My 2002 Ford Focus ZX5 has had a low idle (not unsteady) for about a year. Just thought I needed a tune-up. Recently, it started surging and bucking between 40 and 50 MPH. Seems to run fine under acceleration, but then when backing off the accelerator, it starts the surging/bucking. The check engine light is on constantly. A diagnostic pointed to the EGR valve, which I replaced, but it didn’t solve anything. Any ideas?
I’d check your fuel pressure. You may want to replace the fuel filter. It may be due anyway, it is cheap and could easily cause what you are describing.
OK, fuel filter has been replaced. Problem still exists. It seems to now be surging/bucking as I’m accelerating from a stop. Once underway, it’s fine until I reach the 45-ish mph mark, then pulling back on the accelerator, it starts again.
The problem might be caused from a dirty Mass Air Flow sensor.
Purchase a can of MAF sensor cleaner, remove the MAF sensor and spray down the wire inside the MAF sensor. Let the MAF sensor dry and reinstall it.
Tester
Test, I did that about 6-8 months ago, thinking it would solve my low idle issue. No go. 