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URS4 Boost issue

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 Post subject: URS4 Boost issue
PostPosted: Wed Oct 12, 2011 11:02 pm 
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Location: Colorado Springs, Co
i just got a 1994 S4. i am having some issue with the boost on it. when i start the car. warm or cold. it will boost through all the gears no problem but once i let off and decel or come to a complete stop it will stop boosting. you can hear the turbo spool a little but i feel no boost. now just to throw it out there this is my first forced induction car so im very new to this. i was thinking N75 valve but not 100% sure. any other info you guys have for me?

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 Post subject: Re: URS4 Boost issue
PostPosted: Thu Oct 13, 2011 3:04 am 
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Sounds more like a boost leak that comes out when the hoses are warm.

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 Post subject: Re: URS4 Boost issue
PostPosted: Thu Oct 13, 2011 3:55 am 
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I've never heard of a temp dependent boost leak, although I suppose it could happen under the right circumstances. You may be on the right track with the n75, but usually they aren't intermittent like that. Get a boost gauge as the first step to solving the problem (or use the one on the dash I guess).

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 Post subject: Re: URS4 Boost issue
PostPosted: Thu Oct 13, 2011 4:37 am 
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How is the engine running when this happens? Smooth rough? Im thinking coil packs are probably shot.

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 Post subject: Re: URS4 Boost issue
PostPosted: Thu Oct 13, 2011 9:23 am 
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If the car is really new to you, and if you don't know the history on it, you should baseline a few things to make sure a some things can be ruled out.

Things you should make sure are in good shape:
-all vacuum lines
-boost hoses(especially the "Michelin Man" hose that is under the front left of the car)
-spark plugs(if the car has the stock ignition set up then use only Bosch F5DPOR plugs)
-air filter
-fuel filter
-spray out the MAF with some MAF cleaner
-o2 sensor

There are a number of things that tick these cars off, but the above list is a good short list of things to do that will make your car happier.

One thing you could do is unplug the N75 and see how the car runs. If it runs the same as you describe above(when it stops boosting) then the N75 is a real possibility.

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 Post subject: Re: URS4 Boost issue
PostPosted: Thu Oct 13, 2011 10:34 am 
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Well i have already cleaned the maf with the maf cleaner. Checked the air filter and its pretty new. Ill definetly check the rest of that. Ill see what the car does with the n75 un plugged. The motor runs good it just has no boost. Its not rough or anything. It idles perfect and runs and drives good just doesnt boost. Its odd.

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 Post subject: Re: URS4 Boost issue
PostPosted: Thu Oct 13, 2011 10:36 am 
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One way to test the N75 is to unplug it while the car is cold. If the car exhibits the same behavior as when the car is warm, that could be a culprit. Also check the turbo inlet hose to see if it's torn under the clamp (quite common) and the boost hoses for holes. Contact me with any questions.

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 Post subject: Re: URS4 Boost issue
PostPosted: Thu Oct 13, 2011 10:58 am 
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Ill check the other boost hoses but the inlet pipe to the turbo is a brand new 034 one. i just tried unplugging the n75 and it just made the car not boost at all. any suggestions? i think im gonna check for boost leaks and see what i find.

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 Post subject: Re: URS4 Boost issue
PostPosted: Thu Oct 13, 2011 2:01 pm 
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Have you pulled the codes? Never a bad place to start. Find somebody with a VAG-COM or ProDIAG in your area - bring 'em a 6-pack. Personally I think a visual check for vacuum leaks is a waste - too easy to miss a small slit/tear - you want to pressure test the system. There have been various write-ups for how to do this - sorry don't have time to write it all down now. And the coil packs on these now old cars are starting to fail often.

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 Post subject: Re: URS4 Boost issue
PostPosted: Thu Oct 13, 2011 2:07 pm 
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I've checked the car out, the michelin man hose looks decent, no obvious tears like there were on my 200. And loxx, i'm pretty sure only the '92s came with the trip computer, which i think is a bit odd, but oh well. And tyler, if you want to try my stock coils from the s6 just to check we can, or you can pick up some cheap vw coils and just replace yours. I can't imagine it's a boost leak, it boosts decently on the first pull, but decreases after that. Is there some sort of volatile memory in the ecu that could be decreasing boost due to a bad sensor or something but it resets after the car is turned off? Other than that the coils/fuel are my only guess.

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 Post subject: Re: URS4 Boost issue
PostPosted: Fri Oct 14, 2011 7:39 pm 
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eli lets try that N75 out of the S6 and see if it works. it take like 2 mins to take out. i think that may be the issue. if not i cant try other shit. im getting a boost gauge tomorrow from my friend so i just gotta install that and see what its boosting. that should help me out a little bit.

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 Post subject: Re: URS4 Boost issue
PostPosted: Fri Oct 21, 2011 10:59 am 
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well while installing the boost gauge i was looking at all my vacuum lines and realized one was unplugged. BAM boost problem fixed! i feel kinda dumb not looking there first but at least i didnt buy any parts! and now i have a boost gauge installed so i can watch if anything starts acting up!

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 Post subject: Re: URS4 Boost issue
PostPosted: Fri Oct 21, 2011 9:49 pm 
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Tylerado wrote:
well while installing the boost gauge i was looking at all my vacuum lines and realized one was unplugged. BAM boost problem fixed! i feel kinda dumb not looking there first but at least i didnt buy any parts! and now i have a boost gauge installed so i can watch if anything starts acting up!

:roll:
It usually is something fairly simple...

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