I am getting P0303 code. I've changed all spark plugs to NGK Iridium and swapped coils. Also ran injector cleaner such as techron and seafoam. After i reset the code it usually comes back a week or so later. Its always the same code and the cel comes on a cold start while the car is still warming up. I am out of ideas. What can cause this particular cylinder to misfire when cold?

 

If you replaced the coil to that cylinder with a new one and the misfire persists, pull the plug and check it's condition again. Make sure that all of the connectors are secure.

If you pull the coil off of #3 while the motor is running, does it affect the idle ? Maybe the fuel injector is restricted ? It may be a good idea to have the compression checked if you've isolated the misfire to that particular cylinder.

 

Yes, when I pull the coil for #3 it does affect idle,all 6 have a change in idle when I unplug the coil. I've changed the spark plugs not more than 3 months ago but ill double check. How would i check for restricted injector? I routinely use Shell V power gas and injector cleaner.

 

Did you just unplug the coil or did you physically replace it with a known good one?

The other thing that would cause it is a plugged injector or a bad cyl. Doing a compression test would tell you if the cyl is good.

 

whats the codes you get? they are clues not diagnosis

are you running seafoam at the` cleaning dose` of 2 oz per gal fuel?
thats 1 can to just under half tank

do that twice and the injectors and everything leading to and away from them is spotless
which makes a more accurate compression reading too
Note: always hold throttle wide open for compression testing

no need for extra techron bottles or others if you seafoam and run minimum 91 octane,
prefer tier 1 quality fuel -which contains up to twice the govt minimums that every brand of gas is required to carry.

In this case more cleaners keep our baby happier until midyear maintenance dose of seafoam 1 can in just under full tank 1 oz per gal fuel

note: octane boosters do nothing for non turbo or supercharged engines
just run the 91, or 94 if thats premium in your area,,
the TL was designed for 94,, and will accept 91 if it has too

on coil- did you move suspect coil and code followed the coil? after ecu reset?
plug for that coil looks black -greyish- what????
note cyl numbering: rear= pass to driver 123,,front pass to driver 456

reset ecu via CLOCK FUSE on passenger end of dash after moving coil or install new one

testing coil: remove the power lead to coil - not remove coil from spark plug !!
try running rpm up to 2000 with suspect coil unplugged and watch for symptoms

 

I just unplugged the connectors to test the coils. I also swaped coil #3 with #6 to see if the misfire followed and it still stays on #3. When I unplug them one by one, the idle changes each time. I will remove the spark plug to check the condition. I can never feel the car misfire during normal driving, only runs a bit rough when first started up in the morning and that's also when it throws the misfire code

 

Have you cleaned the EGR passageways, TB and intake ? Sometimes if these are carboned up, it'll cause a poor idle and may even throw a misfire code. Try some more fuel system/injector cleaner......as it may be a lazy injector ? Make sure that you clear the codes after doing any work.