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Old 02-22-2015, 09:10 AM   #1
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Default Leans out/stumbles on acceleration. Broke something?

1994 Miata 1.8L, BEGI S3 Turbo kit. exhintake mod on a BP4W head.

Tune: Fine, Running Fine, no stumbling, no problems.

Gave it a good romp going through Milton Keynes (UK) on a straight.

Afterwards, hit accelerator, engine dies.

Starts normal, idles normal. But under load, stumbles for 1 second before everything kicks in.

Offset the stumble issue by no-load tapping on the accelerator to get the RPM's up, but the stumble is still there.

Problem dissipates at higher RPM's. It's still there, but seems to recover from the stumble much more quickly. Once it's up to speed, it does what it's supposed to do (come on boost, doesn't lean out, etc...) Just shifting and hitting the pedal from idle.

Connected Laptop to test TPS for a flat spot, re-cal'd it, and checked the gauges in Tunerstudio. Goes up smooth, no issues. (With engine off)

So, pulled a log. Not a super long one, and not under load (I can get it to stumble at idle in the same way.)

Log:

accel problem.zip

Tried disconnecting the TPS entirely, tried tapping on it, wiggling it, etc...

Should note that it did this one time previous, on Christmas Eve, and fixed itself before I could get home to get a laptop on it...
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Old 02-22-2015, 11:19 AM   #2
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This sounds lame, but have you tried just increasing the accel enrinch? Both time and fuel? How about the sensitivity?
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Old 02-22-2015, 02:17 PM   #3
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This sounds lame, but have you tried just increasing the accel enrinch? Both time and fuel? How about the sensitivity?
There wasn't anything wrong with the accel enrichment prior to me romping on it. Now when I hit the gas the engine bogs down and the AFR rate spikes for a half a second.
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Old 02-22-2015, 03:53 PM   #4
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That's because it's going lean. Falls on its face and afr goes way up. Lean.
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Old 02-22-2015, 03:55 PM   #5
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What do the plugs look like? We're you driving like a weenie before? I was able to granny drive through incorrect settings. Maybe the same for you?
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Old 02-22-2015, 04:56 PM   #6
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What do the plugs look like? We're you driving like a weenie before? I was able to granny drive through incorrect settings. Maybe the same for you?
I "drove" it before; well into boost as well as used it as a daily for a good period. Other than the Christmas eve bogging (which cured itself), I never had a problem with the tune/settings.

Tune/Settings was done on a dyno by Nick Skuzzle, who knows vastly more about tuning than I do. I haven't changed anything he put into the tune (because as I said previously, this problem didn't exist up until I gave it some right foot going through Milton Keynes. Nothing I haven't done on a fairly routine basis (didn't red-line it, or mechanically over-rev the engine).
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Old 02-22-2015, 05:15 PM   #7
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Fuel injector / pump failure? Filter? Are you using a regulator of some sort? I don't know, I'm downing bloody Mary's watching the 500.
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Old 02-22-2015, 07:57 PM   #8
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Fuel injector / pump failure? Filter? Are you using a regulator of some sort? I don't know, I'm downing bloody Mary's watching the 500.
All fair points to look, but it's the recovery at high speeds that seems to indicate it isn't a pressure/delivery problem... If I keep the pedal down, it'll recover, but the lag between for city driving makes it a bit dangerous going around corners.

Fuel filter is brand new.

I am as "scratching my head" as you are...
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Old 02-22-2015, 08:08 PM   #9
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Save your current tune, and add some fuel to the enrichment. If you F something up, you can always go back to your original starting point.
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Old 02-23-2015, 03:44 PM   #10
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Any logs?
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Old 02-23-2015, 04:32 PM   #11
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Quote:
Originally Posted by ryansmoneypit View Post
This sounds lame, but have you tried just increasing the accel enrinch? Both time and fuel? How about the sensitivity?
i had the same problem.. it was the accel enrich.
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Old 02-24-2015, 12:58 PM   #12
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Any logs?
Sir, the log is posted above; accel_problem.zip

It's a short log, there are two "misfires" represented there where I turned Advanced Accel Enrichment on, and then back off again.

I checked the mechanicals today as best I could in the driveway. Lined up the pulley marks (TDC and 10 mark, going clockwise), pulled the cam cover and checked the gears/lobes. Lobes were in the right place, I and E in the right places, with 19 teeth between them. Checked over the vacuum lines (but didn't use carb cleaner or butane yet), and nothing that relies on vacuum seems out of place either.

I will have a play with accel enrichment, but it doesn't explain the going lean on tip in (like, max lean).
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Old 04-20-2015, 06:49 PM   #13
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Just a quick update on this:

Turns out the Fuel Pressure Regulator had failed wide open, leading to way too much fuel.

Had to turn it over to a local garage to troubleshoot, as I didn't really have the time to do it.
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Old 04-20-2015, 07:48 PM   #14
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Good man on the follow-up / conclusion.
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