Another intermittent boost issue, general experiences

[happy2B5]

So about a month or so ago the car started acting up, after close to a year of hassel free operation. Nothing crazy done with engine mods, just Unitronic chip, exhaust, 034 dv's. But, the car started bucking and cutting power (no CEL) just before 4K rpm about a month ago. Did the usual shit like scan for codes, leak test, search here and other forums (mostly here), etc. Can't remember the codes now (most of them were intermittent), but nothing really that would point me in a definitive direction, without simply replacing usual culprits like EGT's, n75 (one code was for overboosting upper limit), even dv's (if stock, but not). Decided to take care of some maintenance stuff like replacing fuel filter (total PITA, for me anyway - former midwest car = corrosion). Also replaced the 034 dv's with 710n's after all. That was interesting in that the passeger side was loose and wobbly, come to find out the inlet side threads were stripped at the main body. Crappy plastic mated to aluminum. So I am glad to just be running stock dv's now after all. Still not fixed or problem solved. The same retarded power cut for the last month or so mostly under WOT, but some hesitation even under partial when approaching 4K.

So, today I am thinking WTF is different from when this shit started happening and prior to? My exhaust is the Neuspeed Titanium race exhaust (yah I know not everyones cup of tea). The thing is that exhaust has a baffle insert that you can run or not. I did not like the highway drone very much with it out so I have really not drove it like that very much. I'd say 90% of the time it was in the exhaust tip. I did take it out for an extended period of time though because I started getting used to it and figured it was dumb to run the baffle and effectively restrict the flow at the tip. Should perform better, right? Today I put that baffle back in and viola. Got the car up to temp, stomped on it and up the on ramp I went without a glitch.

I was looking at the S4wiki page and saw a statement on there that one possible solution or remedy to over boosting issues is to increase back pressure at the exhaust along with replacing n75 valve (if faulty). I was ready to do that, but thought maybe this stupid exhaust mod is the culprit. It's free. Why not see what happens? It seems to have been my problem and solution. I guess I am a little bummed that my tuning is that stingy to not accomodate a seemingly common next upgrade.

I don't think I am completely out the woods yet, but it seems to have done the trick. The tolerances associated with smooth operation of these cars is insane. Thanks to any that spent the time to read this, and if any of you have some more insight and feedback, I'd appreciate it.

[RolledMySTi]

Sounds like you are masking another problem. Try removing the baffle and swapping N75's with someone.

[happy2B5]

ok. will do. it did skip a bit at first WOT just now during another test drive, then smooth. just need to find another s4 local. few and far between in central CA.

[happy2B5]

broke down and ordered N75 valve from genuinevwaudi. $30 less than from my local stealership that does not stock part anyway. we'll see what happens.

[mys4.org]

Have you checked your spark plugs? Are they gaped between .016 - .024 inches?

[happy2B5]

recently replaced with Bosch sidefires. Don't think plugs are the issue? Maybe though, never know.

[wunderman4]

Dont waist your time with the exhaust, I had the exact same problem. Turned out to be my n75. takes 10 minutes and your done.

[happy2B5]

I am hopig so. N75 is being shipped as we speak. The exhaust is really not an issue to mess with though. A simple baffle insert into the end of the exhaust tip. Really just another type of sound resonator. I'll try and post a pic. It will make more sense.

[Thizz_S4]

man ive been looking for someone with the titanium exhaust post a vid and some pics of it installed if you could. . . love to hear what it sounds like. anyways hope the N75 is the fix.

[happy2B5]

sound http://smg.photobucket.com/albums/v3...t=MVI_0367.mp4

installation from original thread posted here http://www.audizine.com/forum/showth...tanium+exhaust

[Thizz_S4]

sound http://smg.photobucket.com/albums/v3...t=MVI_0367.mp4

installation from original thread posted here http://www.audizine.com/forum/showth...tanium+exhaust

The System sounds good

[megafreakindeth]

overboost can only happen when theres a vac leak between the n75 and the wastegates themselves. commonly it is in the dainty Y fitting that connects the n75 output to the two wategates. also have seen it inbetween the two layers of silicon that the respective vac lines are made of. other than that any other leak will cause underboost.

as for too little backpressure causing overboost...no. something else is wrong. dont forget that you dont need an external vacuum leak to cause problems. if the check valves are bad boost can recirculate around the intake and mess with the computer.

[happy2B5]

check. thank you for the straight forward and understandable input. I will take a closer look at the spots you pointed out. Also expect the n75 to arrive today.

[happy2B5]

new N75 = no luck! Still hesitating and bucking intermittently around 4K! FUCK! The headache continues...

[SRSslpr]

Did you check your O2 sensors? Try replacing the front o2 sensors and see if that has any effect. I'm having a similar boost problem where my car will buck under WOT and I'm pretty sure I need to replace my B1 S1 o2 sensor. I pulled codes and it send the signal to the sensor was intermittent. I also just replaced the passenger side front o2 sensor recently because I was having the same problems and my issue went away for awhile. Does your car idle rough?

[happy2B5]

Pretty smooth idle. None of my initial codes were for O2 sensors? I will pull codes again. I am sure they have popped up again since clearing them and my original post. I just compared and searched the codes in the forum and read up on possible issues and fixes. But, since I am still stuck, I will go ahead and post the codes. Next in line for me are EGT's I think. The first set of codes eluded to them, but nothing definitive. I just did not want to start replacing a bunch of nondefective parts, but low and behold that is what has happened. DV's, N75, (EGT) next....PITA.

[Blake2423]

I am having the same problem, my car will start bucking and cut off at about 3,500 rpm and right before that it will whistle(sounds like the diverter valves are open) but once i get past 5,500 the sound goes away and the car accelerates again. The abs and esp lights come on during this bucking and once I let off the throttle they shut off after 30 sec or so. Im not sure what it is but if you find out what the problem was let me know

[asc_up]

if the check valves are bad boost can recirculate around the intake and mess with the computer.

Any particular way to test them? Pull them out and blow through them?

[happy2B5]

I am having the same problem, my car will start bucking and cut off at about 3,500 rpm and right before that it will whistle(sounds like the diverter valves are open) but once i get past 5,500 the sound goes away and the car accelerates again. The abs and esp lights come on during this bucking and once I let off the throttle they shut off after 30 sec or so. Im not sure what it is but if you find out what the problem was let me know

I definitely will if and when I get to the bottom of it. Actually bit the bullet and took to my local independant shop on Thursday last week. We'll see. Am hoping it is not chip tuning related, because I know the struggles with trying to get them to admit or recognize they may be the issue. I don't think it is though because like I said in original post, the car had been operating flawlessly for the better part of a year.

[happy2B5]

Interesting report on ABC about BMW's with twin turboed engines experiencing "limp" mode related to faulty fuel pumps...Potential fix in my problem? Hmm...My local independant brought up the fuel pump as a possibile item to investigate.

[^@ViVid]

Are you getting a flashing check engine for a misfire when bucking happens?

[damunky]

Interesting report on ABC about BMW's with twin turboed engines experiencing "limp" mode related to faulty fuel pumps...Potential fix in my problem? Hmm...My local independant brought up the fuel pump as a possibile item to investigate.

What codes did you have? Did you replace your EGTs? If you are overboosting, or running lean (maybe your fuel pump is bad as you said) , perhaps the ECU is cutting the throttle. Also, did you check your coils/spark plugs?

[happy2B5]

No EGT replacement, yet. Coils and plugs are good.

First set of codes =
17866 P1458
16805 P0421
17965 P1557

[damunky]

No EGT replacement, yet. Coils and plugs are good.

First set of codes =
17866 P1458
16805 P0421
17965 P1557

Googled "17866 vag" and this was the first result, sounds exactly like your problem:

"W00T!!!! $400 and a couple hours later my car is finally running strong again!!! For several weeks, I have had 2 bad EGT sensors. If I gave the car any boost, it would start to go and then almost buck and cut boost. I found the following DTCs during the last couple weeks and even threw a CEL the day of the CT S4 meet:
17866 - Exhaust Gas Temp Sensor 2 (G236): Short to Ground
P1458 - 35-10 - - - Intermittent"
http://www.audiworld.com/tech/eng73.shtml

[happy2B5]

Yep. Already have the DIY article saved to my desktop...Thanks for looking out.

EGT sensors on the way...

[^@ViVid]

What about ICM's?

[happy2B5]

What about ICM's?

not on my radar as of yet, I guess? Going to replace the EGT's and go from there. I am sure there is already info, but care to elaborate more in this thread on diagnosing the ICM's as culprit?

[happy2B5]

well fuck me runnin! new EGT's = FAIL! I am really fed up with this shit. the car is going to sit all weekend. i don't even want to look at it right now.

[happy2B5]

Are you getting a flashing check engine for a misfire when bucking happens?

not one cel throughout the entire process. just stored codes. after egt replaced, cleared codes. test drove, same shit. scanned, not one code.

[happy2B5]

update: freakin torn tbb. can't believe it wasn't the first thing removed and checked! visual inspection while installed not readily apparent. upon removal, torn near the underside of clamp junction! oh well, N75, 710n's, EGT's and finally tbb later, problem fixed.

[JonnyOnTheSpot]

Oh man, I feel for ya... I bought a boost leak test kit from turboboostleaktesters.com -- that might have discovered your torn throttle body boot in a hurry.

[S4tech2]

Poor guy!

[happy2B5]

Hahahaha, 3 yrs later...