Missing when cold

**PoloGIT** · 03-09-2019, 12:03 PM

Guys, I guess this has been asked a million times over the years, so apologies if someone shoots back with the required search result which I've yet to find.

So, when I first fire it up in the morning, it sometimes runs like a pig, pretty sure ignition related. Sometimes it will idle fine, but as soon as there is any load, it bogs down and, if I'm really lucky, starts to bunny hop down the road until the boost kicks in (not what you really want on a cold engine). The obvious answer I guess is coils, but it's not throwing any codes. I did have to replace one of the coils when I first put Gavin's tune on it, but it has been happy days since (that was probably a year ago). When that happened, it did throw a code and ran like a scooby.

I have replaced the spark plugs, which I think has improved things a bit, but still not perfect.

Thanks in advance for any pointers.

**sambb** · 04-09-2019, 05:20 AM

I'd be looking for obvious air leaks eg in the SAI pipes, any of the vacuum pipes leading anywhere off the plenum, the PCV pipes. I wouldnt have thought it'd be a coil if it only happens at start up but is then ok later when warm but it takes nothing to reseat those coil plugs and change out the spark plugs (28 thou gap). While you are at it examine the plugs to see if they are consistently coloured or not - more later... Another thing you can do is take off the inlet to the throttle body and open up the throttle plate and give the bore of the throttle body a good wipe out. Any crap in there can have an effect. Those are all no cost things you can do.
If its still doing it it could be clogged injectors. I had the same problem recently (but also different spark plug colouration). Gary suggested it could be a mixture imbalance due to dirty injectors. I got them flowed and sure enough two of them were down nearly 10%. This is on a new set of injectors that were just over a year old. Once reinstalled the cold start, warm up and idle dramas I had were gone. That's just an example of what it could be. Don't go jumping to conclusions before you look at the first things.

**PoloGIT** · 04-09-2019, 11:04 AM

Thanks for the detailed response, Sam.

As above, I have already swapped out the plugs, and they are gapped correctly, the old ones all looked evenly coloured and right to my untrained eye.

I do have the blanking plate and Gavin's tune is set up for the SAI delete, so maybe I should push on and do that to take that stuff out of the equation.

Does opening up the throttle plate require a reset of adaptions or anything after?

Will report back. Cheers.

**sambb** · 04-09-2019, 11:40 AM

ha ha yep I see that now re the plugs.
No after you do the throttle plate, put it all back together and then turn the key to the accessories click or maybe one further (cant remember) and you'll hear a high frequency singing noise. Thats the throttle body doing its self check going through its range of motion. Leave it till it finishes and then start the car.
You mentioned adaptations. Do you have VCDS? If so let me know and i'll tell you where to look to get a read on if there's a vacuum leak in the system.

**PoloGIT** · 04-09-2019, 11:44 AM

Originally Posted by sambb

Do you have VCDS? If so let me know and i'll tell you where to look to get a read on if there's a vacuum leak in the system.

Just the lite version at the moment... Thanks.

**Sharkie** · 04-09-2019, 01:44 PM

Most likely a SAI problem. Common symptom of that failing is a troublesome idle when cold.

Never had that problem on my 9N3 GTI, but lots of folks on here did.

**PoloGIT** · 04-09-2019, 02:07 PM

Originally Posted by Sharkie

Most likely a SAI problem. Common symptom of that failing is a troublesome idle when cold.

Never had that problem on my 9N3 GTI, but lots of folks on here did.

Ok, thanks, I might try delete it tonight then. Now to try find a guide that hasn't been ruined by Photobucket...

Edit - this one should do, I think I used it last time: SAI and N249 Removal

**sambb** · 04-09-2019, 05:40 PM

If you can go to engine/measuring blocks/look at block 031 and 032. There will be a field called STFT/short term fuel trim/or additive STFT. It varies in different versions what its actually called. When the car us well up to temp and has been running around for a while, idle it, look at that block and the % should be less than +-3%. Much more than that and you have a vacuum leak.
If you delete the SAI you'll have to plug off the airbox hole. Follow the vacuum pipe from the combi diaphragm which will lead to the SAI solenoid. You can turf the SAI vacuum pipes and the bigger pipes but you'll need to either leave the solenoid plugged in or resister out that part of the loom. Despite the SAI being ticked off in the tune, that just means the second cat sensor isn't looking for a lean condition. However at the solenoid, the ECU still needs to see a complete circuit there. If not, for some reason your closed loop fuel trim adaptations wont work. It'll never go into closed loop. So easiest is in the meantime to leave the sai solenoid plugged in while you rationalise all its pipework, and then attack it with a resister later.