Possible vacuum leak driving me crazy

[chuck_th]

Hello, this is my first post ever here, I am writing you all the way from Costa Rica, to see if someone can illuminate me. I have a 2003 VW Polo 9N 1.6 BAH, even though it's a 17 year old car it has been great all this years (aside from a few electrical problems). But as of today the car refuses to pass the mandatory emissions test, it reports high HC on idle, limit is 125PPM but the car outputs more than 250PPM. I took the car to a local shop, and since, we have change the following:

Sparks plugs
Cables
Ignition Coil
Catalytic converter
Engine breather valve (was missing a o-ring and leaking oil)
Clean the throttle body
Replaced manifold o-rings

The car has a rough idle, not too crazy, but notable, and also it shakes a little. If rev it up on neutral say to 3000 rpm, when the gas pedal is let go the rpms start to drop as it normally would, but then it goes a down under 600rpm and it feels like almost stall but it manages to recover. I have check other things like hoses but non seem to have any cracks. I have removed the canister purge valve but it seems to work ok. Vag-Com shows no error codes.

Does anyone has any other suggestion? The help is highly appreciated.

Cheers.

[RUM4MO]

Brake servo or its vac pipe could be a vac leak area to consider, typically the plastic vac pipe splits where it is fitted to either the one way valve or at the other ends of that pipe, and the splits have an annoying habit of being on the "other" side of the pipe, so you can not see them. Removing that pipe at the manifold end and closing up the opening on the manifold would prove if that was the problem, ie emissions test it with the vac point capped/blocked off.

[chuck_th]

Thanks,

I have checked that hose by removing it and it doesn't seem to have any cracks, what I couldn't test was if the valve is allowing flow in one way only. I will be blocking the inlet of that hose to see if something changes. Another thing I have noticed is oil where the throttle body meets the inlet manifold, almost like if the o-ring is not sealing, checked that o-ring and seems ok, but will be replacing it just in case.

[john M]

Was a compression test carried out?.
Also perhaps leaking fuel injector.

[Harrywilson]

I have a problem with an old ignition coil. There are times when the check engine light in a vehicle is faulty or a rich air/fuel mixture can also cause the emission check to fail. This can be anything from a faulty oxygen sensor to faulty injectors. It is also worth checking the oil pump, replacing the oil filter, and changing the oil, here https://davesoilchange.com/oil-for-3-5-ecoboost/ you can see the top best oil for 3.5 ecoboost, I use Castrol, so far no complaints.