My 2004 Ford Focus (about 230K miles and still going) with the 2.3L 4 cyl is having an intermittent stumble while idling and when taking off. I haven't had a chance to dig into it yet, I'll do that this Saturday but can you guys direct me to the right starting point? I know the fuel filter is over due for replacement but what else could cause it?
Fuel and air filters are a good place to start. How old are spark plugs? You might want to check fuel pressure as well.
Spark plugs aren't too old maybe a year to year and a half. This car doesn't have a replaceable air filter. The gauge on the housing says it's not clogged though.
OK So I checked the spark plugs, all light greyish normal looking. Changed the fuel filter, changed the oil (it was due), cleaned the MAF/throttle body. The result was the same, occasional stumble. I couldn't test the fuel pump because I didn't know how to connect it to the fuel rail due to this big old thing.