A little history. The truck is 173,000 mile truck. Tranny rebuilt 50,000 mile ago(Didn't need it but had the front pump out to replace the front seal and bushing) .. Over the last couple of years it has had a nagging problem which bugs me more than anything else. THe truck runs fun and so long as you do not crowd it(put a heavy demand on it) runs flawlessly. Low end power is great but on the highway especially when towing a boat if you lean on it real hard the truck will lay down some and when you lift off the accelerator you can feel the truck pick up. I am familiar with the EEC SD system and have done the normal troubleshooting. I verified FP at 40psi (No vacuum) and have replaced the Fuel Filter, TPS , ect, O2, MAP. The odd thing I have diagnosed is when the truck is idling if you pull the Vac line off the MAP sensor and slowly reveal the vac nipple on the MAP the truck will start to go rich(normal) but after you pull the vac line completely off the sensor the truck will set and idle fine with only a slightly rich exhaust. I recall the EEC SD in the mustangs that when the vac line was pulled off the MAP it would go VERY rich and chug and huff black smoke and nearly die.. Thihs one in the truck does not.. I have veriefied FP while bleeding vac off the MAP and the FP stays constant.

The only thing I know to do next is try another ECU, unfortunately I do not have access to another one to try without outright purchasing one..

Any ideas???

Rick