EGR Valve problems

[tempforce]

i have the auto, and i leave it in 4th in town. it down shifts as needed but doesn't shift into 5th or 6th. the truck doesn't mind running at 18-1900 rpm.

 

[rscurtis]

That doesn't make sense because the DEF or SCR system is downstream of both the EGR and DPF.



I don't get it.

Actually, you do get it, you just haven't realized it yet. The EGR system adds soot to the system, requiring more frequent regenerations of the DPF. Lowering the EGR rate reduces the soot load on the DPF, reducing the frequency of regens. The SCR is on the clean side of the exhaust system, so it doesn't load up the DPF like EGR does. In reality, it wouldn't load the DPF even if it were upstream of it. I don't know if the SCR catalyst is behind the DPF for convenience or if it has to operate in the soot-free part of the system. Perhaps some other member will know the answer.

 

[HBarlow]

You're saying that with the SCR device in the exhaust the EGR is still present but is set up to do recycle less combustion exhaust so less soot is injected into the system? Or is the EGR eliminated?

 

[rscurtis]

You've got the picture. It would be nice if the EGR was eliminated completely, it is the source of a lot of trouble. The EGR and DEF are both used to reduce NOX emissions, but EGR does it at the expense of soot emissions, which clog the DPF more frequently. An engine like my '03 with a DPF and SCR only would still be 95% clean IMO without all the oil dilution and FE reduction that we have today trying to reach that last 4%, which doesn't change your life in the least as far as air quality is concerned.

 

[HBarlow]

Thanks for the explanation. I think I understand the basics now.