Corsa misfire

I was called out to this 2001 Vauxhall Corsa, Z12XE. The engine was suffering from a misfire and repeating DTC P0300 and P0302.

The garage had replaced the spark plugs and the ignition coil for No.2 and verified cylinder compressions. They were concerned that maybe the fuel injector was the problem, but hadn’t been able to swap it out or check it properly.

I carried out a pretty quick fuel pressure and injector delivery test, using my Picoscope and wps500. That quickly ruled out the fuel injector, as the flow waveform showed symmetry between all 4 injectors. (refer waveform).

During the tests, it was apparent the engine was noisy from the top end, so while I was in the area, I made a quick scope capture of the cam and crank synchronisation and checked it for any deviation. It checked out fine.

I then looked at finding the answer by inspecting the intake pressure and the exhaust pressure, using the wps500. This resulted in a clear pattern anomaly in the intake waveform. There was also a  disruption in the exhaust signal, but honestly I found it to be a little confusing whilst analysing “on the fly”.  The misfire was not significant and would vary in intensity during the running test, but the intake pressure anomaly seemed constant. (refer waveform). My first thought was a fault with the intake camshaft lobes or valve-train, but couldn’t make a definite diagnosis. So I went on to check all 4 cylinder pressures, again utilising the wps500. This resulted in a clear valve-train issue with cylinder No.2 (refer waveforms).

The pressure waveform showed a positive pressure ramp at the end of the exhaust stroke. This meant the camshaft cover had to come off, so that I could inspect the cams. This revealed the fault, as the exhaust cam lobes for No.2 were worn and the tappets dislodged. (refer image).

I was a little confused at first, because I had been thinking intake, however a closer look at the waveforms and a comparison with the other cylinders, you could see the fault with the exhaust valve operation. The exhaust valves on No.2 were opening early and closing early. The closing early resulted in the positive pressure escaping into the intake, when the intake valve opened. This was what could be seen in the intake pressure waveform.

A pleasing result.

The garage actually managed to reset the rocker arms to improve the operation temporarily, as the owner did not want to spend anymore money.