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Zero boost on 1.8T AEB

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18t aeb boost zero
4.6K views 12 replies 4 participants last post by  Kdalt  
#1 ·
Most of the time I can find my answer here with a search but this one has me stumped. I have zero boost from the turbo. I've check the normal things like the DV, boost leaks (small one at check valve for brake booster), waste gate and wastegate actuator. I have not completely removed the turbo yet though. The vacuum lines are good and I went through the vacuum engine diagnostic write ups and they were spot on. Aside from the boost problem the car runs like a top.

So I've narrowed it down a bit. On the intake side of the turbo I think I have a decent amount of oil coke to clean out. The turbine fins are mostly clean though and it spins freely with no contact. There is no play in the turbine shaft in any direction. I can hear the wastegate flapper moving when I blast air into it.

I pulled the N75 valve off and it may be my issue. When I apply voltage to it I can hear the valve click open and close. Pushing air through it without voltage diverts it down one side completely. After applying voltage it clicks but it seems half the air is coming out one side then the other.

Both the puck valve and the DV (710N) are new. Anyone have any other ideas? I can post a picture of the intake side of the turbo if anyone wants to see it.
 
#2 ·
It seems to me that this valve is working properly. If it alternately passes from one side to the other. It cannot pass air all the time, or it will open the wastegate valve.
I do not know if you can see it or not, but if the wastegate is somehow open, then you would get no boost. It could be open because of a failure, or it could be open because the n75 is passing air (that is vacuum) pulling it open. The actuator rod on the turbo must move to open the variable diffusers thereby allowing boost control. If they are closed then no boost. See if you can see this actuator arm moving while revving the engine.
 
#3 ·
I've got the car so far apart at the moment I cannot put the N75 back on to see if the actuator rod moves. It shouldn't move much at all if the car isn't in gear anyways. I may not be describing what my N75 valve is doing properly though. The airflow is coming down to the N75 when it is fully closed with zero voltage and halfway open with 12V applied (IE it doesn't seem to fully switch the flow path through it). I don't know what the normal operation of the N75 should be. Should it be one path or the other or should it be one path without voltage and two paths like a Y with voltage.

If it works the way I think it might it wouldn't have any voltage to it until boost is needed. It would then get the 12V pulse signal and divert air to engage the wastegate actuator. If it doesn't fully switch with voltage then there isn't enough air or pressure to actually engage the wastegate actuator.

It could work the complete opposite way and always has 12V applied to it and the pulse is actually comes from going to 0V. If I can figure that out then I can rule it out as the culprit or move on to pulling the turbo out and inspecting the exhaust side and the wastegate flapper (though I can clearly hear it opening and closing as it should with the air compressor blasts).

I should add with the coked up intake side of the turbo I am not getting smoke or massive oil build up in the intake pathways. I check the hose right at the throttle body for oil and deposits every 3000 miles and there has never been more than a very thin film I can wipe out with my finger.
 
#4 ·
Post a pic of the intake side. Coking is usually a term for oil that has been cooked to hard small bits. Is that what you are seeing? That would not be good.

I think you N75 is working properly, but check with this link to be sure:
quattroworld.com Forums: N75 Wastegate Frequency Valve (WGFV) location, function and PN info

Check the Bentley as well. There may be a resistance test although if there is an electrical issue the ECU would catch that.

Check MAF and air filter also.

Also, yes you are correct, not much will happen unless you are in gear. Load needs to exist first.
 
#5 ·
Thanks for that link. I think the N75 is fine after reading that. I checked the resistance on it last night and it was right where it should be around 28 ohms. The MAF was spotless but I will clean it in any case. Air filter is brand new as well. I should add that I haven't gotten a single CEL from anything on it in quite awhile.

I had replaced the bleeder valve in the crankcase breather assembly just a couple of weeks ago when this started. It was pretty nasty but still seemed to be operating. I replaced all but one check valve (except brake booster 3 way valve) and installed a new suction pump. The metal breather hose has some build up in it as well so I will clean that out. The vacuum lines looked clean and I don't have any oil leaks anywhere.

I'm thinking the turbo is shot somehow in a way I've never seen one fail. I took a couple of pics and zoomed in and can now see some very slight turbine damage on the intake side. I've always thought if it was an oil debris contamination problem it would blow the seals and the bearings would be damaged. If the bearings are damaged then I should be seeing some shaft play and possibly turbine contact with housing. In so far as the intake side goes I don't have any movement and it spins freely but it definitely is not clean as I think it should be.

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#7 ·
If you think the N75 is the cause of the problem, bypass it completely to verify that theory.

Pull and cap the hose going from the N75 to the waste gate diaphragm. Test drive the car carefully. In this configuration there will be no boost control at all so the turbo will always be trying to boost as high as it can. If you put your foot into it and you suddenly boost to the moon, you know it was something with the N75 causing the problem. If you still have no boost, you have a problem elsewhere.

I suspect a waste gate failure. Does or did the waste gate chatter when you let off the gas? That's a symptom of a gate going loose. If the gate doesn't seal, no exhaust pressure builds on the turbine to spin it.
 
#9 ·
I'm doing the timing belt and coolant flush and fill right now. I think I will try it with the N75 disconnected tomorrow if I get it back together. I cannot think of another way of knowing for sure. I am wondering if I have a short in the N75 wire coming down but I would think that would produce a CEL or something atleast? As for running it without the N75 connected to the actuator will it build boost without it in gear?

As for the wastegate I don't think I've heard it chatter before. As far as I can tell the rod is as stiff as it can be. I believe I can hear it open and shut when I blast the actuator with a shot of compressed air. It doesn't sound like it is flapping around in there to me.
 
#8 ·
A blocked exhaust system can also kill boost production. When my CAT core broke apart and clogged up my exhaust, my boost and power went away.

Do you have a boost gauge? What (if any) boost numbers are you seeing? How did it used to boost/behave? as this change sudden or gradual over time? Codes?
 
#10 ·
Now that is something I didn't even think about. I do have a boost gauge and it never gets past the zero between vacuum and pressure (I actually wondered if it was broken somehow but when I did a boost leak test it registered 10 PSI). As sad as it sounds I'm not 100% sure the power loss was rapid or gradual. I lived downtown in a city and there would be months I would never get over 35 mph. I'm leaning towards gradual though and right now I can barely climb some steep hills here without pressing the pedal down to the stop point before the kickdown switch in while going down to 2nd or 3rd gear.

I haven't had any strange noises, no real amounts of smoke, no engine or transmission codes, no strange idling, and no rough revving. Just bad MPGs and massive loss of power.
 
#11 ·
I'm on a different computer now and can see your turbo pictures better. Are those compressor blades showing leading edge erosion?

The N75 bypass test has to be done driving the car. Revving these engines with no load will never get them into boost.

I think you're right that a short or open in the N75 circuit should pop a code. Even if your N75 was bad, you still should be achieving gate pressure. 4 psi or thereabouts.

Verify intake path pressure holding integrity
verify diverter valve function
bypass n75 test


Since you've got everything apart for the timing belt job, now would be easier to yank the turbo for inspection or replacement.
 
#13 ·
Ok. The weather has not been cooperative enough to go outside and work on this thing. My carport doesn't have a door or very good lighting. I ended up pulling the turbo. As far as I can tell it is just nasty all way around. I suppose it failed and I just caught it before the bearings got really bad. It looks bad but the wastegate is still there and operating normally and the shaft has no play in it. I got a line on a new K03S with warranty for $400 with all the hardware. I've ruled out everything else I can think of.

I will probably clean the old one and try rebuilding it for kicks if I don't find anything majorly wrong with it. Anyone know what would be safe to try and clean the coke and sludge off of the inside of this thing? I'm thinking throttle body cleaner and nylon brushs perhaps.

On that same note I'm going to pull the oil feed and returns to replace their seals but also want to clean them for any possible blockages. I'm thinking throttle body cleaner and a very low pressure air blast. They aren't actually that old the dealership replaced them about 20,000 miles ago. But any advice on doing that while I am in there?