Misfire in idle

Chez · Post Wed Jul 10, 2019 8:27 am

So I have a 05 Citroen C3 desire that has a misfire but only when in idle, the car doesn’t lose power or overheat. I have been driving it for almost a year like this and it’s still going fine and I have just got another MOT for a year although I would still like to try and sort the misfire out. The things that have been tried so far are new coil pack, spark plugs, checked injector resistances, exhaust fume test to check head gasket which was fine. Someone else then looked at it and said cylinder 4 and injector 4 were running colder than the others so had the injectors changed, still no joy! The other thing is that the engine management light is on and the reading on the miles/fuel is erratic and sometimes says nothing at all. Could the problem be as simple as needing a new fuel filter? Obviously this car is a certain age so I don’t want to really be throwing any more money at it but at the same time I can’t afford to buy another car. Any suggestions on what the problem could be would be welcome?

C3CAR · Post Fri Jul 12, 2019 9:53 pm

Chez wrote: ↑Wed Jul 10, 2019 8:27 am The other thing is that the engine management light is on

Hi Chez,

If you have MIL on , then the first thing, above ordering any parts in is to do an OBD scan. This will tell you why MIL is ON and you can take the fault finding from there. Use this tool to read engine fault data on the C3 - if you don't have lexia.

Chez · Post Sat Jul 13, 2019 6:14 am

Hi yes they did that, the code was commission fowler apparently

Chez · Post Sat Jul 13, 2019 6:15 am

I forgot to mention that, it was the first thing they did which let to all the other things they tried that didn’t work.

C3driver52 · Post Sat Jul 13, 2019 9:35 am

Chez wrote: ↑Sat Jul 13, 2019 6:14 am Hi yes they did that, the code was commission fowler apparently

Hi Chez

Unfortunately that is not an error code. They usually start with a P and then have four numbers.

After the numbers there is often a description for manufacturer specific codes.

If the original code was misread or interpreted incorrectly you don't gain anything useful from the read.

Read again and start from a fresh page.

Chez · Post Sat Jul 13, 2019 10:06 am

So I had a Citroen diagnostic person with the relevant equipment diagnose the code and still not fix it, so I’m not going to spend more money getting a code on a car that cost £550 when it runs fine apart from this misfire. If there are any old school mechanics out there that know what causes a misfire in idle no loss of power but the mpg to say 600 miles 1 minute then 27 the next and engine management light to be on. I’d love to hear from u and that’s not to say I don’t appreciate the reply I have received so far.

C3driver52 · Post Mon Jul 15, 2019 10:32 pm

Chez wrote: ↑Sat Jul 13, 2019 10:06 am So I had a Citroen diagnostic person with the relevant equipment diagnose the code and still not fix it, so I’m not going to spend more money getting a code on a car that cost £550 when it runs fine apart from this misfire. If there are any old school mechanics out there that know what causes a misfire in idle no loss of power but the mpg to say 600 miles 1 minute then 27 the next and engine management light to be on. I’d love to hear from u and that’s not to say I don’t appreciate the reply I have received so far.

Air leak on the inlet is bypassing the engine management sensors for air flow/volume.

The leak has more effect on the fuel trim at idle than it does on WOT.

But for around £25 you can read engine management codes yourself. Citroen made the ECU put the MIL on for a reason and that reason is 'read the codes'.

Chez · Post Tue Jul 16, 2019 6:27 am

Ok thank you for your reply, can u tell me what is the proper name of the inlet part?

Chez · Post Tue Jul 16, 2019 6:40 am

Also is this a big job to replace?

Arfur Dent · Post Tue Jul 16, 2019 11:39 am

C3driver52 wrote: ↑Mon Jul 15, 2019 10:32 pm Air leak on the inlet is bypassing the engine management sensors for air flow/volume.

Chez wrote: ↑Tue Jul 16, 2019 6:27 am Ok thank you for your reply, can u tell me what is the proper name of the inlet part?

It is the whole metered inlet system, any of that can be a culprit.

This is probably why it has been so tricky to track down.

It is not a single part.

The throttle body leaking is a possibility, but I am not saying that is the cause, just this is the area to look at, along with all of the metered air inlet part of the engine.

Chez wrote: ↑Tue Jul 16, 2019 6:40 am Also is this a big job to replace?

What part?

There is quite a lot to check, and obviously changing parts that are not faulty can get very expensive and make no difference.

This excellent reply to a different topic by Ozvtr has a good explanation of a very similar problem you may be facing.

Ozvtr wrote:I know this is an old thread but it might help someone in the future.
I have issues with some of the posts on this thread and I hope I can clear things up.
The TU3JP engine has a MAP (Manifold Absolute Pressure) sensor and not a MAF (Mass Air Flow) Sensor to determine the amount of air going into the engine.
I wont bore you with the technical details but engines that have a MAP sensor and a vacuum leak will tend to run rich. The leak makes the manifold pressure higher than it should be. The engine ECU thinks more air is going into the engine (the leak changes the pressure not the volume of air) so the ECU increases the amount of fuel. Consequently the engine runs rich. The engine ECU thinks the car is accelerating and over rides the o2 sensors and does not "trim" the fuel. Typically, vacuum leaks only appear to effect the idle but overall fuel economy and performance suffer.
Unfortunately there are no screen caps of the manifold pressure of this car with the engine running.
If you can not find any leaks in the manifold or throttle body then check the purge valve. Without power, it should be closed (blocked). It should only open when the car is at speed and not at idle. Remove the valve and plug the line going into the inlet manifold. See if the engine runs better. If that doesn't fix it, remove the oil vapor hose and block the small tube which is part of the inlet manifold. The small diameter plastic tube of the oil vapor line can crack and break.

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