3 Broken N75 and Overboost.

[Waris]

Audi A4 B7 2.0TQ

I have 3 broken N75 valves and all of them have broken the same way. The hose connection plug on the N75 that connects N75 to wastegate comes off causing overboost and limp mode. Why does this happen? Does the wastegate actuator create vacuum that could pull the hose connection plug off on the N75?

Important information about the car
- Ko3 hybrid
- Upgraded HPFP internals
- Wastegate rod has been adjusted to meet requested boost at higher rpm.

Upgrades have been done by the previous owner.

[Smac770]

https://www.ecstuning.com/b-genuine-...ve/06f906283f/

The connection to the wastegate actuator is the plastic post sticking up sideways towards the side with the connector plug. You're saying that whole gray plastic end is coming out of the solenoid body?

[Waris]

Yes exactly.

[Smac770]

That seems extremely odd. Never seen anyone post of such a problem. But I frequent the B8 forums, not the B7. But it's the same N75 p/n.

Turbo boosting means vacuum at the turbo inlet and boost at the turbo outlet. The N75 simply toggles between the two, connecting both at a high rate of switching to the wastegate actuator depending on if it wants more or less movement of the actuator rod. Vacuum is not going to "pull the plastic out", but excess boost levels inside the solenoid might push it out? Still, people run stage 2 on these things, so not sure what's jacking them on your vehicle.

Are these Audi OE units, Pierburg OEM units, or Chinese knock offs?

[Waris]

All of them have been pierburg units.

I’ll add some extra info.
2 weeks ago when I noticed that the pin had come off again I decided to check all the hoses
leading to N75 to see if they were clogged. All were good. I didnt have a new N75 solenoid but I had fixed a old unit by gluing the pin on. Started the car up everything held up until revved up to 4k on neutral and it popped off.

A week ago me and my dad noticed that the wastegate rod had a slight bend. https://imgur.com/a/qfgIKne Link to pic there.
The wastegate rod shouldnt be bent right? Would a bent rod cause the pin to pop off? Probably not as they are not connected?

We had an desperate idea to adjust the wastegate rod. The idea behind that was maybe the pin comes off because wastegate movement max out before wastegate actuator movement, but the actuator wants to move the rod further causing vacuum that pulls the pin off. (Would that even create vacuum? im not good at physics) (I dont even fully understand is there vacuum or boost in the hose?) So we loosened the adjustment screw by 2 full turns so it makes the rod basically shorter.

Idk if what we did makes sense, but after doing that the pin stayed on for about 200km. Might have been pure luck.

Sorry for the bad explanation, english is not my native language.

[Waris]

Forgot to mention it was a glued unit that held up 200km after adjusting the wastegate.

[Smac770]

Rod shouldn't be bent. But none of that matters to the N75 literally falling apart. There's no physical pulling on the plastic end with the two ports. The N75 passes the vacuum and the boost, the actuator does not generate anything, it only receives whatever the N75 sends it. I think the config simply has too much boost for the units. Or something is so hot it's melting the units.

You keep calling it a "pin", you mean the plastic end with the dual ports, yes? I've never taken my N75 apart; does it appear the gray plastic ports cap is simply glued into place? I think heat would be the concern then. Combined with high boost present inside, maybe that pushes the ports out past the weakened glue.


As I said before, there's both vacuum and boost coming to the N75, by way of the connections to the turbo inlet and turbo outlet. The ECM varies the exposure of the wastegate actuator between the two pressures by varying the duty cycle, depending on if it wants more or less rod movement. Vacuum to hold closed the wastegate position (maintain boost; high N75 duty cycle), pressure to open the wastegate position (less boost; low N75 duty cycle). The failsafe position (aka 0% duty cycle) connects the boost side to the actuator, that way it self regulates, whereby high boost would actuate the wastegate, reducing boost.

If you put a pressure hand pump on the actuator, pump it up and you should see no movement of the wastegate for the first 4 psi, then the rod should move from its rest position to 10mm traveled by time you get to 10 psi. Exceeding 11 psi can damage the actuator.

In physics, vacuum doesn't pull, pressure pushes. Vacuum just means no pressure. But vacuum alone will not pull something; vacuum just means no impediment to being pushed. There must be pressure behind to push. https://youtu.be/V1N6lnm6D5w Notice with a vacuum on both sides, no water moves into the syringe. Once pressure is restored "in the back", it pushes the water into the vacuum since the vacuum is not pushing back with equal force.

[Waris]

Yes the gray part where the hose attaches to. And it breaks from away from the body. (See picture)
https://imgur.com/a/PuTj1D9

So what would be the move here? Wastegate has been tested and it moves atleast 10mm. There isn’t anything hot close to the n75 other than the engine and one rubber hose above it seen in this video at the start.
https://youtu.be/Bo5ZN55d3-M?si=emzD2XpLq1eDKsav

I have bought 6mm (not oem) hose, should i try replacing old hoses with new?

[Smac770]

Oh, so it's just the one port that's breaking off of, at the gray plastic? Not the whole gray plastic end coming out of the black housing. A picture of the actual broken unit would likely have been more informative.

Yeah, maybe the install is simply under too much stress. Making sure that hose comes off straight before bending, and not yanking on the port laterally would be a play. Notice how in the video that line angles over right at the end of the port. Because I doubt the clamp can be overtightened to the point of cracking the plastic. But maybe that's what is going on. Or the clamping is twisting the the port and initiating a fracture?

If you did install a new solenoid, I'd cut the clamp off after a week and inspect the gray plastic closely for hairline fractures or other evidence of deterioration. If it looks clean, reclamp it and maybe check after a month. If still good, should be fine?

You just want to make sure the hose is rigid enough that it doesn't collapse under vacuum, being not OE. The vacuum won't be real strong; you could put a vacuum gauge on the line from the turbo inlet to the N75 and see what the vacuum actually builds up to when the turbo is under full action.

[Waris]

Lateral stress actually might be the reason the port is coming off. I doubt clamping would cause a fracture. I forgot to mention but when the port lasted 200km I didnt have clamp on it. Stayed on fine.

This weekend i will put on a fixed or a new N75 and the 6mm hose that i bought. I doubt that it would collapse as it is sold as Fuel line / Vacuum hose so i think it is going to be sturdy enough.

[Waris]

Replaced broken N75 with old fixed one and replaced the hose going from N75 to wastegate with flexible 6mm SILICONE hose. Port has stayed on for 200km and car boosts normal i think.

I think the port used to broke off because i have bad engine mounts which causes vibration. Non flexible hose and bad engine mounts that vibrate and flex have put stress on the port breaking it.