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1991 Mazda MX5 Miata Automatic Transmission

I have a 91 Miata Auto. I bought it from someone that (I think!) never drain auto trans fluid during its 145K mile life. So last year I replaced all the gaskets/filters and tuned up the car. It runs fine except shifting harsh in cold weather. I have to mention that it seem to have no power and goes very slow on first few minutes.
I used a pump to suck trans fluid out. It was so dark (even I droped the trans pan and replaced filter/gasket). Then I pumped Lucas trans fix fluid in and it seem to improve. What should I do more to reduce the harsh shift condition in cold weather? Or is there a booster module need to be replaced?
Please point me to right direction,

I’m having trouble understanding exactly what you did. Did you replace your transmission fluid WITH the Lucas Trans-Fix, or did you replace the old transmission fluid with new transmission fluid and then add a bottle of Lucas?

Either way, I’m not a fan of transmission additives. Shifting harshly in cold weather isn’t terribly unusual, especially when you first start the car. And you shouldn’t be racing it in the first few minutes anyway, because that’s bad for just about every moving part in the drive train.

Post back with a better description of what exactly is wrong, and what you’ve done so far.

Thanks shadowfax.
I did replace the trans fluid with Lucas Trans-Fix (2 bottles + Lucas trans fluid after replaced new filter and gasket), since I have very good experience with it in my MR2 automatic. I have no issue with the MR2 in cold weather and the MR2 accelerate at ease. For this Miata, I feel like something prevents the torque converter doing its job

Lucas fluid could be fine in one car and bad in another. I suspect the Lucas fluid (which is “generic” meaning for lots of cars) might not work well in your Miata. Before doing much else I’d get all the Lucas fluid out of the transmission and refill with a known compatible fluid, in this case a Mazda brand fluid from the dealer service dept. The few extra $$$ is worth it to know you have the correct fluid in there. Then I’d drain and replace the fluid again in 5 to 10K miles to help get out all the crud from lack of service by the previous owner.

If the shifting problem smooths out, you’ll know the fluid is the reason. If not, then you get into deeper into a resolution. There is a good chance the Lucas fluid is either the whole problem or a contributing factor so that’s where I’d start.

believe that the Lucas oil is very thick, so I would think that if you have TWO bottles(??) in there, you would get harsh shifting when cold… It will flow better when warm, and maybe that is your whole issue. I think @uncleturbo is right… Get the right fluid in there ASAP, and see what happens, some transmissions are VERY sensitive to the fluid in the pan.

I would go to a Mazda dealer, buy the exact transmission fluid specified for your Miata, and replace what’s in there.

Thank you all for the advise. The issue was a vaccum modulator on the driver side of the trans. It was malfunction. I replaced it ($20+ after market part). Car is now running fine,