2.8 idle cough

[205man]

im wondering if anyone knows why when im sat in traffic or where ever with the engine idling, that every so often say 30 sec to a min the engine does what i can only explain as a cough lol.

its like it tryng to clear its throat lol....i ve had the car almost 5 years now and its always done it and it has never affected the running and is fine in traffic and when driving, but i was sat in traffic along time today and i just started thinking i wonder why it does it lol

when it does it the revs drop a fraction and it only does it for a split second then carries ont ticking over fine then a min or so later it will do it again etc etc

if its soemthing simple to fix then ill sort it, but if not ill leave at its done it for 5 years with no problems lol

cheers alan

 

[ScottyP45]

could be the egr valve opening / shutting, or some electrical load cycling mate, try switching everything off and wait for it to happen, if it still does it try watching the engine when it happens to see if it shudders or makes and other noises when it coughs, or may be normal operation. You can get it scanned for stored fault codes to see of anything shows up. AC compressor kicking in? If you leave your ac on auto....

 

[205man]

ive switched everything off and it still does it, no other noise and the negine has a minute shudder when it does it but it almost not that noticeable.
where is the egr situated? and can it bee taken off and cleaned like it can on some other cars, im not buying a new one ive just seen the price lol, and it done this since ive had it and not got any worse lol

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how much ??? lol

 

[ScottyP45]

if it has one, it wont likely be caked up like a diesel one gets, but may have a bad diaphragm or you may have a vac leak when a certain solenoid opens, just fishing for ideas mate. The egr only lets some exhaust mix with the intake to make sure theres no oxygen left in the exhaust gases. I thought maybe yours was leaking vac or maybe not shutting correctly, it will be open at idle but only vag com can show the actual opening percentage.

and no, dont replace anything if you dont know for sure its defective. it may be something and nothing, and like you say it hasnt affected the car for years other than the shudder itself. It seems not to be an electrical accessory load then, so something on the engine is momentarily either robbing air, fuel or electricity from the motor somehow causing a shiver.......can be a vac leak, a sticking solenoid or valve, a vvt actuator, a bad alternator every now and again giving a ripple of ac thru the rectifier pack, or the coils breaking down for a second, making the ecu reboot while running?

Just exploring the possibilities. Vag com logging would be the best thing, to track lots of different engine sensors and look for a deviation that occurs when the shudder happens, some stuff will be secondary effects of the shudder, but one will be the cause I bet.

Imaginary scenario...
Imagine say, the real fault is that fuel pressure blips at the lift pump supplying your injector rail with petrol (we dont know this yet but), so lets say for a brief second the fuel pressure may drop out then back again, just a brief pulse, causing a wave of low injection pressure events and a shudder, the o2 (lambda), maf, fuel pressure etc should all show a deviation in their readings from the shudder, but the o2 cant cause this, leaving the fuel pressure and the maf, now if the maf was faulty it would cause other effects like bad fuel consumption, or cutting out, lack of power at times, so we are left with the low fuel pressure event, which might lead to checking pump flow, connections, pump relay, fuel filter etc etc, see my way of thinking?

Hope this helps mate. You can get an ebay vag (not genuine) lead that works with the free 409 vagcom version and that will do logging etc....just dont be connected to the net or let the software update when the lead is in the laptop or it will kill the "copy" lead


Log everything you can and see what changes when it shudders.

 

[murran]

could also be signs of a ropey injector not opening correctly every now and again. seen that before on a 99 2.8 a6 with similar fault description but it would "cough"/misfire every few seconds.....