Reader Question: ’91 Miata Woes?

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Here’s the latest reader question, along with my reply!

Rob writes: After almost a year of waiting for my local mechanic to replace a head gasket, my subject pickup is just now home but upon start-up is belching thick white smoke. The thick white smoke stops after a minute or two but my question is could this appearance of water in the exhaust be a result of the gas sitting for so long in the tank? I’m afraid the sweetish smell is going to confirm the inadequacy of the machine work and that the head gasket replacement probably didn’t work but I thought I would ask your thoughts. I suspect water in the gas would not smell so sweet.

My reply: My question to you – or rather, your mechanic – is, why did it take a year to do a simple head gasket replacement? A frame-up restoration could have been done in that time!

As regards the rest: I suspect the job was botched. Thick white smoke should not be coming out of the tailpipe and if it is, it indicates what you indicated. A few simple tests can confirm. Look for bubbles in the coolant as well as coolant loss (without any puddles under the car). Check compression in all four cylinders; a vacuum test may show erratic readings.

The water in the gas thing is even easier to confirm or deny; simply burn up what’s in the tank and add fresh gas and see whether the white smoke goes away. I suspect that your Miata wouldn’t be running at all if there was that much water in the tank…

Very sorry about this. And hopefully, we’re both wrong.

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  1. Eventually, hard parts from the head into the pistons will break along with bearings giving up. So many mechanics won’t even check for deck or head warpage when gasket goes, just slap a new one on and pretend it’s a-ok.

  2. I have had antifreeze get into the exhaust before when replacing heads. Didn’t get the block drained down enough and when removing the bolts (which go into the water jacket) some of the remaining AF managed to pour into the open exhaust pipe. oops
    It did smoke for about an hour (of 50+ mph driving), until I got enough heat into it to evaporate the antifreeze. Maybe (hopefully) this is the problem – it does sound similar from your description.
    I wouldn’t have given it back to the customer if it were still smoking…

  3. A year for a head gasket (typically a few hour job) should teach you one thing – do it yourself. I had a mechanic replace axles once on an old Jeep. After seeing the horrible job he did, I thought “I could do a better job than that” and never looked back. This is especially true of an older car, which is often easier to fix. Just pick up the factory service manuals (usually on-line).

    FYI, same goes with home repair.

    • Much of the time a head gasket failure is indicative of another problem (i.e. cracked head). FWIW it doesn’t take a year to magnaflux heads.

        • Pass side head on my (SBC) motor cracked in the chamber, between the I/E valves, on cylinder #2. Another relatively common spot for them to fail.
          I was a little salty that the machine shop charged me $90 just to check them, found another shop with more reasonable rates but they haven’t been around long enough to build much of a reputation. Guess I will find out shortly. Two other local shops wouldn’t even return my calls, kinda hard to do business if you won’t answer the phone.

          The old GM 2.5L 4 bangers like to crack/warp heads as well. I am sure there are others too. It doesn’t help when folks overheat the hell out of them.


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