m3gavin

Hi i have a problem with my 2004 audi 1.8t quattro it has started with a rough idle, car seems to drive fine just when it sits idleing seems to idle rough. i have checked the coils and all four seem to be working fine, i can pull dip stick and oil cap of no problem when running, there is a new cat fitted as well, codes that are coming are for running lean and misfire all cylinders then cylinders 1,3,4. i have tried checking for any hissing noises sugesting vacuum leaks

midrange

Whens the last time you changed the Plugs?

Heres a theory which can result in the issues you are having. Your engine has a O2 Sensor which calculates how much air to fuel is being burned. If your plugs are bad enough to not give you good spark, then your going to have fuel going out your exhaust.. The O2 sensor will pick that up, and trying running the engine lean and cause it to run rough. By all your missfires, i think your plugs may need to be changed.. but this only one of many theories that can cause this issue. Timing, faulty coils (you say there fine but it can be very intermitent)

Does your Cat have an O2 sensor on it? Being a new cat and it does have a O2 sensor on it, perhaps when changning the cat it got buggered up..

jordon

iTrader: (1)

I would almost guarantee you have a vacuum leak. Check the hoses. But also check all those little check valves. When at idle, your system is at vacuum, so a leak will draw in more air than was measured by your MAF, resulting in slightly higher rpm, a rough idle, misfires across all cylinders. When you hit the gas, you are reducing the vacuum, so the condition appears to improve, but when you are accelerating hard, the vacuum leak will become a boost leak (unless it is on the other side of a check valve, then it will not really leak much boost, but will draw on air at vacuum conditions).