Piston Slap: Justification for Higher Miata-ification

Sajeev Mehta
by Sajeev Mehta
piston slap justification for higher miata ification

Darren writes:

I recently took my 1990 Miata to St. Matthew’s Imports, a reputable shop here in Louisville, Kentucky. The a/c was not blowing cold, and a running light was out. I also wanted them to inspect and tighten the undercarriage bolts like it says to do in the manual. They tested the a/c for leaks, it had none but they said the low pressure side had too high a pressure and there were signs of moisture in the evaporator case. They want to replace the compressor, drier, expansion valve, and possibly the evaporator core. Also the radiator is leaking coolant, the right rear brake caliper is leaking, the left rear parking brake does not hold, the front brake pads have no shims, and the master cylinder is leaking fluid. To make matters worse, the front engine seals, the rear main seal and the valve cover gasket is leaking.

Wait, it gets better: the left side of the vehicle sits lower than the right side. They recommend replacing the struts and coil springs. I decided to hold off for now on the struts and springs, but the bill to fix the rest is about $4000. I know this is a 20 year old vehicle, but damn! I think I am going to have all this done, but I wanted to know what Sajeev thinks of all this: does it sound correct? I generally trust them but wonder if they could be jerking me around. I only wanted to fix the a/c and the running light and now all this…. I guess I could limp along with it and just top fluids off periodically, but I love the car and want to keep it.

I think the book on the car is only about $2000. Tough to justify spending $4000. What say you?

Sajeev Answers:

Old cars require time and money to be in a like-new condition. Which is why people in this situation are more likely to fix stuff on their own, buy parts after months of shopping for great deals and—generally speaking—being a tightwad in most of their daily activities.

I am not telling you to change your life and become a mechanic, but meet me halfway: learn more about your car from your fellow owners. Post on their messageboards on occasion, silently research relevant threads quite frequently. Learn where to get stuff for good prices, become an informed consumer of restoration parts, and then go back to a local mechanic. Again, do this as an informed consumer.

Back to the car: what you discussed sounds normal: waves of troublespots come to light when you take a close look at a twenty-year-old car that’s mostly original. Unless we’re talking about a crude truck (that just get better as they go bad) an older vehicle is in dire need for a colossal number of fresh components to return to its former glory.

BUT…the A/C repair sounds kinda nuts. I thought that replacing the “orifice tube” would fix these problems, it’s either a separate part or it’s built into the low pressure line. If the compressor runs quiet, that’s even more reason to second opinion on the air conditioner. Sometimes a new orifice tube and new O-rings at every fitting fixes the problem.

Send your queries to mehta@ttac.com

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3 of 52 comments
  • IronEagle IronEagle on Oct 08, 2010

    Yeah the local DSM club's motto is: Turning ordinary people into mechanics since 1989. I am saving $800 doing the new non sprung puck clutch/lightweight flywheel in the Eagle doing my own clutch swap. Most of these import cars are easy to work on with a few wrenches and a few sockets in 10mm-19mm sizes!

  • 65692 65692 on Oct 27, 2010

    An "expansion valve" and an "orifice tube" are the same thing, more or less. if the suction line is slightly high, try cleaning the a/c coil in the cabin. you would have to remove your dash to get to it, but 20 years without a cleaning would be a problem. I service home a/c's for a living, and neglected care for the evaporator coil is the biggest cause of a/c problems. i doubt the Miata has an in-cabin air filter, so as such, 20 years of dust, dirt, convertible down smogy-air and god knows how many tons of cigarette smoke have passed through that coil. A/c supply stores will sell anyone a mild acid and a metal comb for coil cleaning, but for cars, hot soapy water and an old tooth brush will work great. if you want to know the theory behind it, the compressor pumps liquid into the cabin where it reaches the expansion valve. the valve mists out the right amount of liquid into the coil so that it when the hot air of the cabin crosses it, liquid refrigerant boils off into a gas. if the valve is defective it can open too much and cause the low pressure side to be higher than normal, but then inversely, the high pressure side would be reading a little low. if the valve is working fine, but something (like dirt) is blocking hot air from flowing across the coil, it wont boil all of the refrigerant and therefore give higher than normal pressure on the low side, and a normal to high on the high side.

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