Hello Everyone,

I wanted to share a discovery I made when trying once again to diagnose my misfires problem.

My car runs big boost for a stock engine. I'm making 1.7 bar at peak boost and spraying tons of methanol. For the past year or so I've been changing my spark plugs about every 3-4k miles. Anyone who has done the job knows what a ***** it is.

Anyways, I recently had a dead battery from leaving my radar detector going all night (I know, it should be wired to the ignition switch, but it isn't). I noticed that the voltage gauge was reading pretty low, even after the battery was charged back up. So thats when I got the idea: maybe my battery is causing my missfire? I then googled the issue, and it seems most people disagreed that a bad battery could cause missfire, since the alternator takes over once the car has started. However, lots of people were saying that it could be a symptom of a bad alternator.

Long story short, I test the alternator voltage and get 13.3v which is very low. I replace the alternator and now I'm getting just over 14v, and my missfire has literally disappeared. Apparently the difference in 0.7v is a BIG one when it comes to alternators.

If you're a person struggling with missfire at WOT and you've changed your coil-packs. This is the next step. Test that alternator. The current draw from the alternator is BIG when you're trying to fire a spark through a methanol-saturated cylinder. Blowout happens easily if you don't have the power to fire a quality spark.

Hope this helps someone eventually, I spent so long trying to remedy this problem, when it had such a simple solution.

Cheers.

 

Thanks for the heads up!

 

A bad battery can also produce a low voltage output on a "ok" (not great) alternator as it can't keep up with the load demand on the battery. if you're using a battery tender or similar to recharge the battery, use the tender a few more times over the next week. Automatic chargers don't fully recharge a discharged battery. They read a false charge and go into float, rather than allowing the battery to absorb the charge in a deep charge situation you would get from a manual charger

 

I had this issue when i was running a PC680. changed it out for a pc925 and it odd misfires went away. Later on that year i ended up having another issue though with the line coming of of the FPR...

 

Thanks for posting this, great info. Seems like alternators can wear out pretty quickly in the Porsche world. Glad to hear you're fully sorted now!

 

Interesting! I ended up replacing my battery too.

 

A weak battery can also cause you a misfire if you are heavy boosting.
Having to keep charging the battery will load the alternator taking away from the power the coil needs. If the alternator is already on its way out that just adds to the problem.

 

Good find

 

also make sure the engine ground strap is clean and secure

 

If your battery was a bit weak it could have absorbed any voltage in excess of the 13.3 your alternator was putting out. When batteries start getting long in the tooth they charge slowly and sometimes not fully and will discharge fairly quickly. I recently ran into an issue that looked like a faulty regulator, which it was. Then looked like an alternator was not putting out high enough voltage through the regulator. It turned out to be the battery which would start the vehicle perfectly fine but would not allow the alternator to show a rate of charge in the 14+ volts with the motor revved up. New battery and the output voltage of the alternator immediately showed higher.

 

you'll come be good for a while. If you don't have one, get a ctek or other battery tender and keep that new battery maintained

 

Great post M! After servicing these cars for nearly 20 years I can tell you that although these cars are pretty much bulletproof...The alternators definitely go bad as one of the common repairs like ignition switch, cool packs, seatbelt buckles, wheel bearings and water pump to mention a few others.

 

Were you getting misfires at idle ?

 

so moral of the story is : Check your alternator guys!, i should check mine, probably growing bats in there.

 

No, my idle is sloppy already due to mafless, but I've never had any stalling issues.

However, I did remove the negative terminal from the battery, and the car died after about 10 seconds. The alternator couldn't keep it going.