car won't run anymore
 

I have a 2000 miata with diypnp. It has been running great for about a month then today it would not stay running.
I started the car and the engine rpm would rev up and down very quickly then after a few seconds die. It would not start again unless I used the throttle. I could hold the engine at a consistent rpm if I use the gas pedal.
When able to get the car to run at a consistent rpm (verified in tunerstudio) the tach in the car would bounce between around 300 to 1200 rpm.
So iirc the tach gets the signal from the coilpacks. So the would lead me to believe the coilpacks are going out.
One the other hand I can get it to run if I hold the gas so leads me to the iac going out.

So hit me with your ideas.

whole day and no ideas. Come on guys put your thinking caps on.

so im now suspecting my alternator. I tightened the belt and it started perfectly. shut it off and tried again the issue came back. So I use my multimeter on the battery and it read 12.93v kinda low i think.

12.9 while the engine is off is fine. 12.9 while the car running is low yes.

Yes that is while the car is idling.

i have an idea of what might be happening. I'm thinking my car wouldn't start because the voltage was not high enough. Loose belt not helping the situation. Tighten the belt and clean the connector at the alternator the car starts and runs just fine. I'm still getting 12.98v at the battery when idling but tunerstudio shows 15v. I'm not sure why tunerstudio shows 15v but i'm thinking to trust the multimeter more. I also get a battery light that I used to think was because of a incorrect resistor on the board I got from westfieldmx5 (it has a circuit for battery low light).

What would cause tunerstudio to show a higher than accurate voltage?

Try reloading the firmware and a tune from before the problem happened.

well the voltage thing wasn't the fix. I got the car running at around 13.9v. I'm gonna try changing to a spare set of coilpacks.

Matt can you explain why you think firmware may be an issue. It wouldn't help my crazy tach would it?

This is not necessarily a firmware issue; I just recommended reloading the firmware as a "hard reset" to the ECU. When you have a bunch of weirdness crop up at once, the first thing I like to test is to try and erase all settings on the ECU and revert to ones that previously worked. It's a quick check and will rule out almost any settings problem in one go.

I had a similar problem. Voltage was showing 11.9 volts. My alternator was shot. Just replaced it today and it runs like a champ. Of course my car ran great before the alternator went bad though so i wasnt starting from scratch.

One thing to try is get a jump from another car and keep its engine running to keep your battery at the proper voltage and see if it runs fine.