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Old 08-18-2007, 01:20 AM   #1
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Default 1999 Tranny in a 1990 Miata

My tranny suffered the inevitable bent shift fork and was stuck in reverse, so I grabbed a 1999 tranny ($200) which I'm told is stronger than the 1.6L tranny. After install, I'm having problems with idle. When I push the clutch in, the RPM drops to almost nothing, then hovers around 500. Is this a vac hose problem? the only other thing I did was put a new A/C compressor in. I used all the 1.6L sensors as well since the '99 had different connectors and I didn't feel like chopping them off.
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Old 08-18-2007, 01:29 AM   #2
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what sensors are hooked up to the tranny? I cant see any affecting idle
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Old 08-18-2007, 11:29 AM   #3
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The sensors are for backup lights, neutral, and speedometer and should not effect the idle at all.

Has anyone else had this problem after installing a new transmission? I figure it's a vac hose, but I can't seem to find any that are loose/off. Are there any other possibilities?
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Old 08-18-2007, 07:12 PM   #4
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More information:

So I turn the car on and it idles ok, I push on the gas a bit, then take my foot off and when rpms go back down, it almost stalls and sometimes does, then goes back to a lower idle than the initial. My AFR gauge went out now too. It's getting power, but it doesn't look like the signal is getting through.

Any ideas?

Anyone?
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Old 08-18-2007, 07:21 PM   #5
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It is possible that when you were switching transmissions that you broke the O2 sensor or cut or pinched the wire. If your ECU is receiving no signal, it thinks the AFR is lean, and adds fuel after it should be receiving an O2 signal. The fact that your AFR gauge has no signal could be a hint.
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Old 08-19-2007, 12:17 AM   #6
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The funny thing is, it was working after I put everything back together. I took it out for a ride to see if the problem would go away, put it away, and the next morning I had no o2. I think it had been going bad for awhile though since the reading was never stable unless I was accelerating. I guess it could just be the low quality AFR sensor used.

Time to get a 5 wire and pray it fixes the problem...
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Old 08-19-2007, 12:29 AM   #7
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ground strap? I baked a coil because of this once on another car.
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Old 08-26-2007, 07:03 PM   #8
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Ground straps look good. I disconnected the IAC and the car runs perfectly the way it should. Not sure if I need to clean or get a new IAC, but I figure I'll get some starter fluid and see if I can find any vac leaks. I also bought a PLX 5-wire o2 module and sensor. The exhaust was spitting fuel all over my garage, so hopefully the o2 will help. Still need to get my hands on a megasquirt, but that will have to be next paycheck. I have some minor fixes I need to do on my jeep as well, so it may be a couple paychecks. I think my diff is dying on my miata too. Hopefully I can find a nice torsen.
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Old 08-26-2007, 07:47 PM   #9
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i have a problem in my 240 where once i let off of the gas it drops way low, then bounces up and down for a while until stabilizing, pretty sure its a vac leak (drilled and improperly filled a few holes in my intake), hope this helps any
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Old 08-26-2007, 08:36 PM   #10
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If you disconnect your IAC in your 240, does the problem go away?
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