The symptoms you describe seem to be more indicative of a head gasket leak, which is not uncommon for this vehicle. I had a double whammy with mine a few months ago and replaced the head gasket as well as the IP. The new head gaskets are much more durable than the originals. Though I never enjoy spending big $$, the car runs beautifully now. Also had the head inspected, which is a no brainer, as it is prone to cracking in perhaps <5% of vehicles of this vintage. Fortunately, it is in excellent condition. Head gasket replacement will run ~$1000.
'04 ML500 - 53k, Inspiration Edition, Desert Silver
'11 Audi A4 Avant - Brilliant Black
'87 300SDL sold
'99 C280 Sport sold
'85 190E 2.3 sold