Engine problem

Regen is a cyclic process, so whether it needs one depends entirely on how much soot has built up since the last regen.

Typically there are two types of regen: passive and active. The former happens when there is significant soot load (but not enough to yet need an active regen) and long high-speed (or high revs) driving causes the DPF to get hot enough to start burning without any need for an external input.

Active regen is initiated by the ECU when it calculates that the soot level is above a preset threshold, by comparing the pressure at each end of the DPF. I think on the Boxer there is a separate additional injector, and this is activated when the DPF temperature is high enough? The VW T5 works in a slightly different way, in that it injects extra fuel into the cylinders during the exhaust stroke (no extra injector needed), but the overall effect is the same - the unburnt fuel hits a catalyst at the front end of the DPF and heats the exhaust gases so much that the soot gets hot enough to burn, and most of it burns away, leaving a small amount of ash residue.

There's not really any point in forcing a regen unless it actually needs it. On my T5 it's possible to read the ECU data and find out how many miles since last regen, and what the calculated soot load is - so you can roughly predict when the next one will be due after a while. Yours probably does the same, but it may not be well documented. If you have been doing long-distance high-speed runs recently, it's very likely to have already done a passive regen if the DPF was only partially full.

One further observation: on my T5 I have noticed a "flutter" effect when on the overrun while the DPF is undergoing regen - but there are few other signs that it is regenerating, other than slightly higher idle revs and a lot more heat underneath.

Having said all that... your symptoms seem to point to an EGR valve that is sticking partly open, and is only shutting fully with more throttle than it should need. It would be interesting to find out whether the replacement EGR is a factory part, or an aftermarket one. In the VW T5 world, some aftermarket EGR valves seem to cause more trouble than the factory ones...

One further point: on the T5, I'm fairly sure that you need to run an "adaptation" procedure when the EGR valve is replaced. This allows the ECU to "learn" the fully closed and fully open positions. Do you know if the Boxer requires a similar procedure and, if so, whether it was done?

DaveyA wrote:Would it not show as a fault if it was partially open?

I don't know enough about how it's operated on your specific engine to say. By analogy with the T5, the system changed drastically over the years, starting with a vacuum-operated mechanism which had no way to feed back anything about actual valve position, to a motorised system with feedback about actual valve position. The former probably wouldn't register a fault under any likely conditions, and the latter probably wouldn't if it needed adaptation and that had not been done.

The big problem is that Peugeot sell these parts at an extraordinarily high mark-up, so there is a lot of money to be made by the Chinese aftermarket parts makers - and it seems that some don't care too much whether their copy works very well. I think they calculate that if they charge £60 (versus the ~£200-400 of an OEM part) there's enough money in it for them as long as more than half the buyers can't be @rsed to return a product that doesn't work very well...

One thing that may help with the diagnosis is to temporarily fit a blanking plate. It (obviously) isn't legal for full-time use, but it's a certain way to tell if the EGR valve is open when it shouldn't be. You can either DIY or they seem to be £5-6 on Ebay.

If yours is electronically positioned it probably won't be viable as a full-time solution (leaving aside the legality) because it will *probably* bring up a fault light on the dash which will be an MOT fail. But purely for diagnosis, it may give you some evidence to decide if the new one is duff or not.

You may not be able to see anything wrong. I had something similar on a Transit a few years ago, and visually I could see nothing wrong with it - but it was definitely faulty. I suspect that even a tiny gap can make a significant difference on a diesel.

I would definitely investigate the ambient temp sensor. I can recall this coming up previously, and how it can have a fundamental effect on things like regen. It seems (to me) bizarre that they would use that as an input to the ECU, but it seems that on some models they do. As said earlier, I have no experience of Boxer engines directly, so hopefully others (or possibly other forums) will be able to help.