Piston Slap: Sans Souci or Inquietant Diagnostic?

Sajeev Mehta
by Sajeev Mehta
piston slap sans souci or inquietant diagnostic
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 sajeev@thetruthaboutcars.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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4 of 27 comments
  • James Montana James Montana on Apr 20, 2019

    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.

    • See 1 previous
    • ToolGuy ToolGuy on May 10, 2019

      James, 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.

  • Pwrwrench Pwrwrench on Apr 22, 2019

    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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