Partner low level misfire

Peugeot Partner M59 KFW 1.4L low level misfire condition from cold.

This vehicle was suffering an annoying low level misfire condition, which didn’t caused any DTCs to be recorded, and was generally gone after the engine had been running for about 5 to 10 minutes, from cold start. The temperature appeared to make a significant difference to how long the fault was active, if active at all. The condition was very minor and most people would have ignored it.

The first test I carried out, after a diagnostic interrogation, was an ignition system and fuel system investigation.

This image is of the ignition system, whilst the misfire condition was present.

I then added a channel to sample the inlet manifold pressure.

As you can see in the images above, the ignition system was operating perfectly well. The manifold pressure illustrated an issue with balance and would require further analysis. But I thought a quick look at the injector balance would be useful, if for no reason but to eliminate them.

You can clearly see the injector balance is consistent, indicating good operation.

So with the inlet manifold pressure waveform anomaly present, I was driven towards a mechanical issue. But by this time the fault had just about gone away and so I left it for another day.

I disconnected the crankshaft sensor, in order to perform a relative compression test.

You can see there is a definite anomaly with the battery current, indicating a compression issue, which is backed up by the manifold pressure.

So a compression test was called for:

Now you can see, clearly, that cylinder 3 is low on compression and the other three cylinders appear pretty even. I did carry out this test a couple of times to make sure. The relative compression test didn’t really illustrate this completely, as it appeared that one cylinder was low and two cylinders were not as good as the best cylinder, but I think that was likely due to the nature of problem. That being the compression’s were not always the same.

I then checked to see if there was any pressure leakage into the crankcase, I won’t bother showing that, as there was none. That ruled out piston ring bypass. I also confirmed there was no leakage into the cooling system. That just left the exhaust pressure. Take a look.

The exhaust pressure illustrates the normal signal pattern for a low contribution cylinder and not compression leakage to the exhaust, so I was confident there was no fault with the exhaust valve sealing.

With that in mind, the next check for me, was valve clearance, along with a visual inspection of the cam lobes and rockers.

All valve clearances were set correctly and there were no obvious issues with the valve gear.

I did then setup a running cylinder pressure analysis, but found no obvious pattern anomalies and so inconclusive.

This left me with the intrusive method of inspection; The cylinder head had to come off. My thought process being that we would probably not see any obvious faults, but I was thinking ‘sticky’ valves.

The cylinder head came off. The piston heights were verified and all visual inspections, revealed nothing.

So the head was stripped and valves were all removed. All the valves were measured for straightness and fully cleaned, along with the head assembly. The valves were all lapped in and the head rebuilt. A new head gasket etc.

As you can see, in the relative compression, the cylinder balance is much closer than before. It is still not perfect but the engine is much smoother from cold start and no longer has that annoying low level misfire condition.

In conclusion there were two tests which proved the most useful. Relative compression and inlet manifold pressure. Both tests were really easy to do and full of useful diagnostic information.

This job would never have been a financially viable job, but the vehicle belongs to me and I’m fussy!