Engine idles rough even though we seemed to have checked everything... had one code stating a miss in #4 cylinder..... thought it was the #4 fuel injector so replaced with no change... now no codes... compression & spark ok... yet the engine idles rough. Thoughts please!!!! Thanks!!! Mike

SOURCE: Check engine light comes on. p0171 and p0351 codes found

From Lusty Kid:

I know the economy is bad, but change all of the plugs. Check your PCV Valve hose, 150 rottens them out allowing air to be sucked in, thus all kinds of codes.

Depending on mileage. Dismount the EGR valve and clean passages. If you are really good dismount the throtle position sensor body and clean out passages with Choke Cleaner., dismount where the EGR valves connects to the engine, clean out passages. Also check the hose thats behind this mounting to ensure is not clogged. If clogged clean it out. If not replace it. Is about 1.5 ft. long, connects to the PCV Valve Hose.

When is the last time you change your harness. Can't remember??? Replace it. Good Luck

SOURCE: 1993 Ford Explorer idles rough when engine is warm

try changing distributor cap & coil

SOURCE: 2005 Ford Escape Rough Idle - engine code state lean fuel mixture

I recommend getting a repair manual from an auto store for your vehicle. They run $15 to $20. It will tell you exactly how to do everything you asked and also give you the values that should be expected. Hope this helps.

SOURCE: rough running, erratic spark

Run a compression test on the cylinders.If reading is low on any drop a little engine oil into the pot. If compression rises you have compession ring problems. Remove all plugs before you try this and place leads away from engine.

SOURCE: Car is running rough, afetr spark plug change

make sure no fuel is down in the spark plug ports it will short out the spark