By on April 19, 2019
TTAC Commentator Majda writes: 


I recently bought a 2002 Miata, manual transmission, in silver. The prior owners appear to have been followers of the sans souci school of maintenance, so I have been wrenching on it quite a bit. It has only one problem I can’t solve: a persistent P0301 code, showing a misfire in Cylinder 1. The actual experience of driving the car is fine — I don’t feel a miss or a drop in power. That light, though… it isn’t constant. It doesn’t come on instantly; if I clear it, I might get a few minutes of light-free driving, or an hour, or a day. But after that, the CEL goes blinky-blinky.

Logic suggests that the misfire can only come from spark, air, or fuel, so I went at each as follows:

  • Spark: I’ve replaced the spark plugs (NGKs), plug wires (ditto) and boots. I swapped ignition coils to see if the code would move; it didn’t. The ignition wiring harness has some broken protective tape, but I don’t see any broken wiring.
  • Air: I’ve replaced the air filter and checked for vacuum leaks.
  • Fuel: I replaced the fuel filter, and, to make sure I didn’t have a fuel injector problem, I swapped the injectors between cylinder 1 and cylinder 4. I spilled a bunch of fuel, but the code stayed put.

I’ve been busy fixing other issues (leaking valve cover gasket, cracked radiator, soft top made of cheesecloth) on this Miata, but the P0301 has me stumped. I checked compression, and it’s good across all four cylinders. Forums mostly argue in favor of ignition problems, but I feel like I’ve covered that area.

The big question is this: Do you think new driving shoes will fix the problem?

Sajeev answers:

Duh, of course that’s all you need…but only if they are brown

While you didn’t tell us the compression test numbers, I’ll go right ahead and assume you came to the correct conclusion. Ditto for the fuel and spark tests. So now it’s time for a leakdown test.

That’s right, you and your fancy new Cognac Ligne 24H Le Mans Pilotis must head to the parts store, get a leak down tester (maybe they rent ’em?) and see if all four cylinders have an acceptable percentage of leakage over time.  I’m guessing not, which is precisely why your diagnostic work needs less sans souci and more inquiétant

This also means it’s time for LSX-FTW, SON!!!

[Image: Mazda]

Send your queries to [email protected]com. Spare no details and ask for a speedy resolution if you’re in a hurry…but be realistic, and use your make/model specific forums instead of TTAC for more timely advice.
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27 Comments on “Piston Slap: Sans Souci or Inquiétant Diagnostic?...”

  • avatar

    A leak down test is probably a good idea. I would also try to get my hands on an FSM (schematic) and try back probing both ignition & injector harness’ for cylinder #1.

    Good luck Majda!

  • avatar

    Or, it could be a faulty sensor throwing a bad code, it happens

    • 0 avatar

      That’s not a bad guess; in my previous experience, if the CEL is blinking (instead of solid), the car’s shaking like an unbalanced washing machine because it’s detecting active, rapid, misfiring.

      This would definitely point towards a sensor problem instead of an active issue.

    • 0 avatar

      IIRC misfire detection is done by looking for irregularities in the signal from the crank position sensor.

      • 0 avatar

        On this car, I believe JimZ is correct on crank sensor operation. A cylinder mis-fires, the sensor detects a slight decrease in crank rotation speed, P0301-4 is set.

        I’m inclined to believe that if the CPS were faulty you’d likely get P0300 (random) set.

        These cars have a CAS (cam-angle) which triggers ignition & injection timing, I think this would be a more likely culprit over the CPS. Just throwing ideas out there.

        In any event, driving with a blinking check engine lamp will destroy the catalyst.

  • avatar

    How does this system detect misfires, a scan tool with live data is going to be your next step I think. How does the system detect misfires? That may help to further narrow things down. An excellent source of knowledge is a youtube channel called “ScannerDanner” he’s a wizard at diagnosing issues like with with a scan tool, lab scope etc.

  • avatar

    I had a misfire code once that turned out to be a cracked head. One used-up warranty later it was all fine.

  • avatar

    Similar issue on my olde BMW 325i – would throw “lean” codes randomly, sometimes with a misfire indication. Everything I read online indicated a vacuum leak. I replaced the upper cracked intake tubing after the MAF. Still threw the codes. Replaced the lower intake. Same thing. Cleaned and latter replaced the MAF. Same codes. Drove me crazy throwing parts at it.

    Long story short it ended up being the DISA, a butterfly valve between the long and short intake runners. It was stuck and not operating at all. Once that was replaced, the error codes stopped popping up.

    Probably _not_ your issue but just an indication of how uninformative codes can be, or how complicated the whole rigamarole of engine management is.

    • 0 avatar

      or How BMW uses cheap plastic parts that disintegrate and can potentially be digested by your engine. Mazda wouldn’t do something this STUPID but hey what do I know?

      Have an e90 I’m looking at right now, 30FF code, no boost leaks any where. Boost solenoids and diverted valves test good. I’m leaning towards wastegate flappers made of PLAY-DOH.

      Junk cars.

  • avatar

    What if it’s not an actual problem but rather a sensor or wiring problem? The intermittency suggests the sensor wire, contacts or even ECM board could be realizing momentary breaks or shorts due to frayed insulation, stressed connectors or cold solder joint. It is possible the OP is troubleshooting the wrong problem.

  • avatar

    Problem could be in the lower end. Worn crankshaft/bearings will produce enough endplay to cause #1 misfire. How many miles on it?

  • avatar

    Bad crankshaft sensor on my Miata did this…

    • 0 avatar

      On my previous Nissan 350Z the cam shaft sensor was to blame for the same kind of issue.

      If the sensor is going bad this problem will only get worse. When the sensor complete fails the car will not start! So for piece of mind I’d would replace it. For me it was a low cost and easy to access part. I needed 3 of them: 2 cams and 1 crank. Forum research indicates when one goes the other 2 aren’t far behind. Also replace with the OEM brand, saving a few dollars on aftermarket senors was not recommended.

  • avatar

    Hmmpphh! Never had such problems on my 61 Chevy 6cyl 3/tree!

    • 0 avatar

      My sister had a check oil light come on, intermittently at first, then all the time. My dad fixed it with 3-1/2 quarts of oil. Then she gave the car to me, and a main bearing blew out at 65 mph.

      Only Heaven knows what might happen if OP can’t fix the code in time, but I suspect it’ll make him decide to avoid sellers who practice sans souci maintenance.

  • avatar

    Valve adjustment. On this engine… valve adjustment. Miss at idle? Clears up once rpms rise? Spark first. Vacuum leak second. Fuel third. A miss that changes a tiny bit when cold vs hot (don’t be misled -izzat a pun? – by rich burn during coldstart phase). Better at least check this before moving to other things. Better adjust it soon. If if if the exhaust valve(s) are indeed a tiny bit too tight better get on it now before the dang thing burns a valve. Even modern sodium filled valves burn when an exh valve is a tiny tiny bit too tight.

  • avatar
    Land Ark

    I’m just here to point out that my maternal family name is Sansoucie.

    This is also a good time to plug my cousin Thom Sansoucie’s auto art. He’s an amazing artist who chose to focus on cars. Check his stuff out sometime. I would like to his website but it doesn’t seem to be loading for me at the moment. is his site when it loads… Do a search for some of his work.

  • avatar

    “Swapped” a coil as in another used one? They all go bad on these so start with a new one if you haven’t already.

  • avatar


    I like what you’ve done so far. Sajeev’s leakdown test is probably a reasonable next step, but you might not like what you see from the results – so let’s avoid going there for awhile.


    A) Quantify exactly how bad the problem is – i.e., the CEL is either on or off, but the OBD-II compliant brain of your vehicle knows the individual misfire counts, and a midrange scanner (my latest buy was the second step down from the top at Horrible Fright – and by the way they are fairly dramatically improving the quality of some of their stuff lately) will let you read the exact misfire counts. Cylinders 2-3-4 are going to be zero. Cylinder 1 is going to be a non-zero number on your car – but if the number is small, you don’t have a big problem, we just need to tweak things potentially.

    B) Backtrack slightly – when you say you did a compression test, you did dry and wet?

    C) Hook up an old-school vacuum gauge and see if any clues there (walk through the manual that comes with the gauge – and don’t have the system open to atmosphere while you’re looking).

    D) Throw a new crankshaft position sensor at it (I don’t mind throwing some parts if I plan to have the vehicle for awhile).

    It sounds like the problem is localized to cylinder 1, but DIYer may be on to something because 1 is 1 and 1 is special.

    E) Some vehicles like certain spark plugs – you could become the spark plug whisperer for your particular model.

    Borrow (buy and sell back) a fuel pressure tester if your fuel rail has an easily accessible valve on it – this will rule out fuel issues (but again, overall fuel pressure issue probably wouldn’t affect just cylinder 1). This step is good for general peace of mind.

    F) Controversial recommendation which everyone will hate (but might work) – pick up a can of ‘Restore’ additive and pour it into the crankcase (dear critics: do this with the engine running and listen carefully and you tell me if the engine sounds happier or less happy).

    G) Run a bottle of mid-range fuel injector cleaner through the fuel tank just to be sure.

    H) Next I’d probably throw new coil(s).

    I) Back upstream from the spark plugs and give it a little love with some electrical contact cleaner.

    Again, it comes down to the misfire counts – exactly how bad is the problem, according to the computer?

    • 0 avatar


      The ignition wiring harness has some broken protective tape, but I don’t see any broken wiring.

      That light, though… it isn’t constant. It doesn’t come on instantly; if I clear it, I might get a few minutes of light-free driving, or an hour, or a day.

      *End Quote*

      Just because you can’t see it doesn’t mean it isn’t there. You have a wiring problem that randomly interrupts your spark. And a VOM won’t indicate it on a static test. You need to shake the wire while each circuit is tested for continuity. A test light can be invaluable for this check.

  • avatar
    James Montana

    OP here. (Formerly d/b/a Majda.) I want to update the B&B on what happened since I sent the question to Sajeev.

    A few notes on questions posed above. Yes, I did a compression test. (I appreciate the assumption from SM that I did!) It came back fine across all four cylinders. Tried various oil and gas additives after a call to Ms. Cleo suggested that might work. (They didn’t.)

    Eventually, I gave up and brought it to a good shop. The diagnosis was: Get a new engine. Why? The shop did a leakdown test and the results were very bad. Also, the shop told me that the engine was coughing oil all over the floor. (I know, I know.)

    So, I buckled down and ordered a remanufactured NB engine from AAA Engines in Chatsworth, CA. They’re still putting together the engine for shipment. The hoist and engine stand are assembled and waiting, and most of the engine bay is already clear. Detaching the exhaust downpipe required a Sawzall. It’s an adventure – or, in keeping with the title of the post, a folie a deux. Will update once the new engine is in.

    Thanks both to Sajeev and to the B&B for the helpful suggestions.

    • 0 avatar

      Best of luck J. Montana. I am sure you will be ripping through those twisties in short order!

    • 0 avatar


      I’m sorry, I misunderstood the assignment – I thought we were trying to fix your engine – not replace it – TEASING!

      Enjoy your reman engine – please let us know how it goes and if you are pleased with the process/results.

  • avatar

    Many good suggestions here, Sobro in particular. I have encountered numerous broken wires when the outer insulation looked OK.
    In some instances cars need to be driven with a ‘Break-Out’ box and a Volt-Ohm meter (an assistant is helpful in avoiding a crash while reading gauges and driving simultaneously). Then the bad wire/circuit might be located.
    And, as others have mentioned, mechanical wear can cause these problems. I saw a car that had an intermittent CEL and reduced power. Scan tool showed detonation (knock) and retarded timing. Eventually this was traced to a very slightly loose wrist pin. No noise audible, except by an experienced ear with an engine stethoscope, but it was enough to fool the Knock Sensor that the engine was detonating and the ECU obediently retarded the timing. The Knock Sensor had been replaced without fixing the problem.
    Scan tools are useful, but only pint you in a direction.

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