P0300,0301,0302

Last night my engine started misfiring and my reader gave me these codes. After a little bit of research, I am thinking my path is going to be to check the sparkplugs/coils and also see if anything is leaking into them. If that all checks out, I am going to move on to cleaning the EGR system including the cooler and the intake manifold. Does this seem like a good starting point or should I be looking into other avenues first? I am trying to go for a combination of the most likely cause, and the easiest/cheapest to do first.

Additionally, when I went to check my coolant to see if it looked off due to leakage, it looked fine but there was also a bit of pressure release when I opened the cap. Nothing crazy but loud enough to be easily noticeable. The car has sat all night and is completely cooled off. Is that little bit of pressure normal, or is that something I need to look into?

 

Sorry I left one out. It was 0300,0302,0303,0304. So cylinders 2,3, and 4 misfiring, but not 1. I also just changed the oil about 4 days prior and the oil came out looking normal. I haven't noticed any loss of coolant or discoloration. Before I got home, while I was still driving the car, the misfiring happened while idling, and at very low speed, but went away while driving it at 25+, maybe a bit lower. I just changed the spark plugs and while I was at it, I checked the coils and I looked into the wells and there was no sign of any liquid.

I will definitely look into the head gasket though. That isn't a problem I want to leave there if it is the cause.

 

I think somewhere around 250k miles. I went ahead with the process of cleaning the egr system and intake manifold because i figured even if it isn't the cause, the previous owners havent seemed to keep up on maintenance, so it is long overdue. When I removed the egr cooler, it was almost 100% clogged. Barely a trickle of liquid would get through. I am still in the process of cleaning it because I don't really have $350 to drop on a new cooler right now.

Unless any of the things I can check/test for a blown head gasket are any easier at my current state of disassembly, I'm going to have to reassemble once the cooler is cleaned out and then I'll try to run it again. If the codes pop again, first thing I'll go to will be a leakdown and block test.

 

Update: Got the EGR and intake manifold cleaned and manually cleared the codes with my reader. I was getting worried that I wouldn't be able to get the cooler cleaned but an overnight soak in purple power and a pressure washer did the trick. No misfires on startup and no misfires or codes during the 6 hours I spent doordashing last night which involves a good combination of driving and idling. I am still keeping a close eye on signs of a bad head gasket though just in case.