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VW B5 Passat TDIs This is a general discussion about B5 Passat(>98 (2004-2005 in North America)). Non TDI related postings will be moved or removed.

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Old February 28th, 2013, 08:18   #16
oilhammer
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You can keep throwing money at them, but they are not going to figure it out. If the boneheads cannot figure out a simple boost problem on a system that is essentially been in use since 1998 what makes you think they'll figure it out today?

I'd get it to someone who actually knows what they are doing. These dealers floor me sometimes.

The boost control system is DIRT SIMPLE. Like, stupid simple. And on the BHW, it is even simpler than some of the others, because unlike the ALH, BEW, BRM, etc., the BHW only has ONE vacuum circuit.... to the turbocharger! That's IT!!! The EGR is electric, the throttle valve is electric, it has no EGR cooler changeover flap, it has no twin-path intake, nothing but the turbocharger control coming off the brake booster tube from the tandem pump.

You just have a vacuum line, a check valve, the N75, a vacuum reservoir tank, the VNT actuator, and the vent line. That's it. That's the entire boost control system in one sentence. Simple.

Now the charge air tract is a little more convoluted, but hardly impossible to figure out, especially on an automatic car where a quick brake torque can be performed to find boost leaks literally within seconds.
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Old February 28th, 2013, 09:20   #17
tomsawyer
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Hammer - I appreciate your comments, especially since I have absolutely no technical knowledge of this stuff so I'll ask your opinion.

Now that the car has full power and the code thrown the 2nd time is now gone and hasn't been duplicated, what do you recommend I should do? Let the service manager take the car home for the night to see if the car goes into low power or should I just go pick up the car and take my chances?
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Old February 28th, 2013, 13:35   #18
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Quote:
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I'd get it to someone who actually knows what they are doing. These dealers floor me sometimes.
I think your question was already answered. Save yourself time and money and call Mike Hevner - http://forums.tdiclub.com/showthread.php?t=331284
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Old March 6th, 2013, 06:01   #19
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Latest update -

I got the car back from the dealer after they could't replicate the low power condition and the car did not throw a code after the tech followed VW procedure after the 2nd low power condition after changing the N75. He cleared the code and it never came back and it operates on full power now.

I've been driving the car all week and I too have not been able to get the car into low power condition.

Now, here's the stupid question -

The 2 times the car went into low power, I was entering the highway and was at full throttle. Now that I've gotten the car back, I know it sounds crazy, but I haven't gone above 2500rpm so I've been driving it very gently. Is it possible that the condition only happens on hard acceleration? I don't pretend to know but I would imagine the turbo and its components have to do more when there's more gas being fed?

I plan on taking the car to Hevster, hopefully this weekend and figured I would do the hard acceleration test once I was close to his place (he's about 50 miles away from me).

Thanks to all who answer.
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Old May 1st, 2013, 08:16   #20
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Just wanted to give everyone a head's up and to get this documented just in case it hasn't been.

I took the car to Mike Hevner after getting it back from the dealer. Mike went over the whole system - no vacuum leaks, no frayed wires, acuator fine, turbo good, all hoses good. After driving the car with the vagcom, the only issue he could determine was the MAF sensor seemed to mess up, then reset itself (I'm not a tech guy so I may not be explaining things exact). Finally, he was able to get the car to go into limp mode (for the time he had it, it ran on full power).

After all the diagnostics, he decided to change the MAF sensor as all other parts/vagcom readings/diagnostics all checked out.

I've had the car for 2 weeks since and haven't gone into limp mode since getting it back from Mike.

Thanks to all, and especially Hevster!
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Old May 1st, 2013, 08:17   #21
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Kudos to Mike!
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Old May 1st, 2013, 09:55   #22
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Huh, that actually sounds like a restricted fuel filter. Only vehicle that i have had set that fault though was a V10 TDI. Most of the other cars just won't shift and have low power, with no MIL. Odd...
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Old May 16th, 2013, 09:37   #23
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I guess I spoke too soon.....the effin car went into limp mode this morning, approximately 1 month after changing the MAF sensor. Now what???????
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Old May 16th, 2013, 11:38   #24
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Read my post right above yours ^^^ I would try a fuel filter (cause it's easy and relatively cheap), and check for fuel restriction or lack of supply. Over boost fault can be too much air or not enough fuel. So a restricted fuel filter, pinched fuel line, or flaky lift pump could be cause on fuel side.

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Old May 16th, 2013, 12:19   #25
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Read my post right above yours ^^^ I would try a fuel filter (cause it's easy and relatively cheap), and check for fuel restriction or lack of supply. Over boost fault can be too much air or not enough fuel. So a restricted fuel filter, pinched fuel line, or flaky lift pump could be cause on fuel side.

Jason
What is the simplest way to check the lift pump and N75? My BHW does the same thing as Tomsawyers and I even swapped the MAF sensor off my wifes' BHW and it still does it from time to time. Mine has a new N75, all vacuum lines checked,turbo cleaned VNT moves freely and has a new silicone vacuum line, fuel filter changed. I still get an overboost code sometimes when it goes limp and no code other times. It sure gets old cycling the key in traffic. Mine is more inclined to go limp when the engine is not fully heated up. Temperature sensors all appear spot on in VAG. I just loaded a stage 2 TDTUNING into it and will see if that helps. Is it a no no to adjust the VNT actuator? Maybe a faulty N75 from IDPARTS? I'm a little hesitant to borrow my wife's N75 as I broke my old one getting it off.

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Old May 16th, 2013, 12:36   #26
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Just as an fyi -

When my car went into the dealer for the original limp mode condition, I had them change the engine oil, fuel filter and then they determined it was the N75 valve which they changed.

The car went into limp mode again.

Then, I took it to Hevster and he changed the MAF and like Harley, Hevster told me everythinng checked out fine via the VagCom.
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Old May 17th, 2013, 02:48   #27
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There's something not so obvious going on with these cars. Hopefully Oilhammer or Mike can lead us through it. Was yours fully warmed up your last episode? Mine rarely does it at operating temp.
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Old May 17th, 2013, 08:24   #28
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I can't remember if mine was completely warmed up as I drove about 3 miles to the local gas station to get a cup of coffee, went inside to get the coffee and let the car idle which was about 3 minutes and then as I went to get back on the highway, it went into limp mode under hard acceleration. If it wasn't at normal op temp, I would assume it was pretty close.

I did the same as you....on the highway, I put the car in neutral, shut it off and then restarted and had full power to complete my journey to the train station!

This morning, I did the same thing. Went to the gas station, got coffee, idled, hard acceleration but this time no limp mode. This is gonna drive me crazy!!!!!
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Old May 17th, 2013, 09:05   #29
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Sorry - I had full write up typed and I thought posted. Don't know what happened to it. I am going to get into detail because both people with this problem seem to have it very intermittently. So not an obvious broken part, but maybe a flaky part that is still working for the most part.

Didn't see the part about the fuel filter. I have been burned by assuming the replaced part was good, just a caution. We'll assume it's ok for now.

You can check the lift pump and transfer pump fuel pressure and delivery volumes. The lift pump is about 0.5BAR I think, and the transfer pump is 10BAR. The delivery rate is dependent on the pressure on the system, so you need to restrict the pump to obtain the correct pressure. Also the voltage at the pump is part of the equation when determining delivery rate. Measure it and use the table of volume delivered and voltage at pump.

There are some specific conditions for the pressure reading on the transfer pump, and it is going to be "fun" to get the gauge on the pump on your Passat. There is a plug you can take out and screw a pressure gauge into. I think the RPM is the main factor in getting 10BAR or more pressure.

Like oil hammer said - air side is simple. Vacuum supply, to N75 control solenoid, to actuator.

Basics first - default position is full boost. Start the engine and the ECM takes over. I believe it pulls the actuator into a no boost position. When throttle is applied and torque is demanded, the ECM releases the actuator to full boost. When boost reaches the level the ECM wants it starts activating N75 to control boost to that level. In practice this is being done in milliseconds and moving constantly.

So for an intermittent over boost I would lean to a mechanical fault somewhere. Is there a strong vacuum supply? No leak from vacuum reservoir? Restriction or lack of restriction in appropriate place in vacuum lines? Actuator moves freely UNDER OPERATING CONDITIONS? Is the actuator starting to move at 8(?)in and fully moved at 18(?)in? N75 is new so I assume it is not at fault.

I'll add more as I think of it

Jason
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Old May 17th, 2013, 10:16   #30
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A while back I had a problem (completely unrelated to yours) regarding a fuel pump relay code coming up... Car would shut down, sometimes restart, sometimes not (glow plug light would always come on so I did not feel it could be relay 219 yet again). Replaced fuel pump relay inside car and no improvement. I ended up throwing parts at it and replaced the tandem pump (and relay 219 for the second time). I took notice to the vacuum nipple on the old tandem pump and saw that it would spin with no resistance. The new tandem pump vacuum nipple was secure and would not spin. Since the replacement of the tandem pump, I can count on 1 hand (probably 2 fingers) limp mode overboost/underboost.

Related to your problem - I don't know but it is a vacuum source and I think mine would leak some vacuum there as all my vacuum hoses were replaced and other items always checked out. I remember when I first got the car in 2005, there was a recall on the tandem pump. When I brought it in, they said mine was fine... like everything on this car, if there is a replacement part, it is probably the "new and improved" replacement part. Good luck.
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