Battery boiling over causing misfire cyl. 2 on track.
 

I have a 2013 FR-S with nearly no engine modifications other than a Jackson Racing Oil Cooler (SSC autocross setup).

I was in my third 20-minute track session last night when the car suddenly developed a steady misfire. The car immediately went in to "limp mode" with the SLIP light illuminated, and the CEL blinking.

When I stopped in the paddock I had a little smoke/steam from under the hood. I went looking for coolant or oil, but it turned out to be a boiling over battery. In 30 years of car ownership/shadetree work I have never had a battery boil over. I have never had a voltimeter reading from this car that was unusual, but I also don't look at it when I am on the track.

The battery is a cheap one I bought from Wal-Mart, which I suspect may have been my mistake here. However, even with the boiling-over event, the battery continues to work fine.

I checked the codes and got P0302 - misfire in cylinder 2 (front passenger cyl.). I frequently get this for a bad coil, but bad coils from heat have always run pretty well at idle for me. This was definitely a complete failure to get spark/constant 3 cylinder firing situation. I decided to make the 50 mile drive home and it was on 3 cylinders with CEL blinking the entire way.

I let it sit for a couple hours, and changed Coil 2, and it still ran on three cylinders.

This morning, I decided I better through some baking soda all over the quadrant of the engine bay where the battery is located. There's quite a bit of battery acid juice in that area of the car, but nearly all of it has dried. After doing that, and to my surprise, the car started running on 4 cylinders. Yay! I intend to give it a good rinse later today.

I am interested in whether anyone knows what is causing the loss of spark from something getting wet in that quadrant of the engine bay, or otherwise having liquid or battery acid causing a "short" that would cause the car to lose spark like that.

Thanks in advance!

Since battery acid is for all intents and purposes, an electrolyte, it is by definition, electrically conductive. If that gets on your coil near the spark plugs, you may have a direct path from the conductor to ground, which can definitely cause a misfire.

Did you have to clean any old, dried-up battery fluid from the coil or spark plug when you changed it out?

Thank you - in process of cleaning it now. There was quite a bit of battery acid. There were drops of dried acid on the cyl. 2 coil last night which is some distance from the battery. I am concerned about the wires running to the computer module above the passenger side cylinder bank, which are much closer.

I am going to switch to an AGM battery.

Is your current battery a "maintenance-free", sealed design, or does it have the vented, removable caps?

I think that you swapped out the wrong coil pack. Cylinders 1 and 3 are on the passenger side of a left hand drive vehicle. 2 and 4 are on the driver's side. I had an everstart battery that leaked acid. Change to an Optima red top.

The caps don't appear removable to me but I have not tried prying at them to confirm. The car is reading 13.1 volts when I turn it off, and 13.0 an hour later (off). It is charging at 14.7 with all electrical gizmos off, and 14.3 with a/c, fan max, and headlights on. I think that is OK for this car, although a little higher than I typically see on most cars.

You are right. I mis-read which cylinder was throwing the dtc. Interestingly, it is the farthest from the battery. I suspect wiring at the back of the engine that was covered in liquid was 'shorting' causing the misfire.

This morning, the car started running on four cylinders again. I doused the passenger rear quadrant of the engine bay in baking soda and then hosed it off. The car then started running on three cylinders again but no new codes. Just P0302 and a general misfire code. I let it dry out and it started running on 4 cylinders again, and felt fine while cruising, but it felt weak under load at all rpms.

Over half an hour while driving it seemed to get stronger.
I wonder whether continued driving is drying out an important electric line at the back of the engine that is important for getting good spark at cyl. 2.

I also hope I did not do any damage to the catalytic converter driving 50 miles on the highway on 3 cylinders.

It's not good to do but the potential shortening of lifespan to the cat may not be noticeable.

I drove from NJMP to NYC on a misfire p0301. About 250 miles. A few months later I got a p0420. I cleared it and it after 2 more years i sold the car didn't get a p0420 again.

Double check the coil pack connections. Sounds like one may be loose. Also check the engine ground straps. There is one on each side of the engine. I may be wrong, but I don't think that battery acid will damage modern car wiring.

Thanks for all the responses. I am more concerned about hurting performance than damaging a replaceable cat. I am slow enough as it is and don't want a clogged cat holding me back.