[sbchevfreak]

The 1990 ford EEC system is still OBD I, so no misfire monitor, in fact a misfire will read as a false lean and open up the injector pulse width. A converter is not needed for ECM operation, as the EEC III/IV systems did not use a catalyst monitor. The #1 thing turning on the MIL is the EGR fault. These are known for the EGR valve position sensor failing, as well as DPFE faults. Get the cooling system fabb'd up, and do a full KOER code read, and give us the resulting numbers. This will help narrow the search to the proper system. Also, do a quick visual on the vacuum lines, the early 90's fords had a whole bunch of them. IIRC, the line to the EGR is a green line, with computer controlled vacuum supply.

[DanielC]

I guess a refresher term at a local community college, 2 years ago, studying fuel systems and emission control systems paid off. The last time I was at a community college, cars had carburetors, and most distributors still had points under the cap. But most of the cars had alternators, not generators.

[DanielC]

My 1996 Aerostar van is EEC IV, but because it is a 1996 (or later) it is OBDII. It does have a catalyst monitor.

[sbchevfreak]

Originally Posted by DanielC

My 1996 Aerostar van is EEC IV, but because it is a 1996 (or later) it is OBDII. It does have a catalyst monitor.

DUH, I had brain fart, lol! I forgot EEC IV carried over into the OBD II years, my bad. Once OBD II compliance came into effect, catalyst monitoring became manditory. In the OP's case, he does not have it, and IMHO, should focus his efforts on the EGR and EVAP faults.