High RPM stumble and backfire, insulate plug wires?

mikedamageinc · 10-23-2009, 03:29 PM

99 SS Full bolt ons, plugs, wires, fuel filter, O2s, 28# injectors all under 10K miles. Tuned for all bolt ons and 93 octane.

Starts normal, drives normal at low RPMs and light load. Uphill cruising or heavy acceleration at around 4K it stumbles, hesitates, and starts backfiring. It also wants to die but if I let off and drive normal its okay.

Very similar situation a couple weeks ago, I had the stock metal plug wire heat shields on the MSD wires and half of them were grounding on the headers. (could see the spark at idle). I replaced the heat shields with DEI fabric type insulation sleeves and it was good for a couple weeks and it is doing it again. I pulled all the plugs and gap and condition are good, wires are good.

I thought it might be bad gas but I filled it up with some Texaco 93 and a bottle of Lucas octane booster and no change. (previously used Shell 93 at a possibly shady station so I questioned if it was watered down)

Could the spark at high RPM still be jumping to the headers? Is there some kind of heavy rubber insulation I should try? Thanks in advance!

99blancoSS · 10-23-2009, 04:28 PM

If you burned the wires you need to replace them. That is the only reason a wire will ground out. You can not fix it with a band-aid. That DEI stuff isn't cheap either. I offer 4 packs of plug wires.
http://estore.websitepros.com/2120533/Detail.bok?no=43

Smoke Em · 10-23-2009, 08:33 PM

check for a burnt plug wire, mine was doing the same thing and it progressively got worse and worse until the entire cylinder was out

LS6427 · 10-23-2009, 09:21 PM

Get rid of the heat shields, just make sure no part of the rubber wires are touching the headers/exhaust manifold pipes. Not even the boots can touch.

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shaun · 10-24-2009, 03:51 PM

i would say its the wires.
i had a 98 grand prix gtp with some bolt ons and i put a new set of autolite pro wires.
then my step bro drove the car and hit the rev limiter and right after that it developed a miss under load.

so i swaped the plugs like 3 times and it dident help and i figured it cant be the wires because the are new then i just said shit on it and i went and got the cheapest autozone wires i could find and it fixed the problem so times a wire is bad evev tho it looks good.

mikedamageinc · 10-25-2009, 07:22 PM

I'm starting to think wires too, but I have brand new MSD, maybe one went bad somehow but it has to be a fluke right? These are supposed to be good wires. How can I figure out which wire it is? I dont want to buy a whole new set.

99blancoSS · 10-25-2009, 10:03 PM

try looking under the hood at night, you can see them arc in dark sometimes

bearcatt · 10-25-2009, 10:14 PM

Might want to check your sparkplugs, a bad arcing sparkplug wire can sometimes cause the sparkplug porcelin to crack... Once that happens that sparkplug is done.

I have a cracked sparkplug sitting on my desk.

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mikedamageinc · 10-26-2009, 12:22 PM

Well I checked everything out again and plugs look good physically, gap is good, wires are good except one had a small slit at the end of the boot, I replaced with a stock wire and no change.

This is getting irritating but I dont want to start throwing money at it. If a coil or any other component was bad it would throw a code right? What else can I check?

bearcatt · 10-26-2009, 01:34 PM

I can see how that can become frustrating.

I see in your previous post, that you replaced your fuel filter.
How about your fuel pressure or the fuel pump itself.

Shortly after I installed my Ractronix fuel pump and hotwire kit, I had a low RPM stumble. After I changed my fuel filter ( you did that allready ) and had my injectors cleaned, that solved my issues.
My fuel filter was filthy after 2 years and about 6500 miles.

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mikedamageinc · 10-26-2009, 02:03 PM

As I think about it, it has to be in the ignition since it pops and backfires when it stumbles, meaning the fuel isnt getting burned, not a lack of fuel. If it was a coil or misfire it would throw a code right? So it has to be plugs or wires, but both are new so I dont want to waste money since they both look good too.

I ran it in the dark revving to limiter and no visable arcing. If I could get a code it would be nice but I guess at the same time that should eliminate a lot like sensors and such.

Thanks for all the suggestions, please keep them coming!

LS6427 · 10-26-2009, 02:29 PM

Quote:

Originally Posted by mikedamageinc

As I think about it, it has to be in the ignition since it pops and backfires when it stumbles, meaning the fuel isnt getting burned, not a lack of fuel. If it was a coil or misfire it would throw a code right? So it has to be plugs or wires, but both are new so I dont want to waste money since they both look good too.

I ran it in the dark revving to limiter and no visable arcing. If I could get a code it would be nice but I guess at the same time that should eliminate a lot like sensors and such.

Thanks for all the suggestions, please keep them coming!

You could have a bad "new" plug wire.

For $36.00 you can go get a set at Advance Auto, put them on and see if it fixes it. If it doesn't, take them back for a refund.

.

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bearcatt · 10-26-2009, 02:37 PM

Not neccessarily, sometimes fuel issues can give strange symptoms.

After reading you first post again, it's starting to look like your problem is a re-occuring ignition problem. Did you check the wires with an ohmmeter ?

Did you have issues before changing the plugs to MSDs with the stock metal jacket ? Are the wires touching to headers anywhere ? Would angled sparkplug insulators help ?

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mikedamageinc · 10-26-2009, 03:11 PM

Quote:

Originally Posted by bearcatt

After reading you first post again, it's starting to look like your problem is a re-occuring ignition problem. Did you check the wires with an ohmmeter ?

Did you have issues before changing the plugs to MSDs with the stock metal jacket ? Are the wires touching to headers anywhere ? Would angled sparkplug insulators help ?
.

No problems before the plug/wire swap and old plugs and wires showed normal wear. The DEI insulators are barely touching on one or two so angles boots might be better.

I could be a "bad new wire" I kept the originals that all had around 700 ohms, I havent had the chance to check all the MSD wires and its time for work today so I'll swap wires and check the resistance of the MSDs later.

bearcatt · 10-26-2009, 03:30 PM

Quote:

Originally Posted by mikedamageinc

No problems before the plug/wire swap and old plugs and wires showed normal wear. The DEI insulators are barely touching on one or two so angles boots might be better.

I could be a "bad new wire" I kept the originals that all had around 700 ohms, I havent had the chance to check all the MSD wires and its time for work today so I'll swap wires and check the resistance of the MSDs later.

It wouldn't surprise me if you re-install the original wires and that solves the problem.

I've never tried the MSD wires but I've experienced other aftermarket wires.
I've had the best luck with the OEM wires.

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mikedamageinc · 10-27-2009, 08:14 PM

So I went to put the stock wires back on today and I started with the passenger side and noticed a wire group sitting on the header. It had burned thru the heat wrap and the flex loom but the wires themselves *appeared* to be good. I pulled it away from the header and ziptied it. Took it for a drive and what do you know, it ran great, no issues.

So now I think it wasnt the wires at all, the problem was similar to that which i had recently but the way it was acting, it must have grounded out the wire bundle when the engine torqued over since I have stock motor/trans mounts, I didnt think to replace those when doinf headers so it has a decent amount of engine movement. The bad thing is I had a wire bundle fall on the headers of my last car and it was jerking in almost the exact manner as this one, I should have known. So no I think I'll wrap up those wires good and put the MSD plug wires back on and see what happens.\

Thanks again for all the help!!!

bearcatt · 10-27-2009, 08:45 PM

Glad you got it figured out... but what is a wire group ? ... are you talking about wire looms or seperators ?

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Dal1as · 10-28-2009, 02:19 PM

Glad you figured it out. FYI though, you won't always throw a code with a bad coil. Just went through that.

mikedamageinc · 10-29-2009, 03:01 AM

Quote:

Originally Posted by bearcatt

Glad you got it figured out... but what is a wire group ? ... are you talking about wire looms or seperators ?

.

Quote:

Originally Posted by Dal1as

Glad you figured it out. FYI though, you won't always throw a code with a bad coil. Just went through that.

Yea, wire group=loom, I was just trying to differ from plug wire.

Sucks about bad coil and no code, how did you figure it out, trial and error?

I'm still just waiting for it to return as I'm not totally convinced it is good. I looked close at the loom today and it didnt look like the wires were burned to the metal so I'm going to do some more plug wire swapping and see if I can figure this out for sure.

Dal1as · 11-03-2009, 04:00 PM

Got it scanned. Dead #2 cylinder. Replaced #2 and #4 coil packs and found the problem. I also had another problem at the same time. Bad gas. Had to run the tank dry a couple of times before it got completely better.