a4 problem air delivery

**richard3009** · 09-07-2008,10:47 PM

Audi A4 1.9TDI 110HP 1997

Having had the diagnostics done it indicated air mixture problem, checked out all my intercooler hoses they are fine, where to next?

Would the diagnostic have fagged up a failed turbo?

Uphill it smokes like crazy its like driving into a gale force wind, fuel consumption is through the roof.

What can i try next.

**Crasher** · 09-07-2008,10:54 PM

No fault codes and smoke says a boost leak to me. There were absolutely no fault codes stored?

**richard3009** · 10-07-2008,11:02 AM

Yes there was a code, the mech mentioned something about vacuum and to check the intercooler hoses, it sound really blowy the engine its hard to describe the noise sound like a large fan that increases proportional to revs.

Could the inlet manifold be leaking and how do i check it?

In which areas is a serious boost leak going to occur, wer'e talking about major power loss and getting around 20mpg.

**Crasher** · 10-07-2008,11:12 AM

First of all you need to step back and get the fault code he recorded otherwise you could be chasing your tail round in circles. The inlet manifold itself on an AFN engine can’t leak as it is solid cast ally and a leak from the head to manifold is unlikely as long as the gasket has never been compromised such as being removed and then re-used.

**richard3009** · 10-07-2008,01:00 PM

I dont have a record of the fault code im afraid, boost solenoid has been mentioned what does this do exactly

**richard3009** · 10-07-2008,01:08 PM

One more thing what is the second sensor for on the boost circuit the one after the intercooler mounted on the rigid plastic pipe just before the air enters the engine

**Crasher** · 10-07-2008,01:36 PM

The solenoid you will have been told about is the Boost Pressure Limitation solenoid but is commonly known as the N75 valve and this is opened and closed by the ECU in order to apply a vacuum (generated by the vacuum pump on the head) that operates the turbo vane actuator which itself operates vanes inside the turbo to adjust the speed of exhaust gas flow over the turbo impeller and this is how the boost is controlled. The vanes are usually the issue; they become clogged with carbon and stick, generating code “00575 Intake manifold pressure Control difference” on your cars ECU, 17965 or 17964 on later ECU’s. A faulty N75, a vacuum leak or incorrectly connected hoses can also cause this code. If the N75 valve was faulty you would normally get “01262 Boost pressure limitation solenoid valve N75”.

The sensor on the boost pipe is the combined Intake Manifold Pressure sensor G71 and Intake Manifold Temperature sensor G72. VW also call them a “Thrust sensor”. If it was faulty I would expect to see code “00519 Intake manifold pressure sender -G71" but I have seen them stick at too high a reading, often 2880mBar and not leave a code.