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Thread: Missfire is driving me nuts, even changed set up, please read

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Posts: 1-8 of 8
2008-10-11 23:19:26
Missfire is driving me nuts, even changed set up, please read
Ok, so my car was running great at 11-12psi, i trapped at 111mph with it with s4 cams, 3 inch turbo back, gti-r turbo manifold with turbo, 370cc injectors at 4 bar.
So then my buddy in his mach 1 mustang trapped at 113 with bolt ons and 100shot of nitrous, i couldnt allow such a thing so i upped the boost to 15psi, bam started getting crazy missfires up top in the rpms.
So after alot of searching and questions figured i was maxing out the bored stock maf and the 370 injectors
Changed the set up to a z32 maf with sti 525cc injectors and sent the ecu out to jwt to fix that business
Get everything all wired in and start the car go out for a drive and well hello mr missfire again
Last time this happend im fairly sure i had the gap at .035 or lower on the plugs.
Anyone got any ideas?
i dont know what the fuel pressure should be at, i do have a nismo fpr. Just havent checked that situation yet, anyone have numbers for what i should with this new set up?
I havent changed the wires, They look ok, but that could be the culprit

one more wierd thing is it only missfires at full throttle, at like 3/4 throttle it doesnt

I just want some ideas for when i go back under the hood tomorrow
sorry for the rant and the everywhere ness this post has become lol
2008-10-12 00:13:55
i would deffinately try lowerin the gap down to like .028 or something like that
2008-10-12 00:39:45
Yeah it is a good chance your plugs are getting blown out. Pick up a cheap fuel pressure gauge from Gofastdepot, you are asking for trouble with out knowing what your fuel pressure is
2008-10-12 03:36:38
i actually have one i bought off someone, i just dont know what fuel pressure should be set at
2008-10-12 05:08:31
yup, x3 on the plugs. had the same problem on one of my turbo sers. just gap the plugs down till it doesnt do it anymore. .028-.025 should be fine. might not have readd it right, but if your tune was at 4 bar for the 370 injectors, did you have your fuel pressure set at 4 bar? just curious.
2008-10-12 10:56:35
Thanks guys, I'll regap the plugs and mess with the fuel pressure. The only question i have about that now is i see some people leaving the vaccuum hose connected and some people dont to change the fuel pressure. Which way is the right way lol
2008-10-12 12:11:25
If your setup is for 3bar, you should have ~43psi of fuel pressure with the vacuum line disconnected, and capped off at the manifold. When you reconnect it, it will drop a little.

At 4bar, you should have ~58psi of fuel pressure with the vacuum line disconnected and capped off at the manifold.

This is the correct way to set your base fuel pressure.

You are blowing out your spark under boost. Same thing I went thru, crazy miss in higher RPM around 25~30psi of boost. Like the car basically would fall on it's face in 3rd gear around 6Krpm, because the spark wouldn't ignite the air/fuel mixture. .035" is too high of a plug gap for boost, IMO. I would at least start around .025" and work your way back up to .030", before you start misfiring again.
2008-10-12 16:06:28
thanks for the help guys, .025 was the charm
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