How to test for boost leak

[Technogym]

Hi all!

I am having a issue with an S4 with AAN engine. It has a very unstable idle, and is juddering in cruise speed. This only happens when the engine is warmed up.

Been trying some things like disconnecting the lambda and maf, but with no luck. Been very many shots in the dark, and now I have come to the point where I fell like starting over from the beginning.

Boost leak is many times a cause for unormal engine behaviour, so i guess i will be starting there. Is there any here who has a good description in how to proceed to check for leaks in the vacuum and press system?

Any other tips and thoughts in this matter would also be highly appreciated!

Vegard

[Acidburn]

speak to andyp about a boost leak test kit and if he has any left to sell.

you will need access to a pump/compressor to do it

have you got a boost gauge fitted if so what happens with that when you drive, any high pitched noises when its on boost?

[bazookachicken]

Doesn't sound much like a boost leak to me (although they can cause some strange side effects) because your having a problem at idle and cruising i.e when the turbo is producing none or very little boost. Probably still worth checking but I would be looking towards the coil packs / spark plugs / leads etc from what you've described.
As Acidburn suggested try andyp for a boost leak testing kit they aren't very expensive but they are guaranteed to be useful at some point.
Worst case if he hasn't I could send you mine in the post on loan so long as I get it back

[merlin's S4]

Where in the Southwest are you? Sounds to me like it could be a POS or coil breaking down under the heat?

[Technogym]

Thanks for the replies!

Thanks for the offer bazooka! I am in Norway, so would be to much hassle to send it. Just as well buy me a kit.

Regarding the symptoms i agree that it doesent look like a boostr leak. But I only thought that I had to start in one end.

Is it a way to check the coil packs? without bying a new one?

Inspected the spark plugs, and they are all white, so it doesnt look like a misfire...

[Acidburn]

Also what are the collars of the coil packs like? if they are damaged I think fataudi80 - steve is organising a group buy

[Technogym]

Just to add, also have the vag com. No error codes apear, and cant say i understand anyuthing out from the readings further up in the channels in live mode... So if anyone have any tips checking with vag com I have the possibility to do that...

[merlin's S4]

When the problem occurs does it seem like one cylinder isn't firing, ie your running on 4? If so then remove the injector plugs one by one with the engine running until you find the cylinder that isn't firing. Then you can start to trace the problem from there!

[Technogym]

How can I tell/hear if a cylinder is missing? Is it so that I should pull out one plug and when no change in sound appear this is the one that is faulty?

Vegard

[Acidburn]

from what I read about jas11n's problem when his car was running on 4 cylinders it will sound like a subaru an off beat vw beetle sound.

[newsh]

I wonder if it could be an ISV problem?

[Technogym]

Yes, but, how to test the isv? I have tried to clean it an spray it with crc, and during the test, i can hear the valve run. but can it still be faulty? as in not closing properly?

[merlin's S4]

How can I tell/hear if a cylinder is missing? Is it so that I should pull out one plug and when no change in sound appear this is the one that is faulty?

Vegard

Yes! Unplugging the injectors one by one, 4 of them will make it worse and the other one will make no difference. If so then that cylinder is at fault. If this is the case the first test that you should do is to swap the POS connections over. Start the engine and re-do the injector test. If the missing cylinder has moved then you have a faulty POS!!

[Technogym]

Ok, but i cant disconnect one by one while the engine is running? there are to connection points on the torpedo wall and they connect to and three coils.

Where is the right place to disconnect? and a POS is?

Thank You very much for helping me out here!

Vegard

[merlin's S4]

Ok, but i cant disconnect one by one while the engine is running? there are to connection points on the torpedo wall and they connect to and three coils.

Where is the right place to disconnect? and a POS is?

Thank You very much for helping me out here!

Vegard

You can disconnect the injector plugs one by one while the engine is running. See Picture.

POS = Power output stage. These are the two blocks on the firewall (torpedo wall) that the coilpacks connect to. One will control two coils and the other will control three as you say. They are both identical and each have three channels so it is possible to use the spare channel if one is faulty. It is also possible that by switching the plugs from one to the other that the missfire will go away.