Bucking/No Power at above 3,000 RPMs (No CEL)

[R490]

Stumped on this one.

Just finished up a bunch of work, timing etc and now whenever I go above 3,000 RPMs in 2nd-4th gear, the car bucks back and forth and there's no power. It almost feels like it's in neutral.

NO CEL, no codes in the system. I just replaced a bunch of vacuum hoses, and it idles at 18 inHG.

The car is running an APR Stage 2 tune.

Any ideas?

[CELison]

Unplug the maf and see if it changes anything.

[Bordom]

My money on MAF as well.

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[R490]

Hmm, thanks guys. I'll try that next.

I just thought it was so strange how I didn't have a CEL. It even passed emissions this morning.

[CELison]

My bet is you have a leak and your maf is working correctly. That’s why you don’t have a code.

[RMode]

unplugging the maf is an easy way to check things - it will reveal a bad leak, or a bad MAF

you WILL get a check engine light once MAF is unplugged, and do not give it any boost

[S4James]

concur

[R490]

Well, I appreciate the advice from everyone.

No vacuum, or boost leaks.

The cats are bad. This is strange because I passed emissions two hours ago.

[ReggieNoble]

Also check your cps

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[CELison]

Well, I appreciate the advice from everyone.

No vacuum, or boost leaks.

The cats are bad. This is strange because I passed emissions two hours ago.

describe "bad"

[R490]

I'm about to go talk to who is looking over the car in person......

They told me over the phone that they're evidently clogged.

[S4James]

ah shit. that a recent development? check if actually melted, could be a result of other issues like running too rich etc.

[RMode]

that seems odd

anecdotal, but I recently upgraded the downpipes on mine...161k, and they were the original units. They worked perfectly fine, zero clogs. I was even able to upcycle them to someone and made my downpipe $ back and then some

[R490]

Yeah, I'm not too sure what to think. They're XSPower "high flow" cats, with 50,000 miles on them.

I guess I'll replace them with the same. I just think it's odd how I don't have a CEL, and that this was so sudden. I recently drove the car 300 miles and it was perfectly fine.

[R490]

ah shit. that a recent development? check if actually melted, could be a result of other issues like running too rich etc.

I guess so, I'll have to see.

I thought it could be running rich, but it idles and performs exactly like my other S4. It did fry an 02 sensor in like 40,000 miles though, so I guess that could point in the cat direction.

[RMode]

If they are clogged that means they dropped the downpipes to check. That's not the simplest, have this for you in an hour, type thing. Are there pictures?

[CELison]

Shops can do a back pressure test on cats. Peasants like us would have to pull the exhaust apart and look.

[R490]

I dropped the car off at like 1:30.

I brought it to a German car specialist that does a lot of VW/Porsche work, so hopefully, they'll have some evidence.

[RMode]

Shops can do a back pressure test on cats. Peasants like us would have to pull the exhaust apart and look.

still involves dropping the downpipes on these cars, since those testers use the 02 sensor bungs, and those face upwards IIC. Well, up on on, and vertical on the other.

[ReggieNoble]

describe "bad"

When it smells like cologne, leave it alone

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[R490]

They apparently tested the cats, by driving the car? Nothing else. Tech said there's a "whooshing" sound when the gas pedal is pressed and the RPMs drop off. He also said it smells like it's running rich.

One of them told me to start, by flashing it back to a stock tune. I don't see how that would help, since it's been on Stage 2 software since 2011. If the cats are bad, they're bad. But why would I spend $1,000+ if they can't even tell me with certainty what's wrong?

Here's the exact symptoms

-Car falls flat on it's face at 3,000 RPMS, won't rev any higher, jerks and bucks when it does happen

-Whooshing sound when this happens

-Fuel/exhaust smell in cabin

-Car is hard to start sometimes

Pulled one of my spark plugs to gauge carbon deposits. It's clean.

[NachO_GMC]

check the CPS on the bellhousing for any issues (sometimes the wires get melted from the exhaust heat - that could cause your issue)

Grounds - check and clean them

Are your Diverter valves in good working order?

I'm working on a 4500 rpm engine cut that is almost exactly the same lead up symptoms as yours. I did the above things to rule out the obvious.

GL

[R490]

check the CPS on the bellhousing for any issues (sometimes the wires get melted from the exhaust heat - that could cause your issue)

Grounds - check and clean them

Are your Diverter valves in good working order?

I'm working on a 4500 rpm engine cut that is almost exactly the same lead up symptoms as yours. I did the above things to rule out the obvious.

GL

Appreciate the advice!

Assuming CPS is crank pos sensor?

It's just such a strange issue...

[CELison]

Sounds like a boost leak.

[qcrazy]

TBB okay? Have to remove to fully inspect.

The woosh makes me think TBB.

[R490]

The car has an APR bi-pipe, so the TBB is bypassed.

[R490]

Sounds like a boost leak.

That would be one hell of a boost leak, no?

The whooshing noise when the car is failing to hit boost is what's throwing me. Why would they say that's the cats?

[CELison]

Doesn’t take much of a leak to cause major bucking. It sounds like it’s large enough to be audible. Also, this show sounds pretty incompetent.

[R490]

Where would you suggest I look first? I've never dealt with a boost leak this large. I'm only in town for another few days....

Yeah, I got that impression too. The tech also said it could be my fuel pump failing, as well as both of my cats failing simultaneously. I'm 0/2 on German car repair shops.

[CELison]

Need to do a boost leak test by pressurizing from the Y pipe. No point in guessing. Where do you live?

[R490]

Need to do a boost leak test by pressurizing from the Y pipe. No point in guessing. Where do you live?

I store the car in Virginia, but I (unfortunately) live close to Altoona PA.

[S4James]

that does sound like you blew the TBB, one of the couplers.. something really big.. ive had both happen, similar effect to your issue. Disconnect the maf, and see if you can drive around ok (obviously not trying to make boost)...

clogged cats dont go whoosh

[R490]

that does sound like you blew the TBB, one of the couplers.. something really big.. ive had both happen, similar effect to your issue. Disconnect the maf, and see if you can drive around ok (obviously not trying to make boost)...

clogged cats dont go whoosh

Is it fine to drive with the MAF unplugged above 3k RPMS? That's the only range I have the issue.

I don't have a TBB with my APR Bi-Pipe. What do you mean by the couplers?

[CELison]

No issues driving with the maf unplugged. It will just be in open loop. They should do that ASAP. You’re 3 hours from me. I would tell you to just bring it by.

[R490]

Come to think of it, I stripped a bolt that holds the TBB sensor to the top of the APR bi-pipe and it's not mounted securely up there. Could that cause this?

EDIT: Def not, I can't move it by hand

I'll try it tomorrow morning, thanks again for the advice. Wish there were more B5 people around here, indie shops just don't really seem to know.

[CELison]

“Woosh” noise has boost leak written all over it. Pretty astonishing that a German Indy shop wouldn’t pressure test the car first.

[JustManson]

What a ride. Even took it to emissions like that, that’s amazing.

Definitely a boost leak for the sound of it.

Get your soapy water and a spray bottle. You can make or buy a kit pretty cheap and quickly.

You’ll have a lot more fun once you track it down!






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[ShelbyM3]

Stumped on this one.

Just finished up a bunch of work, timing etc and now whenever I go above 3,000 RPMs in 2nd-4th gear, the car bucks back and forth and there's no power. It almost feels like it's in neutral.

NO CEL, no codes in the system. I just replaced a bunch of vacuum hoses, and it idles at 18 inHG.

The car is running an APR Stage 2 tune.

Any ideas?

Vacuum/boost leak only showing its head under load/boost? Can you do a boost leak test or smoke it?
For what it’s worth, my 2.7 usually idles around 20inHg and when my throttle body boot was torn this past summer I was only getting a soft code and no CEL.


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[R490]

Yup, only shows when I'm getting on the throttle. Also noticed it doesn't occur in first gear. Driving around and idling, it's fine. However, it sometimes has difficulty starting.

Tomorrow I'll try to do a boost leak test. I just think it's so strange to have such a large issue, and no codes to show for it.

[R490]

“Woosh” noise has boost leak written all over it. Pretty astonishing that a German Indy shop wouldn’t pressure test the car first.

Yeah, I was kinda pissed off that they couldn't even give me a straight answer. Appreciate all of your help, I'll update tomorrow.

- - - Updated - - -

What a ride. Even took it to emissions like that, that’s amazing.

Definitely a boost leak for the sound of it.

Get your soapy water and a spray bottle. You can make or buy a kit pretty cheap and quickly.

You’ll have a lot more fun once you track it down!






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I reeeeeeeeally hope it's a boost leak, and not cats like this "shop" said. It's been one thing after another since I did my timing job.