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Old 08-21-2009, 12:27 AM   #1
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Default Hesitation while in boost

When I first turbo charged my Miata it had a pretty bad stumble at higher boost. So Kotomile and I attempted to tune it better. With adjusting the RRFPR the stumble virtually went away. But would appear sometimes at 6-7 psi boost, and go away.

Fast forward to two weeks when I installed the Megasquirt. Sav came down to help tune it. After some hard pulls we encountered the stumble again. Although now the stumble was at about 4 psi boost, violent, and freaked out the megatune software. So as per the recommendations of Braineak I redid the engine grounds. I added two 1 gage engine ground cables, clean the battery, etc. The car still hesitated at 4 psi plus, but the megatune no longer locks up.

So today I took the car to DSR in San Jose, under the recommendation of Rick at PR Motorsports. I know they use a Dynojet, but they are proficient with the megasquirt. Same deal while strapped to the dyno the car hesitates pretty harsh at 4 psi and up. The gentleman tweaks the RRFPR, goes through settings, etc. He tells me the car's AFRs are perfect but it is still having a bad hesitation. So he pulls the plugs, and they are perfect. He tries closing the gap on them from .044 to .035 and this helps a lot. The hesitation is still there, but not nearly as bad. He thinks that the stock 150,000+ mile coil is possibly crapping out. Thoughts on this? Any other suggestions?

I included the dyno sheet, hope it is legible


Last edited by Qckslvr; 08-21-2009 at 12:49 PM.
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Old 08-21-2009, 01:02 AM   #2
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I have my money on coils. I had the same problem once. But im not using any management (if that makes a difference or not). Change the coils and it was gone.
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Old 08-21-2009, 09:55 AM   #3
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yeah, you should have been down to .030-.035 form the start. could just be wires, but my be the coils.
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Old 08-21-2009, 12:49 PM   #4
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Going to borrow a set of CoPs from Sav, and see what that does. I will close the gap further to .032 and see what that does. The wires are NGK wires with only 10k on them. And the plugs only have 500 miles on them.
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Old 08-21-2009, 12:58 PM   #5
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In for updates...I think this may be my 5K+ rpm stumble.
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Old 08-21-2009, 01:37 PM   #6
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when scot redid my spark output mods it cured 90% of this problem for me.
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Old 08-21-2009, 02:04 PM   #7
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his are fine as it stands now...
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Old 08-21-2009, 02:45 PM   #8
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I had a huge stumble at higher RPM and it was pretty much entirely due to plug gaping, mine were .04 and as i know now that was WAY too large, gaped them to .025 and it basically went away switched to

Flyin' Miata : Engine/Drivetrain : Fuel and Ignition : NGK extended reach spark plugs

Which are extended reach, gaped down to .28 and everything was perfect as far as ignition. If .035 helped might try .030.
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Old 08-21-2009, 03:18 PM   #9
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Quote:
Originally Posted by tiber55 View Post
I had a huge stumble at higher RPM and it was pretty much entirely due to plug gaping, mine were .04 and as i know now that was WAY too large, gaped them to .025 and it basically went away switched to

Flyin' Miata : Engine/Drivetrain : Fuel and Ignition : NGK extended reach spark plugs

Which are extended reach, gaped down to .28 and everything was perfect as far as ignition. If .035 helped might try .030.
Would you have the NGK number? I see that FM has conveniently left that off. And I am a NGK dealer
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Old 08-21-2009, 05:50 PM   #10
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Should be ZFR7F11, twice as expensive as regular plugs but the extended reach does have benefits.
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Old 08-21-2009, 06:10 PM   #11
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Don't forget the igniter as well. I've had 2 go bad on me now, and one gave me constant stumbling, and another was random bad ignition. Also, what does your AFR look like? It could also be a failing fuel pump.
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Old 08-22-2009, 12:18 AM   #12
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Small update, changed the plugs out to Autolite 3922s a BKR7E equivilant, and gapped at .030. Still had the occasional stumble, but was able to take it to red line now with out issue. So I dug up a spare coil and ignitor. Installed the coil, with little to no change, was quite dissapointed. Last I changed out the ignitor, DING, big change so far. The low end stumble is completely gone, so far. Boosted to 10 psi and a very slight, barely noticible stumble at 10 psi. So I am eager to try Sav's spare CoP set to see what that does.

Now I have a whole new issue, over boosting. The car ran to 8psi no issues, never crept past, under the hardest acceleration while on the Bipes. Now car hits 8 psi, and just keeps going untill it hits the overboost protection at 12 psi. The AFRs are at about 10.5-11.00 when this is happening. Thoughts?
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Old 08-22-2009, 01:00 AM   #13
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Quote:
Originally Posted by Qckslvr View Post
Small update, changed the plugs out to Autolite 3922s a BKR7E equivilant, and gapped at .030. Still had the occasional stumble, but was able to take it to red line now with out issue. So I dug up a spare coil and ignitor. Installed the coil, with little to no change, was quite dissapointed. Last I changed out the ignitor, DING, big change so far. The low end stumble is completely gone, so far. Boosted to 10 psi and a very slight, barely noticible stumble at 10 psi. So I am eager to try Sav's spare CoP set to see what that does.

Now I have a whole new issue, over boosting. The car ran to 8psi no issues, never crept past, under the hardest acceleration while on the Bipes. Now car hits 8 psi, and just keeps going untill it hits the overboost protection at 12 psi. The AFRs are at about 10.5-11.00 when this is hapening. Thoughts?
dude mine was overboosting just like that with a mbc inline etc no matter how loose i set it. I havent checked yet but i think its either a WG can not working correctly or, well im gonna have to tinker more and pull **** apart before i can go any further. post up when you find it, cuase it could help me as well.
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Old 08-22-2009, 05:03 AM   #14
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god this is like the exact same thing that happened to my car, like last month same order of things just slightly different solutions, at least for my overboost it was an easy fix a stainless line had rubbed a hole in the line from the MBC to the wastegate, so at least as an easy precaution make sure that all of your lines are intact and good.
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Old 08-22-2009, 09:55 AM   #15
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boost creep "solution"

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Old 08-22-2009, 10:44 AM   #16
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Quote:
Originally Posted by Braineack View Post
boost creep "solution"

sad but I had dreams about that last night
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Old 08-23-2009, 11:25 PM   #17
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ok some updates. The stumble reared its ugly head today. Although not in boost, I was just cruising the free way, and got a little stumble once. And that was it, soooo I am going to try Sav's CoPs and see what that does.

As for the boost creep, I was really trying to avoid having to take my turbo off and port it. So I did some checks on my original waste gate set-up, and sure enough it was full of fail. Due to the really bad angle I had it at, it was barely opening 1/8 - 1/4 of an inch. So I made a completely new bracket with the idea of not mounting my wastegate to the turbo. So I made a very stout, and over kill bracket. Now I get more throw in the waste gate, still not sure if it is enough. But the creep is no where near as bad. At least now it stops at 11 PSI, and I have the MBC set for 10 PSI. This is livable until I can pull the turbo off and port it.

original wasegate design
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New design
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I am still thinking the bell wastegate might be too short of throw. I might try to get an original Nissan wastegate, and see how much throw it has.
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Old 08-24-2009, 04:04 PM   #18
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quick hijack for bad *** avatar

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