A frequent complaint is a rough idle, which can originate from the fuel system or from one or more weak cylinders. Follow this procedure:
1. Verify that the disturbance at idle is not caused by exhaust pipe contact with the vehicle frame, a loose air-conditioner compressor, or other mechanical sources.
2. Scan for trouble codes.
3. Perform steps 1 through 9 in the preceding “Slow or no-start” section.
4. If you have access to a Ford factory tester, make a “buzz” test on the injectors, listening for differences in the sound. A bad injector often buzzes at a lower frequency than the others.
5. Remove the valve covers and, with the engine idling, watch the oil flow out the injector spill ports. Each time a HEUI closes, the oil charge above the plunger drains out the spill port. Thus, amount of oil spilling out of an injector relates to fuel delivery. Replace any injector that passes noticeably less oil than the others.
6. Test engine compression.
7. As a last resort, replace the injectors.
