High Rpm miss, boost or no boost

B

Blown95M8

New LVC Member
Joined
Mar 8, 2014
Messages
7
Reaction score
0
Location
Hannond
Hi there! So I acquired a supercharged (7psi stock engine) mark viii last winter. Got it dyno tuned to make sure everything was good a/f wise and got it chipped. But I have a problem with what feels like ignition cutting out at around 5300rpm and progressively worse up to 6500. if I keep my foot in it ill get a miss then hit 6000ish and miss again and so on...It had aftermarket ignition on it, the tuner told me to get rid of it and its junk and the likely source of my problem. (While it was on the dyno we could see it peg rich during this event). So I replaced both coils with brand new motorcraft, new motorcraft wires. still same problem but it felt like it ran a little better. Today I swapped out the new ngk plugs that the tuner put in. for some autolite plugs (one step colder than stock) and gapped to .025. same issue..i did somewhat of a test to see if it was blowing out the spark. I kept it out of boost in 1st gear all the way up to 5300 and it happened less but it was still the same...anyone have a problem with, I dunno....crank signal dropping out? im getting low on ideas...any thoughts, questions, or suggestions are greatly appreciated :)
 
The only thing that comes to mind at the moment: does it make any difference if traction control is on or off?
 
Close the spark plug gap to about .028 to .030 Edit, Sorry I see you already did. I swear mine runs best on Autolite Iridium sever duty and I'm running Screamin Demon coils and Live wires from SCP
 
theres no traction control anymore in the car. But yea I was reading how some ford mod motors having what sounded like the same issue using the ngk plugs. So I put some Autolite 103's in there (coppers) and nothing changed..im starting to wonder if im barking up the wrong tree with the ignition/coils/plugs, (because its acted almost exactly the same with an aftermarket coils/wires and then with brand new motorcraft stuff) and should be looking at something like the knock sensors going off and retarding the timing? or a bad crank signal at high rpm?
 
markviii.jpgyea if I can figure out how to post pics id gladly show you haha..might have to give me a bit..as for fuel pressure its right around 50-55psi. and when this occurs on the dyno he says it jumps rich so it must be ingniton..I even tried swapping ingniton control modules with a known good one and still this occurs

markviii.jpg
 
managed to get the stock hood to fit fairly well...little bit of cutting of underneath but it worked...
 
Looks nice!:headbang:

So you still have an aftermarket module on it?

It's hard to tell but the battery looks non stock... How about the alternator, does that check out? I'm thinking if it drops voltage it would affect the computer, spark etc
 
yea the pics with the red wires are when I 1st got it...ive got a new battery, tested alternator and that tested good, ignition control mod is out of my 94 mark with 60K miles, motorcraft coils/wires..runnin outta ideas on things to throw at it
 
I'm not familiar with the ignition control module on Mark VIII's... Is that like a separate computer just for ignition or what is it?

Is the IAT sensor still hooked up? Is that an extra coolant temp sensor on the crossover plug? You have aftermarket gauges?
 
Did you examine each plug when they came out?
Does the miss stay in one cylinder?
Do a leak down test?
Just some ideas.
 
Well Im not sure if its a specific cylinder dropping but it doesn't feel like it. feels like it just loses all of something across both banks..spark,fuel,something... just for a second at 5300ish rpm..then it revs to 5500ish and it might happen, and then 6000ish and happens...im thinking I might have to put a lab scope on the crank sensor maybe? see if for some reason it gets a bad signal when it happens? I did look at each plug after, they looked totally fine, no fouling, detonation marks, not lean. and I believe the ignition is controlled by a separate module in 93-95. my 96's don't have it. as for the sensor on the crossover tube its for an aftermarket gauge, coolant temp. but hmm iat seems to be missing...ill have to check more into that one..but I wouldn't imagine that causing the problem.. a/f is right at 11.5 at wot when we were doing pulls..and im not sure if it would even be relevant as the air is heated up quite a bit when its compressed by the blower? maybe? kinda guessing at that one...
 
The '93-'95 have an ICM (Ignition Control Module) located under the drivers strut tower cover.

I would say a oscilloscope would help find the issue?
 
is your gap supposed to be that small with forced induction? I thought these engines were around .048-.050.
 
With a big gap you can actually blow the spark out. That's why the plugs are gaped to around .030".
 
Do you know what your timing is at 3k 4k 5k etc?

I know the iat sensor influences spark, but honestly don't know if it's a factor.
 
You must log in or register to reply here.

Members online

Total: 450 (members: 1, guests: 449)
Back
Top