My 1990 245 has a problem. It was running a bit rough, but getting good mileage and had good power and response. Then one morning it had a problem. It hesitates at throttle opening and is low on power, but runs fair above about 2000 rpm, still hesitates a little when throttle is increased, but responds better than at off-idle. The off-idle peformance is really poor, and with a cold engine I can nearly get it to die by slowly opening the throttle. It feels to me like an ovberly-lean situation- no popping, backfiring, or any such symptoms that would indicate an overly-rich state. When I "pump" the throttle I can get it to not hesitate so badly, so it would seem, again, to point to an lean mixture (I realize it does not have an accelerator pump).

At first I thought Throttle Position Switch, but the hesitation and low-power is at least somewhat evident throughout the normal operating RPM range.

I have checked a LOT of things, including (but not limited to):
-OBD cocket 2 and 6 codes (none displayed for some time, but originally was getting a 1-2-1, "no signal to/from AMM").
-OBD socket 2, test 2 and 3. Everything checked out.
-Cleaned and reinstalled throttle body
-Cleaned and rinstalled Idle Air Control
-Readjusted Throttle Position Switch
-Readjusted throttle cable and linkage
-Adjusted idle stop
-Checked for any air leaks between AMM and throttle body.
-Replaced Plugs, ignition wires, and dist cap (rotor on the way).
-Checked every vac line I could find, made sure that they were tight, and trimmed the ends of those that felt even a little loose.
-Cleaned all vac fittings on manifold to be sure they were clear.,
-Cleaned flame trap and related lines. (good vac at oil cap and no significant oil leaks).
-Removed all under-hood connectors I could find, cleaned contacts, silicone greased, and reconnected/disconnected a few times before fastening.

Another oddity (maybe) is tha tthe "Check Engine" light never came on. it illuminates when the switch is in P-II and goes out when the cars is started, but even when the AMM code was set, the light did not come on... if that means anything.

I have a few guesses as to what it might be, but to just start replacing parts until I get lucky can get expensive, particularly with what one or two of the suspects cost! I also have a few more things to check, but thought I would ask here to see if I am heading in the right direction before I give up and take it to an independent service center..

I am going to try the "Spray something on all vac leak suspected places to see if the RPM or behavior changes" trick (use WD-40?), but if you have any thoughts, please share them. I would much rather be driving the car than washing my hands three or four times a day with mechanics' hand cleaner, if you know what I mean..

It is (or seems to be) idling a bit smoother (roughness mostly gone, I think) but the hesitation is fairly severe and needs to be taken care of.

HELP!

... and thanks!