2000 miata runs intermittently bad

I have a intermittent problem with my 2000 miata, intermittently runs bad misses terribly then clears up just as fast. I have had the Mazda
dealer tune it up, plugs, wires fuel filter but it still does it. It has a P0420 cat efficiency code in it when this happens. I have never been able to get it on a scanner quick enough to figure out what the o2s were reading and if it is missing on certain cylinders when this happens. My local dealer said it could be O2's, Cat, or MAF sensor. Im no Miata expert but i thought a rule of thumb was if the cat was bad the thing would be real sluggish and get worse as it gets hotter. Any Ideas? I dont mind putting a Cat of MAF on it just would like to try to narrow it down before I start hanging parts.
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The symptoms match an intermittent bad coil pack.
A possible confirming method is to use a Scangage II to continuously monitor with. This was the only way that I eventually confirmed that my 99 had an intermittent coil pack, and a bad aftermarket coil over ignition system. P0300 is the intermittent code that is the key, along with possible P30x codes and the P420 code. While you may need a new cat, it's failure is likely a result of other problems. A borderline front O2 sensor can also cause related problems. (Been there also!)
Originally, my 99 started out with undiagnosable problems involving occasional hesitation, occasional P0420 codes, etc. Wires and plugs were replaced several times, with subjective evidence of some improvement. The problems eventually returned. The Cat was replaced as part of a turbo install. After the CAT "aged in", the occasional P0420 codes returned, and often under an odd condition-- the next morning after frequent 500 mile interstate runs to/from Florida. The code usually set during initial low speed closed loop operation.
Next the 99 developed an intermittent hard to start, no start, backfire, then maybe start symptom. Replacement of the Cam sensor seemed to have cured this problem until it occurred again some months later. This time, the cam and crank sensor were replaced.
To cut to the chase, I finally started continous monitoring with a ScangageII. This showed that P0300 codes and a P0302 code were occurring on occasion, and possibly accompanied by misfires on one or two cylinders. Swapping parts around partially isolated this to a "Spitfire" aftermarket coil over system, and likely the controller. Replacement with the original coil pack showed that the aftermarket system was at fault.
However, occasional hesitation etc. returned, and more frequent P0420 codes.
Replaced factory coil pack. Problems went away, except for a very occasional P0420 code. If I reset it, I may go months before it sets again.
Other parts replaced, cleaned, etc during the various troubleshooting processes Fuel pump and filter. Used AM pump, due to turbo. Cleaned intake manifold passages and EGR valve. Replaced front 02 sensor, due to P0420 codes. The OEM sensor was a bit slow to respond, and .05v low in output compared to a new sensor. (If the rear sensor is faster than the front, a Cat efficiency code may result.) Also replaced Temp sensor used by the ECU.
On 5/16/2010 3:34 PM, dave wrote:

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dave wrote:

After having the coil pack on my 95 replaced i had similar symptoms of intermittent rough idle, so rough that i had to give it gas to keep it from stalling, even when driving... the trusted mechanic i use was perplexed by this and confirmed that it wasn't a ground problem (a common problem) and knowing the cars history and solid maintenance he could only suggest doing a fuel injector purge (plus a new set of cables and plugs), and it worked. The car had about 125k on it at the time. Not sure if this solution would apply to you but thought it couldn't hurt to mention it. Let us know how it works out.
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On 5/17/2010 1:08 PM, Christopher Muto wrote:

The NAs behave a bit differently than the NBs. The "no return" fuel system is one difference, as is the coil pack on the 99-01 s. Cam sensor failure has been a bit more common. Due to the no return fuel system, a pressure relief valve inside the tank on some 99s was prone to sticking partially open, causing starting problems.
I ended up replacing the OEM fuel pump, not because of any obvious problem, but because my 99 is turbo'd and had some "raggedness" at high power levels. This later went away when I installed a new OEM coil pack.
What was so frustrating was that the problems were very intermittent. One thing that was misleading was that use of Techron or other strong fuel supplements seemed to help.
The intermittent no start, rough start, backfire, and then start normally on the next attempt really had me going for a long time.
Finally, I broke down and bought a fancy timing light that actually worked with the miata's ignition system. This showed what looked like random rather than timed firing, or no firing when I was able to catch things in the act. Again, when I changed the Cam sensor, the problems seemed to go away. In the middle of all of this, I was making 500 mile interstate trips with little or no indications that anything was wrong.
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