C2 1.1 idle missfire / Lexia readings

[ibiscios7]

Hi there

I know this subject has been looked at many times there but I believe I'm experiencing quite complicated situation (at least for my understanding), so I would very much appreciate any suggestions/ideas.

It all started with after cambelt snapped on C2 1.1 only weeks after purchasing it with story that cambelt has been replaced 5000 miles ago, obviously it wasn't but it's just one of those things... before that engine was working like swiss watch

After replacing cambelt kit and all 8 valves this started:
Engine runs absolutely fine when cold, even on idle.
When warm (up to normal working temperature) it works fine but when driving. BUT as soon it stays on idle for minute or two it starts to behave not perfect - sounds like struggling a bit and rattling at bit. When stays on idle for about 5 minutes it starts to struggle much more and the engine error P300 - multiple misfire comes up.

First idea - cambelt was not fitted properly, checked, spot on.
Next step - new spark plugs and coil pack (Bosch) - no changes (didn't have to buy the coil pack as was able to return).
Next step - fuel pump (was able to source one with option to return) - no changes. This car doesn't have a fuel filter so that's off the list as well.

Now I have borrowed Lexia 3 diagnostic tool and trying to get some useful info with that - but no luck so far.
The misfire error is P300 - which doesn't specify the exact reason and fore some reason Lexia doesn't show any engine sensor readings on time of misfire together with error code.
Have ran the Lexia diagnostics but nothing comes up with error.

So I'm thinking to look deeper in the sensor readings - which reading would be relevant to this issue? And how to see if the reading is "abnormal"?

Thanks in advance guys!

[RichardW]

Moved to Citroen forum....

Does it have adjustable tappets on the valves, and are they correctly set?

[ibiscios7]

It does and they have been been checked.

[ibiscios7]

so I been playing a bit with Lexia and have gathered some data - doesn't mean much to me but I hope somebody can enlighten me a bit, thank you very much in advance

when engine running warm on idle and Not misfiring, Lexia was reading following

MM48P / NFX

coolant temp - 85
engine speed between 700 and 800
injection time 5.46 ms
inlet manifold pressure around 450 mb
idle stepper motor around 34 pas
upstream oxygen from 30 till 720, changing constantly
downstream oxygen stable at around 900

Then after few minutes misfire started

readings on Lexia:

coolant temp - 85
engine speed between 740 - 770
injection time 7 ms
inlet manifold pressure around 640 mb
idle stepper motor around 71 pas
upstream oxygen 0
downstream oxygen 0

Misfire continued as I was reading the data and always does that, if starts to misfire, it goes on until turn the engine off and then on again.

Turned engine off, erased error, started - runs fine again.

Error was P300 again - nothing specific, misfire on undetermined cylinders. The engine readings on error - engine speed 740 ; temp 85 ; inlet manifold pressure - 430 ; richness state - closed loop 1

I really hope somebody has got any idea what's going on.

I really don't want to spend another 85 quid at Citroen centre just for their diagnostics....

[ibiscios7]

Brought the car to local Citroen dealership - after 2 days they came up with: we don't know what causes the issue, can number of reasons, diagnostics doesn't show much, let's start with replacing the spark plugs and coil pack with original Citroen parts and let's see what happens... what a joke.


Any suggestions which direction should I go now - sensors? injectors?

[Zelandeth]

The upstream and downstream oxygen both dropping to zero sounds fishy to me...I'd wait for someone who knows these things better than me, but that should be a non-zero value I would think (I'm assuming this to be the pre and post cat O2 sensor readings).

[Mandrake]

Agreed - both the upstream and downstream sensors staying at zero when the problem begins is a major red flag. Unless the sensors themselves or the wiring is going faulty this means the engine is running really lean, or it can be caused by a dead misfire on one or more cylinders - in that case the unburnt oxygen from that cylinder goes into the exhaust and makes the sensor think the engine is running lean even if there is no shortage of fuel and even if the remaining cylinders are actually running rich.

The ECU will then try to richen the mixture which you can clearly see is happening as the injection time has gone up from 5.46ms to 7ms, which is a massive change. If the cause of the "lean" reading is one misfiring cylinder, this can cause all the other cylinders to run rich.

Also extremely suspicious is that the idle stepper motor has gone from 34 to 71% - this is also a massive and unusual change. Because it has opened the throttle so much the manifold pressure has gone from 450mb (about normal for idle on a petrol engine) to 650mb - which is completely abnormal for idle speed. Despite these changes the rpm hasn't increased.

This tells me the engine is really struggling to keep idling.

Normally this would either be a spark or a fuel problem but given the history with the camshaft it could as Richard suggests be a problem with valve clearances, and I would not rule that possibility out yet even though they have been "checked". It could also be that the camshaft timing is out a tooth!! Double check the timing of the belt if possible and make sure the tensioner is set properly.

Unfortunately a Lexia whilst providing these clues is not going to diagnose this problem for you or your Citroen dealer. If their suggestion is lets start replacing parts and see what happens, go somewhere else to someone that actually knows how to diagnose a fault other than reading fault codes from a scanner...(hard to find, for sure)

This engine really needs to be put on an engine scope by someone who knows what they are doing to look at the spark waveforms. This will help identify if only one cylinder is misfiring or multiple cylinders are, and whether its a lean misfire or an ignition related misfire. I'm not familiar with that engine so I don't know whether it's coil over plug, one big coil pack etc, depending on what system it uses it may be easy or difficult to diagnose this way.

Have you tried any basic tests ? Like a bit of propane sprayed carefully into a slightly open air cleaner box while it is misfiring ? If its a fuel related lean misfire it should stop misfiring when you do this if you spray in the right amount - if its an ignition related misfire it won't make any difference. This test should also (if its a fuel related misfire) cause the oxygen sensor reading on the lexia go from 0 (lean) to 900 or so. (rich) This can be a good starting point for basic tests.

What about a cylinder drop test ? Again how safe or easy it is to do this depends highly on the ignition design. If its coil over plug with a 12 volt connector on the top of the coil its relatively easy to disconnect the plug to the coil to induce a misfire - if only one cylinder is misfiring the engine should run worse when any of the other coils is disabled but won't change when you disable the faulty coil...

Remember any cylinder drop test like this will create a fault code as part of the test, which you will need to clear afterwards.

Edit: one more test you can try, when the car is misbehaving, set your Lexia to monitor the upstream oxygen sensor in realtime, floor the throttle for about 1 second (don't rev the engine too high) and see if the oxygen sensor reading changes from 0 to at least 800 - does it ?