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Why does my 2000 Miata 1.8L (104000 miles) struggle to start when shut off for a short time? This doesn't seem to happen as frequently when starting up after being off over a weekend or just overnight. Some mornings it fires right up.

For example, when leaving the grocery store after 30-45 minutes it weakly fails to turn over; I have to turn the ignition off and try again. When it does eventually start, it runs roughly for about a minute, then runs pretty smooth after that.

The last time this happened, the CEL came on, and I got two codes with a Bluetooth ODB-2 dongle:

  • P0302 Cylinder 2 Misfire Detected
  • P0300 Random/Multiple Cylinder Misfire Detected

The battery and starter are both less than four months old, and it got a new set of plugs two weeks ago. Here are the old plugs, which are about 8-9 months old:

Old spark plugs

I've had this problem for roughly two years. The plugs have been replaced a couple times in that period, and the used cylinder 3 plug always looks a bit darker than the others. I've taken it to two different shops but neither has been able to diagnose it.

I'm not very knowledgeable about cars, but a couple enthusiast friends suggested maybe the fuel injectors need to be replaced. I bought it used so as far as I know they've never been replaced. I took it back to the shop to see if they could replace the injectors and they said that's expensive so (to their credit) let's try cleaning the mass airflow sensor first and see if that helps. Unfortunately it hasn't, but I don't want to spend a lot for injectors when it's not even clear they're the problem.

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    Welcome to Motor Vehicle Maintenance & Repair! You can get your current injectors refurb'd for fairly cheap, or you can purchase a refurb'd set off of eBay for under $50US. Fuel injectors might be what you need to look at, but I'm not quite sure. Something to check is fuel pressure during and after running. It should stay solid after running. Not sure what it should be stock, but would bet it should be ~58psi for your Mazda (it's an educated guess). Something to check, anyway. – Pᴀᴜʟsᴛᴇʀ2 Apr 13 '19 at 18:37

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