Jeff got a good idea about the oil. I have seen the problem a number of times and the conclusion I have come up with is that the evaporators on 300Es have multiple passages through them. I believe that they are next to impossible to flush. We use dry Nitrogen set to 150psi to propel the solvent. We only flush the evap with highly volitile flush. The oily flushes may clean well but the easiest passage clears first and reduces the ability to push fluid in the lower passages.

I believe the combination of flush and oil causes these problems.

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Steve Brotherton
Continental Imports
Gainesville FL
Bosch Master, ASE Master, L1
33 years MB technician