What's wrong with my car?!: Misfire Edition
December 9, 2016 9:35 AM   Subscribe

My car is running rough, CE light on, diagnostics show it has a misfire in the 6th cylinder. We've checked all the usual suspects, replaced the fuel injector, and still, misfire, cylinder 6. Help.

I have a 2001 Chrysler LHS, automatic transmission. I bought the car from an acquaintance knowing there was a problem, and expecting it to need some work. At first the check engine light was coming on periodically. The car sat for a while, then when we went to get it registered, the check engine light was off, and it passed an emissions test. Fast forward to now, about a year later, the light is on, can't pass the test to renew the registration.

I don't know much about cars (but my dad, a mechanic, does), so I'll do my best to explain what's going on and what we've tried. When I bought the car, it had just started having the problem, and the previous owner had replaced the spark plugs. So those are fairly new, and didn't solve the problem. Multiple diagnostics have shown a misfire in the 6th cylinder.

The noticeable symptoms are the car running rough - when idling there is a noticeable uneven vibration. The exhaust smells extremely strong, and there is a lot of smoke - especially big clouds when accelerating from a stop. And finally the problem that never seems to happen when my dad drives the car: the car will periodically start shaking violently when accelerating, almost like the engine is about die (but it never actually has). This seems to happen most often when changing speed/gears around or above 40 mph. Usually I've been cruising along at a fairly stead speed around 40, then I slow down for traffic or a turn, and as I begin accelerating again the shaking starts. It only shakes while my foot is on the gas. I usually can stop the shaking by flooring it for a few seconds until I feel the transmission kick into the next gear and it stops, and is usually fine for a while even when I slow back down. Like I said, the shaking also stops if I take my foot off the gas and coast, but usually starts again if I step on the gas pedal again, until I accelerate quickly as I described.

So far here's what we've tried:
- last owner replaced spark plugs, dad says they still look good.
-swapped coils from 6th cylinder and another. Misfire is still in 6th.
-replaced spark plug boots (? the rubber tube insulation thingies)
-compression test. Looked okay was about 140 in no. 6. I don't know if he checked the others.
-finally, we replaced the fuel injector.

No change after all of these things. After the injector was replaced, the diagnostics briefly returned a random misfire, then consistently went back to the 6th cyl. When my dad tried to reset the CEL, it wouldn't clear the first few times, then finally it did, and stayed off for about a whole half a block before coming back on. (Also, if relevant, the CEL alternates between flashing and solid while driving.)

So, please, anyone have any ideas of what could be causing this misfire? What to check next? I'm so frustrated and can't really afford the take it to another mechanic. I'll try to answer any other questions, and I'll be by my dad soon if there's anything that needs clarification. I realize I may not be explaining things quite right.
posted by catatethebird to Travel & Transportation (8 answers total)
Have you checked the wires going to the coil pack? Swapping the coils themselves wouldn't reveal a problem with the wires leading to the coils.
See this article on Chryslers "Coil on plug" spark plug troubleshooting
posted by cosmicbandito at 10:02 AM on December 9, 2016 [1 favorite]

Maybe this will help: "P0306 OBD-II Trouble Code: Cylinder 6 Misfire Detected." It seems to describe your issue pretty closely, including the flashing on the CEL, but recommends some steps you have not taken.

Have you had the code read, so you can get the number, to confirm it is a P0306?
posted by Riverine at 11:26 AM on December 9, 2016 [1 favorite]

This is probably way too obscure and specific to my car (an older Volvo) but in case it's useful anecdata: When I was having mystery misfire issues earlier this year, troubleshooting and googling led me first to the crank position sensor (which I replaced to no apparent effect) and then to a technical service bulletin describing how the spark plug wires can cause interference if the crank position sensor wire is too close to them. Recommended fix: zip tie the crank position sensor wire away from the spark plug wires. In my case this low-tech fix solved the misfire issue immediately and decisively.
posted by usonian at 12:01 PM on December 9, 2016

Did he put a test light on the injector to make sure the new one was firing? The wiring could be damaged to the injector so that it's not actuating.
posted by hwyengr at 12:52 PM on December 9, 2016

Spark plugs are so cheap, that it's really silly to have gotten this far and not already replaced them, regardless of what condition they appear to be in, or when they were supposedly changed. At least the No. 6 one. They *often* look fine and aren't.

And, it's probably plug or coil wires. Which are also fairly cheap*, wear out, and are frequently forgotten / not considered (because they don't look like something that wears out.) They're conspicuously absent from your list.

* if you actually get the cheapest ones, you may only get 10k-30k out of them, if they're not defective when you install them. OEM ones last 30k-100k. But there's just too much variability to say more.
posted by teatime at 2:04 PM on December 9, 2016 [1 favorite]

usonian: That's a really good point, and makes total sense to me. The aggravating part is, it could literally be just about anything, and it's really hard to figure it out without either a clue (like your TSB), or real auto diagnostic/mechanic experience.

That's the kind of problem that costs a confident, hobbiest car guy like me a bunch of money, spent on replacing perfectly good parts.

And OP: This process of replacing possible causes until the problem goes away, is almost disgustingly inefficient. But I understand exactly why you're doing it (you usually win the bet that it'll be way cheaper than a shop, even with the waste). Sometimes you lose, though.
posted by teatime at 2:10 PM on December 9, 2016

Yeah you don't mention wires at all. Usually plugs and wires are the first thing to try with any kind of misfiring issues. They're so cheap that even if they were replaced recently, there's no real reason not to do it again to 100% rule out the cheap stuff. It's a 10 minute job that any person who can turn a wrench can do. Also the fact that the problem started right around the time they replaced the plugs is.. suspicious.
posted by zug at 5:33 PM on December 9, 2016 [1 favorite]

If it happens primarily when the car is cold-started, or when it's humid out, it could be arcing from bad wires. I had this in my Subaru on chilly autumn days, usually cleared up when the car had a chance to warm up completely or I was starting it in the warm middle of the day and it wasn't humid out. Replacing the wires fixed it.
posted by sixfootaxolotl at 1:59 PM on December 10, 2016

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