Strange idle, jerking idle, spark plugs breaking up

[3vEvan]

hi i have a 2006 mustang gt with an auto trans. my mods are an edlebrock eforce kit with a 3.5 pulley, hot rod cams, offroad hpipe, borla ataks, BA5000 MAF, msd master blaster coil packs, autolite HT0 spark plugs, 39lb injectors, walbro 400 fuel pump and 4.10 gears.
I've been having issues for some time now where sometimes on start up the car will have a jerking idle varying between like 200-300 rpms and it'll rock the whole car. Sometimes it will idle fine and occasionally miss or start struggling to maintain idle and the engine will start shaking real bad may or may not come back up and if I rev it up itll idle back down and die... while driving it seems fine 90% of the time other than when the car downshifts and I give it gas the plugs will start breaking up real bad or when im slowing down itll idle weird till it adjusts. It pulls great at WOT ive been to the track and it runs high 11s. I've cleaned the maf/throttle body, I've changed to multiple TPS sensors, and done KAM resets but none of that seemed to work and I dont wanna buy a bunch of stuff to replace if its not the issue any feedback is appreciated. Thanks!

 

[Juice]

You need to datalog the issue to see what the PCM is seeing/doing and why.

 

[Am Speed]

3vEvan - out of curiosity, what did you gap your Autolite HT0 plugs at? I ask because I also have a 3v and getting read to install the eForce kit soon. I'm seeing folks recommend to gap them at .032 to .035 rather than the factory .045 gauge. I've read that the factory gap can lead to the rough idle you are describing - here: https://www.modularfords.com/content/208-How-To-Gap-Autolite-HTO-Spark-Plugs

 

[stkjock]

gap should not effect idle with a blower, at peak boost yes, you gap down to avoid spark blowout. you want to run the largest spark plug gap possible before you get spark blowout.

 

[07 Boss]

Have you pulled the plugs and looked at them? Reading spark plugs is old school but its how we used to do it.

 

[3vEvan]

I have not pulled them out yet but im strongly considering it now

I dont remember what I gapped them to honestly

 

[AndrewNagle]

you want to run the largest gap possible before you get blowout.

Could take this a couple of ways

 

[svanur]

hi i have a 2006 mustang gt with an auto trans. my mods are an edlebrock eforce kit with a 3.5 pulley, hot rod cams, offroad hpipe, borla ataks, BA5000 MAF, msd master blaster coil packs, autolite HT0 spark plugs, 39lb injectors, walbro 400 fuel pump and 4.10 gears.
I've been having issues for some time now where sometimes on start up the car will have a jerking idle varying between like 200-300 rpms and it'll rock the whole car. Sometimes it will idle fine and occasionally miss or start struggling to maintain idle and the engine will start shaking real bad may or may not come back up and if I rev it up itll idle back down and die... while driving it seems fine 90% of the time other than when the car downshifts and I give it gas the plugs will start breaking up real bad or when im slowing down itll idle weird till it adjusts. It pulls great at WOT ive been to the track and it runs high 11s. I've cleaned the maf/throttle body, I've changed to multiple TPS sensors, and done KAM resets but none of that seemed to work and I dont wanna buy a bunch of stuff to replace if its not the issue any feedback is appreciated. Thanks!

I had the same issue with the autolite ht0 plugs so i stopped using them 2 years ago and switced to brisk 3VR12S for when i am using blower+nitrous and 3VR14S if not using nitrous. After swapping the plugs i have not had any of the issues the ht0 plugs generated. I gap the 3VR12S at 0.28 and 3VR14S at 0.32.

 

[06 T-RED S/C GT]

3vEvan - out of curiosity, what did you gap your Autolite HT0 plugs at? I ask because I also have a 3v and getting read to install the eForce kit soon. I'm seeing folks recommend to gap them at .032 to .035 rather than the factory .045 gauge. I've read that the factory gap can lead to the rough idle you are describing - here: https://www.modularfords.com/content/208-How-To-Gap-Autolite-HTO-Spark-Plugs

I've gaped my HT0 plugs at .032 and currently gap them at .030 as recommended for my current JDM tune. Needless to say, I've never ran across any rough idle nor bogging or jerking issues. What you read about the factory gap of .045 leading to rough idles on a FI application is in fact accurate, as the maximum gap for an FI application should be no more than .035 Therefore, if your not sure what your plugs are gaped at? then it would be in your best interest to pull/check them.

I had the same issue with the autolite ht0 plugs so i stopped using them 2 years ago and switced to brisk 3VR12S for when i am using blower+nitrous and 3VR14S if not using nitrous. After swapping the plugs i have not had any of the issues the ht0 plugs generated. I gap the 3VR12S at 0.28 and 3VR14S at 0.32.

If you had idle issues while running Autolite HT0 plugs? it's more than likely they weren't gaped between .030-.035 as required when running FI applications.

 

[svanur]

I've gaped my HT0 plugs at .032 and currently gap them at .030 as recommended for my current JDM tune. Needless to say, I've never ran across any rough idle nor bogging or jerking issues. What you read about the factory gap of .045 leading to rough idles on a FI application is in fact accurate, as the maximum gap for an FI application should be no more than .035 Therefore, if your not sure what your plugs are gaped at? then it would be in your best interest to pull/check them.



If you had idle issues while running Autolite HT0 plugs? it's more than likely they weren't gaped between .030-.035 as required when running FI applications.

They were all gapped at 0.32 and that was double checked and still it happend. So i switched to brisk. I have gone through atleast 4 sets of HT0’s where this has happend not one issue with the brisk.
Might just be my luck

 

[06 T-RED S/C GT]

In that case, I just may have to consider Brisk plugs for my next set, especially knowing they're a one-piece design spark plug

 

[Doug Huggard]

A smaller plug gap would not matter at idle as the engine is in a state of vacuum and not blowing out spark. Your issue is happening at idle so I would start by checking for vacuum leaks. If you have a vac leak post MAF, it will cause idle issues because un-metered air is entering the engine and will cause a misfire or small rpm bump (how severe is dictated by the size of the leak). How do you have your pcv system setup?

 

[06 T-RED S/C GT]

They were all gapped at 0.32 and that was double checked and still it happend. So i switched to brisk. I have gone through atleast 4 sets of HT0’s where this has happend not one issue with the brisk.
Might just be my luck

A smaller plug gap would not matter at idle as the engine is in a state of vacuum and not blowing out spark. Your issue is happening at idle so I would start by checking for vacuum leaks. If you have a vac leak post MAF, it will cause idle issues because un-metered air is entering the engine and will cause a misfire or small rpm bump (how severe is dictated by the size of the leak). How do you have your pcv system setup?

Apparently it does matter, otherwise making the switch from HT0 to Brisk spark plugs wouldn't had made any difference if it had been a vacuum leak.

 

[07 Boss]

I've run Brisk from day one except for one set of HT0's and the Accel ones in there now. I hate the strap design. I don't have any empirical data just anecdotal. I pretty much coast down this pass everyday. I had an unusual amount of popping and burbling during this descent with the HT0's. The only time I got a lot of popping on decel was with the HT0's and improperly gapped or fouled plugs. I do believe the strap design hides the spark and so the flame front is not as vigorous as it should. I came to this conclusion just based on the amount of unburnt fuel making it into my exhaust when coasting. Back in the day when we would run power adders we would side gap our plugs for better efficiency in igniting the air/fuel mix in the cylinders. I truly think this helped so that is why I don't like the hidden spark.

 

[stkjock]

Apparently it does matter, otherwise making the switch from HT0 to Brisk spark plugs wouldn't had made any difference if it had been a vacuum leak.

I suggested above that it should not make a difference, especially with a stock motor, why do you gap plugs down, to avoid spark blow out, not to idle better.

So why would it make a difference?

 

[06 T-RED S/C GT]

I suggested above that it should not make a difference, especially with a stock motor, why do you gap plugs down, to avoid spark blow out, not to idle better.

So why would it make a difference?

Perhaps it's the strap design of the HT0 plugs as 07 Boss brought up in his post that somehow hides the spark rather than the gap? Also don't quite understand how Svanur went through 4 different sets of HT0 plugs and yet still claimed to have the same idle issues until making the switch to Brisk racing plugs. At any rate, just trying to figure out the mystery behind the HT0 plugs as being related to causing idle issue claims

 

[3vEvan]

I ordered a set of brisk plugs and im gonna be putting those in within the next week or so and see what happens i guess

 

[06 T-RED S/C GT]

As mentioned in a previous post, I haven't experienced any rough idle issues with the HT0 plugs and am currently on the second set. Needless to say, I'm not quite grasping how Svanur went thru 4 sets of HT0 plugs, but yet still experienced rough idle issues until finally making the switch to Brisk racing plugs? Therefore, if the purpose of gaping down the spark plugs is only to prevent spark blow out as mentioned in previous posts, then it would appear as though gaping down wouldn't make any difference when the engine is just running at idle, to begin with anyhow. At any rate, the only other conclusion I can come up with for the rough idle/misfire issues, is the 4 sets of HT0 plugs would appear to have some sort of manufacturers defect, despite the odds being very unlikely