Non Mercedes misfire question.

[Deleted member 37751]

Family member has the following problem and it's currently sat at the main dealer and they aren't exactly sure so I thought I'd see if there was any thoughts or ideas as to what you think it could be...

Vehicle: 2006 Audi A6 3.2 V6 (petrol)

Mileage: 99k

Symptoms: intermittent misfire, but (bizarrely) if you put the air suspension up and down three times it will make it misfire. Audi think it could be the cam chain, but they're not 100% and that job has been quoted at £800. Does show on the computer as a misfire on cylinder 4 but the compression test has been done and it's all good there. Checked the usual injectors etc.

Very strange how it's somehow related to the air suspension but as you can see it's a big puzzle and I wouldn't want the family member to start throwing £100's or even £1000's at it.

Any advice appreciated.

 

[Bellow]

When, in relation to driving does it occur (other than going up and down)?

Ignition related mis-fires will invariably appear under load. Fuel related - more random.
I've never heard of a cam chain causing a miss.

If the engine is loaded by the suspension raising then suspect ignition.

 

[C36fan]

Family member has the following problem and it's currently sat at the main dealer and they aren't exactly sure so I thought I'd see if there was any thoughts or ideas as to what you think it could be...

Vehicle: 2006 Audi A6 3.2 V6 (petrol)

Mileage: 99k

Symptoms: intermittent misfire, but (bizarrely) if you put the air suspension up and down three times it will make it misfire. Audi think it could be the cam chain, but they're not 100% and that job has been quoted at £800. Does show on the computer as a misfire on cylinder 4 but the compression test has been done and it's all good there. Checked the usual injectors etc.

Very strange how it's somehow related to the air suspension but as you can see it's a big puzzle and I wouldn't want the family member to start throwing £100's or even £1000's at it.

Any advice appreciated.

Lee IMHO opinion that is related to wiring loom ,and you know it's a audi what would you expect?lol

 

[Deleted member 37751]

It is just random (besides the air suspension trick).

No loss of power though.

 

[Bellow]

I'd run it till it gets worse and the cause makes itself known.
And watch for it missing when you first put your foot in it. If it does - ignition. If it doesn't - something else.

 

[Deleted member 37751]

When you say ignition, can you be more specific please F?

 

[Bellow]

When you say ignition, can you be more specific please F?

Certainly Sir!
Ignition failure on the high tension side (HT) appear when the engine is loaded ie, the throttle is opened. The reason is that it it falls to the HT side to generate sufficient voltage to introduce the spark into the combustion chamber. When the throttle is opened, the pressure in the combustion chamber is higher and introducing the spark becomes more difficult. Any weakness in voltage (the electrical equivalent of pressure) will see the spark falter. Insulation breakdown allowing the voltage to leak away (path of least resistance) the usual cause.

Given that it can be provoked, provoke it in darkness with the bonnet up and observe for any stray sparks (though hard to do with coils over plugs) and listen for tracking. Look also for signs of tracking - thin brown lines on alloy where voltage has found its way to ground/earth.

 

[Deleted member 37751]

Excellent, thank you.

There is also an issue with the actual air suspension pump but I'll leave that question for another day!

 

[Bellow]

HT failure will steadily worsen as the insulation breaks down further (and this can be within a coil). If it is that, as soon as the defective component is identified get it changed. A bad coil can direct the voltage to the low tension side - all the way back to the ECU...

Air suspension pump is offering up a clue I think!

 

[m2287]

I know on my grandads Audi A4 3.2 he had a misfire which was cured with a new coil pack, no air suspension on that though!

 

[Tim203]

Check the colour of the plugs to find the culprit. Swap a coil pack to prove/ disprove that this is the fault as it could still be an injector. Had the same symptoms recently and fault codes on a little Matiz and it turned out to be an injector.

 

[Tim203]

Having slept on it I reckon C36 is on the right track with a wiring fault somewhere.The air suspension is a bit odd, almost as if an earth wire is poor somewhere.

 

[Deleted member 37751]

Yesterday, Audi have recommended changing the cam sensors but this is a gearbox off job and touching a grand before you even know it.

Plus, they can't say for certain that will fix the fault but the decision is once your into it for a grand do you just keep throwing money at it? The age old dilemma I think.