heres the deal, i got a 98 GMC sierra with a 5.7 vortec. its starts and idles just fine and as long as you accelerate lightly it is also fine. if i accelerate hard it will just pick up RPM's a little and then sit there and chug and hesitiate. if im at a higher speed and i try to accelerate hard enough to cause a downshift it will not downshift and it will sit there and hessitate and chugg. it up shifts perfectly also. if i stop and put the truck in park or nuetral and i slowly build up RPM i can get it to redline. but if i just hammer on the accelerator in park or nuetral it will get up to about 2500 rpm and sit there and chug and hesitate(same thing it does in drive). there also is no service engine light on. it just randomly started doing this today when i was out for a drive. i turned the truck off and let it sit for 5 hours and then drove it again and it still doing the same thing. Im wondering it anyone has had the same problem and gotten to the bottom of it? or could point me on a good direction to start looking. thanks in advance

 

Use some mas airflow sensor cleaner on your MAF sensor: CRC 05110 Mass Air Flow Sensor Cleaner - 11 Wt oz.

$10 from autozone.
The sensor looks like this:


The sensor should be between your airbox and the throttle valve somewhere.

Just take the sensor out, DO NOT TOUCH ANYTHING INSIDE THE SENSOR, lay it on a flat clean work place (I used a plastic lid to a 5 galon bucket) and spray the sensor and let it set. Make sure to follow the instructions on the can carefully, because it's been a while since I've done this

If that doesn't work, the sensor is probably bad.

 

so heres what ive done so far. new plugs and wires because i figured i should anyways. that didnt help, the MAF sensor was spotless so i didnt do anything to it yet, but i will clean it tomorrow. i also pulled my front o2 sensors and then ran the truck and it was still the same, so i can check off the cat convertors. im gunna put in a new fuel filter and check MAF and MAP sensors tomorrow and then ill update. thanks for your help

 

Fuel pressure? I had a 98 Vortec 454 leave me stranded and it was doing a similar thing to what you are describing before it just quit. My fuel filter was blocked bad! The weird thing is I was on a 14 hour trip and it went from running perfect to chugging to dying in a matter of 10 miles or so. I had it towed home and replaced the filter and it fired up and ran great.

 

The description of symptoms in the op are classic fuel filter, issues.

 

just put the new fuel filter in and its still doing the same thing. i did a presssure test and i got about 55 when key is on and not running, 48 when running at idle. when you step on it the pressure spikes to 55 maybe 60 and then drops to 48 again........ WTF is this things problem?????

 

I would start testing sensors - specifically the TPS.

 

My money's on the MAF sensor. The Symptoms sound exactly like the problem I had with my mom's van. Apparently microscopic particles build up on the sensor wires, and that's what causes bad readings. Try unplugging the MAF sensor and drive it around. Made a night and day difference for me, but I do acknowledge it could be a different sensor. Just my two cents.

 

i put new MAF in and nothing different, tested map and TPS sensors they are good. i call my high school buddy who is a mechanic and he said 48 is too low for these trucks anything below 54 will run funny because they spider injectors need a really PSI to fucntion properly. so new fuel pump is going in in about an hour. ill let you know how it goes.

 

it was the fuel pump. i was 48psi before and now i running 54psi and it runs like its brand new