Rough cold idle

[tooch]

Just wanted to know if anyone else has noticed this. When I start my car first thing in the morning and it does its 30-40 second cold idle, it can be a bit rough at times, with what sounds like a very slight misfire.

After this 30-40 seconds has passed and it drops to the normal 800rpm or whatever, its fine, smooth as.

Not a real bother, just don't know if I should drop past the dealer and get them to check codes etc?

[seangti]

mine has this too, marginally rougher sounding and sorts itself. Nothing to worry about IMO, all normal cold start/warm-up. almost sounds like an air cooled hahaha.

You think VW's sound bad/rough, hear a subaru.

[noone]

I think of this process as an 'auto-choke'.

There is an airpump for some pollution control or something odd, some people have removed it.

The rough idle is louder if you have a dump pipe, and you feel the vibrations more with solid bushes.

[Catfish]

It's normal - part of the emission control system. When the engine's cold an air pump blows air into the exhaust manifold to get the cat converter up to temp quickly - ie, more oxygen to burn in the exhaust stream. Hence the increased idle/rough running/slight misfire, which seems to be worse on chipped cars. I think most people don't drive off till the idle slows after about 30 sec.

[tooch]

Thanks lads, sounds like it's normal then

I always let the idle settle down before I take off anyway.

[Jmac]

Very normal guys. The engine requires a relatively rich mixture for smooth operation on cold start. For this reason, the ECU operates in open loop mode with a fixed fuel map for the first 20 to 120 seconds of engine operation until the oxygen sensors have heated to operating temperature.Because of this, exhaust gases contain high levels of carbon monoxide and hydrocarbons after cold starts. The unburned hydrocarbons could be further oxidized, except there is no oxygen left after combustion. By feeding air into the exhaust manifold (secondary air), CO and HC are oxidized through afterburning at temperatures over 600°C. When all this switches off she smooths out.
All in the name of emissions
What to you see what things are going to be like when OBD3 comes out, very interesting crazyness in the name of emissions
Jmac

[DarkCloudNebulae]

Some times I wish the rough start up idle could be switched on and off (when warm), it is a lovely sound the little four making a good quality lumpy burble (I miss it from old carby V8s). Of cause it won't do much good for the fuel usage, or the emissions... But imagine being able to rumb-idle at the lights each time you stop, at your pleasure...

My understanding its using an otherwise unutilised variable valve timing also Jmac? the cams are overlapping like a good high RPM V8 cam, but then switch back to normal once the car has a little temp? I find some days when its warm but the car hasn't been started it will idle with the cams shifted still at 700RPM, then it sounds GREAT, that same type of "I'm trying to stall on you" sound of a V8...

[Corey_R]

If it were 2 days ago, I would say that this was not normal. But yesterday I turned on my GTI and for the first time noted that it was a very rough idle, with the misfire sound you noted... I thought it was going to stall a few times, but it then warmed up quick and has been perfect since.

[nikanasz]

good to know it's normal, although when i brought this up at my mechanic he did a scan and found a code that say "over advanced timing" or something like that (can't remember). cleared the code, and since this is normal, the rough idle plus the misfiring sound is still there on cold starts

at first i thought my giac was causing this. glad it's not. phew

[Golf4]

I also have this rough idle with my 1.8 20v (non-turbo) but this morning when I fired her up I checked the exhaust and there was a bit of nasty grey coming out, not too bad but there shouldn't be any, should there? Update day 2 - started this morning, no carbon(ish) output but still rough even though I disconnected the MAF. So maybe its the coils?

Once the engine has idled for a few minutes (3.5) things get back to normal and with warm starts its fine and clean.
Is this the mass air flow sensor or something else I need to get checked ?