a4 problem air delivery

[richard3009]

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. :zx11:

[Crasher]

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

[richard3009]

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]

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]

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

[richard3009]

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]

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.

[richard3009]

how about this for a makeshift boost leak test

http://www.myturbodiesel.com/1000q_how_to/multi/boostleaks.htm

[Crasher]

As long as you don't exceed 0.5 bar. The proper VW kit is £400 odd, I have just ordered one today.

[richard3009]

Any pictures of the N75 valves location and any way i can test it?

[richard3009]

there is a solenoid mounted in fresh air at the front of the engine with some thin pipes going to it, if i turn the ignition on without starting the engine this rattles away like a little ticking machine gun.

Could this be my culprit

[Crasher]

It is right down at the front of the engine in front of the turbo on a bracket. The valve is tested using something like VAG-COM in Output Tests 10 where it should click and you test the passage of air through the valve using a hand vacuum pump. Without VAG-COM all you can do is check the vacuum routing and measure the resistance of the valve which should be between 14 and 20 Ω.

[richard3009]

there is a solenoid mounted on a bracket in fresh air at the front of the engine with some thin pipes going to it, if i turn the ignition on without starting the engine this rattles away like a little ticking machine gun.

If this solenoid was faulty could it cause such a loss in power and smoke from the exhaust

Could this be my culprit

[Crasher]

If it was faulty, yes it would cause those problems but if it is rattling away it doesn’t sound like it is faulty.

[richard3009]

its normal for it to be ticking away sounding like an insecticutor then

[Crasher]

On some software levels they will tick with the ignition on and the engine not running as they would if the engine was running.

[richard3009]

had a good look at the intercooler circuit from the turbo through to the manifold nothing obvious, and all the small pipes seem to be in order , however the rigid plastic pipe (aprox 20mm in size) that comes from a circular thing on the rocker box cover and then drops deep into the engine has a split in it just behind the jubilee clip, is this anything to do with the boost circuit?

[Crasher]

That is the oil breather hose from the crankcase pressure regulating valve. It being split will allow in a lot of unmetered (and unfiltered) air and seriously upset the running of the engine. You should replace this before doing anything else. The part number should be 028 103 493 AF and it costs £12.70 from Audi but check before ordering.

[richard3009]

Looks like its an absolute sod to fit, goes deep into the center of the engine, where does it go to and how is it retained

[Crasher]

Which side do you mean, the one going down to thee turbo or the one going down the other (left hand) side of the engine? If it is the side down the left, that is the wrong part number I gave you.

[richard3009]

If you look into the engine bay from the front of the car, it goes from the rocker cover valve to the right and down into the engine

[Crasher]

That is the one to the crankcase breather rather than the rocker cover breather but still important. I thought it was the one down to the turbo inlet.

[richard3009]

Strange the car ran on full boost on the way home from work tonight, plenty of power slight hesitation on hills but still pulling.
The only thing that was different tonight is that it was raining.
Could this rule out an air leak and point more to a component fault.