jgamelsky

1998 A4 1.8t quattro
136k miles

Symptoms:
Loss of power during a recent trip. Going up a steep grade the car was really struggling.
Did a VAG and came up with codes for N75, charge pressure and throttle body.

17956 - Boost Pressure Regulation Valve (N75): Open Circuit
P1548 - 35-10 - - - Intermittent
17965
17967
17973

So far I have changed Diverter Valve as it did not hold vacuum and changed N75.
Current CEL is only for N75 ( even though part is new).
Car idles ok, but stumbles on way back to idle if given some throttle.

I have checked the wastegate actuator with a bicycle pump. The rod moves and returns to position but I do not think it is actuating upon WOT.
I removed the intake tube to the turbo and the vanes spin freely (not seized).
I need some help figuring out where to look next.

Thanks

Jeff

turbo510

If you have access to a boost pressure gauge, "T" into the short line to the fuel pressure regulator, and see what boost you are getting. At that mileage, I'd suspect the wastegate isn't closing. Happened to our Passat.

jgamelsky

"At that mileage, I'd suspect the wastegate isn't closing. Happened to our Passat."

Did you have to pull the turbo and replace the wastegate actuator or was it something else preventing the wastegate from opening?

Thanks,

Jeff

turbo510

In my case, the wastegate never closed because the hole in the turbine housing for the lever wore too much. Sometimes the wastegate's stainless steel disk valve actually burns away, or comes off completely. The best way to check this out, I think, is to disconnect the CAT at the turbo flange, and move it back enough to look in there. The disk is supposed to be pre-loaded against its seat, so if the lever's "bushing" is worn, you can get more life from it by readjusting the jam-nuts on the push rod. You've got to take the turbo out to do this. Some time ago I posted about this problem, and showed some pictures.

jgamelsky

I pulled the cat off and looked at the turbine. The wastegate flap looks normal as does the turbo. That led me back to the N75 error and I discovered a wire to the N75 was indeed broken. Soldered that back and that error is gone, but a new CEL appeared - MAF - low signal. Car still has no power.

Can a bad MAF limit the boost? I've tried disconnecting the MAF but it does not help.

I've ordered a boost gauge but it has not arrived yet. I am also going to do a boost leak test. Any other thoughts?

turbo510

Did you check that that wastegate was tight against its seat? Does the turbin spin easily? If the wastegate disk is loose, exhaust takes the easy way out and the turbine doesn't spin fast enough.

I think that a malfunctioning MAF sensor will cause the ECU to limit boost as a safe mode.

jgamelsky

Yes, the wastegate was seated correctly, and the turbo spins freely. There did not appear to be any damage to the turbo vanes.

I rigged up a boost leak tester and there do not appear to be any leaks. I had it up to 10 psi and no leaks.

I am now waiting for the boost gauge - should be delivered tomorrow.

jgamelsky

So, I installed the boost gauge and a new MAF. The MAF was bad, as now there are no error codes or CELs, and it idles a little better.
The new gauge shows 20-25mm Hg vacuum at idle but when I rev the engine I get to about 0 mm Hg or maybe 1-2 boost max. It appears the vacuum is good but the turbo is not boosting. What boost should I see with no load and 3k rpm ???

I hear a slight mechanical slapping when I rev the engine and let is back to idle. I'll have to pull the turbo next. I think it is gone.

turbo510

Don't expect boost by revving in neutral. The turbo works by capturing energy from the flow of exhaust, and throwing a rev won't give enough to accelerate the turbine. Only check boost when the engine is actually doing the work of moving the car.