No Power On Acceleration

mhart · September 23, 2008

I have a 99 nbs 5.3 Z71, 140,000 km. For some time now I have noticed the when the truck is cold or has been sitting for a few hours it has no power when I try to accelerate. At first it was random but now it appears to be happening more frequently. The truck starts fine, I can pull out of the driveway and up the lane for 100 feet under light throttle, then when I pull out on the main road and try to accelerate hard it has no power. The more I try to accelerate the worse it gets, I have to let my foot off then re-apply slowly and then it works OK after that. I do not hear any backfiring or odd noises when it happens. I have cleaned the MAF but other then that the truck is original except a K&N filter.

Sorry, I have performed a search but can not find the same symptoms. Thanks in advance to those who reply.

azchevy99 · September 23, 2008

I'm having the exact same problem and would also love to know the answer.

nbscanuck · September 23, 2008

could be fuel filter with those miles

abracadabra · September 23, 2008

Check fuel pressure.

C6Racer · September 24, 2008

Yea, fuel pressure is where I would start. It should read around 60 psi at idle. I was having the same problem and it turned out my fuel pump was bad. It was only putting out 20psi. If pressure checks out ok replace the fuel filter because that should be done every 30k-40k miles anyway.

mhart · November 2, 2008

Added a new filter and no change.

Checked pressure, key on up to 60psi then cycles back and holds 56PSI

(Haynes spec 55-62)

Pressure with engine running 50psi (49.5)

(Haynes spec -3 to -10 below recorded reading above)

Turn the truck off pressure falls to 46psi (Haynes; should not drop less then 5psi for 5 minutes)then slowly rises back to 56

Drive test, average no load situation about 51-52psi, up to 59 under hard acceleration.

Vacuum at idle going to regulator is 18-20 in-Hg, cycles between 18 & 20 with engagement of the air compressor. (Haynes says 12-14in-Hg ?What should the proper vacuum be?)Pressure rises to 59 - 60 with vacuum disconnected.

Have not tried to pinch the return, I am still trying to figure out the best way to do this. Also I have not been able to record the pressure when the loss of power symptom, yet.

For those looking for a cheap but good way to test fuel pressure, order " Hose with Fitting & Relief Valve for Fuel Pressure Tester Kit CP7818" from http://www.actron.com/product_detail.php?pid=16222 for 10$ (add shipping) and stick a pressure gauge on it. I bought a liquid filled gas compatible 0-100 psi quality gauge and thread adapter for another 14$.

Sorry for the delay in my reply, but would someone be kind enough review my finding and let me know what you think.

Thanks

mhart · November 7, 2008

Finally able to test during no power symptom and there is 60PSI.

Had some advice off another board to check TPS, O2 and Temp sensors. It was suggested that the truck was running rich by checking the tail pipe. Mine appears to be running rich.

Z71GMC · November 7, 2008

I also have the same truck and problem. I posted a while back but never got an answer. Anyone?

thecraft · November 7, 2008

Finally able to test during no power symptom and there is 60PSI.
Had some advice off another board to check TPS, O2 and Temp sensors. It was suggested that the truck was running rich by checking the tail pipe. Mine appears to be running rich.

If it was running rich, it might have clogged your catalytic converter, which will cause the sluggish acceleration you described.