have passed through a catalytic converter, further oxygen is used up in the process of breaking down more completely the oxides of nitrogen and carbon monoxide, and this no longer is able to tell us about the mixture.

So, back to the cat efficiency errors. If the pre-cats are working, and the front lambda sensors are, through the fuel trim process, keeping the engine's mixture correct, there should be no or very little oxygen left in the exhaust after the cat. A chemically correct mixture means that there is exactly enough oxygen flowing into the engine to fully break down the fuel. Most of this breakdown happens in the engine itself, as the fuel is burnt, and the rest occurs in the pre-cat as some of the harmful products of part-burnt fuel are removed, leaving no oxygen at the post-cat lambda sensors. Due to the principle on which a lambda sensor works, a sensor seeing no excess oxygen will always read "rich".

So, the ECU can expect the pre-cat lambda sensor always to be "cycling" between rich and lean as the continuous process of adjuting the fuel trim occurs. For a while the post-cat lambda sensor will follow this cycling as the exhaust gas passes straight through the cat. As the cat reaches working temperature, however, the oxygen will start to be removed from the gases and the post-cat lambda sensor will start indicating "rich" for a greater proportion of the time until it is reading "rich" 100% of the time.

The ECU can look at this pattern of response from the post-cat sensor, and if it does not revert to reading "rich" after a reasonable warmup time, a cat efficiency fault is raised.

You might well ask why the car still has a main cat after the pre-cat? Well, this is because the pre-cat is not sufficiently large to cope with the flow of exhaust gas under full load. It is a small device introduced because it warms up much quicker than the main cat (Hence the fact that engines with pre-cats don't require secondary air injection to warm up the cat). The ECU will therefore ignore the post-cat lambda sensors above a certain level of engine load where it knows the pre-cats aren't going to be effective.

This also explains why, on 3.2 litre engines where the pre-cats have become tired, MOT emissions failures are not common. Anything that passes through the pre-cat gets cleaned up by the main cat, and, of course, the MOT emissions test is only performed when both cats are fully warmed up. It is quite likely that a tired pre-cat means the car no longer meets the emission standards that it was designed to during cold-start - but that's not what gets tested at MOT time - hence the requirement to have an emissions light and monitoring on the car.

Moving the post-cat Lambda sensors to after the main cats can also resolve this issue, since the main cat now gets the opportunity to break down any compounds that have passed through the pre-cats, and in doing so, excess oxygen is removed from the exhaust gases.