1989 244DL w/EGR - LH 2.4 Fuel Injection, EZ-116K Ignition system

I've done a lot of work on my ol' girl recently but I have this lingering issue. When the engine is warm it idles around 1200rpm in park or neutral. OBD is saying "Compensating for rich/lean fuel mixture at idle." No CEL. Part of the mystery for me is that almost every major component that could be involved is new/clean.

I did not have a blown head gasket but decided to do the job to breath new life into my car after 250k miles. The car now has a brand new head! The machine shop I went to had a new head that had been in storage for 30 years. I bought that instead of cleaning and resurfacing my old one. He used my old valves with new seals.

ALL BRAND NEW: head, vacuum hoses, AMM, belts, fuel injectors and seals, spark plugs and wires, distributor cap, harmonic balancer, header, catalytic converter, all involved gaskets, throttle cable (new and adjusted correctly), oil separator box, flame trap. Everything was torqued to spec. 15k miles on oxygen sensor and engine coolant temp sensor. 25k miles on crank position sensor. Throttle body cleaned and TPS adjusted.

I've tried swapping in a spare ECU (used) with the same result. Same thing with with the idle air control valve. OBD tests of the IAC and TB look good. Examination of the air hose from the AMM to intake manifold does not show any cracks and hose clamps are secure. No exhaust leaks.

The valve cover is weeping a little oil. I installed it without using any RTV silicone on the gasket, but also the cover mating surface did not look perfectly even. I'm going to replace the valve cover and gasket in the next few days. This is the only thing I can think of that is not currently optimal. Could that allow air in and throw off the mixture?

The car drives down the road great and does not stumble, hesitate, or seem to misfire (which I would expect if it was a timing belt alignment issue right?). I took it to get a California smog check, but the guy didn't want to test it due to the high idle speed.


Any ideas?