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1999 GMC Work Truck

I have a late 90’s GMC 3500 SL with 275k miles and a 5 speed manual tranny. I drive it about 80 miles a day going around town for my job. Every day it starts and runs fine until noon or so when it begins to have trouble accelerating. It seems like the more gas I give it the slower it wants to go…just feathering the throttle will get it to accelerate…but barly, almost jerking it forward…it almost sounds like it’s backfiring all the while. It sometimes gets to the point where I need to pull over because it just doesn’t want to go, but if I turn it off for 5 mins and restart it’ll run fairly well for awhile. It doesn’t seem to have anything to do with the temperature of the engine…it can sit for 2 hours in the middle of the day and start acting up right after I start it up. I took it to the shop…they replaced the fuel filter, but it didn’t help.

There is another sock type fuel filter inside the tank, protecting the fuel pump which is also in the tank. The next time it’s running poorly, check the fuel pressure at the injector rail. There will be a fitting for that purpose. A fuel pressure gauge is not that expensive. Most vehicles need 40-50 psi. Check what yours should be. You may have a failing fuel pump…

It’s just weird to me that the fuel pump can deliver the required fuel to keep the truck going at 70 mph for hours, but as soon as I pull off the interstate I can’t accelerate onto the main road. It seems almost like it’s flooding the engine rater than starving for fuel.