GM Techs need some help please
 

My little economizer finally died so I have been driving my truck this week. 2000 GMC Sierra with a 5.3. I noticed yesterday that it didn't seem to have much power on the highway. The more throttle I gave it the less power it would have. This morning on the way to work it seemed fine. Had power on the highway at any throttle position.

On the way home it would run fine as long as I do not go over quarter throttle. If I went past 28% it would fall on its face. 50%-100% and it would barely chug almost not even run. If I bring the throttle back to 25% it runs great and I'm back to 70mph.

In the driveway in park I can rev it up to 5000rpm no problem, but if the throttle passes 28% it dies and barely pulls 2000rpm. I have no codes or faults stored in my ecu that I am able to read.

I am reading all the stats from my phone using my OBD2 dongle and an app. I can see what % my throttle is, my O2 voltage and my MAP reading. It usually it works pretty good but I think this might be over its head.

Thanks

fixx
 

your fuel pump is probably on the fritz,,low fuel pressure will caus a back fire in the intake..do the filter first if that dont take care of it then go to the fuel pump...

ONLY install a delfi unit,all others dont last,your local napa has the cheapest price....if you have over 70k then its time..

+1 for the fuel pump, buy AC Delco or Delphi, most of the others are less than reliable (the AC and Delphi units aren't even that great, but better)

Would this turn on and off like a switch with just throttle position if it was a fuel pump? The truck has 280,000 new fuel filter 1 month ago and fuel pump 15,000 miles ago.

throttle position sensor

Kinda same problem I ran into borrowing my sisters brand new '03 GMC at the time.
Was fine at city speeds, Get out on highway (above 40-50) and it slowed down the more gas I gave it.


Try this next time above 40 or so, wherever you notice it drops off... drive by your cruise control.
Set it, then hit your accel button and see if it does. I ended up driving all the way from IN to OK towing a trailer empty there, then back with my boat. Only way I made the trip was by driving with the CC anytime over 40 mph
She took it into dealer as soon as I returned and they determined the sensor/positioner??? at the gas pedal was defective. The CC test will tell all

I'm gonna go with the fuel pump and don't replace with an airtex pump unless you want to do it again

fixx
 

he mentioned he replaced it 15 k ago,,maybe it was a airtex :lolhit:


on anothe rnote if you did replace it check what mfr it was,,what was the fuel trim at?

clean the mas airflow sensor,a spec of dirt on it and it will act the same as a bad fuel pump..

clean your mass air and clean your throttle body. is it electronic throttle???

been a GM tech for 27 years and agree you will most likely need a fuel pump..early 2000-2003 tahoe suburbans also had issues with the fuel pressure regulator,,would not hurt to pull the vacoum hose and make sure its not leaking...

if you think its the mass sensor then just unplug it and drive it..it goes to a set value,,check engine light will come on but truck will drive ok if it a bad mass..

a dirty throttle body will not give you the issue you are having,,once you step on the gas pedal the throttle plate opens and you are getting all the air your engine needs..

I would not think its a Throttle position sensor either,,trans gets input from the TPS,if that was the problem you would also notice hard/firm trans shifts..

hard to diagnos over the internet with out a Tech2 and a fuel pressure gauge hooked up but it does sound like a fuel pump.
and like the others have said use a GM pump..

Converters plugged ????

Thanks everyone. So it still runs fine in the morning when it is cool out (15miles to work) to work. But after sitting 9 hours while at work it acts up within the first 1/2 mile after sitting in the 85*F weather. Might just be a coincidence.

Boatnt... It does have some different shifting points when it is acting up. It seems to shift out of the gears faster, I will be in overdrive by 35mph. Some times it will not shift into 3rd until 3000 kinda erratic.

Will have that checked out.

Thanks all for the suggestions

Check the coolant temp with you dongle to see if it is accurate. Put a fuel pressure tester on it to verify fuel pressure.

Quick update:
The last few days the truck has been running pretty good, slight hesitation above 60% throttle.
I disconnected the mass air, put in a known good mass air and TPS with no change.
This weekend I will hook up a fuel pressure gauge and do a back pressure test on the cats.

I hooked it up to my brothers Snap on Solus and it had no stored codes and all checked good.

As always, thanks

Another update:
Fuel pressure checked out good.
There was a small measurable amount of back pressure on the passenger side cat.
I replaced the TPS.

Today I replaced the battery and now all hesitation has gone away. I can put it to the floor and it runs
great.

Thanks everyone for all the suggestions. I hope this was it.

Rookie: Sometimes all it takes is a battery. These vehicles are sensitive to that and it seems strange that a battery will "function" but not allow a vehicle to run right.

Wannabe