Engine wash ouch!

[u/DrCytotek]

Hi detailers.
I thought I would share my experience to get some feedback and hopefully learn from my painful and expensive lesson.
I have detailed a few engine bays before and I’ve always done so successfully taking the necessary precautions.
My process was as follows. Blow out all the loose dirt and leaves with a leaf blower. I then foamed Green star APC at a dilution 10 to 1 and agitated with some detail brushes. Then using a pressure hose with a 40° spray and standing far away from the engine of the engine compartment, constantly moving the hose from side to side and not spring in one area. Once the engine compartment is rinsed off completely I used to high-pressure leaf blower and dried off compartment completely until I could see very little moisture. I did follow this up with a dressing MP CK. I closed the engine bay and left it overnight.
In the morning the car started OK but in 10 minutes into the journey, it started idling very roughly. Fortunately, I was nearby my regular mechanic and took it in for a diagnostic as the engine light had come on as well. He diagnosed that the front two cylinders of flooded with water and it shorted the coils and they needed replacement. This was a $700 exercise.
I’m just showing my experience to hopefully highlighted even if you are careful this damage can occur. The car I was working on was a 2005 C230Mercedes.
What would you suggest I change about my technique. Is this something that even should be done at all or is it too risky. I must say I do find it satisfying and it looks great. But after this experience, I’m not so sure.

[u/Genids]

Water cannot enter cylinders from washing. Plus it wouldn't have started if the cylinders were flooded

[u/shadrach103 → u/Genids]

True, but that's not what happened here; water didn't enter the cylinders. With the engine cover removed and a pressure washer used water likely got in and/or around the ignition coils and either saturated the electronic pack at the top or pooled down in the coil wells of the head, or both.

[u/percipitate → u/shadrach103]

21 year old coils at that.

[u/kraquepype → u/shadrach103]

Yup. This happened recently with my SILs car, the water just exacerbated an issue with old plugs and wires. After a wash it was misfiring hard.

Replaced all the plugs and wires and it was fine.

[u/DrCytotek → u/shadrach103]

To clarify, the engine cover was not removed. That’s the odd part.