2002 Mazda Protégé LX, 2.0L engine, automatic transmission, 240,000 km

Recent Work:

Thermostat replacement


  • Owner reported that the car shook and stalled on startup. She parked the car and drove another vehicle.
  • The next day, the car started up fine. I went to see if the car was driveble in order to bring it to my house to diagnose it. The car shook, there was a nasty vibration from the engine (at idle), giving it partial throttle (still in Park) made the engine rev smoothly.
  • Inspected the spark plugs, and they were black and smelled like motor oil (see picture)
  • One of the tires was flat, so one of the neighbours offered a can of puncture sealant, he put it in and then I drove the car back and forth to spread the sealant around inside the tire. The engine sometimes ran rough, and sometimes it "cleared up" and ran nice (good torque).
  • The car was driven to my place and I removed the spark plugs to do a compression check, lo and behold, the spark plugs are a normal tan colour.

Tests Done

  • compression was even on all cylinders (175-185 psi)
  • did a resistance check on the fuel injectors, they were all 40-80 Ohms.

  • did a resistance check on cables and coils, they are within spec

  • check engine light was blinking (that can't be good), codes read were "random misfire" and "warmup catalyst efficiency below threashold". Codes were cleared, light did not come back on during a brief drive.


What causes an intermittent misfire that coats the plugs with oil but then clears itself?

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Update #1

Put everything back together intending to go for a test drive, but the engine misfired massively on startup. I shut it down and took the plugs out, and here they are. enter image description here

Plugs #1 and #4 (extreme left and extreme right) smelled like gas, and they are both fired from the same coil (#2 and #3 are on a separate coil). Although the coil passed the resistance tests, I do find that the lead seems too short to actually reach the plug (#4 sits right under the coil without an actual spark plug cable). I suspect that running like this may have damaged the coil. Thoughts?

Update #2 I needed to get the car out of the garage to put winter tires on my daily driver (surprise! snow outside!). I reassembled everything, started this car up, it misfired massively and stalled. Twice. The third time, I revved it while in park, and the misfire cleared. I drove the vehicle to a nearby parts store (the car drove fine, good power) to get the codes read, it's the same 2 again: random misfire and warmup catalyst efficiency below threshold. So, what causes a misfire but disappears when you rev the engine briefly.

Update #3 I had rented a fuel pressure tester, but this Mazda's fuel lines don't play well with others and I couldn't get a good seal. I needed a break from staring at this thing, so I put it back together and drove it around the block to park it out front. The battery light comes on occasionally at low(ish) rpms, and clears if you rev it high enough for long enough. Do I have a charging problem? Or a voltage problem?

  • 1
    That sounds like the effects of an "Italian service" i.e. a spirited drive - cures many known ills... Not saying you abused it, but may be used more revs, wider throttle etc...
    – Solar Mike
    Oct 29, 2017 at 6:39
  • 1
    As an aside : My mum had a motorhome and it only did short trips for years - ran well enough though. I took it on an extended road cruise for 3 weeks - without thrashing it, just getting it properly hot and working it. Ran beautifully after that - idled well etc... Even the mechanic who looked after it for years wanted to know what had been done... laughed when we told him!
    – Solar Mike
    Oct 29, 2017 at 6:45
  • 1
    @SolarMike I hear you, but this is unlikely in this case. I had this car under my care last month and I ran it hard to see if the cooling issue was resolved. 5,500 rpm and full-throttle on a 3.6 km uphill.
    – tlhIngan
    Oct 29, 2017 at 7:20
  • 2
    Is that an "actual" spark plug from the car? If so, that's not what I'd call "tan". That spark plug looks like it's fuel soaked to me. Oct 29, 2017 at 12:49
  • 1
    I still wouldn't call that "tan". A whole lot less black, but not tan. Nov 1, 2017 at 1:23

3 Answers 3


I've seen this behaviour on a car with a hairline crack in the fuseboard. The result was that sometimes the car would run beautifully and sometimes it would all but die and bog down on the throttle.

This was because the ignition system wasn't always receiving a good strong input power at it's low tension side. Sometimes it was, sometimes it wasn't.

I'm not saying that this is certainly what is wrong with your vehicle but it would certainly be one thing that could explain the behaviour you're seeing.

  1. You mentioned the coil and leads, but I would inspect the leads for cracks etc. They may test within spec when removed from the car, but when vibrations start or they bend and flex, they could leak voltage. Black plugs - as you know - can be from one or more bad leads.

  2. I would remove the fuel filter and inspect it vigorously...I would shake it and bang it gently to see if there is any sediment. If you see anything at all, replace it. This can cause very intermittent problems, especially black plugs and inconsistent idling, throttling, etc

  • 2
    The fuel filter is in the fuel tank for this model. Oh boy.
    – tlhIngan
    Nov 12, 2017 at 19:35
  • Oh thats nasty...
    – Grantly
    Nov 12, 2017 at 20:36

Something wrong in the fuel enrichment for a cold start. Too much fuel during the start and warm up, but in normal operation the mixture is correct and it burns soot from the plugs. The ignition is an" innocent bystander ".

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