audi a4 b6 misfire & rough idle advice


Registered User
hey all

I started a "help me" thread recently but didn't get too much advice that wasn't readily available on google, see my post:

so, if you're having the misfire or rough idle issues please read below, before you go out and spend your hard earned cash!!!!

basically my car had a misfire issue at idle (and throughout the rev range, but less noticeable when rev's high).

But I noticed a lot were having this issue throughout the rev range so i though i'll write this guide....

obvious & most expensive things to check are the coils and sparks, I can't say "don't buy new sparks" straight away, as this is never a bad idea to get new one's, but what I can say is "Don't buy new coil packs"

On the 1.8t models the loom wires that go to the coil packs are exposed to the engine heat as they're not insulated... even if you haven't had an issue with your 1.8t, i bet, if you peel back the rubber on the wires going to the coil packs you'll see damage to the wires!! - PLEASE NOTE Heat insulate these wires ASAP, before they cause you issues in the future.

I replaced all 4 coil packs(1.8t) only to find.. that if the engine is running you can "pop" them out and see the actual spark off the coil, when they're out, this will confirm the pack is okay ans also the wires aren't excessively burnt). BE CAREFUL not to electrocute yourself but if any aren't working/ sparking, don't replace with the cheap ones use Trade Part Specialist (TPS) offer all 4 for about £120 - gen Audi parts, so don't be temped by cheaper one's on ebay

replacing those didn't work anyways......

If you've tried these no go/ expensive steps already then...

The air system is the next thing to check...

After the above I checked the "check valves", you can google "audi a4 b6 check valves" & also the "brake booster valve" to see what i mean

I went to the scrappers, and managed to replace this brake booster valve and the check valve that sits above it for a tenner. (got from a 2.5 B5 model)

replaced these and at the same time i checked the "check valve" above the air intake box..

the valve above the air-intake box & the one above the brake booster valve was f##ked. air blowing both ways. so I replaced these (and the 2nd hand brake boosted valve") and started car.

it run good for about 5 mins.

previously, the main symptom had been multiple misfire across all ports, however it would idle steady.

after I sorted out these broken valves the misfire issues had stopped, but the car was idle'g at 1000rpm but then jumping to 2000rpm, then nearly choking itself and very nearly cutting out but would recover and jump to 3000rpm and settle back at 2000rpm.

I read then about a "crankcase breather valve" causing this issuse...

So at this point, I've changed the coil packs, changed the sparks, changed 2 check valves and the brake booster valve but still no luck.

changed the crankcase breather and all is well again, A very easy swap, I'm not going to post instructions as you can google search it and it's V easy,

so, I don't think there was ever an issue with sparks or packs.

cost a tenner to replace the valves (2nd hand) and a tenner to replace the crankcase breather valve (brand new) and all sorted.

if you're getting rough at high rev's then it's most likely the PCV valve as this operates at high rev's. again not gunna post pic's as redily available on google,

any Q's please let me know & i'll get back to you asap

how this helps ppl out




Registered User
Oh and in addition to the above, a faulty coolant temperature sensor can cause all of the above so this is a good thing to check and also to have a good clean of the throttle body.

forgot these :)