99 Gmc Loss Of Power At High Rpms

[gssieg]

I have a 99 GMC Sierra with the 5.3. 162,000 miles. Lately Ive been experiencing some issues with loss of power when passing, towing, or under a load at high rpms (4,000 +). It seems more common when it is cold. The truck seems to shake, and sputter as if it was misfiring. I do not have a check engine light or anything though. And it seems to only happen when the truck is colder, or at least more consistant. The other day it did it to me after being on the road for over an hour, I went to pass a vehicle and it downshifted and started acting funny. I couldnt get it to do it again. Today coming home from work I could get it to do it almost every time I put the throttle down, yet taking off and normal driving not only did the truck run great but had power. When I was going 50 put the throttle down and it downshifted I was able to get it to act up. I did bring it to a friends house and put it on a scan tool, I found a code for the Crankshaft Possition sensor, the code was created when the vehicle was driving 60, at 3800 RPMs in 3rd gear, which sounded close to what was happening when I was having the problems today at least, but the code was not turning the check engine light on. Any suggestions at all would be great. Someone thought maybe I was having transmission issues, I checked fluid was clean and full.

 

Thanks in advance,

Greg

[Ron98]

i have a 1998 1500 5.0 110k. at wot 4000+ rpms the truck shudders and loses power. ive replaced cap rotor wires plugs distributor gear new mpi fuel injection from linder. ive checked my fuel pressure. key on engine off 62 psi till pump shuts off after 2 sec then it drops to 56. engine at idle 54-56. wot 62 then fluctuates rapidly during misfire. any help would be appreciated

 

Ron

[OUMike]

I have a 99 GMC Sierra with the 5.3. 162,000 miles. Lately Ive been experiencing some issues with loss of power when passing, towing, or under a load at high rpms (4,000 +). It seems more common when it is cold. The truck seems to shake, and sputter as if it was misfiring. I do not have a check engine light or anything though. And it seems to only happen when the truck is colder, or at least more consistant. The other day it did it to me after being on the road for over an hour, I went to pass a vehicle and it downshifted and started acting funny. I couldnt get it to do it again. Today coming home from work I could get it to do it almost every time I put the throttle down, yet taking off and normal driving not only did the truck run great but had power. When I was going 50 put the throttle down and it downshifted I was able to get it to act up. I did bring it to a friends house and put it on a scan tool, I found a code for the Crankshaft Possition sensor, the code was created when the vehicle was driving 60, at 3800 RPMs in 3rd gear, which sounded close to what was happening when I was having the problems today at least, but the code was not turning the check engine light on. Any suggestions at all would be great. Someone thought maybe I was having transmission issues, I checked fluid was clean and full.

 

Thanks in advance,

Greg

 

Alot of times a problem won't always throw the SES light, because the problem has to be repeated in series a lot of times. Also, just because your fluid is clean and full doesn't necessarily mean there are no tranny issues. What specific behaviors is the truck doing when you say "acting up"? Have you checked the Crankshaft Pos. Sensor? I would start by fixing the code, and then have your trans serviced. Most places want about $500-700 to diagnose the trans, and I just had my 4L60-E rebuilt for about $1300.

 

Throttle position sensor and Idle Air Control valve come to mind. Both are less than $20 fixes, and would take you probably an hour to replace, but if your already throwing a code, that issues should be attended to first.

 

Hope this helps