I initially thought I was having a bad case of the 'herky jerkies' but it suddenly got much worse and I realized the car was misfiring. My Ultragauge beeped and it showed 3 pending codes (P0300 P0302 P0303).
I cleared those and drove the car gingerly to see if I could diagnose the behavior.
The bucking starts at around 2200RPM and disappears around 3000RPM. It is periodic and dependent on pedal pressure (slower acceleration often yields no bucking at all).
After driving it a bit (was at work when this happened), I saw a pending P0303 two more times, but the P0300 and P0302 didn't reappear. The car is driving fine the majority of the time.
If it matters, the car had been sitting for a week while I was on vacation.
The spark plugs were replaced 40K miles ago, but I think it still has all the original coils.
Any tips on where to start looking? Could the EGR plate or valve cause something like this? (I suspect they are in poor condition)
Finally had a chance to look at it today and quickly found the problem. It was as simple as it was odd.
I had removed the engine cover to replace the spark plugs awhile back and had left it off since I was planning on changing the coolant among other things. I never bothered to replace the cover and didn't think anything of it.
There are two bolts on the rear of the engine cover where it mounts onto the top of the engine. What I found was that these bolts also hold in place the rail that has the wiring to the coils attached.
The back right bolt remained with the cover when I took it off and the rail was no longer firmly attached. The back left bolt kept the rail loosely in place, but the right side was flapping around freely with the vibration of the engine. I'm guessing it was losing its ground connection when it lost contact with the top of the engine.
I fixed the problem temporarily by simply sticking in a random bolt I found at work. I'll reinstall the engine cover in the morning to make the fix permanent.
It is always nice when a potentially expensive repair (I was envisioning having to replace two different coils) turns out to be such a simple and cheap (free!) solution, even if it was a bit embarrassing when I discovered what the problem was.
I had thought I had fixed it, but it continued to appear on a sporadic basis(work fine for weeks then misfire badly for 5-20 minuted then be fine again).
Last night it reappeared again but this time it was persistent. My regular mechanic was booked solid so I had to take it to the dealer.
Even though I told them it was electrical, they insisted on doing all the mechanical tests first and everything checked out fine.
After talking with me again they finally traced it to corroded wiring in the main harness. They said it would $900 to replace (is that even possible?) But they could repair via soldering for *only* $300. Guess they are including the cost of all the unnecessary testing they did.
I really hate going to the dealer, but my only other option was renting a car for the weekend.
On an upnote, I guess you know all that other stuff is good now! So I'm curious if there is a common point on the wiring harness that might affect others, that's why I'm curious where they performed the harness repair.
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