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Old 10-11-2016, 02:57 PM   #1
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Default S.O.S. Need diagnostic help with 1990 Miata

Completely stock 1990 1.6

At first, car ran a little rough, would rev freely in neutral, miss if under load.
After it reaches operating temp it would just bog down and cut off while driving, hard to restart, the more you drive the faster it bog down and harder it would be to start. Eventually it would just not start again until it sat for a few hours, then it was all back to normal, until it wars up again. So I did the following.

New NGK plugs and wires
New fuel filter
Replaced lots of vac hoses and checked for leaks
Cleaned throttle body
Re-calibrated TPS
Cleaned all grounds
Checked, reset timing, several times.
Checked continuity from igniter to coils etc.
Tested igniter.
Cleaned CAS and replaced o-ring
No rust on any of the connectors.
Tested resistance on coils.

Now seems to run fine, idles good, revs freely, even drives good.
Occasionally, while driving under normal conditions, after it warms up, a problem appears and gets worse the longer I drive it.
After about 15-20 minutes of any type of driving it will occasionally bog down. It feels as if spark is cut.
If I lift of the accelerator pedal it will go back to normal, if I stay on the pedal it will bog all the way down until it stalls most of the time.
But sometimes, it feels like spark kicks back in and it resumes to accelerate, but the problem occurs repeatedly. Like an on off switch.
I can recreate this problem even before it warms up if I drive the car hard, will not happen if I drive it normal.
It seems that the problem is more likely to occur if I make a hard turn while accelerating, happens almost every time.
Had a code for AFM.
Slight lag in throttle response.

Replaced AFM, tested what I could.

No change in behavior.

What am I missing? I have a feeling it is something very simple that I am just overlooking.
Or maybe that's just wishful thinking and I have an intermittent electrical issues?
Should I replace the CAS? ECU? I threw so much money on the car already that my butt hurts just thinking about it.
Any input is appreciated!!!
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Old 10-11-2016, 03:10 PM   #2
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This is stock ECU?

Have you tested / replaced the Coolant Temp sensor? (the two wire one that goes to the ECU)

I say that because bad CLT readings can cause the ECU to go very rich/lean.
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Old 10-11-2016, 05:20 PM   #3
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Could be bad coils. Can you borrow known good coils to test? Also spark plug wires.
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Old 10-11-2016, 06:08 PM   #4
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Onyx,
Yes, stock ECU, stock everything.
I tested the Coolent Thermosensor at the plug, get 2.21Kohms in resistence. At roughly 60F which sees to be on par.
Buuuut I can't seem to get any VDC at the ECU plug with key on, should be about 2.2VDC, The ground at the sensor plug also tests good.
The blue/white wire has continuity. This makes even less sense now.

Stefan,
Two sets of old NGK wires, same result, now have set of brand new NGK wires, no difference.
Tested resistance between coil posts and it was in range, do not have spare coils.
Seems highly unlikely that both coils would intermittently fail at the same time, but Im open to the possibility if someone has a spare set I could borrow.
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Old 10-12-2016, 01:09 PM   #5
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I replaced the coolant thermosensor just in case, no change.
Is it possible that the AFM is bad and not give a code?
I still have that slight lag in throttle response, almost feels as if it gets flooded for half a second, but then revs freely.
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Old 10-12-2016, 01:14 PM   #6
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The AFM can fail and not throw a code. Door can stick open. Check the wiring to your O2 sensor and the sensor itself.
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Old 10-12-2016, 01:42 PM   #7
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Quote:
Originally Posted by olderguy View Post
The AFM can fail and not throw a code. Door can stick open. Check the wiring to your O2 sensor and the sensor itself.
AFM (old one and newer one) the doors are moving like they should, no sign of either sticking, the spring does its job just fine it seems.

O2 sensor wire (red/blue) at the ECU reads within spec, jumps anywhere from .3V to .7V while running. Spec is .2 to .9V according to Mnet garage ECU pin-out...
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Old 10-12-2016, 02:27 PM   #8
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So, I changed the water thermistor and re-installed the old AFM.
Drove around for 5 minutes and the problem is way worse.
Now I got two codes, 8 and 9.
Code 8-AFM
Code 9-water thermistor
Cleared the codes and re-installed the newer AFM.
Drove around for a few minutes, seems much better but the problem still persists just not as freqently.
Now I get no codes.

Starting to think that my new AFM is not that good.
Anyone near me have a known good AFM I can borrow and possibly buy?
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Old 10-12-2016, 06:19 PM   #9
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After lots of driving and swapping out 4 different AFM, I am now convinced that the AFM is not the problem.
I now suspect the Crank position sensor.

Thanks everyone for your input, I will report back with the results.
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Old 10-13-2016, 02:19 PM   #10
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Turns out I had a spare CAS from a 1.8 so I went ahead and switched it out.
There is no change in behavior.
Still get code 1.

Any advice on what to do?
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Old 10-14-2016, 04:16 PM   #11
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Issue is resolved!

Pulled the fuel pump out even though everything tested good, found the connector at the pump (inside the tank) to be muddy looking, no rust on the contacts, just really dirty.
Disassembled the entire pump, cleaned it, reassembled, everything is good now.
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Old 10-14-2016, 04:36 PM   #12
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Excellent. Thanks for posting the resolution.
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