Car starts and idles fine, but light throttle input causes it to stall - Miata Turbo Forum - Boost cars, acquire cats.

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Old 04-21-2013, 05:07 PM   #1
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Default Car starts and idles fine, but light throttle input causes it to stall

After doing some WOT tuning yesterday, I went to start my car and found that it would stall with very light throttle inputs off idle. I could barely get it back to my garage, yet thankfully this happened in my neighborhood. I didn't change anything in the tune besides the boosted region of the VE table.


Fuel: I first suspected a fuel issue, as when I tap on the throttle the wideband pegs at full lean while it stumbles, and then if it recovers, it goes back to normal. I changed the fuel filter, checked the in-tank strainer (the fuel tank is spotless), and checked fuel filter. With the car off and f/p jumped to gnd I had 42psi right after the fuel filter. Idling at 1300rpm I had 35psi, and as it falls on its face it jumped to 45psi. The injectors are RX-8 injectors and I double checked the harness and they are plugged in.

Spark: The spark plugs were not overly fouled. With timing fixed at 10 deg, I got 10 deg. HOWEVER, when in table mode, according to tuning studio the DIYPNP is demanding 15 degrees. It appears to only be at 13 degrees with the light.

Air: I found nothing abnormal in the air path. Clamps are tight, I don't hear a vacuum leak, and the filter is clean.

Not sure what else it could be. Are these symptoms consistent with a slipped timing belt? Or is this a tuning thing? Is it worth trying a reflash of the firmware? Could there be something wrong with the DIYPNP itself? It was running great all day yesterday, felt strong until this issue appeared.

Any ideas? I've attached two logs and my tune. Watch for when the RPM drops off high idle and AFR pegs lean. All the sensors appear to be putting out normal data (even though they are a bit noisy...). Also note that when I start, the RPM oscillates a bunch before stabilizing. IAC is in PWM warmup.
Attached Files
File Type: msq CurrentTune.msq (84.6 KB, 39 views)
File Type: msl 2013-04-20_19.34.04-stalling.msl (196.6 KB, 35 views)
File Type: msl 2013-04-21_14.35.53-stall.msl (1.56 MB, 36 views)
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Old 04-21-2013, 05:30 PM   #2
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what happens if you add a bunch more enrichments to the accel wizard mapdot?
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Old 04-21-2013, 06:03 PM   #3
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It gets better, but still it pegs lean. I noticed while I was in the AE wizard that MAPdot (I havent wired up my VTPS yet) never really exceeded 50 kpa/s. I looked at my other logs and saw that MAPdot goes way higher then that when pulling away from a stop. So I checked the map line and found that it was pinched. I shifted it a bit to unpinch it and it works normal now! <that's my dunce cap for the day.

Sorry for wasting a precious thread in here... but at least it's fixed now! Too bad I missed my autox today and spent $40, precious gasoline, and a shitload of time trying to fix what wasn't broken...

While you're here, do you know why the timing is off by 2 degrees in table more but is dead on accurate when set to 10 degrees?
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Old 04-21-2013, 06:14 PM   #4
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is your table set to 12?
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Old 04-21-2013, 06:26 PM   #5
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Quote:
Originally Posted by Braineack View Post
is your table set to 12?
No, 15.
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Old 04-21-2013, 06:37 PM   #6
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Never mind.
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Old 04-22-2013, 02:06 AM   #7
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Sorry if I missed something, but what you're saying is you were doing WOT tuning.
Everything was FINE and dandy when you turned the car off and all of a sudden it goes
dead lean?

Has this happened before or it always started right back up perfectly until that day?
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Old 04-22-2013, 11:18 AM   #8
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He found a pinched hose. The delay in the MAP signal was causing the car to lean-stumble. If you look at the logs, you can see MAP rising slowly while RPM falls. You'll notice that the RPM signal becomes the dominant force over injector duty cycle (less fuel is sprayed).

Without the pinch, the MAP signal will rise quickly (it will look more like a step than a slow rise). The MAP signal will then dominate injector duty cycle.

His description of doing WOT tuning was just a coincidence.
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Old 04-22-2013, 11:42 AM   #9
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Quote:
Originally Posted by event View Post
No, 15.
So when your spark advance in TS is displaying 15, you see 13 or 17 actual.
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Old 04-22-2013, 02:03 PM   #10
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Quote:
Originally Posted by Braineack View Post
So when your spark advance in TS is displaying 15, you see 13 or 17 actual.
TS says 15 degrees, and the timing light says it's at 13 degrees. If I fix timing at 10 degrees in TS, the timing light shows 10 degrees.

Quote:
Originally Posted by hornetball View Post
He found a pinched hose. The delay in the MAP signal was causing the car to lean-stumble. If you look at the logs, you can see MAP rising slowly while RPM falls. You'll notice that the RPM signal becomes the dominant force over injector duty cycle (less fuel is sprayed).

Without the pinch, the MAP signal will rise quickly (it will look more like a step than a slow rise). The MAP signal will then dominate injector duty cycle.

His description of doing WOT tuning was just a coincidence.
Perfect explanation. I had no idea what to expect for typical MAPdot readings for throttle blips, but If I had a VTPS then I wouldn't have this issue, or at least it would be a much smaller issue. TPSdot AE is better in every way from MAPdot.
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Old 04-22-2013, 02:04 PM   #11
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what happens if you change fixed timing to 15 or even 20?
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