Having researched posts and posted on this previously the consensus was there was a vacuum leak allowing unmetered air into the combustion cycle.

Here is a recap of tests, results and questions. I am thinking that I must be checking something the wrong way since with the list I have available I think I have checked everything.

1) Throttle knob 'O' ring is intact. It is tight enough to require some force to pull out of the Idle Control Knob socket.

2) I did an all around block of air hoses test on the IAC (IN and OUTlets), the Oil Vapor trap (PCV) at the accordion pipe, and the large hose to the EVAP at the Throttle body, the small EVAP tube holds pressure. I removed air blocks one at a time to see which caused the engine to stall for lack of air. Only the air block to the IAC stalls the engine.

3) The Throttle plate was visibly inspected to make sure it was closed.

4) Brake Booster: The line passed the break pedal test ... is there more to this line holding vacuum?

5) Climate control ... when I disconnect the vacuum tube and blow back into the vacuum tank I do not get a build up of pressure. When running a warm engine and disconnect the climate control vacuum tube the RPM climb by maybe 6-800. When the inlet to the intake manifold is blocked the RPM return to normal. ... Is there another test?

6) Cruise control ... (does not exist on my car)

7) Ignition advance ... ? ... My '88 has no vacuum line to the distributor for ignition advance. I thought this was managed by the Fuel ECU. ... Is there something else?

8) FPR is new and the new vacuum hose is clean an holds pressure.

9) Injector seals,
1. I checked with a hose (stethoscope) and hear nothing other than engine noise.
2. I blew propane over each injector and there was not change in RPM.

10) Cracked and/or brittle manifold gasket.
1. I checked with a hose (stethoscope) and hear nothing other than engine noise.
2. I blew propane over each cylinder port and there was not change in RPM.

11) Ignition ECU ... the tube pressurizes when I blow to the ECU.

I have been tracking MPG and oddly it has gone up from a normal 20+ to 21+ to 23+ to 20+ to 22+ to 19+ to 23+. I never have noticed such fluctuations in MPG and recently have used the car for much less varied purposes and think the MPG would be more consistent.

Insights appreciated.
Thank you
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1988 244 DL; B230F; LH-2.2; Manual 5-speed (M47)