This seems to be a common problem and I have tried every recommendation that I have seen posted. Around 4000rpm when the boost gage is over 0 psi, the engine hesitates. Push the boost above 0 bar significantly (5 to 10 psi) and random misfires occur and the boost pressure fluctuates. The car is basically undrivebale under boost. I have searched for leaks after the turbo, none found so far. Also have done the following:

1) Coil packs were new about 6 months ago.
2) Changed the pugs anyway but no improvement.
3) Change the MAF to brand new Bosch OEM part. It seemed like it ran better with the MAF unplugged but now its just as bad with or without the MAF. Wasted $180 on a new MAF.
4) Checked the diverter valve. This is a Turbo XS model and probably came with a Techtonics chip kit that the previous owner installed. Seems to open properly. Can this device cause the symptoms I am having?
5) Replaced some cracked breather hoses for the PCV which helped with a problem of reliable starting but didn't help the boost problem.
6) CAT converter was replaced just before this all started. Old CAT was plugged.
7) Replaced fuel pressure regulator. Fuel pump was replaced about 3 months ago.
7) VAG-COM (basic free version only) says the Mass Airtflow numbers are ok (90g/s at 4000 rpm). Misfires are all random.

Never get a CEL because of this problem.

Right now I am at a loss as to what to try next. Can the intercooler have a leak and cause this, that's about the only thing I can't really check visually? How can that be tracked down? Can I bypass the intercooler and see, or will that cause damage?

Looking for some inspiration!!

Thanks

 

Do a search for "boost leak checker" - you'll find info on how to make and use one. That'll at least show you the integrity of your charge piping and vacuum system (a vacuum check gauge would help too). In addition, try a compression test to be sure all your cylinders are sealing correctly. A minor rupture in a ring might not become symptomatic until you get a ways up into boost. Also, you don't mention if you verified the proper gap on your plugs (.032" if stock boost and .028" if your boost is elevated). And yes, a malfunctioning DV could cause you to see the issues you're dealing with - if it has a weak spring/diaphragm or isn't up to the task of handling your boost level it may be opening up under boost and dumping air away from the cylinders.

 

Ok. Thanks for the pointers. The plug gap is set to 0.032. I will check the diverter valve again. Is there a simple test for the spring rate? I can manually depress the spring fairly easily.

thanks for your help!

 

If you think you have some sort of chip then gap the plugs down to .028" and see what happens.

 

Is the DV stock? If so it doesn't have a spring, it has a diaphragm that is prone to tearing. The stock DV also isn't meant to handle boost pressure levels higher than original.

 

No the diverter valve is not stock. It is made by XS Turbo and it has a spring, not a diaphram. I guess I can put some shims behind the spring to stiffen it up and see if that helps. Any other suggestions?
Thanks!

 

even though the coils are newish, it could still be them, i had a set of coils go after around six months, now its been a little over a year and i think im loosing one...they are a pretty common problem. also when it starts to miss just keep your foot in it, if it is the coils it'll throw a cel and youll be able to tell which one or ones it is, since you said its pretty much undrivable, i'd say its more than one.

 

ok, I have some extra coils. I'll check that out. Thanks

 

did you ever figure out what the problem was?

 

Ok, so I am narrowing down the problem.

1) Changed the coils and checked plug gaps. No difference.
2) Installed some shims in the diverter valve to increase the pressure required to open it. No difference except the valve does not open quite so swiftly when you take your foot off the gas.
3) Tried unplugging the MAF and once more it behaves very well with default values rather than MAF feedback. Car is driveable under boost.

My conclusion is that there is an intermittent connection between the MAF and the ECU or the ECU is bad. But the ECU controls the engine well without the MAF, so I am tending towards the intermittent connection.

Does anyone have a wiring diagram for the ECU to the sensors? Or where might I find that. I only have the Chilton Manual. Is this stuff in the factory manual?

Thanks for the advice so far.