October 9, 2021

NB Miata project - Problems I've had during my first six months of ownership

In my last couple of posts I covered the performance and cosmetic modifications I have done to my NB Miata during my first six months of ownership. I was able to improve the experience of my car. Things like improving the exhaust and intake sound, along with changing the color of the car from silver to yellow using a wrap.

My Miata has been very reliable overall, but you can't expect a 20+ year old car to be perfectly reliable. While none of my issues left me stranded, I thought I'd share them anyways. Some of them have been resolved, while others still need to be fixed.

Speedometer fun

NB Miata gauge cluster

One of the first odd things I noticed while driving my Miata happened when I was coming back from a car show that was about an hour away. I was driving on the interstate, so the engine was continually running at 4000+ rpm due to the gearing. Running at higher rpms puts out a lot of heat. This was the first time I saw the speedometer signal come on and off. I didn't really think much of it, and still haven't put any effort into resolving it. It can be reproduced by driving the car long enough on a warm day. My normal 20 minute drive to and from work doesn't trigger the issue.

Sometimes it will be a single blip where the speedometer drops to 0 and then immediately back up to where it should be. Sometimes it go on and off, and sometimes it can stay at 0 for quite some time. From my research it's a fairly common issue with NB Miatas. The speedometer signal wires that connect to the sensor that goes into the transmission have weak solder joints. Heat causes solder to weaken, so after a certain amount of driving on a warm day the solder connection becomes unreliable.

From what I've read just resoldering the wires with better solder can resolve this issue for good. I'll have to try fixing this at some point. Since it isn't really something that will leave me stranded, and doesn't occur during my normal driving I'm not in a rush.

Alternator failure


The first time I had an issue that could potentially leave me stranded also happened driving back from a car show. My alternator went out. It wasn't a sudden failure, it slowly failed over the course of 20-30 minutes. It would stop charging and the battery light would come on in the dash, and then start charging again some time later. I just kept driving since the battery voltage wasn't too low. I knew restarting the car would use up a lot of battery power that I may need.

The good news is that I managed to get the car home under its own power. I wasn't 100% sure the alternator failed. It could be a loose alternator belt, so I tightened the belt a little more. It could also be an issue with the ECU since the NB controls the alternator output using the ECU. Or maybe it could be an issue with the wiring. I did try to drive the car a few more times after trying to adjust the belt, but didn't have any luck. During the last drive the alternator completely stopped charging so I ordered a replacement.

After swapping in a new alternator everything was back to working like it should. I guess my 20+ year old alternator just wore out. I did consider replacing it preemptively when baselining the car, but chose not to. Replacing working parts can be a slippery slope, and it was also easy to access down the road if needed.

Stripped oil pan drain plug

Miata oil pan

When I first changed my oil I noticed that there was this weird conical oil drain plug with some sort of thread sealant gunk. I also noticed that the replacement original style oil drain plug wouldn't thread into the hole in the oil pan. I ended up putting the conical plug back in and researched the issue later down the road.

Turns out that the oil drain plug hole in the car was stripped and one of the previous owners put in an Eco-Plug universal tapered self tapping oil drain plug. The plug is supposed to be permanently installed, and a magnetic insert can be pulled out to change the oil. Of course not knowing this my oil drain plug had a slight leak after cleaning off all of the thread sealant and reinstalling the plug. I ended up installing a new Eco-Plug with fresh thread sealant for the time being.

There are other solutions like Timeserts or Helicoils, but didn't end up using them. I was worried that I may mess things up considering I have no experience with them.

Leaking rear main seal

Miata oily transmission tunnel

Not long after getting my Miata I noticed that there was a small oil leak. Some amount of oil was on the bottom of the transmission bell housing, with some more blown onto parts downstream from there. It was engine oil, not transmission oil, and appeared to be originating from the bell housing weep hole. This all meant that I most likely had a small rear main seal leak.

Right now the leak is manageable. I just have to regularly check the oil and add more as needed, but eventually it'll need to be repaired. My current plan is to replace the rear main seal over the winter when I'm not driving the car. The plan would be to pull the motor out of the car, replace the rear main seal, and do a bunch of other things while I'm in there. For example replace the oil pan so I'll have a regular drain plug. Doing all of those things are easiest when the engine is out of the car, and would set me up well for the future.

Startup issues

NB Miata

I've had a few of issues related to starting my Miata. They may be related, or maybe not. None of them have left me stranded or caused any issues, they are more just more oddities I'd like to mention. Some of them may have been resolved, but it's hard to say at this time.

Intermittent rough idle after starting

First I'd like to mention what happens right after I start my car. Sometimes, but not always, it will start up and run fine for a few seconds, then it will lean out a little bit and run rougher for a few seconds, and then go back to normal. I didn't notice this during my first couple of months of ownership, but did notice it during cold starts on warm summer mornings in particular.

Oddly enough it doesn't seem to happen when starting my car after work. After installing a wideband O2 sensor I was able to see that it was leaning out with an AFR of about 16-17 instead of the optimal 14.7. I have no idea what is causing this, but it isn't really impacting my use of the car so I haven't looked to much into fixing it yet.

Click, but no cranking issues

I've also had some issues relating to actually starting the car.

Sometimes I'll turn the key, hear a click, but the car won't do anything. It isn't a super common occurrence, maybe once a month. The first few times this happened I was able to get it to start after just cycling the key once, so I didn't think anything of it. More recently it took maybe 5-8 key cycles to start. After that happened I ended up doing a couple of things and haven't had the issue since.

  • Soldered the clutch safety switch wires together. This bypasses the switch which could have failed.
  • Replaced the ignition switch.

I also ordered a replacement starter to have on hand just in case. I'll probably end up putting it in when I pull the engine. Every time I have the click issue the car starts up normally once the key is cycled enough times.

Cranking for longer than it should

My Miata has always taken longer than I'd expect to start, but it was always fairly consistent so it didn't really bother me. It may just be me being used to my Hondas starting pretty much instantly. More recently it took a long time to start, as in 10+ seconds of cranking. Once it did start it ran perfectly fine.

This particular issue happened when I started up my engine in between autocross runs. My theory is that it was an issue with the cam angle sensor being too warm. I left the hood closed in between those runs so the heat didn't have an easy way to escape. I replaced the cam angle sensor and haven't had the issue since.


Miatas are known for being very reliable cars, and my NB Miata has been pretty much trouble free during my first six months of ownership. I've had some small issues, but none of them have left me stranded. You can't say that about every 21+ year old car that you drive hard on the track.