I have a 2014 Focus ST2 with 69000 on the clock, It has an Airtek Stage 3 intercooler, RS airbox with a K&N filter and a Forge recirc valve and a Collins CP1 map. It has had oil & filter changed every 6000 miles using 5/30 Castrol (Ford) and a gen Ford filter. It runs fine once warmed through and pulls as it should do. Its had V Power or similar since I have owned it (8 years) and in general gets pampered, Its always warmed up when pushed and always allowed to cool down before turning off, but once warm gets driven as it should be 😁

However lately over the last few months has developed a slight misfire at low throttle openings in higher gears, its much more noticable when cold. I have a fairly good OBD2 Code reader and Forscan on my laptop , both show no DTC's but do show a small number (between 6 & 9) of misfires on cylinders 2,3 & 4 on the last run. My plugs are a step cooler NGK Iridiums as recommended by reliable tuners and when inspected show the usual deposits of a healthy running engine, but have put a new set in anyway as a process of elimination etc etc.

Once warmed up the misfires are only noticable (car jerks slightly) when at low speed in a higher gears, but once more throttle is applied they seem to disappear. The car idles as well as any other ST250 does and a "sniffer" test shows no signs of head gasket issues....

My initial thoughts as a purely DIY type are that the intakes could be well overdue a walnut blast having never had it done, the coils could be showing their age or an unmetered air leak somewhere rather than the known issues with the pistons.

How would I test for air an air leak and where would I connect it?  I will probably think of getting the intakes Walnut blasted anyway, simply due to its mileage and being modified. Any advice would be much appreciated.

20 minutes ago, efiste2 said:

I have a 2014 Focus ST2 with 69000 on the clock, It has an Airtek Stage 3 intercooler, RS airbox with a K&N filter and a Forge recirc valve and a Collins CP1 map. It has had oil & filter changed every 6000 miles using 5/30 Castrol (Ford) and a gen Ford filter. It runs fine once warmed through and pulls as it should do. Its had V Power or similar since I have owned it (8 years) and in general gets pampered, Its always warmed up when pushed and always allowed to cool down before turning off, but once warm gets driven as it should be 😁

However lately over the last few months has developed a slight misfire at low throttle openings in higher gears, its much more noticable when cold. I have a fairly good OBD2 Code reader and Forscan on my laptop , both show no DTC's but do show a small number (between 6 & 9) of misfires on cylinders 2,3 & 4 on the last run. My plugs are a step cooler NGK Iridiums as recommended by reliable tuners and when inspected show the usual deposits of a healthy running engine, but have put a new set in anyway as a process of elimination etc etc.

Once warmed up the misfires are only noticable (car jerks slightly) when at low speed in a higher gears, but once more throttle is applied they seem to disappear. The car idles as well as any other ST250 does and a "sniffer" test shows no signs of head gasket issues....

My initial thoughts as a purely DIY type are that the intakes could be well overdue a walnut blast having never had it done, the coils could be showing their age or an unmetered air leak somewhere rather than the known issues with the pistons.

How would I test for air an air leak and where would I connect it?  I will probably think of getting the intakes Walnut blasted anyway, simply due to its mileage and being modified. Any advice would be much appreciated.

I'd recommend getting a cheapish compression tester and testing all your cylinders if you believe it to be an air leak (that at least rules out anything to do with compression) 

I have a compression tester, and have used it on other cars, but not the ST. Im assuming it would be the same procedure....as in remove the lower pressure fuel pump fuse and the let the car stall, then do the test. I dont need to do anything with the high pressure pump etc etc

I would change the plugs before you go any further

Just see how it runs on standards