1.8NB - old FMII - 'judder/shudder' when releasing the gaspedal - Page 2 - Miata Turbo Forum - Boost cars, acquire cats.

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Old 07-09-2011, 04:02 AM   #21
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Thanks Doppelgänger, that's exactly what happens. I'm pretty sure the bov blows when releasing the throttle, I just haven't noticed it that much because I was more focused on the problem.

I have a very simple and small bov and I don't think it can be adjusted, will have a look today.

Thanks!
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Old 07-09-2011, 12:29 PM   #22
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You thinkin' he goes lean in that condition?
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Old 07-09-2011, 12:36 PM   #23
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How bout this -

OP, you are metering airflow with the stock MAF instead of with a MAP sensor. If you have a BOV (vent to atmosphere) then your engine is not dispensing fuel correctly anytime you are under a reduction in load to a reduced boost pressure (i.e. when you let off the throttle). You will requre a BPV (vent to intake tract) which vents to a location in between the MAF sensor and the turbo compressor.
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Old 07-09-2011, 02:37 PM   #24
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Quote:
Originally Posted by fooger03 View Post
How bout this -

OP, you are metering airflow with the stock MAF instead of with a MAP sensor. If you have a BOV (vent to atmosphere) then your engine is not dispensing fuel correctly anytime you are under a reduction in load to a reduced boost pressure (i.e. when you let off the throttle). You will requre* a BPV (vent to intake tract) which vents to a location in between the MAF sensor and the turbo compressor.
I beg to differ* On my GF's MSM, it's running the stock MAF and a VTA BOV and never stumbles when lifting either full or partly at any rpm.

*Not all cars, tunes and turbo setups are the same.
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Old 07-09-2011, 04:09 PM   #25
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If you have the billet aluminum FM BOV of the same vintage as that kit (it was an extra-cost option vs the plastic one that came from a Mitsubishi I think), then it's a hybrid bypass/VTA 2-stage valve. There are two exit ports on it, the first to be uncovered as the piston moves goes to the recirculation port, but if the piston moves another half inch or so then it also uncovers a VTA port on the other side.

On mine, the VTA port would only open when I closed the throttle all the way with significant boost also pushing on the piston -- at that point, the bypassing of the MAF isn't a problem because the factory computer isn't running the injectors under those conditions anyway. If yours is open VTA under part throttle when the injectors aren't off, then that would invalidate the MAF reading and could go either too rich or too lean.

IIRC, there's a screw on one end to adjust the preload on the spring. Here's a picture of my BOV from when I was selling the old FM2 kit, the recirc port is at the bottom and the VTA port (not visible) is just a cutout in the body of the BOV at the top.



--Ian
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Old 07-10-2011, 10:07 AM   #26
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Ian,

I currently have the small plastic one indeed, but I think I do have the aluminum posted above too, just not currently installed.

Might be worth a try to install that one? I think the previous owner didn't use that one much because sometimes it would came loose and then it blocked and damaged the fan/radiator.
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