Can anybody make sense of this for me?!

[chud]

1997 A4 1.9 TDi AFN Quattro:

Car runs great and has normal power until 70-80mph then hits limp mode. I've cleaned out the turbo so no sticking VNT, swapped N75 and replaced all vacuum hoses and blanked off EGR.

After it hits limp mode I get '000575 Intake Manifold Pressure - 17-10- Control difference - Intermittent'

I measured block 011 in VCDS between 2500rpm and 4000rpm in 3rd gear and got this:



Shouldn't the Actual MAP follow the Specified and the duty cycle boost valve % increase with RPM?

Any help greatly appreciated!
Thanks

 

[aragorn]

N75 duty will vary with boost pressure and rpm.

Your log suggests that the engine makes no boost whatsoever, which makes no sense!

 

[chud]

Solved it! I had a boost gauge connected to the pipe that runs to the MAP sensor inside the ECU. The boost gauge was showing 26psi but the MAP sensor was only reading atmospheric pressure and not correcting the boost levels. I took the boost gauge out and fitted a new pipe and hey presto, no more limp mode!

 

[aragorn]

26psi is waay high!

Presumably because the ECU couldnt see any boost, it kept the N75 closed and effectively requested everything the turbo had to give.

Probably worth trying to T the boost guage back in properly?

 

[chud]

Presumably because the ECU couldnt see any boost, it kept the N75 closed and effectively requested everything the turbo had to give.

That's how I interpreted it too.

After a quick blast this morning before work the actual MAP just about matches the specified so im not sure where the gauge was getting 26 psi from? It was the cheapest one on eBay so it could just be naff.

Would putting it between the N75 and the turbo actuator give the correct boost pressure?

 

[aragorn]

it was running 26psi because the ECU couldnt see the boost. On a working engine, as the boost climbs to the target pressure, the N75 adjusts the turbos wastegate to maintain the desired pressure.

Because the ECU was only seeing atmospheric pressure, the N75 would have been constantly demanding "more boost" from the turbo, as the ECU tries to get the pressure up to requested. As a result, you ended up with 26psi.

The guage should work fine where it was, perhaps there was a blocked pipe or it was just badly installed?

 

[chud]

I think i've realised my mistake. I recently replaced the vac lines with 3mm silicon ones but the boost gauge came with its own tubing which is a thicker diameter. Presumably the vacuum found it easier to go up this pipe to the boost gauge and not carry on to the MAP sensor?

Thanks for your help with this Aragorn!

 

[aragorn]

Unless the boost gauge was leaking, both ports should see the same pressure.

Yes the larger pipe might "see" the pressure momentarily quicker as theres less resistance, but it would quickly balance out.