very low boost. Pressure deviation low. specific N75 question im confused

ok 2001 Alh 280,000 miles. Intake has been replaced once in its life. Turbo is original. I have checked the vac system and N75 valve. I replaced the N75 with a good working one and this is what im finding. If you log with VagCom it says that the % for the N75 should be around 45%-97% or so with the car idling. This car is at 24% at idle. My question is what would cause the N75 to have a lower spec then what its supposed to? Im finding also that the Vane Actuator is not moving at all when I run the VagCom test. I thought it may be the actuator so I replaced with a used one with no change. Im thinking it has something to do with the low % the N75 is putting out. When you run down the road full throttle it will only boost to 1150 mbar when the ecu is requesting 1900 mbar. Another thing I was thinking is stuck turbo vanes, when I remove the actuator rod should the lever on the turbo that moves the vans move completely free or should it have some resistance when pulling on it? This one just flops back and forth so I was thinking the vanes could be stuck part way.

Sorry if its a little hard to follow. I have checked all the normal things and im running out of patience with it. Hoping someone can answer my questions about the turbo vane lever and N75% really so I can continue on with my diag.

Thanks in advance.

You should be able to apply a vacuum to your turbo actuator to check movement. At 5 psi it should start. To move, and by 17 to 20 psi you should have full travel. You should have fairly easy movement of the turbo vane lever if you disconnect it from the rod. You may find there is wear on the internal vane ring and the lever arm causing the vanes to not fully travel.

Thanks for that. I will double check my operation of the actuator to be sure its working with a vac pump hooked up. I did a little more digging and it appears Im having a vacuum problem after all i think. I cannot get vac out of the N75 when commanded on from vagcom and the vac canister does not have any residual in it when I remove the line after the car has been running. I have vac coming from the pump and check valve on the booster line but it gets lost after that so I guess I need to trouble shoot from there. Could the EGR solenoid be bleeding off the vacuum before it can get to the N75? I see the first vac line goes from the pump to the EGR then out and to the rest of the lines.

OK so I found out that the EGR solenoid is at 84.5% duty cycle at idle so its above spec and the N75 is at 24.7% below spec. What would case this? Vac leak?

You can swap the N75 with the N18 valve, they are very close to each other on the firewall above the brake booster and toward the middle of the car. Just be careful with your hoses coming off of each one and relocating them on the other.

You can take the N75 out( mark the vacuum hose locations) and check air flow through the ports, then apply 12 volts to the valve and listen for it to actuate, and see if the air flow changes from one port to the other.

The EGR duty cycle is independent of the turbo actuator.
I would check the actuator at start-up and see if it moves (2.5cm?)
If not check to see if the hose is providing vacuum.
If not check where that hose exits the N75 and see if the unit produces vacuum.
There is a large vacuum hose that's runs to the brake booster which often seems to fail, causing low vacuum in the sytem. If you still have the factory clothe covered vacuum hoses, you may want to replace them

When that break booster vac fails. You get a hard brake pedal.

I get no vac from the N75 valve. I have vac to the valve but it will not put out and move the actuator. I have good vacuum from the vac pump and my main line to the brake booster is good. I have replaced the N75 with a know good one and with either N75 the value is always 24.7% at idle when it should be around 40%. So I have gotten to the point I know the valve is good but im just not getting the correct amount of vacuum. My hand pump broke today so hopefully the new one will be here tomorrow and I will check all the lines for leaks. Does anyone know if low vacuum would cause a low value % at idle for sure? Also what are everyones thoughts on the vacuum canister possibly being bad? Not sure if it needs the extra vac stored in the can when the engine is running or not. I would say this is for sure a vacuum problem but its just funny how much of a pain its being, I swear I have gone over each line 10 times now lol.

Thanks
Cris

Check the "t" fitting going to the N-75. Mine was clogged, it would produce vacuum but not enough volume could make it through as you describe. I had to use a small drill bit to open it up. It works great now.

johnsongti said:

I get no vac from the N75 valve. I have vac to the valve but it will not put out and move the actuator. I have good vacuum from the vac pump and my main line to the brake booster is good. I have replaced the N75 with a know good one and with either N75 the value is always 24.7% at idle when it should be around 40%. So I have gotten to the point I know the valve is good but im just not getting the correct amount of vacuum. My hand pump broke today so hopefully the new one will be here tomorrow and I will check all the lines for leaks. Does anyone know if low vacuum would cause a low value % at idle for sure? Also what are everyones thoughts on the vacuum canister possibly being bad? Not sure if it needs the extra vac stored in the can when the engine is running or not. I would say this is for sure a vacuum problem but its just funny how much of a pain its being, I swear I have gone over each line 10 times now lol.

Thanks
Cris

Click to expand...

r72gsaol

Thanks for that reply. I did check those last night and found one clogged going to the N75. cleaned it out and all the others still no dice. I think im going to replace all the vac lines so I know I have no leaks for sure and see what happens. Never had a vac leak this hard to find lol. If only I had a smoke machine. I think I will try to swap out a good vac reservoir as well.

24.7 % is the same value what i have, and I'm currently having boost problems my self. did you manage to fix the problem?

As am I ,The issue that johnsongti sounds excatly like my issue.

I feel like i ahve done everything, to no avail.

Put the vac gauge on the output of the n75 and cover the bottom (Rtn to airbox) fitting with your finger. If it builds vaccum, the valve is usually toast.

Also use a vacuum pump to check the actuator holds vacuum and the arm going into the turbo starts moving at 4 inches vacuum, full movement 18 inches, and moves smoothly through the range.

You can also take the actuator off to check for rust inside it. (VW bulletin about this)

While its off move the arm going into the turbo to see if it moves smooth.
If not move it back & forth for around 5 minutes to see if it clears it up.

Tie a piece of thread or dental floos to the clip when you put it back on so you can find it if it flies.
Ask me how I know to do this.

I'm wondering if the N75 is only at 24% because the ECM has "shut it down" due to the original low boost code. Have you reset the code? If not, try it and see if the N75 duty cycle goes up...

Also, have a look at the small nipple where the small vacuum line system begins, just after the vacuum pump. Rust from the actuator has been known to move up the vacuum lines and restrict this opening, the same as r72's T did. That can be a point of low volume as well.

-Rich