cold start issues...

ok so i was sure that the cold start issues i was having was due to a bad water thermo sensor. well swapped it out, and :punchout:, i'm still having a issues with really rough idle before the engine comes up to temp. and it stumbles on throttle tip in and damn near dies. once it get moving it runs fine. it's throwing code 8 and 15 and 17. which leaves me puzzled as i just replace the o2 sensor. code 8 is the Airflow meter open or short circuit.

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FAULT CODES ('90-'93)

1 Ignition pulse (No Igf signal)
2 No Ne signal
3 No G signal
8 Airflow meter open or short circuit
9 Water thermistor open or short circuit
10 Intake air thermistor in airflow meter open or short circuit
12 Throttle position sensor open or short circuit
14 Atmospheric pressure sensor open or short circuit
15 Oxygen sensor <0.55 V 180 sec after 1500 rpm
17 Oxygen sensor unchanged after 1500 rpm
26 Solenoid valve (Evaporative canister purge)
34 ISC Valve

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so would a bad would a bad air flow meter cause cold start issues? i figure if that was bad then i'd have problems all over the rpm range, not just at idle on a cold start.

help me obi one kenobi your my only hope...

If you're up for swapping out parts, I have a '90 ECU and AFM that I have sitting around collecting dust. They both worked when I pulled them from my car back in 2003, and have used them to troubleshoot a car here and there over the years. They both have ~35k miles on them. You're more than welcome to try them out for a few days if you get out here to pick them up.

Sounds like the O2 wire has a break in it somewhere though, whether in the casing causing a short, or the actual wires inside the casing are broken.

thanks for the offer rob, but i have no clue when i'll be headed out that way agian. do you by chance know what i need to hook up my multi meter to to figure out if my unit is bad? if it is i may just see if i can find an old rx-7 afm to swap in for a bit more air flow and see if that clears code 8 at the same time. as far as 15 and 17 i'll check the o2 sensor and see if it's output is up to spec. if not i'll take it back to o'rileys and get a new one.

I don't know offhand how to test the AFM, no. I'm not near my manual either. I'm sure someone here knows though, so hopefully they will chime in.

The sensor might be OK, the wiring to it might be the issue. I'm assuming you're still using the 1-wire unit?

Can you run a vacuum test on the motor when its running? The condition you described is similar to a vacuum leak. Hard to start and runs rough when cold but wakes up when it gets warm. Its not unusual to have a higher than normal idle, too. Just a thought.

Is the exhaust manifold stock or is there a header installed that relocated the o2 sensor? +1 on the 1 wire or 4 wire sensor question. Getting both codes for it indicates a problem with the sensor or the install, and I'd chase those before moving upstream and swapping any parts.

Check all of the harness connections to the AFM, TPS, O2, ISC (and CAS while you're back there) to make sure they're good and solid, then verify there's no issue with vacuum as noted above. If all connections and the vacuum test are good, the problem could be with the ISC valve that's mounted below the throttle body or with the AFM itself.
Does the car hunt at idle? +-150 RPM with the occasional drop lower?

I have the same symptoms and have narrowed it down to the ISC valve (I never got an AFM code, so YMMV) - if the problem doesn't go away once the car goes into closed loop, it's likely the AFM, if it does (mostly, but not 100%), it could be a stuck ISC valve letting through bypass air all the time that the ecu is trying to correct for based on o2 sensor data. The root cause of the idle hunting and stumbling on tip in is unwanted air, so it's something in the system in front of the intake manifold that's causing it.

HTH,
Chris
Chris

looks like the afm modernbeat loaned me at this weeks wed night gtg solved all the issues. cranks and runs without a hitch, no bogging doesn't stumble and the check engine light has yet to come back on.


as far as the 15 and 17 codes, i'm a complete and total idiot and forgot to clear them after i replace the o2 sensor.

i've got an rx7 unit on order i found for 25 bucks, supposedly it's less restricted than the stock miata unit. doubt it will actually make much difference, but for 25 bones it's worth a shot.

Glad you found the problem. Let us know how the Rx7 part works out.

How do you clear the codes ?

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Originally Posted by miata_B_racing

How do you clear the codes ?

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Disconnect the battery and step on the brake pedal for 5-10 seconds.

Disconnect the battery and press the break pedal until the tail lights go out.