E46 M3 - jerky/hesitation when accelerating

[Toilet Duck]

Evening all

My 05 E46 M3 convertible sometimes sufers from a hesitation when accelerating, when it does it its like you are coming off and on the throttle really quickly. I can't get it to do it at will, it just "happens" seemingly randomly. I believe its doing it in the lower half of the rev range, and I've experienced it on both full and partial throttle. Thinking back, its been doing this for many months, but due to the infrequency I've not given it much thought. Its only recently that I have come to the conclusion that there is something "wrong". It is NOT the traction control activating, this is a lot more violent, its literally like coming on/off the throttle very rapidly. I have always used quality super unleaded petrol from when I first bought the car, so I don't think for one minute its dodgy fuel causing this. If I had to have a guess, I would say its an electrical issue, but I'm not a mechanic.

The car has done less than 30K with full BMW history, its due an Inspection 1 in around 5500 miles. Its still under AUC warranty so I can take it back to the stealer to look at. However, because it does it intermittently, I can't show the problem to them, knowing my luck they won't be able to replicate the fault. Would an intermittent fault like this store a fault code on the ECU which they could then read? If there isn't a fault code stored, I doubt they will be able to fix it as they seem to rely on the magical BMW diagnostic computer telling them what to do. Also, If its not something covered under warranty I don't want to take it to a main stealer as they will no doubt pull my pants down, if its something I've got to pay for to fix I will take it to a reputable indy.

I would appreciate any help offered as this is starting to spoil my enjoyment of the car. The straight six engine should be silky smooth, when it does this jerking its like I've got Parkinsons in my right leg

Many thanks in advance

[StreetDragster]

Any record of the spark plugs being replaced in the history? Maybe you have a dodgy plug, or a coil pack thats starting to break down?

Violent on/off type problems in my experiance have always pointed to an ignition problem, dull on/off symptoms being fueling or compression issues.

Thanks

Matt

[Toilet Duck]

thanks for the reply Matt

I don't know if its ever had new spark plugs. I made a mistake in my post above, the next service due is an oil service, the last one was an inspection 1. I'm not sure when the plugs get changed, I think maybe at an inspection 2 service? Obviously plugs aren't covered by warranty. Maybe its worth me changing them myself to avoid getting raped at the main stealer? I will have to see what the access is like for the cylinder furthest away towards the bulk head as it might be tricky to get to or need specialist tools. If its a coil pack, I'm hoping that will be covered under AUC warranty? Would an intermittent dodgy coil pack store a fault code on the diagnostics? Would all six need to be changed (if ones on its way out, will the other five be as well)?

Cheers

[Chessers]

is this just from cold or when it is fully warmed up as well?

[outnumbered]

They are all prone to doing that when cold. If it's happening a lot when the car's hot, either you have a remarkably insensitive throttle foot, or something is wrong.

[Toilet Duck]

Chessers said:

is this just from cold or when it is fully warmed up as well?

It does it when fully warmed up. For example, it did it a couple of times the other day after I'd driven 30 miles. That's when it finally dawned on me that something must be wrong. I then started trying to re-create the issue but I can't, there doesn't seem to be a definative way of driving that triggers it. That makes it very frustrating . I know they can run a bit "rough" when cold, but I'm very light on the throttle and use minimal revs until fully warmed up (usually no more than about 2.5K revs).

[Toilet Duck]

outnumbered said:

They are all prone to doing that when cold. If it's happening a lot when the car's hot, either you have a remarkably insensitive throttle foot, or something is wrong.

Its doing it when fully warmed up. Its not my foot causing the problem, I think something is wrong with car.

[oli_quick]

do you have the 'sport' button on...

I find often it can create a feedback loop - the first jerk (ahem) causes the foot to resonate therein jabbing on the throttle..causing another jerk etc, etc...

Best cure...plant your foot solidly into the carpet

[lamby_gti]

I've seen this problem before on cars - A change of all leads and plugs should solve the problem. Don't let the problem go on - if it is a fault lead/plug you could be allowing unburnt fuel into the cat converter.

[Toilet Duck]

oli_quick said:

do you have the 'sport' button on...

Occasionaly, but its not this that is causing the problem. I normally drive without that activated and have experienced the jerky throttle/hesitation then.

[Toilet Duck]

lamby_gti said:

I've seen this problem before on cars - A change of all leads and plugs should solve the problem. Don't let the problem go on - if it is a fault lead/plug you could be allowing unburnt fuel into the cat converter.

I agree with what you are saying, but I'm unsure about the best way to proceed. As the car is under AUC warranty, I'm hoping that they will fix it for free. However, I know spark plugs are not covered, so if the dealer will just start changing stuff until its fixed I don't want to get shafted for £120 +VAT per hour labour for them to do the plugs (I should be able to do them myself). If its a coil pack, would this be covered under ACU warranty? Again, if its an intermittent spark plug/coil pack issue, would this store a fault code? If I have to pay for a diagnostic check its not so bad if it tells me exactly what is wrong. I don't know what's the best/cheapest course of action. Main stealer labour rates are extortionate, especially if they want to just change everything hoping it cures the issue.

Cheers

[E30M3SE]

Toilet Duck said:

lamby_gti said:

I've seen this problem before on cars - A change of all leads and plugs should solve the problem. Don't let the problem go on - if it is a fault lead/plug you could be allowing unburnt fuel into the cat converter.

I agree with what you are saying, but I'm unsure about the best way to proceed. As the car is under AUC warranty, I'm hoping that they will fix it for free. However, I know spark plugs are not covered, so if the dealer will just start changing stuff until its fixed I don't want to get shafted for £120 +VAT per hour labour for them to do the plugs (I should be able to do them myself). If its a coil pack, would this be covered under ACU warranty? Again, if its an intermittent spark plug/coil pack issue, would this store a fault code? If I have to pay for a diagnostic check its not so bad if it tells me exactly what is wrong. I don't know what's the best/cheapest course of action. Main stealer labour rates are extortionate, especially if they want to just change everything hoping it cures the issue.

Cheers

Dealer will/should run a diagnosic first, that will/should tell if it is coil, plug, airleak, sensor or other item.

If it's under warranty then take it back to a dealer.

Edited by E30M3SE on Tuesday 14th July 14:47

[scz4]

Forget plugs etc, take a look here - http://www.zeckhausen.com/CDV.htm

I did this on my Z4 and it helped with the occasional kangerooing.