C280 Intermittent Missfire

[Gee82]

My 95 C280 Sport is still got issues regarding an intermittent missfire that crops up usually once the car heats up it's generally ok when the engine is cold although it is hard to start after short journeys but this is definetaly not a battery issue.

I have already been advised on this forum to replace the Mass air sensor but this has made no differance so it looks as though something else on the HT ignition system is playing up.

I have already changed the spark plugs without a difference what should I change next?

Has my W202 C280 got HT leads and if so should I replace these?

Also is there any way of testing the ignition coils for faults?

Thanks

 

[kth286]

Gee82

It is likely one of the 'cable plugs' as Mercedes call them.

There are 3, and they are the rubber tubes which fit between the coil and the spark plug on 3 of the six cylinders. Inside the tubes there is a wire coil and this looses it's fexibility and sometimes does not sit fully onto the plug top connection.

On your engine the 3 coils supply 2 cylinders each, and if a coil breaks down you will usually lose 2 cylinders.

The fact you feel it is just missing on one cylinder points to the above item.

You were also asking about HT leads, and they are indeed the 3 leads from each coil to the other 3 plugs.

You could swap a coil from one position to another and see if the misfire moves with the coil. If it does, that points to a faulty coil.

 

[ALAN@ECUCLINIC]

if you still have the problem after changing the coils , it may well be that the coil has blown the ecu oh so very common!! i must repair 3/4 a month

 

[kth286]

Gee82

If one of the output drivers in the ECU has indeed blown as Alan has suggested in his post, then that points to the engine wiring loom insulation disintegrating and shorting out the coil.

BUT as mentioned that would cause 2 cylinders to fail and the car would be most uncomfortable and difficult to drive, so is unlikely to be the problem in your present situation.

For peace of mind, you need to snip a couple of inches of the outer fabric covering the actual wires going to the coil input connections. Inside you will see if there are cracks and crumbling insulation on the wires, if there is, you need a new loom at a cost of some £550 plus fitting, although if you are hands-on you can do it yourself.

It's a known problem on the 104 engine and maybe yours has been replaced.

 

[Gee82]

Hi guys, I have just replaced the 'cable plugs' from the coils to the spark plugs and it seems to have made a difference.

I've not drove it much on them yet but the car now seems to start a lot better when its hot and feels smoother on the move although I still get a slight stumble on acceleration just after changing up a gear!!

Any suggestions on this? Ignition timing problem?

I see that the engine has a cam sensor on the front can this be moved at all to adjust the ignition timing?

I've also checked the wiring to the coils and it looks fine so I am starting to run out of ideas as to whats causing my problems!

 

[kebo57]

Quote "I've also checked the wiring to the coils and it looks fine so I am starting to run out of ideas as to what’s causing my problems"!

You can check if HT leads are tracking and all high tension by running engine in the darkest place you can find, like in garage at night with all lights out get your head under bonnet and you might see sparking - tracking along the HT leads, coil etc. But keep your fingers away from anything that’s rotating you could even lower bonnet to enhance the effect.

 

[Gee82]

I tried this last night and all looked OK.

Any other ideas?

 

[Myros]

dare I say the OVP relay

there I said it. Unless you have already changed yours, you will have one of the lovingly crafted, don't know when it will fail, but it will, Siemens original versions. Costs about £50, takes about 5 mins. Search for a surprisingly high nuber of hits on the subject.

 

[big x]

Sounds like a faulty crank sensor,these typically play up when hot.
If the car is using more fuel check the coolant temp sensor is not open circuit
leading to overfueling when hot.
Always start with the simple things first.

For the life of me I can't understand why the moderators here allow
persistent commercial sales posts like alan@autoclinics.

adam

 

[kth286]

Gee82

At this stage I would get the codes read by a merc specialist that understands your car.

It's probably not the crank sensor because they normally either work or not, there is no half way measure.

It may be the coolant temperature sensor as they often degrade and are cheap.

Has the Lambda sensor been changed do you know ?

They are really a service item as they degrade over time and cause the cat to expire early also because of bad fueling.

My 104 engine at 154000 idles and drives very well and on original cat, just passed MOT easily.

 

[big x]

Gee82

It's probably not the crank sensor because they normally either work or not, there is no half way measure.

In fact like many Hall effect sensors the winding around the magnet suffers small cracks which expands with heat.Over time this leads to total failure,however it's normally preceded by erratic output which can be measured at the crank socket lead situated close to the ECM. As I said misfires when hot but fine cold running are an indicator.

adam