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Old 10-03-2016, 06:43 PM   #1
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Default Hitting premature rev limiter? AFRs spiking! Scary!!

Hey guys,

So a bit of background: It's an NA with a C30-74 on a 80mm pulley.

It's got an intake, header, exhaust, tiny Mazdaspeed intercooler, ID 725cc injectors, Walbro 190 fuel pump, MTX-L, and a MSPNP2.

It was tuned in a previous life, but I bought the car with a blown motor and rebuilt it with a forged bottom end.

With a more conservative timing map loaded, it fired up to life and drove around without a hitch up here in Colorado. It pulled strong and felt great.

I recently just trailered it down to Laguna Seca and it was stumbling and surging at idle. I had to give it gas until it warmed up so it would idle (poorly and surging from 900-1.2k rpm).

The big issue was that it would hit what felt like the limiter at 5.8-6k rpm? I'd watch the AFR spike from 12.5 to 22 and it would stop accelerating. It didn't have an issue running all the way to 7k at partial throttle, but would slam into a limiter every time I was wide open at 5.8k or so.

Here's a link to a log of one of the sessions. I'm sure I slammed into the limiter (if that's what it is) by accident a few times.

http://lols2k.com/stuff/logs/2016-10-01_12.24.45.msl

Anyone have any ideas as to what is happening?

Gonna feel pretty dumb if I pop the fresh rebuild. I've stopped driving it for the time being.
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Old 10-03-2016, 07:09 PM   #2
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Post your tune
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Old 10-03-2016, 07:33 PM   #3
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Quote:
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Post your tune
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Old 10-03-2016, 11:00 PM   #4
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Sounds like you're misfiring. What coil-packs are you using?
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Old 10-03-2016, 11:36 PM   #5
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Quote:
Originally Posted by Barton View Post
Sounds like you're misfiring. What coil-packs are you using?
Stock coil packs. It wasn't backfiring so it didn't seem like there was excess fuel?

For what it's worth, the car is back home at 5280' elevation and no longer stutters. It runs smooth all the way up to red line.

That being said, I still want to get to the bottom of this because I'd like to track near sea level again soon.
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Old 10-04-2016, 08:17 AM   #6
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So I just battled a very similar problem. Fine in low boost, any rpm. Then at about 4k and more than 7 lbs, it would pass out. It was a teeny tiny microscopic hole in the spark plug boot. Every once in a while, the spark would jump through the boot, over to the valve cover bolt.
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Old 10-04-2016, 08:31 AM   #7
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I do not have tunerstudio here at work to look at the tune, but did you check the overboost protections? It is possible that you are reaching your set boost limits when not at altitude and causing the car to cut power. Just a thought. Good luck.
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Old 10-04-2016, 08:31 AM   #8
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I just had a similar issue. It ended up being a bad fuel pressure regulator.
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Old 10-04-2016, 08:37 AM   #9
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Wait, do you have the bar correction correct? Set to reset with ignition on?
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Old 10-04-2016, 10:00 AM   #10
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Quote:
Originally Posted by MX5RACER View Post
I do not have tunerstudio here at work to look at the tune, but did you check the overboost protections? It is possible that you are reaching your set boost limits when not at altitude and causing the car to cut power. Just a thought. Good luck.
Ding Ding Ding. We have a winner.

Your overboost is set at 150kpa. I when you came down to sea level you made "more" boost. Because you were starting at a higher base MAP.

In your logs you hit 151kpa and then fuel cut overboost kicks in.

Turn up overboost, and it wont happen.
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