So I thought I knew how to work on Miatai by now but this is making me perplexed.
Crapbox 1992 bone stock been sitting for several years. Replaced the gas tank, fuel filter, and injectors. There was a crack in the intake snorkel so I replaced that today. Removed and cleaned the throttle body. Replaced the air meter with a known good one. Checked and cleaned connections to the TPS and air meter. Plugs are new and I replaced the plug wires with known good ones as well.
So the car acts as if it has a HARD rev limiter of 2,000 rpm. Absolutely will not rev past that and momentarily shuts off (just for a beat, enough to flash the CEL if you try to rev it and it bounces up and down).
What next? I'm running out of ideas. It's OBD1 so I tried the paperclip jumper to pull any codes, nothing in it's little brain.
Help please.
IIRC this is what happens if the TPS is not adjusted correctly. The 1.6 TPS is really an idle switch and a WOT switch in the same housing, and it can be rotated to change the trigger points.
Pop the cover off the TPS (it pries off) and you'll see how it works. Loosen the mounting screws and rotate until the idle switch disconnects as soon as the throttle plate starts to move.
Another possible thing you can do to screw these up is damage the wiring on the AFM if you try to remove the connector by removing the two very tempting screws. But that's usually the "fires up and runs for 5 seconds before shutting down" failure.
Or the cat could be clogged?
In reply to BlueInGreen - Jon :
Any metallic sounds from below? My cat broke up and plugged tailpipe but you could hear it bouncing around when reving and then would cut out around 4k in similar fashion.
Exhaust flows great. Yes it all rattles but it's normal Miata rattle.
The TPS thing is interesting, it's in such an annoying location!
A hard cut at 2000 is most likely to be the ECU calling time instead of something clogged. The car thinks the throttle is closed but the engine speed is way to high. IIRC we used to run into this in the early days of programmable ECUs. The Link had a super-handy idle switch indication on the keypad.
The TPS is in a much less annoying location than the cat :) If you want, check continuity between the lower two terminals. You should have continuity with the throttle closed and it should go open as soon as you move the throttle. There's a procedure in the manual, let me pull it up...
Insert a 0.016" feeler gauge between the throttle stop screw and the stop level.
Loosen the mounting screws.
Rotate the TPS approximately 30* clockwise, then rotate back counterclockwise until you get continuity between the lower two terminals.
Replace the feeler with a 0.027". You should no longer have continuity.
Tighten screws.
Repair update. Popped the cover off the TPS and inspected/tested it. Perfection.
That left us with either a bad wire or bad ECU. Since chasing wires is my least favorite thing in existence and I have a perfectly good NA6 ECU on the shelf I swapped it in. Carpet padding was sodden and the ECU was really rusty.
New ECU installed and voila, a perfectly running Miata!
Thanks gang, next up is getting it registered and a new top installed. Put some miles on this beasty and see how it feels.
Glad you found it. People are usually very quick to blame the ECU because it's a mysterious box, but it sounds like yours had actual damage in this case!
It took me years to learn that we tend to skip over all the simple stuff we understand and assume the problem is in the complex part we don't understand. I spent months as a teenager swapping and tweaking carburetors only to eventually have a shop tell me the pickup in the gas tank was clogged. In this case, though, I'd also swap an ECU I had on the shelf before chasing wires.
Bringing this thread back up for a weird issue. I've not been driving the car due to reasons, but this weekend I had to move it out of the garage and the battery wasn't in the mood. Put the jumper cables on it and it started for a moment but then died and wouldn't restart. Didn't act like it was getting any fire at all. Eventually I shoved it into the driveway and used my working garage for the project and yesterday I tried again. At first, nothing but cranking but then it suddenly figured out where it was and fired right up. Ran it for a bit in the driveway and moved it around but before I could put it away again i stalled it and (surprise surprise) same thing happened, cranks great but wouldn't fire again.
Potential ground problem? The seller had said something about "sorting out a ground problem" when he'd messed with it a couple years ago. Where would one add or replace grounding straps on a 92 Miata?
Other thoughts/suggestions? When it runs it seems to run pretty darned well for an antique that hasn't in years. 
Grounds: check the multi-ground point just below the brake booster. That one is prone to corrosion. There's an important one on the PPF back near the diff but I wouldn't expect it to show up like this.
I'd do the following:
- clean up grounds, always a good practice
- change out the main relay because while they usually fail completely, I could see a dodgy one working like this. It's not making contact immediately but once it does, it's good. Check the condition of the connectors when doing this.
- check the condition of the connectors at the ECU, make sure all wires are fully seated and the connectors are free of corrosion. Given the rusty condition of the previous ECU, there's a good chance you have a bad connection there. The ground wires (there are several) are black.
- with the car idling, do a wiggle test on the wires on the cam angle sensor. They can break internally from flexing and that takes away the main "engine is turning" signal to the ECU
