T4 transporter limp mode issue.

My van is 2002 2.5TDI with ACV code ,recently it started falling over @ around 2.5K,feels like the brakes have come on,change up a gear and it pulls away again and has the same issue because of what I think is overboost causing the ecu to go to limp mode.
There is no question that the turbo isn't boosting as it can be heard till it gets to that RPM,had a friend follow me and he said it wasn't blowing out excessive smoke.
I have checked the wastegate actuator and it appears to be working,I replaced several small vacuum lines which were perished.
My mate that has a eurocar workshop suggests it may be the N75 valve or a blocked cat converter,he mentioned he would scan it for me but it may not be read as the exact problem.
It has a bad rear engine mount so I'am not driving it till that part arrives ,thought I might delete or punch out the cat and see what happens as the van has 240 km on it.
When I got the van I was really quite impressed with the power for such a heavy lump,I fitted a quality power box soon after and received a really good gain in torque and could certainly notice a handy increase in power.
Any help would be appreciated as I don't want to just start replacing parts to no result,the bad thing is the parts for the T4 in my country as close to impossible to find unlike the T5 and T6,have been on the transporter forums but can't find a complete solution to my issues,cheers.

 

Code scan would be best. Blocked cat would be underboosting. There is a small hose inside engine ecu to pressure sensor that tends to split, not related to your problem but worth a check.
On a lt35 acv found the pressure feed off turbo broken so gave a limp at 3k rpm

 

Thanks David,you are certainly correct on the under boost with a blocked cat but I thought they had a limited life and don't think mines ever been changed.
Could you please explain further "the pressure feed off the turbo " ? cheers.

 

n75 is fed with pressure from turbo to control waste gate actuator. The hose broke off so wastegate never opened and gave limp mode.

 

I will take off the intercooler and have a look as soon as possible,you could well be right,hose could be crappy,is the vacuum hose on the ecu external and just pushes on ?,I will pull the battery out and check today anyway.
What you say makes sense ,my test of the actuator was only to test free movement of the wastegate to make sure it wasn't mechanically sticking up as suggested on a T4 forum,apparently the spindle can wear and this will overcome the actuator movement.
The forum guy mentioned on high mileage vans a worn spindle for the wastegate generally requires the turbo to be replaced,don't expect this to be the issue with my van.
Will report back,cheers.

 

The boost sensor is inside ecu.
https://forums.tdiclub.com/index.php?threads/replacing-vacuum-line-to-ecm.181758/

 

Pulled the ecu (vw # 074 906 018 AL ) this morning and it has no nipple for a hose on the outside of the casing,don't see any point in opening it up @ this stage ???.
I think I will replace the house from the turbo to the N75 valve anyway as it looks a bit ratty @ the turbo end.

 

You have newer edc15 ecu, this has external map.

 

Oh dear not the result I wanted to hear ,appears you are referring to a different model ecu off another diesel VAG car.
Nevermind will move on to the next test,appears I can change the N75 valve for the N18 off the egr system , the valves are not identical but serve the same function and are a good test method for the interim and apparently have the same electrical plugs.
Rechecked all vacuum lines and they are ok enough,if I don't do the above I will get it scanned.

 

I believe n75 and egr valve are not the same, one works with pressure the other with vacuum, it would have worked if the turbo had variable vanes.

 

Yes appears that way however T4 forum says otherwise,whatever,been going into the system and appears I have an egr block off plate @ the top end of the pipe ,elsa shows gaskets only but I have a 3mm steel closed flange, question is the system is otherwise intact and will this throw a code with this block off plate in situ ???
Research indicates this single block off is a "hack job" to little result and issues from doing so ?,...when I got this van the past owner mentioned the code couldn't be cleared,think that's rubbish as believe the garage that did the last major service ( 241K kms ) are the ones that fitted the blanking plate,all the rest of the service history is VAG or approved centres so reckon I might be on to it.
Not for a minute thinking this will fix my issue but with all as per factory I should have a better baseline for a scan.
Will report back on progress,cheers.

 

Been picking away @ this issue,found the reason for the EGR block off plate,the diapham unit is buggered and the valve could never open,not something that can be replaced and the EGR feed pipe is now completely blocked,so will blank both ends and remove it.
Also found the EGR trunk from inlet manifold 40% bunged up with carbon so will clean out.
Ordered a hand vacuum tester so I can test operation of wastegate valve and other parts,will put the van off the road for a while as the lower engine mount I ordered came back as out of stock and I got a paypal refund

 

The egr can be blocked only on one side, no problem.
Get a pump that can create negative and positive pressure.
https://www.amazon.com/Powerbuilt-6...words=hand+vacuum+pump&qid=1623097142&sr=8-11

 

That's the same pump kit I have ordered should be delivered in a couple of days,found a rear lower mount finally and seller has stock but could only find one from the US so won't get it for a while.cheers

 

Been picking away @ this issue,put everything back together and issue still there,didn't expect to fix it with what I have done but worth a crack as they say lol.
Tested the N75 pressure control valve and it seems to be ok however it seems I have an issue with pressure from the turbo supply hose,with the engine running @ idle I have slight vacuum not pressure,I can only assume I have an air leak @ the suction side (inlet) on the turbo,which considering I have a bad lower engine mount this movement may have unseated the "o"ring on the plastic trunking to the air cleaner ??.

 

Turbo barely spins idle, no pressure there until engine is under some load.

 

Yep had a "blonde moment " there you are totally correct,cheers

 

Had to move the van to mow the lawns and it started well but under throttle it backfired and stalled,really not sounding right,managed to move it but not easy.
Got the lawns done and decided to have another go @ it,pulled the intercooler and found the turbo plastic inlet pipe loose on the turbo air side,the sealing ''o" ring was pretty much toast and to make things worse the PCV sealing grommet was siliconed together @ some stage and that just feel apart lucky the end didn't fall inside the valve cover.
Now I have something to go on ,cheers.

 

Want to add to this with a reply but comes up with an error message ??? can someone explain .