A MD11 Crew noted fumes during preflight which intensified after engine start and during taxi so the aircraft was returned to the gate; removed from service and had extensive air conditioning component cleansing to remove oil residue. This aircraft previously had an APU oil system failure which allowed oil into the air conditioning system.

Narrative

The Captain and I have been flying for the past six weeks together on a base line trip with literally dozens of legs as fellow crewmembers. We completed a four-day pairing and both elected to stay at our domicile (both of us are commuters) in order to catch up on sleep and get a decent rest the following evening prior to this flight. Fatigue was not a factor for either of us in my opinion. We arrived at the aircraft for a morning round trip turn. I noticed upon arrival that the external air cart was hooked up and was very noisy; along with both doors open to the courier area. It is uncommon for the right courier door to be open. The APU was deferred and could not provide air conditioned air nor could it provide air for engine starts. As we arrived in the courier area; there were four ground employees attempted to clean the aircraft and rid the smell that was present. We both completed our normal pre-flight duties and waited for the loading to be completed. Since the loading took longer than normal (40 minutes late); I elected to leave the cockpit area and go outside in attempts to obtain fresh air. I postulated that the fumes were the result of the external air cart exhaust; and that I received 'more' of the exhaust since it was directly outside my window. When the load supervisor arrived with the Weight and Balance forms; I proceeded back into the cockpit. The Captain and I discussed the fumes and theorized that the fumes would subside when we were under normal aircraft pack operation and the external air cart was disconnected. The right engine was started; the air cart disconnected; and the aircraft pushed back and subsequent engines starts were normal. During the ensuing taxi out to the Runway; (runway change from planned); both the Captain and I missed minor items which we caught during normal cross-checks. This was out of the normal behavior for both crewmembers. The Captain is a very thorough; professional aviator who normally does not make mistakes. After several minutes of taxiing to the runway; it became apparent that the fumes were not diminishing; but rather; intensifying in noxious form. With our eyes burning and smell of fumes in the mouth and nose; we both elected to don oxygen masks on the 100% setting. The Captain informed the one Additional Crew Member (ACM) of our donning the masks and encouraged him to do as well. We informed Ground ATC of our desire to return to the ramp. We decided against declaring an emergency since we both felt we were able to successfully navigate our taxi back to the ramp. The taxi back to the ramp was uneventful with both crewmembers and ACM on 100% oxygen. Upon engine shutdown; several Maintenance personnel and Assistant Chief Pilot (ACP) noted the strong odor and noxious fumes. We both elected not to continue with the turn on a different tail number based on our inhalation of the fumes for an extended period of time.