I have owned my '05 S-type 4.2 for a little over a year now. I replaced the thermostat and thermostat housing unit about a month ago and made quite a mess in doing so. I went ahead and hosed out the engine, despite always being taught growing up that you never spray water on an engine. But at this point there was so much coolant all over it that I figured a little water wouldn't hurt. Of course I was wrong and the engine began running really rough after the wash. After a couple hours it evened out and seemed to idle fine without dying and seemed to run fine, EXCEPT when I tried to accelerate. It would start hesitating and jolting if I applied too much pressure on the gas pedal.

A few weeks later I took the car in to have the water pump replaced and had them run the codes because the car was still having the acceleration problems (let me point out that the engine light had been on since before I replaced the thermostat due to the coolant temperature being too low, which I fixed by replacing the thermostat). They got codes that three of the cylinders were misfiring and said I needed to replace the coil packs on them. I told my mechanic about me washing the engine and he said that it was probably just moisture in there and they could be sprayed out with compressed air (which I told them I would do myself, rather than paying $100+ to have them do it). He went ahead and reset the codes and the engine light went off. But not only did the engine light go off, but the car ran great! Perfectly normal again for the rest of the day.

The next day I got in it to head to work, and once again the engine hesitation was back. This was a huge letdown since it was running great the day before and I could gun it and it would take off with no hesitation. The engine light is not coming back on though. It has been a week since he reset the codes and it ran good for one day, and about a month since the issue first started. My question is; if there really is moisture in the coils, then why would resetting the computer make it run good for one day? I plan on spraying them out with air this weekend, but that doesn't seem to make sense to me since it worked for one day, then went right back to having issues again. And, wouldn't the engine light come on again if I was having misfiring issues still?

Any help or insight on this issue would be much appreciated, thanks!

 

Welcome Ian. Thought the CEl was re-set and she ran fine then collapsed with only low speed driveability, moisture in the spark plug well can continue the coil misfire as well as moisture about the throttle body electrical connector. Pull each coil and inspect and dry the spark plug wells, coils and throttle body electrical connector.

 

Hard to say in detail why you got an apparent short term fix but if it IS misfiring then you're busy damaging the cat conv

Fix may have been that it was running open loop, when it runs rather rich. Even worse if misfiring as the extra ful burns in the cats.

These are very expensive things to run with codes when you can read them free at AutoZone or get a $10 elm327 and DIY.