Figure 3

The whistling and wheezing coming from the back of the Fiat Punto being delivered to the workshop was a sure sign that the exhaust was blocked. The owner had complained of a gradual lack of power on the motorway and this symptom fits the initial diagnosis by the recovery agent.

Once the car was up on the ramp the back box was removed this was due to the exhaust gas trying to escape from the joint just prior to it. The engine was then started to evaluate the reminder of the exhaust.

Vacuum gauges have formed a part of the diagnostic tool kit for many years, they were used to tune engines and diagnose all manner of faults. These simple gauges are often overlooked when modern cars are presented with running faults. But do they still have a place in the modern tool kit?

The engine management system seems to think so, more and more management systems are using Manifold Absolute Pressure (MAP) Sensors. So if the vehicle has one fitted it can be used in place of the traditional vacuum gauge. The diagnostic process for checking blocked exhausts is to hold the engine at fast idle and observe the vacuum reading. If the vacuum starts to decay then you have a problem.

The live data from the MAP sensor can be used in the same way.  At idle you should observe around 300-400mbar and when you snap open the throttle it should read 1000mbar (normally aspirated engines). This can be confusing as a vacuum gauge reads 600-700 mbar at idle and 0 when you snap open the throttle. This is due to the MAP sensor reading from atmospheric pressure and the data is simply 1000mbar minus 600-700mbar which is 400-300 as you can see in the diagram

The Fiat had a higher reading 641mBar (poor vacuum) at idle and this crept away to atmospheric values during the fast idle test. This proves the blockage wasn't confined to the back box. Further investigation showed the catalytic convertor was breaking up and causing the poor performance. The catalytic convertor is part of the exhaust manifold, and as such is unlikely to have been damaged to cause it to break up, so the cause must be excessive heat. Catalysts start to melt at around 1000?C and the most common reason for this is an ignition misfire. This fools the engine management into adding more fuel via the fuel trim, making the problem worse.The cause of the melting catalyst must be established and rectified otherwise the same symptoms would soon return, and result in another expensive catalyst failure.