2001 Honda Prelude SH.
About three weeks ago while travelling 10-15 mph I pulled off the side of a dirt road to park and it turns out there was a row of a few railroad ties laying in the grass there. The ties ran just inside the right tires, with most of the weight of the car on them, and my car stopped when the front side of the muffler hit the leading edge of the tie. I backed off of them, took a cursory glance underneath, wondered how my muffler was still attached, admired the exhaust pipes' fancy new wood veneer for a moment, then decided to pretend that didn't happen.
The ties hit the exhaust system pretty hard. Here is a picture, this is somebody else's car with an aftermarket exhaust but it at least shows the routing (railroad tie path is orange arrow):
(image source, also thanks @Myself for finding that).
The car was squeaking a lot after that which turned out to be a bent heat shield rubbing against the exhaust pipe (circled in green above), an easy fix, I corrected it later that day. I didn't notice any other damage on a closer visual inspection. The cat is on the centerline, I didn't hit it directly, but I'm sure I stressed the whole system hitting the pipe in the back.
Now, starting exactly the same day, I am seeing random misfires on cylinders 1 and/or 3 (it's an inline 4) one to three times daily, without fail. That's been going on for three weeks now without getting better or worse. I have yet to see even numbered misfires. The codes are P0301 and P0303 (misfires), P0300 (which is just a consequence of multiple P030x codes), and P1399 (which for Hondas is just another misfire code). I'm not noticing any power loss or other engine issues. I have not yet been able to determine any specific conditions under which the misfires happen (and I have tried).
So here's what I know:
- Car has brand new spark plugs, ignition wires, MAF sensor, and air filter is clean.
- Prior to run-in with railroad ties there were zero engine issues. Also in the 15 years I've owned the car the only misfire I've ever seen was a failed spark plug about 12 years ago, it's not a usual occurrence on this car.
- Starting the same exact time as hitting the ties, random misfires on odd cylinders on a daily basis.
- I'm not noticing any new exhaust noise, but I have no info on additional smells or smoke, I have an extremely poor sense of smell and it's been cold out so lots of water vapor disguising things. I will keep an eye out.
- I have not checked the exhaust system for leaks but probably should.
- No noticeable power loss, no rough idle.
So my question is: Could these be connected? Is there anything that could be damaged in the exhaust that would cause a misfire? Damaged cat / o2 sensors / connections / muffler? I didn't notice any loose brackets or visibly broken pipes or connections around the muffler or any other components. Something in the engine? Could these two things be connected?
What about, is it perhaps possible that something beyond my understanding happened that is causing it to falsely detect / report misfires?
The next step I will be taking is to continue to view these as independent events and change out the fuel injectors. That's the only thing I can think of but I'm wondering if there's some obvious connection to the exhaust.
Update, about a month or two after the above post:
Shortly after this post, I cleaned and reseated the fuel injector connectors. I also ran some injector cleaner through the fuel system. I cleaned the intake manifold and throttle just for fun. I visually inspected the O2 sensors amd cat but all the welds and sensors and wiring looked physically fine. The problem stopped for about a week then returned (a month or two ago). It continues in the same pattern: Odd cylinders only, every day or two, no other noticeable symptoms or engine weirdness.
