I'm trying to track down a misfire, and it unfortunately wasn't the obvious stuff like plugs or wires. I'm trying to figure out why the misfire is occurring (to much fuel, not enough fuel, not a consistent spark, etc). That will help me know which system to look at more carefully.

Because it's a somewhat old car (17 years), I'm suspecting the fuel filter may be causing issues. If that were the case, I would expect a lean misfire. Would a lean misfire give off the smell of unburnt fuel?

  • In a word, "No." – Pᴀᴜʟsᴛᴇʀ2 May 27 '17 at 17:03
  • @Pᴀᴜʟsᴛᴇʀ2 Why is that the case? For the sake of simplicity, let's say that 1/2 the standard amount of fuel is being injected into a cylinder. Would that small amount of fuel be burnt up (without producing enough energy to push the piston down noticeably), or would it fail to ignite and pass through the exhaust system? – Jacob Jones May 27 '17 at 17:53
  • @JacobJones Even on a 2000 Toyota, that fuel is passing through a cat before it leaves the car, which will oxidize most if not all of the unburned fuel. To the OP, I would check the OBD codes, which may help in isolating the cylinder. – SteveRacer May 28 '17 at 5:12

There's lots of things that can cause misfires.

If it's an occasional misfire:

  • fuel pressure: conduct a fuel pressure test. It could be the fuel pump getting weak, the fuel lines being obstructed, the fuel filter being plugged or the fuel pressure regulator not doing it's thing.
  • spark quality: you said it's not the plugs or wires, check your distributor cap and rotor (if equipped) and your igniter.
  • airflow: check your air filter, air meter (if equipped) and test for air leaks downstream of your air meter.
  • compression: perform a compression test, make sure all cylinders are even.

If it's a constant misfire ("dead cylinder"):

  • identify the dead cylinder: run the engine until the exhaust system gets hot (don't touch it!), then drop a little bit of water on each tube of your exhaust manifold to find the one that doesn't sizzle.
  • fuel injectors: perform an electrical test on the fuel injector of the affected cylinder.
  • compression: test the compression of the dead cylinder and compare it to the others.
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  • Excellent information, but this doesn't address the question. – Jacob Jones May 27 '17 at 19:54
  • @JacobJones You are correct, but this will help you fix your issue more than "what does a lean misfire smell like" :) – tlhIngan May 27 '17 at 20:00
  • @tlhlngan I am the OP. I appreciate the info. Do you know if a lean misfire will smell like unburnt fuel, though? – Jacob Jones May 27 '17 at 20:02
  • @JacobJones In my experience, unburnt fuel is the smell of a rich misfire. – tlhIngan May 27 '17 at 21:40

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