T-Bucket

Flew back home to take my S2000 out of storage for the summer. Finally diagnosed the small parasitic battery drain that had been plaguing me for a while. (Turns out the alternator diode was bad, and was leaking current to ground). So, swapped out the alternator with no major issues. Went for a test drive, and after about 15 minutes of driving, the car started misfiring quite badly. At low RPM and under load (pulling away from a traffic light, etc) it would buck and you could hear the occasional pop from the exhaust. Code scanner found the Random misfire code along with misfires in cyl 1,3,4. At higher RPMs it ran perfectly fine. (Anything over about 2500 was without issue).

Anyone have any ideas? Searching the internet has people with anything from bad injectors, to coil packs, to MAP sensors, to bad valves. The gas tank had some Sta-bil in it, but had been sitting for ~5 months now. (Only driven a couple times over the winter).

Oh, and it's a 2002, no mods to the engine/exhaust other than the K&N FIPK.

Soviet

Its always hard to suggest anything specific for these types of problems because so many things can cause misfires. I've personally seed bad injectors and coil packs happen.

I mean this as sincerely as possible, this thread is pretty much all you need: https://www.s2ki.com/forums/s2000-un...thread-659930/

HarryD

+Agreed with Soviet.

Per my own experience with popping out of the exhaust with the misfire at low RPMs, IFF we're banking on this being a minor problem and not a big catastrophic engine problem, it's likely just a coil pack.

andrewhake

Definitely recommend doing a valve adjustment if it hasn't been done in awhile. Valves clearances tightening up can definitely cause random misfires at lower rpm and inconsistencies driving the car around at low rpm in first gear.

afzan

What alternator did you get? I would try swapping the old one back in

Slowcrash_101

Spark plugs should be changed every ~7-10 years regardless of mileage.

windhund116

The car has been sitting for 5 months, but how old is the gas that is in it? Any chance you got moisture into it, during the car's down time?

T-Bucket

Going to try some fresh gas today. Is there any way to test the alternator at home without taking it in? I'm leaning towards that at least contributing to the issue, since it was literally just changed.

windhund116

All you need is a good working DVM meter. Check this out:


Good luck, with the fix!

T-Bucket

Ok, as it turns out, I'm an idiot and the battery drain wasn't caused by a bad alternator, but by my dumb ass leaving my bluetooth OBD scanner thingie plugged in and forgetting about it. Swapped back the old alternator and it checks out electrically. Still has the misfire at very low RPM under load. Next working hypothesis is bad gas. (It HAS sat for 5 months with the same tank of gas in it). Going to go get some HEET/seafoam and see if that helps. (Unless you guys know of an easy way to drain 3/4 tank of gas out of the car.) I have a fuel pump and a hose, but can't seem to get the hose down into the tank through the filler.