Issue with my stock 1.6 ECU and/or my TPS after removing Megasquirt - Miata Turbo Forum - Boost cars, acquire cats.

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Old 07-13-2009, 09:12 AM   #1
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Default Issue with my stock 1.6 ECU and/or my TPS after removing Megasquirt

Hey guys,
I just removed my MS from my 93 (California model) Miata to get it back to stock to sell it and now it's not running right. I replaced the BMW VTPS with a junkyard one at the same time since I used the plug from my old TPS to make a harness for the BMW VTPS when I was running MS. Anyway, I've adjusted the stock TPS according to the FSM and the sensor reads correctly (continuity checks). I also ran down a vacuum leak at the TB to IM gasket. No others to be found. Now the issue I have is that my car runs very poorly at part throttle for the first 3 minutes or so after cranking it up. Doesn't matter if it's cold or hot. It will idle pretty much fine if not a bit high (occasional hiccup, but mostly OK). The problem is that for the first few minutes part throttle input will cause it to sometimes bog. Sometimes when it's bogging, I can even press the throttle all the way to WOT without any response. After a few minutes it will begin to run fine and I'll have no problems the rest of the drive. Thinking it felt like a TPS problem I checked the sensor and it tested fine. I then checked the signal at the ECU thinking it might be an open circuit or short. According to the FSM pin 1N (red wire at TPS) should be 0V with the throttle closed and 12V (Vbat) at any other setting. Checked out just fine. And pin 2L (Gr/Wht signal wire at TPS) should be 5V (Vs) with the throttle closed and 0V at WOT. I'm getting 12.5V (Vbat it would seem) at closed throttle as well as part throttle and 0V at WOT (when the TPS shorts it to ground). Obviously this is weird as the 12V is apparently coming from the ECU so there is not that much I can do to fix it. Could having my BMW VTPS hooked up (both correct and backwards at one point) have damaged my stock ECU? Does this problem sound like it would be the cause of my symptoms? I'm thinking I need to get another 1.6 ECU and plug it in to be sure. Anybody got one they want to sell me or let me borrow? I assume if I get another 1.6 ECU I'll have to rewire the injector wires in batch as my California ECU puts out sequential injection. Any other suggestions or thoughts?

Just a few other tidbits:
I checked continuity from ECU to TPS on both wires and all is good.
I never cut the TPS wires at the TPS as I had a small harness there. Both connections have been soldered back together at the ECU.
Fuel pressure is good, plugs are new, wires are new. Self diagnostics are clear. Checked for intake leaks with compresed air.
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Old 07-13-2009, 05:03 PM   #2
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Try unplugging the TPS... it should work fine with the stock ECU (but won't cut AC at full throttle). If not, something else is wrong (possibly lambda sensor connection), or not returned to stock spec (timing, AFM/MAF, COP, etc.)

I have a very similar car (93 NA Californian Spec, MSPNP, even same color...) and ran it the stock ECU and no TPS for the Winter without any issue.
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Old 07-17-2009, 10:30 AM   #3
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I just wanted to do a final update in case anyone is interested. A fellow forum member was kind enough to send me a 1.6 ECU to try and it fixed it (thanks rharris!). It seems somewhere along the way when I was hooking up my BMW VTPS I must have shorted something and caused some damage to my stock ECU. I opened it up and there were some obviously overheated resistors, but they didn't appear to be in the TPS circuit. Still, when you are getting 12V out of a pin that is supposed to only have 5V there is obviously a problem. I hooked up the non-CA spec ECU and checked the TPS pins and I was getting the proper voltages. I had to jumper the injector banks since the CA spec wiring harness was wired for sequential. Otherwise, no extra steps were required. I was a bit nervous as the ECU's look very different internally withe different chips, and many different resistors and capacitors, but it's running great and I'm not getting any CEL's or anything. Thanks for all the help from everyone and the many offers for ECU's to help figure this one out.
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