323, 1999, stuttering - only when hot
 

Mazda 323, 1999, 1.6l DOHC, 380000 km, manual, 91 octane When cold, motor starts right away and runs at around 1200, later goes down to 900 or such. After some time of driving (sometimes 100 km, sometimes less than 10), the engine starts stuttering. It seems to be running on all four cylinders, and the spark plugs looked dry and pink on the outside cylinders and white/grey on the inside cylinders. After having cleaned the MAF Sensor with a MAF cleaner, and after having driven the car with and without the MAF connected, I think I can rule out that the MAF sensor is the problem.Once the problem occurs, I have to wait for several hours at least (outside temperature here at the moment above 20 Celsius / 70 Fahrenheit). It will start again fine though next morning and again run well for a while.Also, the coolant temperature indicator seems to be volatile, but not obviously related to the problem (car often runs fine with temperature down and problem often occurs when coolant temperature is normal).I learned that the IAC valve regulates the idle, and only the idle. Idle is ok in general and it does still idle around 500-600 once the problem occurs.I resealed the air intake parts that are made of plastic/rubber.I “retrained” the ECU.When the problem occurs and I stop and restart the engine, I smell fuel.The air filter is and looks new.I have gone through forums and have read some major threads regarding this but I seem to not even be able to say if this is related to let me say “air, fuel or spark”.The problem never occurred when the engine was cold, or better, it never occurred in the morning, once the engine rested over night.All ideas and suggestions will be appreciated.

These are common for either Cam or crank sensors ( can't remember), the fact that you need to let it cool down is the tell tale sign.

Also ignition coils are common to play up/pack up on these engines. Go for a good drive get it well warmed up, get a small hammer & gently tap the ignition coils, listen to the engine & see if the engine stutters, this will indicate a internal fault in the coil.

Thank you very much, Grim Reaper. I will follow your suggestions tomorrow and let you know about the outcome as soon as possible.

water in tank
 

Sorry for the late feedback. It improved over the next days, and occurred in intervals. I then added some meth to the tank and the problem was gone. So, most likely, in this case it was simply water in the (metal) tank. Thank you for your help, grim reaper.