greatwhiteshark

I am bound and determined to figure this out. A few posts down I wrote saying I had a hesitation/stumble when accelerating hard at certain RPMs. Air filter was clean, no codes, doubt (but not sure of) a carbon issue, so I chalked it up to the AC compressor cycling on and off during acceleration. That's not it.

Last night I had some time to investigate. It seems the stumble only happens the first time the car is turned on. The hesitation occurs at 2500 rpm in S mode and 5500 in normal mode, both when accelerating at wide-open throttle. As soon as the stumble occurs, any subsequent attempt to re-create it doesn't happen, and the car pulls strongly, until I restart it and the exact same thing happens. (The feeling can best be described as an anticipation of the normal surge in power, but instead the rpms climb much slower.)

So just for the hell of it I reattached the rubber exhaust valve hoses (which were plugged with two phillips screws and wrapped in tape) back up to the muffler flappers. This morning on the way to work, it didn't stumble or hesitate once, and I tried about half a dozen times.

When the rubber hoses aren't connected, can the system sense it, causing the stumble, and then compensate so it doesn't happen again? I know those rpms are right around when the flappers are supposed to open, but I am baffled as to why this phenomenon only happens during the first acceleration after the engine being started.

Any ideas?

bruised rs4

opens and closes at different RPM's and when the car is started the flaps are in one position regardless of button orientation. I can't remember if they remain open or closed on cold start-up.

Nicht ein A4

If not, then you have a vacuum leak which is being noted by the ECU. Not huge, but it is real.

When I had the car, I disconnected the hose up high, at the plastic tube end, capped with real vacuum caps and used the golf tees to just keep dirt and crap out of the hoses. I did not rely on a golf tee to provide and air tight seal.

cit1991

Some engines test the EGR valve by stabbing it open soon after startup, and looking for a drop in measured fresh air flow. If it doesn't happen (or the drop isn't enough), then it throws a code to indicate failure of the EGR valve.

Maybe the RS4 closes the flaps for the test so there's back pressure for the EGR test? Without the extra backpressure, maybe there isn't enough EGR flow, and more fresh air flow than expected, and it goes lean for a second. But it can't tell that it's lean becuase the O2 sensors aren't hot enough yet and it's in open loop mode.

I know it's a stretch, but I can't think of anything else that would happen only once after startup, and be affected by open exhaust flaps / lower backpressure. And the stumble you describe sounds alot like a temporary lean-out.

greatwhiteshark

Hoses were plugged tight with philips screws, screwed into the tube. No leaks.

greatwhiteshark

It appears that the S mode opens the flaps at an earlier RPM based on throttle position. That would explain why it stumbles earlier (2500rpm) versus later (5500rpm) within normal mode.

If I pulled over, turned off the car, turned it back on, and then accelerated (in second or third, since first is way too short) the stumble would happen each and every time on the first acceleration run. Then it wouldn't stumble again until I restarted the engine.

Perhaps there's another vacuum issue that's being masked/highlighted by this 'discovery' and the issue is really the shorter intake runner flaps sticking. But that still doesn't explain why it only does it on the first acceleration run after restarting the engine.

Silver Fun

Do you have a vacuum leak with your mod?

greatwhiteshark

The general hypothesis here at work, where we have nothing better to do than talk about cars all day, is that the system (ECU) is expecting certain checksum values at startup, regardless of engine temp.

So, if it tests vacuum pressure and gets an unexpected value, it may retest, or go into a fail-safe timing table (resulting in reduced performance). After an initial acceleration run the system may think 'oh, flappers are stuck open' and will compensate and go about its business, which may explain why it only does it the first time after restart.

Since the computer is expecting it to take a certain amount of time for the flaps to open, and with the lines plugged, its getting that max value immediately and messing things up, at least temporarily.

It would be helpful for someone who has their exhaust vacuum lines pulled and plugged to test this out:

Make sure engine is at normal op temps.
Pull over safely, turn engine off, Turn back on.
Accelerate in second or third with foot to floor.
Check to see if the power band has a dead spot at 2500 in S-mode, or 5500 in normal, on the first pull only. If you can reproduce this, then the theory that it's ECU/vacuum related is probably true.

craiglieberman

two years with lines capped, no problems.