Removed from storage because winter is over? Lucky you - I expect that won't happen here until mid April.
CTSC with an aftermarket ECU or do you still have the OEM ECU with a tack-on FIC? If you still have the OEM ECU and you are getting legitimate misfires, you should be generating misfire codes. The OBDII ECU has two sets of misfire codes. One set seems to be generated by the sensor connection in the coil and the second set seems to be generated by the more traditional method of detecting crank speed fluctuations. I see you are from Rotterdam - when did the EU cars get OBDII style error code reporting?. Even if you have a pre OBDII car you should still have the older crank speed detection system. If the misfires are bad enough to trigger the codes the individual codes should help you pinpoint the cylinder.
If you have a CTSC, by any chance do you have front and rear wideband O2 sensors to monitor AFR? If you do, true misfires will show up as spikes in the AFR when the misfire occurs. The front / rear sensors may help you identify the cylinder bank on which the misfires occur.
Misfires can be caused by fuel mixture problems (too lean) in addition to ignition failure. A cold engine needs a slightly lower AFR to run smoothly and may have misfire issues when cold if its not getting the proper enrichment. This misfire problem will be random - not cylinder specific. This kind of cold engine misfire will likely disappear as the engine warms up. Also, this type of cold idle misfire may not be 'big enough' to trigger the crank speed detection codes and wont trigger the coil detection codes if you have an OBDII ECU. The cold misfire problem can be caused by an ECU engine coolant sensor going out of spec because the ECU uses coolant temperature to determine the cold enrichment. Checking the cold resistance of the sensor as per the service manual is technically easy to do (getting at the sensor is a bit of a struggle) so inexpensive to rule it out as a possible cause. This problem disappears as the engine temperature warms up because the engine does not need the lower AFR when warm plus the ECU goes into closed loop to control the AFR to target values.
Poorly flowing fuel injectors can also cause a lean misfire when the engine is cold. When the engine is cold the ECU is running in open loop and will not try to correct the AFR if it is running below the target value. Once the engine warms up and the ECU goes into closed loop the ECU may be able to correct the fuel mixture making the misfire go away. At high engine load / wide open throttle the misfire may reappear on a warm engine because the closed loop control may not be able to provide enough fuel correction - normally this would cause an O2 sensor mixture error code. This type of misfire will be cylinder specific if it is just one dirty injector. Unfortunately fuel injectors involve removal and shipping to a specialist to have them cleaned and flow checked.
An out of spec fuel pressure regulator (fuel pressure low) can also cause similar symptoms. If the regulator pressure is slightly low closed loop control can fix the problem when the engine is warm; but, may not provide enough fix at wide open throttle so the problem can reappear. A failing fuel pump can cause low fuel pressure; but, a fuel pump problem usually shows up at high engine load, not as an idle misfire. Fuel pressure is relatively cheap and easy to check. Low fuel pressure induced misfires affect all cylinders so will be random until it gets so bad that all cylinders are misfiring.
Random misfires can be caused by internal insulation failures in the individual ignition coils. If you have an OBDII car this should show up as misfire error codes. This type of failure normally does not go away as the engine warms up. Ignition misfires can also be caused by external flashover to ground at the coil nose / sparkplug terminal. This misfire will show up as tracking on the coil nose or spark plug insulator. This type of misfire can be random until the tracking gets so bad that it is always misfiring. This type of misfire is usually not temperature sensitive unless it is due to moisture / condensation forming on the coil nose in which case it may disappear when the engine warms up. If you search on Prime you can find posts with photographs of coils displaying contamination and tracking on the coil nose and spark plug insulator. The coil contamination / tracking problem is more common on the rear cylinder bank because the rear cylinders are more susceptible to water ingress into the coil wells - particularly if you like to wash your engine regularly. Coil nose tracking / spark plug tracking will be pretty obvious when the coils / plugs are removed for inspection.
Out of spec O2 sensors can cause fuel mixture problems which can cause misfires. But, O2 sensors are not 'in the control loop' when the engine is cold so they should not induce a mixture related cold misfire problem.
Start with the easy / cheap stuff first
- if you have an OBDII ECU, scan for misfire codes. If you have an OEM pre OBDII ECU use the service check connector to see if there are any stored misfire codes.
- remove the coils / sparkplugs to examine the noses for contamination or tracking. Start with the rear cylinders first. If there is any sign of tracking replace the coil and plug. Cleaning the coil nose sometimes works; but, is hit and miss.
- check the coolant temperature sensor resistance when cold (as a hint if you have an OBDII car a good code reader can actually read the ECU coolant temperature which might tell you whether it is out of spec).
- check the fuel pressure at per the service manual
- last step would be the fuel injectors just because that involves removal which is a fair amount of work and spending Euros to get them cleaned and flow tested.
After carrying out the above, everything else gets more complicated and expensive.
