By on November 19, 2013


John writes:

Dear Sajeev,

Thank you for much good reading and practical knowledge for a very amateur do-it-yourselfer. My auto repair and maintenance skills are very limited, but I enjoy doing what I can myself. Even just the oil changes and having control of the materials used to perform it.

So you are looking for subjects, and here goes-this may resonate with any number of Miata owners. For about a year the CEL has been popping up a code (0126) that I read with a simple device purchased online that evidently means the engine is running too cold, which I have never even heard of, but why not? Insufficient combustion temp?

Anyway, I see that it must be the thermostat which is seriously buried in this car and beyond my meager skills to get to, much less reassemble.

Do you think there is any urgency to getting this repaired? Car runs just fine.


Sajeev answers:

P0126 always takes me back to my time as a wannabe car designer at CCS in Detroit: if I wasn’t spilling venom on the vellum I’d dabble in auto repair consultation. To wit, a good friend spoke of his Merkur XR4Ti that he left in Florida, missing it but hating how it always ate “engine sensors”. Now, as a self-proclaimed expert on all things powered by Ford’s EEC-IV fuel injection system, I found that rather odd. Further questioning lead to this comment: “Oh, I never drove it with a thermostat. It’s Florida, you don’t need a thermostat!”


If only I knew better back then, I coulda put him in check: EFI systems run at a certain minimum temperature to ensure the motor’s ideal health and efficiency. If not, you run the system in open loop, instead of listening to inputs like the Oxygen Sensors, MAF meter, etc to keep emissions down, power up, liquid smooth idle, etc. Take the thermostat out of the system and the sensors are never consulted.

Is that happening in your Miata? It could be running in open loop. Or not: modern EFI systems are somewhat more intelligent than Ford’s antiquated EEC-IV, but this needs attention. My advice is simple, this code is normally produced by a faulty engine temperature sensor or a…like my friend’s Merkur…a problem with the thermostat.

Instructions on removing the T-stat are here, and this suggests that 2006 models are plagued with T-stat problems. So perhaps it’s time for a new Thermostat, or perhaps you should re-install your thermostat and NOT RUIN YOUR MERKUR, SON!


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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57 Comments on “Piston Slap: The Too Cool Miata?...”

  • avatar

    I never heard of this problem, interesting. Also, get some winter tires for your Miata. I hear they do quite well in snow with the right tires

    • 0 avatar

      re: Miata and winter. Ya, some Blizzak’s and keeping your petrol tank near full all winter,and you’re good to go. I got by for two winters with just the stock skinny tires and the full tank all winter, and had no problems as long as I didn’t get overconfident with speed and imagine I was in an Explorer. Could not say the same for my S2000, worst winter car ever(but, absolute best nine-months-out-of-the-year ride I’ve had, to date).

      • 0 avatar

        Going back a few years, but over 15 years North of Albany, NY in an MGA, 3 different Fiat 124 Spyders, an Alfa Spyder, and a Bavaria, I always got by just fine with snows on the rear, tank near full, – and a 50# bag of sand in the trunk. The Europa stayed in the garage all winter.

      • 0 avatar

        No problems with any of my Miata’s regarding Winter driving conditions. I run Continental ‘all seasons’ in the Winter on my daily driver and fill the tank when it hits the half way mark. I carry chains, but have never had to put them on, even when going over Mount Hood or up to Timber Line or Meadow’s ski areas.

  • avatar

    Had a boss with the same “problem” code on an S-10 pickup. He drove it 4 miles round trip to work. I read the code for him and told him he needed a thermostat. The truck never ever got hot.

    Of course, being my boss, he didn’t believe me. He took it to a mechanic who sold him on a complete cooling system flush. The next day, same code. I put a thermostat in it for him after work in all of 10 minutes. No more code.

    Todays cars are almost too smart for our own good. Too cold, too hot, a leak here and stuck solenoid there a slightly fried ignition wire causing a miss that you can’t feel…

  • avatar

    Variations of this code pops up on many different cars from time to time and is usually due to a stuck open thermostat. Like Sanjeev said, it will either take much longer to reach closed loop operation or never get there at all. Running your engine that way is much less efficient, can cause excessive fuel use, carbon buildup maybe even wash down the cylinder walls. Check out that thermostat and flush the cooling system while you’re there if it hasn’t been done in a few years.

  • avatar

    Getting “hot” or coming up to 180 deg is a good and bad thing. It must occur of the engine collects sluge and eventually dies. Driving only four miles on per run is a problem even when your thermostat is good. When you make short runs like this constantly, you would be doing your car a favor to step up to a warmer tstat.

    That being said, heat is a contributor to early engine death also. As ECU’s become more intelligent, we could easily change to tstats tha work under variable conditions more efficiently. That is, the ECU could sense over time that teh car is not heating up enough to burn off deposits and it could go through a heating cycle or in the case of someone driving four miles one way, automatically adjust.

    Our problem in my estimation is that the industry has done too good a job with emissions controls to make the govt happy and not used enough of their wiles to make cars last longer. Now why would they do do that?

    Click on my name to read my blog on a setup that would do this part much better.

    • 0 avatar

      Why would they do that?

      Why is it no longer a miracle when a car makes it to 100k miles?

    • 0 avatar

      I’m not sure what you are talking about regarding heat being a killer of engines. Most wear on a engine occurs when its cold, so the faster it gets up to temp the better. From an emissions standpoint, the OEM’s want it to warm up ASAP so it gets into closed loop. How can an ECU do a heating cycle or adjust for short trips? They can’t, the only option is to get the engine up to temp as soon as possible, shutters in the radiator, redesigned coolant systems are all helping with this. In modern automobiles the rotating assembly of the engine is the most reliable system, its very rare for it to fail absent neglect.

      • 0 avatar

        My 41′ Cadillac had radiator shutters and I have never owned another car that sent heat to the heater core as fast as that car. Nearly instantaneous even on freezing days.

        Some OEM’s are bringing back radiator shutters.

    • 0 avatar

      Already been done. BMW uses a computer controlled thermostat that can vary engine temp quite widely and dynamically based on conditions. Of course, when it fails it doesn’t cost $10 to replace.

      As to the POs issue, I had the same thing on my Grand Cherokee. Some idiot replaced the required 195f thermostat with a 180f thermostat, and on cold days it would take too long to reach temp and throw that code. All fixed with the right part in it.

  • avatar

    So this is the best question you got this week?

    Jesus – just put a thermostat in it, for god’s sake. Either you or a garage. There – done.

  • avatar

    Having replaced the thermostat on my 99 Miata, I can tell you its not at all buried and is actually rather easy to replace.

    Where the upper radiator hose connects to the engine is where the thermostat is located. Remove the hose, then unbolt the 4 screws on the piece that the host is connected to on the engine and you will find the tstat under it. Just be sure to get a new gasket, and to scrape the old gasket off with something that will not scratch the mating surface (don’t use a screwdriver, try a plastic paint scraper). I believe I may have also used some type of rtv as well but check the Miata forums for the recommendations.

  • avatar
    DC Bruce

    Thermostats fail all the time; they’re cheap and usually not at all difficult to replace.

    Engine cooling is nothing like the dumb simple system it once was. Even my 13-year old BMW has a cooling system that runs the cooled coolant through the cylinder head first, then down to the lower block. The idea being that you want the cylinder head cool, but the bearings, etc. warm. Running with a stuck thermostat is just never a good idea.

    • 0 avatar

      Unless your car was designed by idiots and you have to disassemble half the exhaust system to pull the thermostat out. :P

      • 0 avatar

        what car is this NoGoYo

        • 0 avatar

          My Buick. You have to yank the exhaust crossover and probably the airbox duct in order to have enough room to pull the thermostat.

          Which is why I was very glad when the family mechanic told me I didn’t need a new thermostat…oh the labor costs.

          • 0 avatar

            your mechanic is on crack or just trying to get money out of you. You do not have to yank the crossover to do it.

          • 0 avatar

            One of these days I’m going to confiscate that Skylark…

          • 0 avatar

            You must have a 3.1 liter, as does my MIL in her 2003 Century.

            I have changed the thermostat on it without removing that exhaust crossover. It’s not easy, but it’s doable (IMO and well worth not risking breaking off rusty exhaust studs and making it a far more complicated job).

          • 0 avatar

            I changed the t stat and all the hoses on a 2002 Century with the 3.1. You don’t have to take the crossover out. Makes sense to change all the little bitty cooling hoses too. It was not that tough a job….BTW, why so much hate for the 3.1?

      • 0 avatar

        Or if you own an Audi.

    • 0 avatar

      Although I’m not sure if they were the first, Pontiac did that on their V8 in the 50’s. Heads first then block.

      GM started doing it again in 92 with the introduction of the LT1 in the Corvette.

      Now I believe BMW uses electric water pumps on some (if not all) of their engines, and I know that GM uses electronic thermostats on their 1.4T engines. We’ve come a long way from the day of the basic mechanical thermostat and water pump…never mind siphon coolant (ie no pump) through the block like in the dark ages.

  • avatar
    The Heisenberg Cartel

    E46 BMW thermostats are known for breaking in the open position. Dumb BMW fans are also known for putting cooler therms in their cars, unwittingly gumming them up. both contribute to early engine death so I would get it replaced ASAP even if it’s not seemingly a prob right now.

  • avatar

    Not that uncommon, I believe replacing the thermostat for one with a higher temp rating would probably fix this. I recommend hitting the forums and doing a search. That forum is an awesome source of Miata information.

  • avatar

    Yep, most likely needs a ‘stat.

    If only there were a cooling system engineer available to explain this… but you’ll have to have me, a former cooling system engineer.

    Here’s the thing: the “thermostat” is really a thermally-activated coolant diverter valve. It will not, ultimately, govern the temperature of the engine. That comes when equilibrium between the heat going into the coolant and the heat dissipated by the heat exchangers (radiator, heater core) are equal.

    A typical automotive ‘stat uses a metallic (copper) wax motor. Being a Newtonian fluid, the wax expands as it changes from a solid to a liquid. This volume is contained by the wax motor housing and pushes (through a rubber diaphram) a pushrod that’s fixed to the ‘stat bridge, opening the poppet against a spring.

    The only time the ‘stat regulates flow is between full closed and full open. When fully closed, all flow goes through the bypass. When fully open, all flow goes through the radiator in most modern engines. In between those two states, the flow is proportioned between the two. Virtually all inlet control ‘stats and newer engines with outlet control ‘stats also have a bypass poppet on the back of the ‘stat, so that as the flow to the radiator starts, the engine bypass circuit is closed off. Inlet control is better for preventing thermal shock — the slug of cold coolant from the radiator is mixed with the bypass flow in the ‘stat housing before it gets to the engine. With outlet control, that slug of cold will get to the engine. Not usually a big issue, but can be. Inlet control, alas, creates a restriction on the pump inlet (well, yeah…), which, if the inlet geometry is already compromised due to packaging requirements (sadly typical…), can require some careful design to keep the pump cavitation point outside the operating range of the engine.

    The temperature on the ‘stat is the start-to-open (STO) temperature. Full-open is typically ~10 degrees F higher. Most outlet (hot) side ‘stats on production cars now are 192F/89C STO. Most inlet (cold) side ‘stats in production are 180F/82C STO.

    That said, the ‘stat is useful in that, in low-load situations, sometimes you don’t need (all of) the radiator, but your engine needs to be within a particular temp window (the design intended temp, typically around 205-210F coolant temp for most production cars) for best operation. The fuel and spark maps are most optimized there. Plus that means the oil is likely over 212F and thus hot enough to boil off residual blow-by water, extending the oil’s service life.

    Some people put cooler ‘stats in as a performance modification. On older pushrod designs, where the combustion chamber geometry and spark plug placement isn’t quite optimum, this can increase the octane tolerance of the engine, and thus allow more spark advance, producing more grunt. But it causes other problems. On more modern cars, with better chamber geometry, this is far less of an issue.

    If you’re getting an operating temp too low code, the ‘stat is the first place to look. That code comes in two ways: time-to-temperature exceeds the designed intent, and/or intended operating temperature is never achieved. Both point to a ‘stat that’s stuck at least partially open.

    Oh, and don’t rely on the OEM temp gauge. For the US market, most are glorified idiot lights. They’re programmed to move to the low-to-middle of the “NORMAL” range (if a range is indeed marked on it) and sit there until things go horribly, horribly wrong. This is because engine temperature will vary under different operating conditions, but the typical US driver doesn’t know that and will bring their car back for warranty service if the needle moves from left of the “R” on a 30 degree day to right of the “R” on an 70 degree day, or gets higher in traffic vs. highway cruise, or moves almost imperceptively on a cold morning as the ‘stat cycles normally.

    (True story: one of my colleagues spent months chasing high ‘stat warranty on a certain vehicle. They couldn’t find any problems with the returned parts, but some owners were bringing their cars back multiple times. After talking with a few of the owners, they learned that the temp needle moving slightly on cold days was the reason they brought it back. Changed the gauge calibration to a flatter response curve, warranty claims plummeted.)

    • 0 avatar

      Now that’s a great explanation. Thanks.

    • 0 avatar

      I’d like to add to the praise, well done.

    • 0 avatar

      It seems that modern oil pressure gauges behave like your description of the temperature gauge. I’ve seen RAM and Ford trucks where the pressure gauge reads the same at hot idle as it does when cold at 3000rpm. Same reason (reduce warranty claims from ignorant customers) I suppose.

      • 0 avatar

        On my old Merc, it’d go from 3 bars/45psi when cold or moving to a little under 1 bar at hot idle on a hot day, with synthetic oil.

        That was Perfectly Correct And Normal for the car, but I can see how it might be a “training issue” (as we call it around here) with the Average Driver, to get them to realize that *was* Perfectly Correct And Normal.

        Most people seem to expect – and it’s not unreasonable, honestly – that the various gauges that aren’t speed or tach will “tell them when something’s wrong”, ideally in an unambiguous way…

      • 0 avatar


        Ask your average US auto buyer: they want gauges, dammit, not idiot lights! But give them gauges that work like gauges and they bring the thing back for specious warranty work.

        The evolution of the temperature gauge in the US has several stages. First, there were gauges that worked. Some even had numbers on them. But US drivers, who a) can’t be bothered learning that a 50/50 ethylene glycol/water mix boils at 224F at atmospheric pressure, and b) that the boiling point goes up 3F for each PSI, would get all sorts of panicky if the temperature gauge read over 200F and bring it back to the dealer for warranty work.

        So the automakers scrapped the numbers and just put cold/hot markings. Well, then customers wanted to know what part of the gauge’s range was trouble, and brought anything “too low” or “too high” by their subjective judgement back to the dealer for warranty work.

        So the “NORMAL” band was added, typically with letters. Now we get into the same problem as with numbers: customers expected it to be in the middle of the NORMAL range, right between the R and the M. And never move.

        So the automakers started putting huge flat spots in the gauge’s response curve. And under most conditions, that helped. Except with some heat-challenged engines in cold climates, where the coolant temp would dither around the point where the flat spot started, and the needle would move slightly in normal operation, causing customers to bring the car back for warranty work.

        So the automakers did two things: one, they removed the “NORMAL” lettering again, and two, they increased the flat spot on the response curve.

        And thus was gestation of the idiot light disguised as a gauge.

        The oil pressure gauges had similar things happen, except of course the average consumer has no idea how much oil pressure is normal.

      • 0 avatar
        johnny ro

        The oil pressure gauge on my NB miata is really an idiot dial similar to an idiot light. It has two possible positions, no pressure and yes, pressure.

        The miata forum has a mod to plumb it differently and rewire it internally so it shows actual pressure and the dial is correctly damped. That car shipped with a fully functional gage instrument that needs two mods to actually be a gage.

      • 0 avatar

        Yes many oil pressure gauges are not actually gauges but glorified idiot lights that will display the same reading as long as the pressure is above 5-8psi. The reason is because the average owner doesn’t know that it is perfectly normal for the oil pressure to vary depending on the engine oil temp and rpm.

        On some Fords they did use an actual gauge but put a resistor on the back of the instrument cluster and instead of a oil pressure sender they put in a switch that closes at 5-8psi. So if you want you can turn it into a real gauge by soldering a jumper to bypass the resistor and replacing the oil pressure switch with an oil pressure sender.

    • 0 avatar

      Very nice job.

    • 0 avatar

      Is there any way to retroactively install an actually functioning temp sender? I’m glad to have read this because I’ve always found the very flat response on my cars temp gage unsettling because I don’t trust a gage that can’t see variation. I’d rather not “fix” it if it will mess with the data the ECO gets though.

      • 0 avatar

        You can just add your own gauge that comes with it’s own sending unit. You leave the stock one in, just find a plug in the head to remove where you get to a coolant passage. You will likely need some sort of adapter to between the head and sending unit. Then just wire up the gauge and place it somewhere desirable. There are many solutions for gauge placement on many different cars.

      • 0 avatar

        Slap on a scangauge II, I mount mine on the steering column cover. Coolant temp is one of the 4 things I constantly have it monitoring (avg mpg, instant mpg, intake air temp are the others).

      • 0 avatar

        Installing an aftermarket gauge works, but with temp senders, the tip of the sending unit probe needs to be in a flowing stream of coolant. Adding a T-fitting to keep an existing gauge will cause both to be inaccurate.

        This is not true for pressure sending units. T away!

    • 0 avatar

      Thanks, all.

      I can talk about cooling systems until everyone will MEGO, but I’ll stop. :)

      18+ years in a specialty that even other engineers think is mighty close to witchcraft… there is math that defines it, but it’s very nasty math. I learned the ins and outs from a guy who was one of the half-dozen real gurus in the industry (he’d been doing it 33 years when I hired in), and somewhere along the way, I became one of the Old Guys myself. Couple of my former young ‘uns are approaching Old Guy status themselves now.

    • 0 avatar

      ‘AutoJim’… Could you clarify this comment…> “this is far less of an issue.”

      ‘Less issues’ or greater ‘Octane tolerance’

  • avatar

    I had a 2002 Golf TDI for a brief time. On a long trip in frigid December temperatures the car never really got warm and it was blowing cool air even though the heat was cranked. So I suspected the thermostat. Never got any sort of code from the computer. I connected a scan gauge to it and it showed that the coolant never got anywhere near the 210 F range that it should have been in. So I replaced the thermostat and it actually made heat because the engine warmed up properly. It was a bit of a PITA to change, but well worth it. Broken thermostats will affect fuel economy too.

    • 0 avatar

      Diesels won’t necessarily have the same OBD-II codes, so yeah. Also, in winter, diesels don’t hold on to as much heat unless they’re operated at high load. This is due to the fact that diesels are “throttled” by reducing the amount of fuel injected, while the airflow remains pretty constant through the engine for a given operating speed. The higher airflow takes away more heat.

      On an SI engine, the airflow is metered via the throttle plates (or variable valve lift system), and thus is held proportional to fuel burn at a fairly steady A:F ratio (and thus proportional heating to airflow). CI engines have widely variable A:F ratios, with them being extremely lean under low load (this leads to the diesel’s NOx emissions problem, actually).

  • avatar

    Here’s what I dont get—why the heck doesnt car set of a series of loud alarms when the temp goes a fair bit past operating temps? You might be driving along and not notice an overheating gauge until its too late—steam from the engine, coolant hoses burst, headgasket gonzo or worse.
    It seriously couldnt be that difficult to add an emergency sensor for overheating coolant.
    On our Audi I get a BEEP and a Faulty TMPS code that flashes, but no overheating icon.
    Or at least none that I know of!!

  • avatar

    My girlfriend’s 2.3 MZR mazda 6i (2006) throws this code. research shows it’s common on the 2.0 and 2.3 MZR models, the t-stat goes bad. a 2006 is a 3rd gen Miata right? That has a 2.0L MZR. I imagine it has the same thermostat-and-housing combination that’s about 60 bucks to replace dealer cost. Should solve it.

  • avatar

    @redmondjp: Yeah, if you choose not to pull the crossover, you have to find a way to get the one bolt out of the housing despite all the crap GM crammed into that general area…there’s the exhaust crossover, of course, but also hoses and the throttle body making access difficult. A Century probably has a larger engine bay than my Skylark does, too.

  • avatar

    I own a 1991 Miata. Changing the thermostat is a piece of cake. Remove the plastic intake pipe (3 screws) Soak the thermostat bolts with WD40 and let soak overnight. Slowly remove the thermostat housing bolts and remove the old thermostat. Do not replace with an aftermarket thermostat!
    Go to a Mazda dealer and buy the factory part. I know it is overpriced but it is worth the trouble. If you have any questions go to the site. Go to their garage section and it will answer all your questions. Good Luck.

    • 0 avatar

      Thanks Cabriolet,

      I neglected to mention that mine is a 2006 model, and the thermostat is very much buried way down under a lot of other hardware that will take considerable effort to get at-unlike my old 1995 Miata which was a cinch to work on.

      Seems like a lot of effort and expense for a small, relatively simple part on an engine that is running just a little bit cool and seems fine otherwise in terms of power and gas consumption.

      I just had to bitch about it a little!

  • avatar

    Drove a 90 Miata year round at Fort Drum. Only issue I had was at 30 below one morning the rear plastic window shattered when I hit a bump. I had non-winter tires and would break the rear loose with the handbrake on occasion to get it to turn. The car also tended to high center on the rear diff when I backed out of the driveway into the street if I didn’t wait for the snowplow before I left for work. Good Times.

    As to your issue, the 1.6 cars had the thermostat right on the top front of the motor. I assume by your use of OBD codes however that this is a 1.8 car or an NC.

    **EDIT – just saw it was an 06 so NC. Take heart though, my wife’s old Saturn VUE had this issue and changing the thermostat required removal of the intake. This started a long string of repairs that ended with the “trade it in on a new Hyundai fix”. Good luck man, Ineedless complication is frustrating.

  • avatar

    Down here it seems like every non-dealer mechanic removes the thermostat from every car they touch.

    Every one of the cars I have bought used in the last 10 years, except a truck I bought just off lease that was never serviced out of warranty, was missing a thermostat.

    100% of those cars had failed emissions. Half of those only required a new thermostat and a week of spirited driving and were passing the test with flying colors.

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