One problem related to gas recirculation valve (egr valve) has been reported for the 1997 Ford Expedition. The most recently reported issues are listed below.
In idle the engine can not hold a smooth idle at 1400 rpm. The dealer service dept. Claims it's the egr valve opening and closing. When driving, this problem does not allow the engine to run smooth and affects power at a range of 1400-1700 rpm depending on loading on the engine (I. E. Towing a trailer, driving up hill. . . ). This is occurs constantly and affects the performance of the vehicle in all driving conditions. The dealer has attempted six times to correct the problem without success. The korum Ford service manager mike powers admitted it being a defect since they weren't able to correct it. Ford has chosen not to take any action to commit to fixing it or providing a fair replacement of the vehicle. I have found one other documented case of this problem on the web.