Loping Driving Me Nuts!

[Classic-00-Crew]

Ok, so I've searched and searched and maybe I'm missing something but nobody else seems to have this particular problem. 2000 5.7/4L80E. When the truck is in gear and I'm on the brakes at a stoplight or even if I just shift from park to drive and am holding the brakes the truck starts loping like it's got a cam. Especially if the a/c is on. I can throw it back in park and it stops immediately so obviously it only does it under load. It doesn't drop rpm's low enough to shudder or try to shut off but it fluctuates between about 500-750 or 800. Plus it almost feels like the TC is trying to lock up when it's doing it. If I'm not on the brakes a reasonable amount it will try to lurch forward every time it lopes. I just got this truck and it's got a new cap, plugs, wires and just put a fuel filter on it. Could this be a mass air issue or what? I'm kinda lost on this one, any help would be appreciated. I can fix most anything its just the troubleshooting I have issues with haha.

 

[1sickcrewcab]

My crew cab also wants to go foward when im holding the brake down, but its not looping. Never really looked into why it wants to go foward , maybe a mass air issue. Maybe yours is a fuel problem, when mine started doing something similier (looping) it was the fuel pressure regulator. Sorry i couldnt be more helpfull

 

[phule]

I've noticed a alight pulse on both my 99 cc and in my father inlaws 98 1500 I figured it was a vortec thing never put much thought into it doesn't throw codes or anything.

Adam

 

[Classic-00-Crew]

My crew cab also wants to go foward when im holding the brake down, but its not looping. Never really looked into why it wants to go foward , maybe a mass air issue. Maybe yours is a fuel problem, when mine started doing something similier (looping) it was the fuel pressure regulator. Sorry i couldnt be more helpfull

Yeah, good point about the regulator. That never dawned on me, maybe somebody will chime in on that. Talking about wanting to move when I'm on the brakes tho, you have to hold them harder than should be normal. Idk

 

[Classic-00-Crew]

I've noticed a alight pulse on both my 99 cc and in my father inlaws 98 1500 I figured it was a vortec thing never put much thought into it doesn't throw codes or anything.

Adam

My dad used to have a 98 as well and I remember it fluctuating some too, but this literally sounds like the truck is camming. Sounds retarded at a stop light or a drive thru haha.

 

[SixPointTwo]

Just tell people it's got a mean cam in it and go about your day!

 

[Keepinitoldskool]

You may have a brake booster that is on its way out causing a vacuum leak when you apply the brakes. You can test this by unhooking the vacuum line from the booster and plugging it. Then see if your "lope" goes away when applying the brakes.

 

[Classic-00-Crew]

Haha that would be a interesting grind to have on a cam since it fluctuates. I tried blocking off that line already and it didn't help. I'm liking 1sickcrewcab's idea about the fuel pressure regulator. Is there a way to test it or is that a "replace and see if it works" part? Maybe put a actual dial type guage on it?

 

[Classic-00-Crew]

Haha that would be a interesting grind to have on a cam since it fluctuates. I tried blocking off that line already and it didn't help. I'm liking 1sickcrewcab's idea about the fuel pressure regulator. Is there a way to test it or is that a "replace and see if it works" part? Maybe put a actual dial type guage on it?

 

[Keepinitoldskool]

I'm not well versed in vortecs but I'm willing to bet that the fuel pressure relief valve is after the regulator on your truck. If it is then just stick a gauge on it.