225 1.8 Question

[lyleswk]

Posting here since there is a lot of experience with the 1.8t.

I have a new to me (actually my son) TT with the 225 1.8t. For those unfamiliar, it has a K04 turbo and I assume mapping similar to a stage 1 APR tune (maybe not, you guys can correct). Anyway, checked for codes this weekend with a VAGCOM, and there were none. But, it has an odd behavior that I would like to have some insite on. While at speed (60 or 70 mph), if you downshift and get on the gas, there is a stutter, like it looses boost. Have any of you seen this behavior on either tuned or stock A4's with no error codes showing up?

[old guy]

Possibly a DV problem. I would start by logging the actual vs requested boost to see if the two get out of whack. If they do then check the DV for a diaphragm leak. I'm sure you already know how to do it but I'll throw in the link anyway: Clicky click®

[lyleswk]

Thanks OG. The link just keeps me from looking it up to make sure. One that is slightly off won't throw a code?

[old guy]

Thanks OG. The link just keeps me from looking it up to make sure. One that is slightly off won't throw a code?

Nope. You will only get an underboost code for a significant and continuous variation between actual and requested.

I think what you are describing is a short blip or boost loss right after a downshift and then the boost returns to normal. What I'm thing is a sticking DV or one with a small leak. When you let off the gas the DV gets full vacuum and starts dumping any pressure into the charge pipe. Once you get back on the gas the intake manifold goes under boost and sends the boost signal to the DV to ensure it closes properly. If you aren't getting a full boost signal or the DV sticks a little the charge pipe is going to dump pressure through the DV back into the TIP until the DV catches up and closes. After that the boost feels pretty much normal.

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[walky_talky20]

I'm going to go another way on this one. If the "stutter" the OP describes is noticeable, physical vibration through the car, I don't think it's the DV. A torn DV will cause low boost and some audible compressor surge on throttle lift, but it isn't something you'll normally feel at WOT.

I'm going to assume the stuttering is actually misfiring. And the cause is a weak coil pack. When they get weak, it is at maximum engine load where they will have a problem first. Dropping a gear and hitting wide open throttle/full boost at 70mph is when you're taxing the coil packs to maximum. The RPMs are too high for misfire recognition to be active, so no code will set and the CEL will not flash or illuminate. But you will certainly feel the engine kicking.

[seanf86]

I'm going to go another way on this one. If the "stutter" the OP describes is noticeable, physical vibration through the car, I don't think it's the DV. A torn DV will cause low boost and some audible compressor surge on throttle lift, but it isn't something you'll normally feel at WOT.

I'm going to assume the stuttering is actually misfiring. And the cause is a weak coil pack. When they get weak, it is at maximum engine load where they will have a problem first. Dropping a gear and hitting wide open throttle/full boost at 70mph is when you're taxing the coil packs to maximum. The RPMs are too high for misfire recognition to be active, so no code will set and the CEL will not flash or illuminate. But you will certainly feel the engine kicking.

this^^ I've had luck logging the missfire counter for each cyl in the measuring blocks (which one eludes me right now) but one or two missfires won't set a code but will show in the counter, log that and if there is a miss move the coils around and see if it follows, probably a good time to change the plugs while you're at it.

[lyleswk]

I will check both when I get a chance. Thanks.

[walky_talky20]

If the diverter valve is stock (black plastic construction), you can quickly check it using the [patent pending] Strawberry Milkshake Method™ :

The quickest way to check a diaphragm-type DV is to remove the vacuum line at the manifold and suck on it. If it holds vacuum, you're good.

The best way I can describe how to ascertain whether it is "holding vacuum" is this:

You know when you're drinking a strawberry milkshake and you get one of the strawberries stuck in the end of the straw? You try your best to suck the berry through the straw, but it just won't go. During one of your attempts your tongue inadvertently gets stuck to the end of the straw. Due to the vacuum applied and the perfect seal your tongue makes on the end, the straw is quite well stuck there. In fact, it is stuck so well you could probably remove the straw from the milkshake with just your tongue.

It is quite easy to feel this suction effect with the vacuum line as well. You simply suck on the line, then stick your tongue on the end. If it sticks and holds - the chamber is sealed, the diaphragm is holding vacuum and all is well. If it sucks through easily and no vacuum can be created - then the diaphragm is torn or the vacuum line is ripped somewhere.

*In case you were wondering - yes, Walky does indeed like his strawberry milkshakes. Yes, he is 12.

[lyleswk]

That is a good one. Suggest you write Bentley and get that added to their troubleshooting guide for DVs...