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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 October 2nd, 2019, 15:02   #1
GmasWagon
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Default Underboost nightmare

Okay so this is a saga, but I NEED your help. I have an 05 passat TDI Wagon. First off I am a homeless vet living out of this car, was spending my summer in the white mountains hiking, and had planned on heading back home to Va in the fall. Then it tarted kicking into limp mode. Any demand for power is met with limp mode. Car still revs, and when on flat ground can gain speed. I researched my a** off for the last two months and replaced N75 and all my vaccumn lines first. When this did not solve the problem I broke down and took it to a shop (who originally claimed to have VCDS, but in fact did not) cost $66 to hand me a list of codes. Based off the codes I have bit the bullet and replaced MAF sensor, thermostat, coolant temperature sensor, intake flap motor, glow plugs(7v), and even spliced in new glow plug wires. All codes are gone minus underboost and constant/immediate limp mode. Egr was deleted by the previous owner, so that code is always there. Went out and got a vaccumn tester. Vane actuator tested somewhat low, so I removed the turbo and adjusted it to start moving about 3 and hits the stop right about 20 (did not move stop screw) as I have read on here. I'm headed to Home Depot now to get some stuff to make a boost leak tester (I have one of those 12v tire air pumps). BTW I'm doing all this in the Biddeford, Me Walmart parking lot where Ive been parked for a month and a half. Beyond that, I'm lost. My dog and I live off of $1,200 a month from my disability and have already spent over $800 from my Oct check trying to fix this. MAP sensor maybe? Wouldnt that throw a code? Tandem pump? Hoping I can find what I need to do a boost leak test and that's it, even though idk how I'm going to afford new vw hoses. Any other suggestions or help would be GREATLY appreciated. My daughter's birthday is at the end of Oct and I promised her I would be there. Plus it's starting to get really cold at night.
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Old October 2nd, 2019, 17:07   #2
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Did a boost leak test. It can build enough pressure to blow off of the turbo inlet, but at the same time bleeds off to nothing. I'm in a parking lot, so there is some noise. Didn't hear any leaking air. Lost daylight so I'm going to get a spray bottle tomorrow and try to spray everything down with soapy water.

Again, can my MAP sensor be bad without throwing a code?

I am not a mechanic, if not evident from my posts.

Thanks in advance.
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Old October 2nd, 2019, 17:34   #3
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Also put the vac tester on the source and it went above 25.
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Old October 2nd, 2019, 19:59   #4
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Have you looked at the turbo outlet hose? Perforation due to contact with a ground cable is a common problem. Also common is a break in the short 90 degree hose connecting the upper crossover to the intake flap. O-rings can also be a problem. I have experienced all of the above. I think you will need a bigger air source than a 12V compressor to find a leak.
I also think a leak small enough to need soap bubbles to find would not trip a limp mode.
Building enough pressure to blow something off the turbo inlet suggests a very small leak but you didn’t say where the other end was.
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Old October 3rd, 2019, 04:08   #5
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Check for a boost leak. My first boost leak (a few years ago) came from the hose off the turbo with a hole worn in it by a poorly placed zip tie near by. I bit the bullet and paid just over $200 for a new hose. I am currently experiencing a leak somewhere - but have not had the time to pin point the location. No limp mode or CEL, but I slowly lose power under demand (going up a long hill). When I find this one, I am considering a tape until I can get a new pipe. Any recommendations?
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Old October 3rd, 2019, 04:25   #6
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I have had good success using smoke machines to find boost leaks that are not readily obvious.

However, on any of the PDs, careful systematic removal, cleaning, and close examination of the boost hoses (there are a total of four on the BHW) paying special attention to any oily areas, splits, worn retaining tabs, and split o-rings, will usually get to the source. The intercooler also hangs low and can be damaged from front end crashes. Yes, even a little "bump" into a curb is enough.
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Old October 3rd, 2019, 05:27   #7
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Did you try anything as simple as just unplugging the maf and leaving it unplugged? You mention you bought a new maf and I found that unless you bought it from VW for the high price they don't work. After going through a few duds, I left mine unplugged and drove it for five years with no problem except the light was always on.
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Old October 3rd, 2019, 10:44   #8
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I have a cap on the elbow directly in front of the intake flap motor. Other cap is on the turbo inlet. I can build up 20psi, and it takes about a minute to a minute and a half to bleed off. Can't hear any leaks, nothing with soapy water. Wouldn't think a boost leak this small (if its not in my cap fittings) would trip a CEL and kick it to limp mode, especially as quickly as it does. I mean as soon as the turbo should be kicking in it goes into limp. 2kish rpms. I got a run in late last night where I rolled super gently into the throttle and it boosted strong 2nd into 3rd, but then I ran out of road. I'm soo lost. I filled the system with smoke with a transfer pump and a cigar, absolutely 0 signs of smoke exiting anywhere.

Still wondering if a MAP sensor can cause limp mode/underboost code.
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Old October 3rd, 2019, 10:57   #9
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Going to put everything back together and try to unplug my MAF.

Last edited by GmasWagon; October 4th, 2019 at 06:03.
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Old October 3rd, 2019, 11:07   #10
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Unfortunately there really isn't much we have to go on here. We have no idea what the actual DTC(s) are. Lots of things can send the ECU to go into limp mode. Overboost, underboost, etc. You said you have an underboost DTC, I assume P0299. That pretty much is always a boost leak, or the turbo is not working.

You're going to need a scan tool to get any good information. The BHW's boost control system is brutally simple. It is the only thing under the hood that the vacuum system supplies (beyond the obvious brake boost assist).

Unfortunately, on the BHW, the N75 valve and its related connections and vacuum reservoir are located in a difficult to access spot down in front. You can more easily get to it with the air cleaner assembly removed. If you have a proper scan tool, you can do the test for the VNT actuator, and with the engine running watch the VNT control rod move up and down. That simple, quick, easy, do-not-even-get-your-hands-dirty test will verify the vacuum control side's integrity.

Then from there, if you cannot find any boost leaks anywhere, I'd take the turbo inlet off and check the condition of the turbine wheel itself, and see if it is trashed. Although a typical failed turbo will whine and blow massive amounts of oil into the intake, exhaust, or both.
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Old October 3rd, 2019, 11:26   #11
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No dice, limp mode and worse drivability.
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Old October 3rd, 2019, 11:36   #12
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The code is 16683
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Old October 3rd, 2019, 11:52   #13
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Not getting any oil blowing out anywhere, small amount in intake, but very negligible as I have removed, inspected, and cleaned all intake tract piping. Turbo spins freely and seems solid. Could I have reinstalled the cartridge incorrectly when I disassembled and cleaned it? It functions "properly", as in moves freely with applied vacuum to vane actuator. Is there a "reverse" direction I could have assembled it to? I don't feel like that's mechanically possible since I can boost normally as evident from my run last night. I have since repeated this test. It will boost fine up to about 3k rpms if I dip into it slowly.
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Old October 3rd, 2019, 11:56   #14
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Should this intake system hold applied psi indefinitely? 2 minutes to bleed 20psi. Going to try to building a smoke generator from a soldering iron, baby oil, and a jar to see if I can pressurize smoke throught the system.
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Old October 3rd, 2019, 12:19   #15
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Did this problem start after your took your turbo apart?????
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