Fortunately I have two of these in the workshop at the same time, can anyone explain the tailpipe pulse trace on the one that is failing an MOT emissions test, it has no fault codes.
The top trace is from a good known vehicle, the bottom trace is from the vehicle is question.
The anomaly appears on every pulse so I am assuming the issue is not an individual cylinder and I have compared the cam/crank traces on both vehicles and the timing is a match.
I don't know if you have one or not, but if you use a piston chart with your trace above you will be able to identify each cylinder and compare data visually. Use the engine cylinders firing order with your trace and you should be in a position to compare cylinder data from your exhaust pulses.
As many have mentioned there could be a number of variables at play here, with temperature being the obvious one. As the engine will be at operating temperature during the MOT, is the known good vehicle at the same temperature when you took the capture?
EGR will of course play a part in this. If possible can we remove the EGR as suggested, or try to active test the EGR closed whilst running. This isn't always possible of course. Typically EGR will come in around 60C so if the how do the waveforms compare when both are engines are cold?
Does the exhaust systems have a split tail pipe? I know that I had some odd waveforms from my vehicle which had dual tailpipes and there was some odd patterns depending on the operation of the vehicle. We had assumed some form of turbulence within the back box but it's difficult to prove.
You do have very different waveforms so if we can be confident the tests are done under the same conditions then this could be a real event.
What are the results of the emission test? Did it fail badly or only just?
Lots of questions I know but hopefully between us we can help understand this waveform better.
Tail pressure pulse measured at the same time with CH2 absolute pressure sensor and CH1 delta pressure sensor.So, different pressure transducers result in different signal shapes.