E39 520 2001 Misfire On Deceleration

[Davo5774]

Hi Folks, My name is dave and I am a new member. I have a y reg 520se with 2.2 engine and manual transmission. The car returns the expected fuel consumption with no bother. It is also fine at speeds of up to say, 50 mph. However, when on the motorway, when I let my foot off the accelerator at speeds above 50, the car judders as if it has a misfire. Sometimes if I dip the clutch during this juddering either the revs rise to around 3500 rpm then drop to tick over or they will drop to tick over at around 700 rpm as normal. It also does the same when I use cruise control. It is not a noisy misfire in that there are no pops or bangs. At tick over it is as smooth as anything and revs freely up and down at tick over. I have done a diagnostic with no faults shown whatsoever. Does anyone have any ideas? (sorry for the chapter and verse story!!)

[Guest Mister Dabsy]

This is straight forward... A misfire happens when either the timing is out or the ECU/DME tries to compensate for another issue... I think its the latter of the two.(Disregarding any plug/coil/lead issue at this point).

My advice to you is clean the MAF and Throttle Body. When was her last service and what was done?

[Guest DazH]

This is straight forward... A misfire happens when either the timing is out or the ECU/DME tries to compensate for another issue... I think its the latter of the two.(Disregarding any plug/coil/lead issue at this point).

My advice to you is clean the MAF and Throttle Body. When was her last service and what was done?

+1

If you can't hear any pops, I would imagine it's to do with the ECU/DME doing something it shouldn't normally be doing ^_^ and as DabsyBoyo says, something is giving the ECU wrong intel!!

Maybe....

Daz

[Davo5774]

Thanks for your speedy reply, Its last stamp in the BMW service book is at 80000 and it was for just an oil service. the car has now done 87000 miles. I was going to look at the throttle body as I thought that it may be sticking but what you say makes perfect sense so I will do a full service including cleaning the trottle body and I will also check and clean the MAF sensor.

Many thanks for your help, I will post the outcome.

Dave.

[Guest Mister Dabsy]

Looking forward to it ;)

[Davo5774]

Hi Jack, Hope you and yours are well. I think that I have sussed the problem!!!

I removed the MAF sensor and housing which was absolutely spotless looking almost new but while it was off, I sprayed it with maplin electrical contact cleaner as it is safe with plastics and evaporates leaving no residues but takes with it any small debris into the atmosphere.

I thought what I would do also is check all connections throughout the induction section. This is what I found and it makes perfect sense.

Just after the MAF sensor, underneath there is a branch of about 40mm in diameter which was perished with small chunks having fallen off at the connection which I think has been letting in air as the engine 'relaxes' causing the afore mentioned symptoms.

I am a plumbing lecturer and in my plumbing tool box I have an amazing 'all purpose putty' called milliput (Its more than worth the 3 quid it costs). Its a two part epoxy putty and when set can be tapped, filed etc. I filled the cracks with this, put everything back together and took the car for a test drive.

I drove it on the motorway took it to about, 80mph and when safe to do so, let my foot off the throttle and it slowed down nice and smooth then I floored it still in fifth and it went up to 70 with no problem whatsoever. The throttle response is back and dave is a very happy man. So if it was in fact an air leak that would explain why the diagnostic would read absolutely fine. Whatever it was its now cured so thankyou very much for your help

Dave. :) :) :)

[Guest Mister Dabsy]

Well, there you go ! Very rare we miss the old "air-leak" chestnut, but well done and Happy days once more ;)