All of a sudden the truck is now breaking up on the top side. I changed plugs, checked all wiring everything. I can't find a single problem. It idles great, cruises great, pulls great so long as you are part throttle. If you floor it it starts breaking up. If you stab it hard down low it does it sooner; if you roll into it then it only does it on the top side 4500+

Truck is a built motor, heads, cams, KB, full exhaust, all the extras. Was running great and has been 6.9@99mph several times which is equivelent of upper 10's with no problems.

Only thing that has happened to it since is was sitting for about 2 weeks, and the weather has turned cooler(upper 30's) BUT, it has ran great in similar weather on this combo.

I pulled the blower belt and it ran great just slow as a dog.

 

Casey, I bet it's a plug! I was out messing around in 40 deg weather and got on it and felt a slight bog. I live 72 miles away from my job so I had to take it easy going home. Ran great and idle fine just on a stab or at WOT it bogged. Got it home and found out I melted a plug!

Peace,

Suavy

 

Well; I changed plugs and had 2 melted ones due to a prior screwup on my part. Changed them and still have the same problem the very first time I got on it.

It pulls like a MF'er till about 4500 rolling into it then just kindof breaks up. If you stab it from a stop it breaks up worse and sooner.

 

Probably not the problem, but mine did that when a coil pack was not plugged all the way in. The next thing I'd be thinking is a coil itself.

Jody

 

Hmmmm; well all of them were unplugged and plugged back in for changing the plugs. Now if one was messing up..................but, could it just be a WOT problem?

 

Do you have a wide band? Has the A/F leaned out a in the cold weather. The only time I've had that problem is when my MAF was maxed out, and the truck was leaning out.

Your truck is a different animal then mine. I still have stock bottom end/heads.

 

A/F did fine. It started jumping around a little bit when it starts breaking up, but that is to be expected.

It's got a Superchips BA2400 MAF so it should max till about 650HP and I don't think I'm making that. Even if I was it would just start to go lean. This is like you are unplugging something really quick and then back in really quick in a series.

 

have you tried pulling a degree or two of timming out and trying that? cold weather could be causing a slight unaduable miss

 

I tried 4 degrees up top and went back to a sloped style timing table as opposed to the locked that I had before for race gas use.

 

Had the same problem early this past spring. Drove me crazy. My problem ended up being one of the fuel pumps. It would idle fine, cruise fine and even launch fine but at about the 330' mark it would start breaking up. Changed plugs, did a compression check and checked for vacuum leaks to no avail. Jim checked the fuel pressure and it was all over the place so Shaun pulled the fuel tank and due to Race Related's hack fuel pump install, one of the pumps had come loose from the support and was swinging around in the tank. He correctly installed 2 new pumps and all is good now. I would check your fuel pressure at wot and see if maybe anything looks off. Hope this helps....

 

Those are the suggestions I am looking for! But, We watched the FP and it was almost up to 60wot and holding steady. Tank was full of fuel too.

I think I'm going to try pulleying down to about 8~10#'s of boost and seeing if it still does it. Then work from there.

 

When my 01 expy with the 5.4 in it had a coil pack going, it would only miss fire when the converter locked up, so coil packs can fail intermittently(sp). They make a coil tester tham wjm5806 bought off a link on lightningrodder.

 

Mine did exactly as yours, it only misfired under wot. I was actually heading to a dyno session and did a "test" on a back road and found the miss. It cruised and idled fine. Took me a while to find the loose plug. If you've had yours off/on for plug changes and had the problem before and after that I doubt yours is having the same issue. So then I start looking at the coils themselves, because under load is where they'd have the most trouble if going bad.

Jody

 

Probably gonna be a relay to fuel pumps or line coming off in tank !

 

Just put it down to 10#'s of KB boost and all is GREAT! Could @ 19#'s it be pegging a SCT MAF???

About to step it up little by little.



On the fuel. I had plenty of FP. The pumps are going into (2) -6an lines out the top of the tank into a big aftermarket filter. SO, I eleminated that issue of the "y" fitting and hose problems.