engine misfires on rough road
 

Pardonez moi if this topic was brought up before--it didn't show up on a quick search. Many years ago (2013) my wife and I took my year-old FR-S on a road trip to Colorado. Somewhere in South Dakota there was a section of highway with pavement stripped down to a corrugated surface. I kept up with the traffic and the engine started running real rough. I took the next offramp and then the engine ran smooth again. I opened the hood and all the visible wire connections looked ok. I began worrying it was a gremlin Toyota and Subaru had overlooked. But when we got going again I kept the speed a bit lower, and I never had that problem again.

Just last week I read how some cars have a "rough road" sensor (not ours, I think). Apparently without this sensor, vibrations from a rough road can fool the knock sensor into causing engine misfires. If the problem doesn't persist, the misfire record gets erased from the onboard diagnostics. I don't know for sure this is what happened, but at least it sounds reasonable...

I highly doubt this is true, to get the correct frequency of detonation into the engine block from the road surface (via tyres, suspension, bushes and engine mounts) has got to take the perfect set of conditions, if it's even possible at all. Even then, it shouldn't cause misfired.

OK then I guess what a rough-road sensor must do is prevent OBD indications of misfires that aren't actually there (and not actually cause misfires). In my case, I guess the easiest explanation is bad gas that by coincidence made the engine run rough I was on the corrugated road.

Overpipe is slapping the engine and giving the knock sensor a run for its money? If you have ECUTEK, hook up your cable and look for knock correction when going off road again.

I'd just chalk it up to ...... an aberration ...... ;)


humfrz

What makes you think that the engine was misfiring? Did you get a CEL for a misfire code? If so, which cylinder?

What did the car actually do whenyou drove down that road?

Were you on cruise control or did you have your foot on the gas?

I hit a bump and got a coilpack CEL once, I think what happened is my body bounced over the bump causing me to lift off and get back on the throttle really quick, got some tip-in funkiness on the early '13 tune and CEL. Cleared it and it hasn't been back since even though I know the coilpack is bad, it only shows up under track conditions.

We were on a road trip and I didn't try get the problem diagnosed. It went away when I was on the off/on ramps, and when I was on smooth roads. I don't recall if the CEL came on or not (this was many years ago), but if it did then it didn't stay on for long.

I was probably jumping to a conclusion about misfires, but the engine was running very rough, meaning I needed more accelerator than normal to maintain speed. Because the road surface was a bunch of periodic bumps, the vibration was periodic and felt like the mechanical equivalent of a buzzing sound--very disturbing, I've never felt anything like it. The the vibration might have had a component related to the cylinder firing rate (= RPM/120, or around 20 Hz at 60 mph) or a harmonic. In these conditions is it possible the knock sensor was getting a signal that fooled the ECU into doing something to the engine timing?

I'll guess the nannies were kicking in pulling the throttle and the buzzing souind was the nannies applying brakes to individual wheels. It's a distinct buzz that rattles the whole chassis.

Yeah that sounds more likely. I don't think it was an engine misfire. Did you notice the brake pedal feeling any different?

Those sorts of bumps sound like they may cause wheel spin, I think Ultramaroon nailed it.

I don't recall using the brakes until I was on an offramp, but that's definitely possible. Thanks.

No driver input involved. When VSC kicks in and starts grabbing the rear brakes it sounds/feels like a giant fart.

Understood. I just meant I didn't notice any difference in brake feel. Thanks.

Here's a crazy coincidence. I was trying to think of a clip where I've heard the VSC kicking in. Then out of the blue @HotLeopardMama pointed me right to it.

Must've had your ESP cranked up to 10. :thanks:

Anyhoo, check out @soulreapersteve's first clip. It's kicking in like crazy. Does that sound familiar?

http://www.ft86club.com/forums/showp...postcount=9277