Hello everyone. I have a 95 GMC Sierra, 1500 series, 5.7ltr(350)tbi, air, auto, 2wd. It just turned 300,000 km (186000m), but my problem has been happening for a long long time now, and I cannot find an answer anywhere. I have only ever seen one other post online describing the same problem, and he didnt get the problem resolved.
I have been experiencing a "hesitation/bog/stumble" when I lightly step on the throttle. It happens either from a standstill OR while cruising. It happens ONLY when I give LIGHT throttle pressure at the low end of the rpm range of the gear(ie: very early into the gear, very light throttle at low rpm). The truck will hesitate and buck slightly(it has even backfired through the tbi once or twice, but that is not a common symptom) unless I step harder on the gas to get it to smooth out.
Associated with this hesitation, is a general loss in power/acceleration/pickup, the truck feels VERY sluggish. If I step on the gas in any gear to smooth it out, the acceleration will SLOWLY smooth out( with not much power) , ending up in a sudden downshift/surge until the next gear is found.
Now for the kicker......this ONLY happens after the engine reaches a certain temperature, it will NOT behave like this when the engine is cold first thing in the morning!! It only happens at low throttle pressure with a warm(doesnt have to be fully warm though) engine. If I step on the throttle harder, there isnt an issue.
Also happening, is an rpm "pulsation" at lower speeds(aprox. 5okmh/30mph) of aproximately 50 rpm as i drive with continuous steady pressure on the throttle. Up and down, up and down it pulsates by about 50 rpm.
I have replaced cap/rotor, plug wires(all Accell products), PCV valve, air filter(K&N), throttle position sensor, MAP sensor, O2 sensor. I have also done a reset of the ECM a few different times (key in the "on" position when yanking the fuses or disconnecting neg cable). Resetting the ECM seemed to help slightly for a very brief period of time, but the problem soon came back. The TPS and MAP sensor were both replaced this summer after a non-related (I think) incident on the highway, and the OBD1 code showed a problem with one or the other, so I replaced both.
The truck is in great shape, and runs very good except for this particular problem, which is driving me insane!!!!
If anyone has some insight into this weird problem, I would be forever grateful and owing of many many beers
Thanks so much

 

Check the fuel pressure. Add some fuel injector cleaner to the gas maybe on a couple of fillups and see if it helps. I'm thinking byou may have a dirty TBI. If the RPM starts limiting out above 2500-3000 it may be a bad fuel filter. That was a problem with my 91 chevy about every 25000 miles.

 

Hey there, thanks for the reply
I haven't checked the fuel pressure, but the one and other post I've seen on this problem said that the guy did check it, and it was within specs. I do add fuel injector cleaner to my gas every couple of fills, and I have rigorously cleaned the TBI with spray cleaner. The rpm doesn't limit out, it is quite fine up at the higher rpms when on the highway, or when I really step on it. Thanks for your input!!

 

My old 89 C1500 did the same thing. My fuel pressure checked at 8 PSI, which was technically in spec, but the issue came when the pump couldn't provide the volume of fuel needed, and it would bog down. I used the Actron fuel pressure kit. When I replaced my pump, fuel pressure went to 13 PSI and it ran the best the truck had ever ran.

Also, your symptoms tend to make me lean towards a possible ECT (Engine coolant temp) sensor issue too. Use your OHM meter and check your ECT sensor against this chart while the truck is warming up to make sure that the temp sensor stays roughly equal to the actual engine temprerature



Use an infrared temperature gun to verify engine temperature.

 

NullHead, thanks so much! I now have another area to check out. I will post back my findings!!

 

Update: Replaced the coolant temp sensor since was a cheap replacement, but unfortunately it had no effect. My next step is the fuel filter, and if that fails, fuel pump. As you can see, Im leaving the hardest fix to the last...haha

 

Egr valve or egr solenoid

 

Did you replace the fuel pump and if so - by dropping the tank of lifting the bed / box ????