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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):

enter image description here (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.

  • Great question +1 – DucatiKiller Oct 18 '16 at 20:00
  • Does your prelude have the odd even manifold split like in the picture? I wouldn't replace the fuel injectors unless a scope test or injector balance turns up something. Also a damaged o2 sensor/exhaust wouldn't cause a misfire unless you're running so lean your getting detonation. If the pipe was so severely restricted to cause misfiring it'd affect the whole bank. – Ben Oct 18 '16 at 20:34
  • @Ben It is split similar to the picture. hondapartsguys.com/auto-parts/2001/honda/prelude-coupe/… part 7 at the bottom left is the split, that connects to the manifold. The fuel trims are highly negative but the car has always been that way, although I don't have enough data to know if it's changed since the damage. I'll have to dig up some old logs. – Jason C Oct 18 '16 at 20:45
  • Maybe take a look at the down pipe. If there was a restriction on one side and not the other it might explain what's going on. Though I'm not sure if the manifold is truly split odd even or cyl 1 & 2/3 & 4. – Ben Oct 18 '16 at 20:48
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    I know some of the Hondas had EGR ports that would clog and sometimes cause random misfires. Try cleaning them if you haven't yet. – Derek Apr 2 '17 at 0:54
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If the muffler hitting the railroad tie stopped the car, you very well may have snagged something loose. Visually inspect your exhaust and pay special attention to the O2 sensors, they may have been jarred loose.

I wouldn't replace the fuel injectors just yet, but do visually inspect the engine bay, and for sure reseat the fuel injectors' connectors. I doubt the exhaust could have moved the engine block, but have a look and see if anything could have been pinched by the exhaust or engine being pulled backwards, look for something that has a tight fit as it is.

  • I took a look underneath. The O2 sensors seem fine. I didn't notice any visibly damaged welds. I didnt see any major looking dents or anything, at least not from the railroad ties. I did discover some old damage on one side of the exhaust fork, looks like an old speed bump hit maybe, and I wonder if this latest thing was enough to cause a hairline crack there or something. I'll have to check for leaks. I've reseated the fuel injector connections and will see if the problem persists. I didnt see anything that could be pinched or damaged by the engine moving backwards. – Jason C Oct 18 '16 at 23:03
  • So, I cleaned all the oil and crap out from around the connectors, sprayed them down with contact cleaner and reseated them. I also ran a bottle of m.autozone.com/fuel-and-engine-cleaners-additives/… through, and, unrelated, cleaned out the throttle just for good measure. I drove it 120 miles with no misfires yet. So perhaps something got knocked loose, or the injectors were dirty, or who knows. Gonna give it a few days to see if the problem stopped. – Jason C Oct 19 '16 at 16:40
  • I forgot to post back here. Unfortunately, while the problem seemed to have stopped for about a week, it started again, and continues to this day. Same pattern. Once every day or two, always the odd cylinders, no noticeable symptoms... – Jason C Dec 22 '16 at 22:15
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So, I don't have an exact answer to the actual problem, but I will say and an exhaust leak would be an unlikely culprit for misfire. Also, if impact damage had enough force to do that it'd probably be pretty visible.

That said, it's most likely that something else is the culprit which could (mostly) unrelated.

What We Know

We know the exhaust system is fine.

We know the O2 sensor is ok.

We know that doing the work you did resulted in a temporary fix.

What We Can Extrapolate

It is doubtful that the exhaust or an exhaust leak is the fault. If this was the case, doing the things you did would have had no effect. That and I think it would take some serious damage to cause a misfire.

There is likely a fault coming from your injectors or O2 sensor. Most of what you did targeted these parts. Cleaning and reseating everything could help a partially faulty component work correctly.

It is possible that one of the connectors is partly damaged. It is entirely that a sharp jar could damage a conductor, pin or clip in a connector or the leads for you O2 sensor. I this happened the vibrations of driving could cause intermittent codes to be thrown, however I don't know that it would actually cause misfire without being a somewhat consistent problem.

I'll keep an eye on this thread and if anything more certain comes to light I'll update my answer.

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