Surging Turbo 1.8T...

[randall977]

It's an 05 Cabriolet 1.8T quattro sport, 163bhp (BFB) manual - 58K. 50% of the time when I accelerate hard the power pulses when the turbo kicks in - particularly on the first drive of the day (normally 30 minutes to work). I have illiminated a few possibilities already; the turbo itself, the DV, boost pressure sensor, coil packs, spark plugs and intercooler pipes. There are no fault codes when I plug my code reader in. So this leaves me a couple of options unless somebody can suggest anything else; 1. The N75 - but surely there would be a fault code present? 2. A vacuum leak - but how can I test this?

I would be REALLY greatful if somebody could suggest what could be causing this problem which doesn't create a fault code as it is the one area of disappointment in this great car. Thanks in advance!

[docurley]

From what I have read it may be the N75 valve but I would also replace the DV while your at it and drop in some new sparkplugs.

[randall977]

It's got new plugs and a new DV - the N75 is original. Wouldn't there be a fault code if the N75 was malfunctioning?

[docurley]

Not always

[randall977]

I've just got it back from the Audi specialists down the raod and they've had it connected to VAG com - no fault codes at all. They say that almost any fault would bring up a code...except the air mass meter. SO could the air mass meter cause surging?

[elPaulio]

Hi there,

Yes, the AMM could cause surging. Bet way to check is to remove the plug from the AMM and see how it drives.
Also check for any vacuum leaks or split pipes as these are quite common causes also.

Unfortunately, I think the only way to diagnose the AMM for sure is to replace it. Not ideal!!

Paul :biglaugh:

[randall977]

Thanks Paul. If I disconnect the AMM won't it drive terribly??? I've read a bit about being able to clean the AMM...

[randall977]

Okay - here's what I know. The boost pressure pulses under heavy load - ie acceleration up a hill in 4th or 5th. It actually over boosts and then drops the pressure - so the acceleration s better than it should be when the fault shows. There are no fault codes. I believe that when I connected a foot pump to the wastegate actuator to check if it worked (which it did) this may have made the problem better than it was (it did it all the time before). I have replaced the DV, boost pressure sensor, plugs, coils and checked the turbo itself. I am seriously running out of avenues!

Could the wastegate actuator be sticking under heavy loads?

I am very appreciative of any advice.

[randall977]

Reading some threads on the VW Vortex site it seems that this surging problem is fairly well know in the 1.8T...with not solid conclusion! However they suggested the N75... So I have ordered one from VW (058906283F - £61) - as well as a new genuine Bosch MAF (£70 - Hong Kong!). To date I have replaced;

The DV (£35)
The coil packs (£95)
The plugs (£25)
The boost pressure sensor (£70)
All genuine correct part numbers apart from the coils and DV which I upgraded to more reliable revision numbers.

I really hope the MAF or N75 solves the problem as it's getting expensive!