Not running good under boost

**High_Rider** · 09-09-2016 09:39 PM

So I just purchased this car about a month ago it's. 2000 with bw k04s apr bi pipes and a mtm or mtm? Tune I think stage 2?? Not sure how to tell any ways it had a cel for random missfire and bank 1 sensor 1 o2 bad so I replaced it and not it will go to redline(before it would studder and go nowhere) but it still doesn't feel right if I ease into it and hold at 3k for 10 second then mash it this thing pulls like a train if you don't wait it kind pulls but not hard....no cel but I was told vagcom could help? Is any one near the Akron canton area? Or have any clues on the problem?

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**TheDarkness** · 09-09-2016 11:26 PM

not running "WELL" under boost. jeez.

**getslideways** · 09-10-2016 12:12 AM

Its probably not running well due to the high elevation.... of your ride height

A vagcom cable will pay for itself. Without any data beyond "kind pulls but not hard" it really could be anything. Also keep in mind the car can have codes without illuminating the CEL. Could be still misfiring under load, EGT sensor failure, etc. Until you get it hooked up it will be hard to rule anything out. Also if you dont already have a boost gauge, get one, as that helps narrow down the potential suspects in a quick glance. Aaaaaalso, you need to contact the previous owner and find out what hardware and software is being used for certain (tune, injectors, etc)

**S4tech2** · 09-10-2016 08:16 AM

Clean car! Sounds to me like you have a boost/vacuum leak. Throttle body boot (likely on the under side of it) is popular, also crank case hoses are bad about cracking and breaking.

**slowSfaux** · 09-10-2016 12:20 PM

Put me down for a boost leak as well. Get a vagcom, it's completely, entirely, a necessity when you own a b5 s4. In the meantime you can unplug your maf to see if the car runs better. If it does, it's very likely a boost leak.

**zillarob** · 09-10-2016 05:23 PM

Limping home with the maf unplugged is one thing, but rallying with it unplugged as a diag tool can be danger zone.

+1 on the vcds cable and boost leak tester. Both mandatory for b5s4 ownership

**james 408** · 09-10-2016 05:45 PM

#1 read codes with vagcom, clear codes, drive 1 tank gas, reread codes, fix
#2 boost/vacuum is a good guess, (smoke test/boost guage)
#3 you need to spend a bit of time searching. stage 2 tune on k04s? hrm.
#4 report back and plenty of people will help you out

**slowSfaux** · 09-10-2016 06:46 PM

Originally Posted by zillarob

Limping home with the maf unplugged is one thing, but rallying with it unplugged as a diag tool can be danger zone.

+1 on the vcds cable and boost leak tester. Both mandatory for b5s4 ownership

I guess I should clarify, I didn't mean to unplug and assuming it ran well to leave it unplugged. I merely meant it as a way to verify a boost/vacuum leak until you can build or buy a boost tester to track it down.

**zillarob** · 09-10-2016 08:46 PM

Yup, just be careful.

A stock car is usually safe to do this with. They run pretty good and might not even notice it except for the xmas tree on the dash.
Had one come in the other day with the maf unplugged. No idea how long and didnt seem to bother them. Had to think it was a rocket ship after I plugged it back in

It is with the modded cars that you need to be careful. Some tuners dont take the time to dial in the maps that handle running without the maf. Fueling might be off.

Unless I knew it was accurate with the maf unplugged, I wouldnt unplug it and boost the car for diag purposes.

**vavJETTAw36** · 09-11-2016 06:15 AM

Stage 2??? Hmm that could be your issue. Might want to check on that. You are probably running lean in WOT if that's the case. Previous owner probably Popped a turbo, threw k04 in for the future and never got the tune. Or it's stage 3 and everything I just said is moot.

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**zillarob** · 09-11-2016 10:46 AM

Or even 3-.
Either way, dude needs to get the vagcom cable and boost leak tester sorted. And do some homework while he waits.

**christianb5s4** · 09-12-2016 10:20 AM

Yeah, first thing is to get VCDS lite and an OBD2 port cable to scan the codes. If you don't want that, at least get the Xtool 401 (search it on Amazon). Clear codes after noting them down, then I would see what codes come back. You should never run a tune that isn't properly matched to the hardware on the car, if it's a stage 2 tune you should first of all confirm he actually put K04s in and if he did tell the truth, get a proper stage 3 tune.

**erykv1** · 09-12-2016 10:33 AM

I been driving for a while with the MAF unplugged (long story) but I keep a really close eye on the wideband to make sure everything sin check.

X2 on the xtools, does almost everything the average B5 owner will need, and some.