RobertKim99

At about 88,000 miles, my 95.5 S6 has developed a low RPM surge & stumble problem that appears to be increasing in severity and frequency. I would appreciate any pointers that anyone can provide on what to focus on in diagnosing the problem.

The problem evolved as follows. It began at around 86,000 miles as a mild surge at just below 2,000 RPMs, when mildly accelerating at part throttle. The surge would be enough to jerk the car forward a bit. Obviously, the situation would only occur while creeping through slow-moving traffic. Since then, the problem has increased to similar surges in the 2,000-3,000 RPM range, and a lumpy, stumbling idle. The stumble at idle appears to be at its worst when the engine is still warming up; it appears only now and then when the engine is fully warmed up, but it still happens occasionally even then.

Has anyone experienced a similar problem, and if so, what has been the source (e.g. spark plugs due for replacement, worn-out sensors, etc.)?

RobertKim99

P.S. The engine and drivetrain are 100% stock, no mods, and with all scheduled maintenance up to date.

Sloooo4esT

I think a failing Power Output Stage terminal could be the culprit to intermitant misfire, if that is what your stumble is, does the whole car shake during this stumble? can happen while at idle or while driving, comes and goes.

if not, maybe just an O2 sensor?
Forrest

RobertKim99

The stumbling varies in intensity. At idle, the tach reading fluctuates and the engine sounds uneven, but otherwise the car is unaffected. With the car in gear and the engine at below 3,000 rpm, the stumble is sometimes enough to make the car shake slightly as the power sent through the drivetrain varies, but over 3,000 rpm the unevenness disappears. I hope that these details help.

bluevr6

Or intake hoses that have come off partially.

UrS4boy

move on to the tougher and more expensive things. I agree with starting by checking all the air lines. If it was only at idle, it would most likely be air lines because the idle stabilizer valve would be trying to hunt for the right amount of air to keep the idle constant and a air leak would make it crazy (hunting and surging). The same thing could happen off-idle to some degree.

Just for laughs (or tears), I would check the spark plugs and the connectors next. You need to check to see if the plugs are a) F5DP0R Bosch plugs and b) they are torqued in properly (22 lb ft) regularly anyway. And this may be the problem. Check the plug to coil connectors for appearance (not ripped or deteriorated) and continuity (using an ohm meter). There is a correct resistance requirement but I don't remember it off the top of me little head.

This may sound real dumb but when was the air cleaner changed? If it is clogged or dirty you could be having an air problem. If it is a K&N, did you over oil it and get oil on the MAF wire? (Stick with the OEM paper - no real gain with the K&N anyway).

Are the injectors working? If you pull the injector electrical connectors off, one at a time, you can check to see if the engine runs the same or worse at idle. If it runs worse, the injectors are at least functional at idle and that cylinder is fine. If it runs the same, i.e. bad, either that injector is f***ed or that coil, power output stage or plug is not working properly (see plug issue above).

If you haven't found some improvements or likely suspects by now, you're left with the hard bits: O2 sensor, coils and power output stage (POS) modules. The O2 sensor is relatively inexpensive to just change on a preventative basis IF you get the universal Bosch 3 wire unit (and splice it into the existing connection). Coils and POS's are more difficult to diagnose.

The injector test should have helped you figure out which cylinder was acting up. There is a procedure to use the sixth (unused) POS position instead of the apparently poor position. (The two, 3-position, POS's are under the black retangular cover on the fire wall)

Coil diagnosis is tough. Almost easier to "borrow" a good complete coil pack and see if it helps. Again, the injector test should have lead you to guess which is the malfunctioning cylinder. If someone local had a good spare coil, you could try it in the apparently malfunctioning position and see if it is the cure.

If by this point you haven't found the solution, you may be in deep doo-doo, i.e. worn cam lobes, bent valves, cam belt off a few teeth, etc. (Something internal). At that point, its gonna cost you good "wrench" time to find it. Hopefully, you will solve it before it comes to this.

Good luck,

Dave F.

UrS4boy

Robert: See my post below ("As usual,....)

germantoy

man urs4boys posts are all valuable thanks for knowing your isht

UrS4boy

Uh, Thankyou, thankyou very much (I try to be helpful even if sometimes I can be harsh)

RobertKim99

The problem seems to have been nothing more than a loose clamp on the big air intake hose that connects the air cleaner to the turbo. 30 seconds of tightening, and the stumbling at idle disappeared. The low RPM surging has not repeated either, although I will continue to watch for it.

Now that the engine runs like new again, I can once again entertain thoughts of it lasting another 88,000 miles.