Misfire on throttle but not at idle? - The 1947 - Present Chevrolet & GMC Truck Message Board Network

12secondnova · 06-13-2017, 02:33 AM

Like title says. I have a 4.8 in my truck with Holley terminator efi, stock coils, and msd plug wires. At idle and light throttle(like 30%) it runs perfect but as you give it more it starts to sputter and pop which gets worse the more throttle you give it. Computer shows nothing is wrong or out of the ordinary with the tune. Wires are clear and clean, all grounds are good. Ran fine before. What would make it do this? 1 coil going bad? ?? Any help or suggestions is appreciated!
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BR3W CITY · 06-13-2017, 10:23 AM

How much boost are you running? You can try tightening the gap on your plug a touch. Low boost shouldn't be blowing out spark, but its not impossible.

akdg87 · 06-13-2017, 12:31 PM

What plugs do you run? Have you checked them out, could be that one was cracked on install?

I would get some BR7EF plugs (NGK 3346) and gap them at .028, if those arent available you can use the TR6 plugs.

davepl · 06-13-2017, 12:54 PM

Definitely going to be an ignition problem. Here are the basics:

At idle, barely any air is being allowed in. That is squeezed down by the compression stroke, and those molecules float around in your plug gap. But there are few of them, so it's easy to fire the spark across that gap.

Now go to WOT, and not only is the cylinder filled, but the compression stroke compressed them down by a factor of 10. The plug gap is STUFFED with air molecules and it's hard to jump the gap, so the spark will take alternate paths it would not have at idle.

Basically, the more power you're making, the more air in the cylinder, the harder it is to fire the spark, and the more willing that spark will be to find anywhere else to go. That's why someone above suggested shortening the plug gap, which can help if it's really a marginal case.

Bad plug boots are a good place to start. If you have anywhere the headers/exhaust get close to a plug, suspect that as well.

solidaxel · 06-18-2017, 12:53 AM

Re check plug firing order and plug ends

Jayred · 06-27-2017, 12:31 PM

Wow, nice twin setup, I envy you. I do not have turbos but I had the same problem and it turned out to be my Maf. I'm sure your running speed density tho. Can you data log with Holley? If you could see what all your sensors are reading when it stumbles you could probably narrow it down. I've had random issues like this and it's alway because of a loose map, maf, iat sensor or I forgot to plug one of them back in. My bad maf was because I thought I needed a new one for no good reason and the new one I bought was bad so I went back to the original.

Custom 68 · 06-29-2017, 10:37 PM

Quote:

Originally Posted by akdg87

What plugs do you run? Have you checked them out, could be that one was cracked on install?

I would get some BR7EF plugs (NGK 3346) and gap them at .028, if those arent available you can use the TR6 plugs.

Mine was doing this and swapped plugs like said here and tightened up the gap. It a whole new truck now

06-13-2017, 02:33 AM	#1
12secondnova Registered User Join Date: Mar 2013 Location: norcal Posts: 673	Misfire on throttle but not at idle? Like title says. I have a 4.8 in my truck with Holley terminator efi, stock coils, and msd plug wires. At idle and light throttle(like 30%) it runs perfect but as you give it more it starts to sputter and pop which gets worse the more throttle you give it. Computer shows nothing is wrong or out of the ordinary with the tune. Wires are clear and clean, all grounds are good. Ran fine before. What would make it do this? 1 coil going bad? ?? Any help or suggestions is appreciated! Posted via Mobile Device Attached Images

06-13-2017, 10:23 AM	#2
BR3W CITY meowMEOWmeowMEOW Join Date: Sep 2009 Location: MKE WI Posts: 7,128	Re: Misfire on throttle but not at idle? How much boost are you running? You can try tightening the gap on your plug a touch. Low boost shouldn't be blowing out spark, but its not impossible. __________________ *'66 Short Step / SD Tuned / Big Cam LQ4 / Backhalfed /Built 4l80e / #REBUILDEVERYTHING* MY BUILD THE H8RDCPTR //\\ MY YOUTUBE CHANNEL REV J HD

06-13-2017, 12:31 PM	#3
akdg87 Registered User Join Date: Sep 2013 Location: fairbanks, ak Posts: 339	Re: Misfire on throttle but not at idle? What plugs do you run? Have you checked them out, could be that one was cracked on install? I would get some BR7EF plugs (NGK 3346) and gap them at .028, if those arent available you can use the TR6 plugs. __________________ build thread 1955 cameo turbo lsx 1993 K1500 Blazer 5.3/4L80e 2009 CTSV Sedan

06-13-2017, 12:54 PM	#4
davepl Registered User Join Date: Aug 2006 Location: Redmond, WA Posts: 6,332	Re: Misfire on throttle but not at idle? Definitely going to be an ignition problem. Here are the basics: At idle, barely any air is being allowed in. That is squeezed down by the compression stroke, and those molecules float around in your plug gap. But there are few of them, so it's easy to fire the spark across that gap. Now go to WOT, and not only is the cylinder filled, but the compression stroke compressed them down by a factor of 10. The plug gap is STUFFED with air molecules and it's hard to jump the gap, so the spark will take alternate paths it would not have at idle. Basically, the more power you're making, the more air in the cylinder, the harder it is to fire the spark, and the more willing that spark will be to find anywhere else to go. That's why someone above suggested shortening the plug gap, which can help if it's really a marginal case. Bad plug boots are a good place to start. If you have anywhere the headers/exhaust get close to a plug, suspect that as well. __________________ 1970 GMC Sierra Grande Custom Camper - Built, not Bought 1969 Pontiac 2+2 427/390 4-speed Coupe 1969 Pontiac 2+2 427/390 4-speed Convertible

06-18-2017, 12:53 AM	#5
solidaxel Registered User Join Date: Dec 2002 Location: Cactus Patch So. Az Posts: 4,749	Re: Misfire on throttle but not at idle? Re check plug firing order and plug ends __________________ 53 TuTone Extended Cab 350 4-Spd 3:08 (SOLD) 53 Chevy Moldy pearl green ZZ-4 4L60E 9" 3:25 55 GMC 1st Black Mll (ZZ4) ZZ6 TKO 600 5 sp 3:73 62 Solidaxle Corvette Roman Red (327 340hp 4spd 3:36) C4 & C5 suspension tube chassis LS 3 4L70E 65 Corvette Coupe 327 350hp 4spd 4:11 78 Black Silverado SWB (350/350) 5.3 & 4L60E 3:42 2000 S-Type 3.0 (wife cruiser) 2003 GMC SCSB 5.3 4L60E 3:42

06-27-2017, 12:31 PM	#6
Jayred Registered User Join Date: Mar 2015 Location: Phoenix, AZ Posts: 30	Re: Misfire on throttle but not at idle? Wow, nice twin setup, I envy you. I do not have turbos but I had the same problem and it turned out to be my Maf. I'm sure your running speed density tho. Can you data log with Holley? If you could see what all your sensors are reading when it stumbles you could probably narrow it down. I've had random issues like this and it's alway because of a loose map, maf, iat sensor or I forgot to plug one of them back in. My bad maf was because I thought I needed a new one for no good reason and the new one I bought was bad so I went back to the original.



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