Cold idle problems plus P0401 EGR insufficient flow

[Zimboe]

Hi all,
It's been a while since I've been on the forum but I'm hoping some of you guys can help as I'm puling my hair out with this fault! Longish post but trying to give as much info as possible.

Car is a 2012 W204 C220CDI (149k miles) and was running fine until one morning I started it and it was idling very roughly, sometimes stalling. If I tap the throttle as it starts to stutter then I can keep it running but if I leave it, it stalls. As I was trying to drive it and keep the revs up, the engine light came on and went into limp mode. The only error code is P0401 which is EGR insufficient flow. I'm thinking this error may be more of a symptom than the root cause asit came up after the rough runningbut I could be wrong.

Once the engine warms up after about 5 mins it runs normally apart from being in limp mode. I have been able to reset the error when warm and after a few off/on cycles and get it out of limp mode but it comes back after rough running from cold again and goes back in to limp mode.

Things tried so far after Internet research!
- Tried disconnecting as many sensors/valves that I can reach in case of a faulty one. These bring up a new error code which go away when plugged back in, so I don't think I found one not doing it's job.
- I've done an injector leak off test - not a lot of flow but got a similar amount coming out of each one although two had slightly more than the other two. I've only done this cold, not with a warm engine.
- Changed fuel filter - no change.
- Fitted a (used from breaker) injector rail incase pressure valve or pressure gauge was faulty but this made no change at all. (I really struggled to start the engine after doing this, possibly air lock - flattened the battery a couple of times but eventually managed to bleed the system by sucking diesel up into the rail using a suction pump onto o e of the injector pipes!)
- I then convinced myself that the problem was the Quantity Control Valve/Flow regulation valve Y94 as the car starts fine and then starts playing up after around 30 seconds. Anyway, I spent most of yesterday stripping everything to access this valve and replace it and then reassemble. There was a lot of carbon build up in the EGR pipe to the inlet manifoldand throttle control valve which I cleaned up. Note that I didn't go as far as removing the EGR valve as it looked like even more of a pig of a job and I was running out of time.

It did not fix the problem! It is doing exactly the same thing. Roughidle and P0401 error after a couple of minutes.

I'm now at the end of my tether and may have take it to the professionals. I just don't want to throw money at the car as it's not worth a lot!

Any help/advice gratefully received!

One last question - does anyone e know which sensor triggers the EGR insufficient flow error? I assumed it was the small one on the inlet manifold near where the metal EGR inlet pipe enters? Seems to connect to a venturi hole which I checked was clear after cleaning the manifold.

 

[pcthrillrider2]

i had the same code put wynns extreme diesel cleaner in tank within 11 miles light went off

 

[Zimboe]

i had the same code put wynns extreme diesel cleaner in tank within 11 miles light went off

I’ve had diesel fuel system cleaner in for about 50 miles during all my activities so far with no improvement.
Did you have rough running too or just the error code?

 

[Supernoodle]

There isn't a single sensor or actuator that directly sets the insufficient flow fault. The software is but more complex than that.
For every emission critical system or component there will be a plausibility check that uses some other inputs to cross reference against.

EGR flow is not directly measured. At each operating point the volumetric efficiency is known, this allows you to calculate the expected air flow into the engine. The actual air flow into the engine measured by the MAF. EGR displaces the fresh air into the engine, so if MAF is less than calculated flow , then the delta is assumed to be EGR. It knows the pressure difference across the EGR valve and the position, so can determine what the flow through the EGR should be. If that flow is higher than that determined from the MAF, it will assume insufficient EGR flow.

So if think about how a fault is determined and what it's telling you, you can work out where to focus your attention. The fuel injection equipment is not the right place to be chasing after for an EGR flow fault.

 

[Zimboe]

There isn't a single sensor or actuator that directly sets the insufficient flow fault. The software is but more complex than that.
For every emission critical system or component there will be a plausibility check that uses some other inputs to cross reference against.

EGR flow is not directly measured. At each operating point the volumetric efficiency is known, this allows you to calculate the expected air flow into the engine. The actual air flow into the engine measured by the MAF. EGR displaces the fresh air into the engine, so if MAF is less than calculated flow , then the delta is assumed to be EGR. It knows the pressure difference across the EGR valve and the position, so can determine what the flow through the EGR should be. If that flow is higher than that determined from the MAF, it will assume insufficient EGR flow.

So if think about how a fault is determined and what it's telling you, you can work out where to focus your attention. The fuel injection equipment is not the right place to be chasing after for an EGR flow fault.

Thanks for taking the time to reply @Supernoodle.
The reason I was heading down the fuel route was because of the symptoms that first showed up I.e. the rough running until warm and Google diagnosis! The P0401 came a bit later and I assumed this was due to the rough running upsetting pressures throughout the system.

I took it for another ride this evening and after multiple error resets and now have a new error “P227962 - Leak detected in the intake air system. The signal comparison is faulty”.

P0401 has now gone away. Unless P0401 shows up when cold and P227962 comes in when warm.

That is unless I’ve introduced a leak when stripping it all down to change Y94 valve!
Although I’m not sure if this would result in rough idle/stalling?

 

[Supernoodle]

I'd be thinking about how to determine if the rough running was linked to faults you're getting. To suddenly have two unrelated faults would be unlucky.
Lack of EGR won't cause rough running, but too much will, especially when cold . So could it be it thinks there is insufficient because the MAF isn't reducing when opening EGR, but in fact EGR is flowing. Disconnect the EGR, yes you'll get another error, but is it still rough when cold?

You can check the rail pressures, pressure and quantity control valve demands in the live data. If you graph them you can see erratic behaviour easier.

 

[Zimboe]

Thanks. I don't have access to Star but I now have an iCarsoft MB which allows me to display pressures etc. Fuel system pressures look ok although I've not graphed them during the rough running to see what happens. I can read some of the other pressures but I don't know what ratios I should be seeing between MAF, MAP, boost etc
I will try disconnecting the EGR to see if it makes a difference to the rough running. That's if I can get to the connector as it's very tight in there!

 

[Supernoodle]

If you have icarsoft you should be to control EGR which will override the ECM demand. If you do it before start with a demand of off or 0% that would deactivate EGR temporarily.