Rough running 2.0 HDi

[Rob_E]

Recently my C4 (2005, 2.0 HDi) has developed a problem, it seems most pronounced on low throttle; if I accelerate harder it seems to smooth out, although doesn't have full power but I think that might be a separate issue as I did find a split in an intake hose so reduced power may just have been due to boost leak. It also doesn't happen at all on the overrun.
When it does happen it feels like someone has suddenly and sharply closed the throttle such that there is a dip in the power, then immediately got back on it again. it seems to happen at a pretty regular frequency if trying to drive at a steady pace with light throttle.

A few months back I had the turbo vacuum solenoid fail, so at first I thought maybe the replacement had quite quickly gone the same way. But having taken it for a drive with lexia logging some parameters it seems the requested turbo position and copy signal followed each other very closely throughout, so I'm thinking that this now seems unlikely and that is likely functioning correctly. The boost was down based on the measured vs requested. But when I looked around for leaks I found a split in the intake hose that bypasses the intercooler and goes direct to the air doser valve on the intake manifold.
I'm still waiting on the replacement hose, apparently still in France 5 days after I ordered it, from the local Citroen dealer. But as I'm due to go away in the car tomorrow I tried to make a temporary repair to the hose to seal the leak.
I think the hose is holding at the moment, but despite this it made no difference to the symptoms.

There are no error codes logged against the injection ECU (well not related to this, there is one about pre/post heat not taking power or something like that, which has been there for ages).

The airflow meter readings looked like they fluctuated quite a lot when idling or being revved a little, with the car stationary; so I disconnected the Airflow meter (MAF) and was actually surprised the car seemed to run fine without it, took it for a test drive and the symptoms were gone. Unfortunately that was with the engine warmed up, and having left it disconnected it just about started from cold the next day, but ran very roughly and kept stalling. From a bit of googling it sounds like that is to be expected as without a MAF sensor it switches to a default engine map that wouldn't really be suitable for a cold start?

I thought at this point I had found my problem, ordered a replacement second hand genuine MAF from ebay (exact same part number); this turned up today and I fitted it, but the problem is practically the same as before, the engine only smooths out if when warmed up I disconnect the MAF again. Though even then on a warm start it can be a bit rough and stall initially.

So, now I'm at a bit of a loss, am I just unlucky and the replacement MAF is also faulty, or could something else be the problem, if so any ideas, as I'm running out of them?

I did think the flap in the air doser that connected to the intake that bypasses the intercooler (air intake heating?) felt like it had a lot more movement than the other inlet from the intercooler; the later of which seemed to be sprung, where as the heated bypass one seemed to be forced closed but not firmly held open, so I imagine the air flow would push it closed; I thought maybe this was the problem and the air doser valve was faulty. But then I'm thinking this wouldn't make sense with the disconnection of the MAF suppressing the symptoms.

I think I read somewhere that if the ECU was in limp/default mode as a result of no MAF signal that it wouldn't operate the EGR, so is this true for this case and so is it likely that it's not actually the MAF that's faulty, but the EGR and the MAF disconnection just also causes the EGR to be out of the equation resulting in the symptoms going away?

Any help gratefully received, as I'm running out of ideas

Thanks,
Rob

[PaulC5]

You could also check the MAP sensor, it might be bunged up with oil.
When was the fuel filter last changed ?

[Rob_E]

It looked pretty clean inside, no signs of any oil, though I have tried cleaning the sensor with a bit if electrical contact cleaner, then drying with compressed air. But didn't make any difference. The replacement I fitted also looked pretty clean.
Fuel filter was only done less than a year ago in an effort to help it get passed the emissions test, so should be a long way off being due. But worth a shot, to see if it helps. Won't do any harm.
Especially as I have had a few water in diesel fuel warnings intermittently, though not for a month or so.
Although I'm not too hopeful as when I had issues with a slightly clogged filter before it was the other way, caused fault codes under heavy throttle and was OK under light load. Which I guess makes sense as less fuel needed.

[citroenguy]

Is it happening on "low load" acceleration?

If the hose doesent help i'd have a look at the EGR, it could be sticking slightly. But not enough for a code to set.
Had that on a -07 1.6HDI

[Rob_E]

I'm still waiting on the new hose to turn up from the dealers. But yes it does seem to be on low load only, so I think you may be on to something. I thought I did read somewhere that if it is running in limp mode because it has no signal from the MAF sensor that it would not use the EGR valve; so if that's true that would explain why disconnecting that removed the symptoms yet replacing it didn't make a difference.

Unfortunately on the 2.0 I don't think the EGR is very accessible, tucked behind the engine. But I will try and do some more investigation with Lexia and look more closely at what is happening with the EGR, and whether it does indeed not try and do anything with it with the MAF disconnected.
It has had a new EGR in the past, but it's done more than another 100k miles since then, so plenty of time for the "new" one to start sticking a little.

[citroenguy]

Yeah it is probably not so fun on a C4, only done them on C5 II's, where there is a lot more space.
Probably need to remove wipers and scuttlle to access it

[Rob_E]

If I do take it off to try and clean it, can the gaskets be reused, or do they need to be replaced each time?

[aerodynamica]

Hi, your trouble sounds similar to mine but on C5 2008. I also suspected the EGR valve being stuck slightly open. I decided to use a blanking plate (at least temporarily) to see if it changed but no difference. I also just fitted a new vacuum solenoid but no change. I cleaned the MAP and intake temp sensors and the MAF and cleaned the very grotty throttle housing and part of the intake manifold and have a new diesel filter, air and oil filter. Same symptom: starts fine, idles (although lately in the heat it was stalling - that seems to have stopped) and at speed it is fine and seems to have a fair bit of boost. There is a real lag to throttle response though at all temperatures. Annoying if someone flashes you out at a junction. Also seems to happen when the engine is already at speed - there's a jerk often pressing the throttle but it does seem to always respond. Just makes a fuss doing so.

There seems to be a quite loud pinking sort of diesel noise when moving the throttle that reminds me of the timing going out briefly. Does yours do anything like that?

I also have no error codes except a new one that seems to have appeared since blanking the EGR - about pressure different to what's expected or something. Categorised as a temporary disappearing error.

There was a thread once that mentioned a similar thing about this engine and the culprit was some solenoid on the the diesel pump. I thik it was concerning fuel pressure but I am not sure. I'm keen on looking into that next but i'm not wanting to keep buying new parts until I hopefully cures it..

Also, what actually is the air doser - is that just the EGR by another title?

[aerodynamica]

I think the component on the fuel pump I was thinking of is the fuel flow regulator. I just ran an actuator test via lexia and it buzzes. I assume that means it's OK?

[Rob_E]

I'm not sure if mine is exactly the same, I wouldn't say there is lag, it was more a rhythmic dip in power when at a constant fairly low throttle position, but overrun or harder acceleration it smoothed out. Also for some time there has been the odd dip just as you initially open the throttle, I guess this is sort of a little bit of a lag, as it then picks up normally after that little blip.

In terms of sound it sometimes sounds like the revs are changing more than they actually are, like the engine is hunting a bit to hold constant revs.

Have you tried running your with the MAF sensor disconnected? May need to be warmed up to be able to run without stalling or having no power. Though I'm a little confused on this front, as it seems quite inconsistent, the other week I unplugged it to find the symptoms went away, which initially let me to believe switching that would fix the problem. But then the following day it would hardly run without it. At this point I assumed it could just only run like that once warmed up. But then when I was about to go away for a longer trip, it started and ran find without it disconnected even from cold (although maybe at reduced power, but the engine felt smoother than it had for a long time). It was also fine like this for the return journey, although took a bit more to get it started. Then on the Monday I tried to go out and it didn't quite stall, but felt close to it with very little power, at which point I have plugged it back in again.
Subsequently, after reconnecting the MAF the symptoms haven't been as pronounced, although it is not as smooth as it was at points with it disconnected, so I think the issue is still there just not quite as significantly as before; Which does reinforce the idea of something sticking to varying degrees at different points.

I haven't had another chance to look at it again, so will try and plug lexia in again this weekend and check a few more values.

In terms of the air doser (I'm assuming that is the correct name for it) but it is the plastic bit just before the intake manifold with two intake pipes on, one more directly from the turbo that bypasses the intercooler, and the other from the output of the intercooler. There are flaps in each of these intakes controlled by vacuum via solenoid valves (either identical or very similar to the one that controls the turbo) that the ecu controls. I believe it is at least partly about helping speed up the warmup by taking the non intercooled air when cold. IIRC I read somewhere it is only the 2.0L that has this and the 1.6 doesn't. I could see the one on the bypass pipe did move when I did the actuator test in lexia (which referred to it as something like intake air heating control)

[aerodynamica]

Thanks for more detail, I think you're right - mine might be a different symptom. I have not yet disconnected the MAF sensor. Often the symptom goes away on its own, or is just not present. But more often than not it is. Still only fault code is P0401.