AWD - Running rough, but only at 2600-2800 rpm ?????????

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Running rough, but only at 2600-2800 rpm ????????? S70 1998

Pardon the "book" to follow, but after researching many threads I'm still puzzled. So you Sherlocks out there (and others like me with WAY too much time on your hands), thanks for reading!! I could have been more to the point, but with this "long-winded" version I have offered a few things I learned along the way, which may be beneficial to you. So anyway ...

I recently noticed some vibration coming from the front left (between engine and wheel well), on hard acceleration - was "sure" I had destroyed the left wheel bearing during replacement of halfshaft (didn't realize shaft nut was bottomed out on hub, so I kept pounding on it - ouch - don't do that!). Replaced bearing ... to no avail.

My next thought, maybe the new halfshaft (DSS) is out of balance? So I put the original back in as a test (which had been running decent, but had a torn boot). And here's where it gets weird: as soon as I let the car down and start it, the CHECK ENGINE light immediately comes on. Coincidence? Somehow I doubt it: I had all kinds of tools clanging around in there trying to get that stinking DSS shaft out - so I figured I had disturbed a vacuum line or wire ... but the only thing I see in that area are the manual tranny cables? (side note: DSS circlip has SHARP edges, compared with rounded edges on OEM clip - that's what makes the DSS shaft so difficult to remove, so beware).

Well, with the original shaft, the vibration is still there, but I have now pinpointed it to 2600-2800 rpm, regardless of what gear I'm in. A low frequency humming sound/feel, from the left side. I have no problems at idle, and overall it runs great - except that 2600-2800 range.

Regarding the CEL, Autozone found a P0455 (large evap leak), and a P0133 (O2 sensor1 bank1 slow response). I checked the obvious evap stuff - fuel cap / filler neck appears ok. I visually inspected the evap lines from cannister to purge valve to manifold - all seem ok. For a "large" leak, wouldn't I expect to smell gas fumes? Because I don't.

Is it possible that this 2600-2800 rpm thing could be related to the ECU/emissions system? (At first, I was concerned that "pounding" the halfshaft nut (described earlier) may have toasted the tranny output bearings - but that doesn't make sense, because my problem appears to follow ENGINE rpm, versus HALFSHAFT rpm)

Thanks to all of you who have "hung on" to the end! I appreciate any feedback.

1998 S70 T5, 5-speed, 140k


New Running rough, but only at 2600-2800 rpm ????????? [S70][1998]
posted by  Bill1959  on Fri Jun 8 19:56 CST 2007 >

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