I'm sure some of you have seen my rants about rubbish diagnostics.
This truck exhaust was a prime example.
Intermittent SCR efficiency faults. Had been in another workshop twice where they'd played some rather expensive parts darts.
First step of diagnostics, perform a visual check.
Heat shield on the underside bent up, which raised the first flag. Unbolt it, and find this dent.
The photos are taken with the complete exhaust aftertreatment unit removed and sat upside down, but exhaust gases come in top right of the photo, through the oxidisation cat, then through the DPF, before passing left to right along the dented pipe, before passing back right to left through the SCR catalyst.
It's removed in the photo, but the AdBlu injector bolts on the left hand side, so it injects down the centre of the pipe with the dent, giving the gases/urea a chance to be well mixed before they hit the catalyst.
Quick look with a bore scope showed a white streak up the dented section, where the AdBlu had been hitting the tube. It would probably be OK when really hot and good gas flow, but lower temperatures with slower gas flow were probably causing poor/delayed mixing resulting in poor catalysing.
So a couple hours later, with entire exhaust unit removed and turned upside down to get access with a slide hammer, and the pipe no longer has a distinct dent.
Whole lot back on, and road test showed all NOx values doing what they should, so it'll be back out tomorrow seeing what the drivers can destroy next.
This truck exhaust was a prime example.
Intermittent SCR efficiency faults. Had been in another workshop twice where they'd played some rather expensive parts darts.
First step of diagnostics, perform a visual check.
Heat shield on the underside bent up, which raised the first flag. Unbolt it, and find this dent.
The photos are taken with the complete exhaust aftertreatment unit removed and sat upside down, but exhaust gases come in top right of the photo, through the oxidisation cat, then through the DPF, before passing left to right along the dented pipe, before passing back right to left through the SCR catalyst.
It's removed in the photo, but the AdBlu injector bolts on the left hand side, so it injects down the centre of the pipe with the dent, giving the gases/urea a chance to be well mixed before they hit the catalyst.
Quick look with a bore scope showed a white streak up the dented section, where the AdBlu had been hitting the tube. It would probably be OK when really hot and good gas flow, but lower temperatures with slower gas flow were probably causing poor/delayed mixing resulting in poor catalysing.
So a couple hours later, with entire exhaust unit removed and turned upside down to get access with a slide hammer, and the pipe no longer has a distinct dent.
Whole lot back on, and road test showed all NOx values doing what they should, so it'll be back out tomorrow seeing what the drivers can destroy next.
