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Old 06-24-2014, 01:32 PM   #1
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Default Car only runs with AFM unplugged!

I've done a ton of research into this, and for the life of me I can't figure out what's going on with this car.
I've basically done a full rebuild on this car, everything sans engine internals. Long story short, everything is back together, and the car will only run with the AFM unplugged. Everyone keeps telling me I have a bad AFM but I have tested them by the shop manual. I have also had some recent suggestions that it might be my ECU. While I plan to go MSPnP eventually, that's an expensive dice roll to throw at a car, which may actually end up further complicating an issue.

Any help is appreciated!
If I press the gas pedal too quickly when the AFM is unplugged the car will die. With the AFM plugged-in using throttle seems to make it die faster than the measly 1-2 seconds it'll "run".

Some info:
- Car has no error codes
- Vac gauge reads about 22 in/Hg at idle (I am at about 1,300 ft elevation)
- O2 sensor reads rich at about 11 or 12 ratio
- I have NOT timed this using the CAS yet as I cannot get it to idle under normal conditions. (I moved the CAS until the engine sounded good)
- New aftermarket fuel pump and filter within last 500 miles
- Injectors, fuel rail, Fuel Pressure Regulator, CAS all pulled from a motor that was working about 4 weeks ago. They sat on a shelf until I put them on about 1 week ago.
- Head has been gone through by a local race shop: Cleaned, pressure checked, resurfaced, valve job
- Engine has ALL new seals and gaskets

Things I have done so far to try to fix this:
- Cleaned and re-tightened all grounds
- Re-Torqued Intake Manifold (thinking it was a vac leak)
- Swapped Fuel Lines to confirm proper routing
- Confirmed spark on all cylinders by pulling plug wires individually and observing spark
- Jumped the fuel pump in diag box
- Pressed the AFM flapper by hand (with AFM plugged in) to confirm the fuel pump kicks on: and it does
- Tested the AFM per the shop manual and resistances are within spec. (I even bought another one on ebay and it didn't help)
- Sprayed starter fluid in places I would suspect vac leaks (none)
- Tried unplugging my wideband O2 and plugging in the factory O2 into the harness (no change)

I am SURE that one AFM has never been tampered with (as it worked before I took it off the car 1 week ago). The new-to-me used one I recently purchased could have been, but from my closest visual inspection seems to have not been tampered with. I tested the units per the shop manual as follows:


I also referenced this thread: properly testing an AFM / acceptable values - MX-5 Miata Forum

But couldn't make much sense of the "reference voltage..." and whatnot. Not sure if my testing methods were proper or not but my readings on my AFM's are nearly identical to what the OP in that thread posted.

Here is the Image of the graph that the "output voltage" of the AFM should be....but I have no idea where/how to test these numbers in the car (since the car won't actually run).
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Car only runs with AFM unplugged!-airflowmetertesting.png   Car only runs with AFM unplugged!-airflowmetermiata16.png   Car only runs with AFM unplugged!-miataafmvoltagegraph.png  
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Old 06-24-2014, 02:31 PM   #2
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The only thing I can think of that changes when you plug in the AFM is the AIT sensor. Try jumping the fuel pump in the diagnostic box, and seeing if it'll run with the AFM unplugged. That's probably why it won't stay running with it unplugged, as the fuel pump isn't triggering.

Then I'd check timing, it's a common one to **** up and no amount of adjusting the CAS will fix it.
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Old 06-24-2014, 03:27 PM   #3
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Looking at this pic makes it look like my exhaust cam is too far anti-clockwise...but turning it one tooth the other way would make 20 teeth between the marks, it looks a lot more "right" in person FWIW.




And with the car assembled I can't really get a clean pic of the bottom mark, but the "V" notch is lined up on the bottom gear.

Also- I have tried jumping the F/P multiple times, it doesn't change anything (other than hearing the Fuel Pump kick on when I turn the key to the "start" position instead of when I actually turn the starter).
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Car only runs with AFM unplugged!-img_20140624_131715.jpg   Car only runs with AFM unplugged!-img_20140624_130618.jpg  
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Old 06-27-2014, 03:40 AM   #4
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Bump.
About to start throwing parts at it!
ECU first, then ignition coil, ignitor, then ENTIRE wiring harness (not looking forward to that.)
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Old 06-27-2014, 08:06 AM   #5
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how did you unplug the AFM? did you unscrew the connector or just pull th plug off it?
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Old 06-27-2014, 04:48 PM   #6
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Quote:
Originally Posted by Braineack View Post
how did you unplug the AFM? did you unscrew the connector or just pull th plug off it?

Quote:
Originally Posted by Snow View Post
I am SURE that one AFM has never been tampered with (as it worked before I took it off the car 1 week ago). The new-to-me used one I recently purchased could have been, but from my closest visual inspection seems to have not been tampered with. I tested the units per the shop manual
I am curious: Even though I have put up all this info saying that I have two AFM's and that I've tested them per the shop manual people have still suggested my AFM's might be bad. Are these testing procedures known to give false readings or something?

Short answer is no, I have never unscrewed either AFM. I unplugged the harness from it....the connector has an annoying metal clip that runs around the outside of it.
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Old 06-30-2014, 04:36 PM   #7
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Are the Mazda shop manual testing procedures for the AFM a load of bull?
Anyone ever tested them good but had them not actually WORK in the car?

Also, threw an ECU at it, didn't help any.

Bump for knowledge. Next step is to purchase some electrical tester thingamabobs and test each side of the wires (ECU Side and AFM Harness Side) to see if there are shorts or something.

I have no idea where the wires actually go into the ECU though, lots of research in my near future. If those check out fine I guess I'll have to check the outbound signals from the ECU to the other components of the car that the AFM affects: Does anyone have a complete list of what changes based on signals from AFM?

1. Fuel Pump
2. ?????
3. ?????
4. ?????
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Old 07-05-2014, 07:34 PM   #8
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Diving into it again with a friend today, bump for knowledge.
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Old 07-06-2014, 09:42 AM   #9
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Well, Last night/this morning was the moment of truth.
I had my buddy over and we dove knee-deep into electrical problems. Armed with my Mazda Shop manual we had every spec for resistance/voltage/favorite color of EVERY part on the car.
And so we tested EVERYTHING that connected to the motor, CAS, AFM, F/P, ISC, TPS, F/P Relay, we swapped in new relays, checked fuel pressure, Ignitor, Ignition Coil, etc etc....just everything....and everything was PERFECT...but the car still would not run.

So in a bit of desperation my buddy decided to try to Manually open the AFM with his hand while I stared the car (a trick I could not perform alone when I was diagnosing the car previously). And behold, the car ran and idled for the first time on its own WITH the AFM plugged in.

But WHY? Bad AFM? We just sat there in confusion for a second, pondering why the car would run like that...and then it hit me, "Is the AFM backwards?"

He looked down at the AFM, then he looked at me, smiled, and shook his head in disbelief.

So this ENTIRE time, the only thing that had been keeping me from a smooth running car, was my own stupidity....I had put my AFM on backwards. Mazda even stamped a little arrow on the darn things so I wouldn't get it wrong...and I still managed to screw it up. I had rebuilt the entire car and that one little mess-up had me perplexed for nearly a month.

I appreciate everyone's help. Hopefully I'll see you guys at the track sometime!
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Old 07-06-2014, 09:45 AM   #10
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i dont even see how you can install it backwards...
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Old 07-07-2014, 05:21 AM   #11
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Quote:
Originally Posted by Braineack View Post
i dont even see how you can install it backwards...
Me either, unless it was a MAF, but the MAF doesn't have a flapper...
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Old 07-07-2014, 08:18 AM   #12
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but they also say people have installed clutch discs backwards, so you people are capable of amazing things.

completely makes sense though and at least you figured it out and posted the solution.
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Old 07-07-2014, 08:31 AM   #13
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Quote:
Originally Posted by Braineack View Post
but they also say people have installed clutch discs backwards, so you people are capable of amazing things.
I can see the clutch disc thing, as it's round on both sides. The AFM on the other hand
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Old 07-08-2014, 03:59 AM   #14
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One thing I failed to mention is that this car does NOT have the stock intake setup on the car. There's quite a few reasons that aided in me putting the intake on backwards:


1. This side of the AFM is way too small for the air filter flange, notice how much slack had to be taken out of this clamp.
2. AFM plugs into the side further from the fender/rest of wiring harness.
3. This coupler is much larger than this side of the AFM
4. I never owned a car with an AFM before
5. When I worked on this car before (when my friend still owned it) he had a Greddy Kit on it and I was scavenging intake parts from it to make the car work for now.

Rotate the AFM 180 degrees and I am sure it's much easier to imagine it reversed now. I guess in a way it is a "noob" mistake since I am new to the 1st Gen Miata's, but in general I am pretty far from a noob when it comes to general car/Miata knowledge. Sadly all of my experience has been relegated to cars that all ran a MAF or MAP so the AFM was a new one to me.

Still, all those are just excuses...but now I know that every electrical component in the car works perfectly, and that all of my wires have exactly 1.3-1.4 ohm resistance.

Soon I'll Megasquirt the thing and won't have to deal with that awful AFM anymore. Cheers!


And yes, I know...there's a damned arrow on the thing....i honestly thought it was a casting mark...and even my buddy who owned the car and had it turbo'd didn't notice it after working on the car for like 6 hours.
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Old 07-08-2014, 02:37 PM   #15
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I've seen people install the MAF from later models backwards, but this is an impressive first for me.
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