Random rough running...any ideas?

[adrianwillis]

Audi A4 B5 1996, 1.8 ADR petrol.

I've been having intermittant problems recently where the car runs absolutely fine, then runs really rough (as though it's misfiring, popping and spluttering with very little power) then it runs smoothly again, then goes through a rough patch...and so on.

I've changed the following parts already:
Spark plugs
coolant temperature sensor
air filter
throttle body (it adapted perfectly)

I took it out for a run after changing the throttle body and all appeared well (there were a couple of isolated stutters) until I sat on the drive with the car at idle...then it progressively started to run rougher and rougher, and was "missing" when I blipped the throttle, or increased engine revs slightly, then it would smooth itself out for a while before it started to run rough again.

I'm not sure where to look next.
VAG-COM shows no error codes at all.
I replaced the camshaft, and spark plug oil gasgets the other day and also checked all of the breather system...all OK.

It seems as though another sensor might be on the blink, sending wrong messages to the ECU every now and again?

Has anyone got any ideas what could be wrong?

Thanks in advance for any replies.

[Chesp123]

I had similar symptoms with rough idling and it turned out to be the cambelt. Its worth getting it checked to rule it out.

[fourringsrus]

Fuel filter and lines good ?

[adrianwillis]

Fuel filter hasn't been changed for probably 10 years.

Would a blocked fuel filter cause this random rough running?

[zollaf]

yes

[adrianwillis]

Where is it located on a B5 saloon?

[fourringsrus]

Don't know the layout on the B5, but quite often if not in the engine bay there will be a filter can positioned up against the floorpan on the outside of the car, somewhere around the rear axle and underside of the fuel tank area.
If a decent constant supply of fuel can not get to the engine, it " aint gonna work".
Not expensive or difficult to change
Good luck

Stuart

[zollaf]

when trying to get to the bottom of a problem like this you need to do the basics first, all filters and other service items. when you know they are right then you start looking deeper. sometimes the silliest and most overlooked things can end up with you emptying your wallet and pulling your hair out.
iirc you will find your filter just in front of the rear axle as 4x0's says.

[adrianwillis]

Fitted a new fuel filter...the problem persists!
When I started the car from stone cold it ran smoothly...the problem of very lumpy running appaered after about 1 mile and got worse...so it seems that this problem is associated with the engine getting warmer.
It was so bad that it struggled to get up a slight hill near our house.
This problem was less apparent when I took it out yesterday...it started behaving badly when I was sat on the drive with the engine at normal temperature.
What should I investigate next???
I've got VAG-COM so I can look at measuring blocks etc and this might highlight the problem, but I don't know which blocks to look at.
There are no fault codes on the engine after this morning's outing.
I'd really appreciate some sage advice.
Thanks in advance.
Ade

[zollaf]

does it have coil packs or ht leads ?