1999 bucket trucket L21 question

[squirrel64]

Hey Guys,

New here so I hope I'm asking in the right section. I have a 1999 GMC C7500 bucket truck with L21 7.4l engine. This thing is pretty sluggish sometimes. I was wondering if there are any upgrades I can do to the motor without going internal. I will be going in the spring to hopefully upgrade exhaust system. But right now I was just wanting to do some small upgrades (breathe better, fuel delivery) .

There aren't many, if any forums on these medium duty trucks and I'm having a hard time finding out if there are some things I can do to help get a little more power. I'm not a mechanic and have limited skills and tools but if there is any help or information you can give me that might squeeze some power out would be a big help. I do know some good mechanics though. With this being in a commercial application, would there be any websites you may direct me to that might help. Thank for looking

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[RyanMerrick]

There isn't much information on the L21 specifically, but that engine does share a lot with the L29 454 in the lighter duty trucks. I think the biggest differences are the forged crank and pistons as well as a different PCM.

Do a search on the site for 454 modifications and you'll find a bit of information. Without digging into the engine definitely look for a better breathing air intake and exhaust system. I'd also check to make sure there's no clogged fuel filter or anything else impacting performance.

 

[454ss]

go to fullsizechevy.com and search the 454 vortec thread, its huge, like 600 pages. there is a guy on there with a l21 in a motorhome that he has made run a lot better than stock. gap plugs at 45, not 60 is one of the common tricks. new injectors from fiveomotorsports.com, new fpr, as with all vortecs with 100k or so, a new distributor gear atleast, I would replace it with a complete dui unit. run a 180* thermostat.

 

[squirrel64]

Thanks guys. I saw that thread but it is huge and alot of misc in between the real good info. 454ss I will def try the things you mentioned. Just put plugs, wires and new fuel filter in. That did help somewhat, but still wants to sputter some at high end of each gear. It has good pickup at lower gears but 4th and 5th are terrible.

 

[GMstrong]

Id do full tune up,plugs,wires,etc then replace that air filter with something better flowing like a afe/k&n,it looks the same air filter I had on my c2500.Also Id clean the throttle body.Not sure what kind of exhaust system they have but due to its size probably has a muffler the size of a 275 gal oil tank.