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SP61GT Powered
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Discussion Starter #1
Well the other day I turned up my boost to 18psi to see how it feels and everything was going perfect. O2's were good, everything looked fine. However, when I came to a stop I noticed that it was misfiring quite a bit at idle. I was told that it could possibly be bad plugs so I pulled the old Autolite's out and put in new 3923 coppers gapped at about .025. The car is still missing at idle but not while driving. Anyone know perhaps what this could be and how I'd go about fixing it?

Thanks,
Cameron


P.S. it's runnig on an SP61, 550's, MAP-ECU, supporting mods if that helps
 

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Are your valves shimmed properly? I know lots of 7m's seem to misfire at idle, and I'm led to believe that some of it is due to improper clearances on the valves.
 

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SP61GT Powered
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Discussion Starter #6
I don't know if it's because of improper valve shimming. It's misfiring a LOT at idle, and I know it's not because of the .025 gapped plugs. Is it possible I have a bad coil or igniter?
 

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SP61GT Powered
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Discussion Starter #7 (Edited)
boosted-bird said:
I don't know if it's because of improper valve shimming. It's misfiring a LOT at idle, and I know it's not because of the .025 gapped plugs. Is it possible I have a bad coil or igniter?

:EDIT:
This may help, I've noticed that the car only does it once it's warmed up. While it's cold it won't misfire at all. Also it's not throwing any codes at all. :rant2:
 

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mine does that too, pretty sure its my plugs, but they are gapped stock. Havent started it in a while though, i think i fixed it when I adjusted my tps, either way im interested in seeing how you fix it.
 

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SP61GT Powered
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Discussion Starter #9
SuprAng said:
mine does that too, pretty sure its my plugs, but they are gapped stock. Havent started it in a while though, i think i fixed it when I adjusted my tps, either way im interested in seeing how you fix it.
Well I'm trying to figure out what it is based on a few things:

1. It only does it while idling, not while driving

2. It only does it once the car is warmed up

3. The car is not throwing any codes


Anyone have any more ideas???
 

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TE44 NA-T
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Do you have a wideband? Sounds like it's idleing too lean.
Get some propane and spray a little into the intake filter. If the backfiring stops, than you are indeed running too lean.
 

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Discussion Starter #11
adversary said:
Do you have a wideband? Sounds like it's idleing too lean.
Get some propane and spray a little into the intake filter. If the backfiring stops, than you are indeed running too lean.
I myself don't own a wideband but a friend of mine does and we tuned it with that. O2's read completely normal at idle. around 14.3~14.7 at idle
 

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well i have the exact same symptoms. Cold start is fine, only when its fully warmed up it pops once in a while. I had this problem with the 440s and the 680s, so for me its not injectors. I dont know if this pertains to you, just sharing my experience.
 

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Discussion Starter #14
SuprAng said:
well i have the exact same symptoms. Cold start is fine, only when its fully warmed up it pops once in a while. I had this problem with the 440s and the 680s, so for me its not injectors. I dont know if this pertains to you, just sharing my experience.
Ah yeah we're having 2 seperate problems. Mine is misfiring a lot at idle
 

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sg02 said:
Won't the .25 gap cause the misfire at idle?
Id change the cap to .32. Not quite sure how much its supposed to change with higher boost levels. My car did that.... what was happening was coolant was breaching my gasket and getting into cylinder number one = BHG. Drove me up the fucking wall cuz I tried every other explanation other than BHG. lol Also keep in mind I had a stock HG then and a gouge on the head allowed the gasket to separate. It wasnt a BHG by boost.

just my .02

Good Luck man :bigthumb:
 

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way over budget
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my car was doing that. I did a compression test and found the hg blown between 3 and 4 and the head was melted. You could try a compression test just to see how your motor is doing just for piece of mind if nothing else.
 

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it's your plug gap. i'm having the exact same problem right now. i was at .31 gap with my NGK's; had a super good idle but anything over 20psi and id get bad misfires. i put some 3923's in at a .26 gap and the idle is total shit. try a .28 or .29 gap and see how the idle is. i bet a wide gap will solve your problems.

FYI. i made 550 rwhp on a .28 gap. and yes .3 thousandths make a difference
 

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TE44 NA-T
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boosted-bird said:
Thanks for all the info guys. I'll definitely be doing a compression test sometime today just to make sure
What kind of fuel control do you have? I have an Apexi SAFC and just richened it at 1000rpm low throttle. Solved the problem.
 

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Discussion Starter #20
I know it's definitely not the gap. The car was doing it before I changed the plugs and the gap on them before was ~.028

Alright well I did a compression test just now and the numbers go as follows:

1. 157

2. 162

3. 155

4. 155

5. 156

6. 155

I'm searching right now to find out what the compression figures are supposed to be. Lemme know how these figures add up
 
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