Does your car have an EGR system? If so, I'd blame it. The sole purpose of an EGR system is to reduce NOx output by dropping combustion chamber temperature.

The EGR valve on my 90 740T (b230ft engine) is attached to the block with the same bolts that secure the oil trap.

The easiest test is either the OBD self-test (this is present in 1990 LH2.4, not sure you have it), or just sucking on the vacuum line to the valve. Nothing should come out, but your idle should drop significantly if the valve and tubing is clear and working.

Seafoam was a great idea, distilled water might not hurt either (I use both, alternating between the two) because piston deposits are a big cause of too-high compression and high chamber temps.

I'd say more but my laptop battery is dying.

Good Luck!