Fuelling around

This Rover 75 had defeated the garage that had carried out a headgasket job but would not start afterwards.
This is a common failure and is due to the fuel tank design, and how fuel is pumped from one side of the tank to the other. The accumulator often comes apart. This results in no fuel to the engine.
All that is needed is to screw the two parts back together and prevent them coming apart again with a self tapping screw.
Once this was carried out the engine spluttered into life and revved up and down on its own.
A sure sign of an air leak.
Finding leaks is often a task undertaken with a smoke generator. But this one was so bad it could be found with your hand, a hole in the plastic manifold.

Oscilloscope Testing

This can appear to be somewhat of a black art.
However with a little practice, it becomes second nature. If you can use a multi-meter you can use a 'scope'.
An oscilloscope displays a picture of Voltage over time.
The waveform or pattern produced can tell far more about the circuit being tested than a changing display on a multi-meter.

keeping in Trim

Decoding fuel trims can be a key part of engine diagnosis.
But what is a fuel trim?
The fuel trim is the ECU's adjustment of the fuelling based upon information received from the oxygen sensors.
A positive trim is the ECU adding fuel to the normal amount injected for the conditions reported by various sensors.
A negative trim is the ECU removing fuel.
They can be a used to diagnose various running problems, from air leaks to fuel pumps.

One such problem had baffled a number of garages, a Fiesta with a running fault.
On the road performance was much reduced, and it posted an air mass meter fault code.
The component had been replaced (twice), and wiring checks proved that no wiring faults existed. Live data showed only 2 volts from the AMM during acceleration tests.
However fuel trims showed -18% at tickover.

So despite low outputs from the AMM the ECU was removing fuel, this seemed to stump the previous garages. New O2 sensors were fitted before the ECU was sent away for testing.
The ECU returned with a clean bill of health.

So the question remains what fault would result in low AMM voltages and minus fuel trims?
Restricted engine breathing, so the usual suspects were checked. Air filter, intake trunking, and exhaust inc CAT condition.
A blocked CAT was found to be the cause.

Rolling road tuning

Having access to a chassis dyno is vital, if you want to know if you are improving the performance of your engine.
A 205 gti road rally car recently came to us for some checks following a head gasget replacement. A skim of the head was required and the owner wanted to make sure this had not upset the previous settings.
A base run to check fuel ratios, detonation, and power output showed a healthy spred of power but with some detonation. Slightly rich mixtures across the rev range and no thermostat resulting in low operating temps.
Adjusting the fuel pressure reg showed increased torque readings with slightly less fuel pressure. However these 'gains' were reduced by retarding the ignition timing to prevent detonation.
Repeating power runs with a set operating temp closer to 86 degrees C than the previous 58 degrees C showed a slight loss due to heat soak.
Advancing the cam timing showed a little improvement but not enough to keep testing.
Rolling road time is expensive and knowing when to stop is important, otherwise repeated tests cost more than they bring in improvements. But if you want to wring the last few bhp or lb/ft out of your engine you have to test all the possible settings to see if they can make the difference.
The importance of repeatable results is paramount, otherwise you are wasting your time.
We use a dyno that gives often depressingly accurate results time after time, with the ability to overlay previous runs to where or if you have made that improvement.