B4 limp mode

Ok, I've read all the posts about other people's problems with b4 limp mode under acceleration at highway speeds. I have had this same problem for a little while now and have been trying to remedy this.

What I have done so far is:
Replace N75
Replace all vacuum hoses (except i think the last segment of the blue one that goes to the turbo, i couldn't get down to take it off on the turbo end). I also wasn't able to replace the one that goes into the firewall near the heater hoses (anyone know what that one is?)
I removed and cleaned the vacuum source hose that goes to the brake booster and verified the check valves are clear.

I just installed a boost gauge to see what was happening with my boost. The car is stock.
It has an aftermarket (generic midas, etc [yuck]) exhaust and it just had the intake manifold replaced under the recall.
With the gauge installed (t'd off the line that goes to the ecu) I have been seeing sustained 20psi under WOT (SIC) and when I hold it at 20psi in 3rd for a few seconds, that's when the limp mode kicks in. After limp mode, i see 5-8psi under full load. Recycling of the ignition restores full boost. I am thinking that I am somehow seeing too much boost as 20 psi is more in the range of chipped tdis. It doesn't feel like i'm getting 20psi of power (probably fueling). I also just replaced the fuel filter.
Does anyone have any ideas as to what my next move should be?
I had initially thought that the exhaust was too restrictive and the turbo didn't have enough pressure differential to make enough boost but now it seems that i'm getting too much boost. Could those last 2 vacuum lines be the problem?
I would appreciate ANY advice that anyone could give me as I am pulling my hair out over this at this point.
thanks
greg

Check the hose to the wastegate on the turbo. also check and see that the wastegate actuator rod is free to move. this can be done from below. not much fun I've done it. If you come in from above you will need to remove the plumbing from the air filter housing to the turbo. this should give you enough room to work.

20 psi sustained is even high for a chipped car. /images/graemlins/eek.gif

Back to basics: your wastegate is not opening when it should. This means that either it is stuck closed or it is not getting the message to open. IIRC the "blue" hose is the one that tells the wastegate to open so maybe that's your problem. You may end up having to replace that one.

Follow GoFaster's advice on diagnosis and hook the boost gauge up to the blue hose to see what signal the wastegate is seeing while driving. This may help. /images/graemlins/smile.gif

The one that goes into the firewall by the heater hoses is the one that tells the ECU how much boost is actually being generated. Usually a failure there will be an underboost situation, so it's probably OK. However, there may be a failure in that one, because there's no way a stock ECU would allow that much boost without forcing limp mode. Though, if the blue hose has a problem, the turbo might not be getting the message.

[ QUOTE ]
The one that goes into the firewall by the heater hoses is the one that tells the ECU how much boost is actually being generated. Usually a failure there will be an underboost situation, so it's probably OK. However, there may be a failure in that one, because there's no way a stock ECU would allow that much boost without forcing limp mode. Though, if the blue hose has a problem, the turbo might not be getting the message.

[/ QUOTE ]

Isn't the one that goes to the ECU the one that tells the computer how much boost is actually being generated? This seems like it's a vacuum hose, as it goes directly from one of the check valves on the vacuum source to the firewall.
I will try to replace the one last hose (blue one) and see if that helps. Unfortunately, I don't have an easy way of getting under the car. Should I also try to lubricate the wastegate actuator? Is that something that can be removed/replaced? Thanks!

No, you're looking at something wrong, because there is no vacuum involved anywhere in the boost control system on these cars - the whole thing operates under positive pressure. Vacuum is used for operating the EGR valve - a different subsystem. Have another look ...

Black hose from upper intercooler pipe to ECU is boost pressure to the sensor in the ECU; a leak here will cause ECU to read lower than actual boost pressure and if condition persists, will cause limp mode.

Red hose is positive pressure straight off the turbo compressor - it's the feed hose for the N75.

blue hose is wastegate control signal from N75.

Black hose from the N75 valve is a vent hose to atmospheric pressure.

[ QUOTE ]
No, you're looking at something wrong, because there is no vacuum involved anywhere in the boost control system on these cars - the whole thing operates under positive pressure. Vacuum is used for operating the EGR valve - a different subsystem. Have another look ...

Black hose from upper intercooler pipe to ECU is boost pressure to the sensor in the ECU; a leak here will cause ECU to read lower than actual boost pressure and if condition persists, will cause limp mode.

Red hose is positive pressure straight off the turbo compressor - it's the feed hose for the N75.

blue hose is wastegate control signal from N75.

Black hose from the N75 valve is a vent hose to atmospheric pressure.

[/ QUOTE ]

Brian, as always, thank you for your help.
I thought I had read in some other thread that a leak in the vacuum system would be a possible cause of this. In any event, it looks like that final hose (blue) as well as the actuator should be my next step. Can I just clean the actuator/wastegate assembly or should I replace it? Can I do this without removing it from the car? If not, how difficult is this to take out? Does anyone have a part number offhand if this should be replaced?

I'm sorry, I think I confused you. Not a surprise, I confuse a lot of people. /images/graemlins/laugh.gif First off, I'm surprised Brian doesn't just post this link over and over for boost issues. How to troubleshoot low boost

I know it says low boost, but everything in it would apply to any boost related limp mode. I put together a picture at the bottom of the page to identify some of the various different culprits. The hose to the ECU is shown in purple lettering on that image. It heads toward the firewall from there, so I assumed that's the one you were referring to.

The wastegate itself is usually a matter of working it back and forth by hand, and then applying lubricant to any pivot point you can see. In most cases I don't think it's available as a separate part from the turbo.

Edited because I re-read and saw you replaced the N75