:confused: anybody run accross this anything similar?? my daily driver, i bought a 91 ext cab 2 wheeler with the 5.7 and 86K back last march, truck got reasonably good mileage (15-16) around town, did a brief tune-up with plugs-cap-rotor-wires, shortly after that mileage started going downhill late summer, all winter now the damn thing gets 10-11, but occasionally jumps back up to 14 ish, had an alternator quit one day and put on a used spare i had, now is where things get interesting, below is the list of issues i am trying to sort out.

when i try to get timing to 0 degrees truck pings when it warms up, fuel grade and or supplier makes no difference.

voltage guage jumps back and forth from 14+ TO 12 intermittently, only after truck has been running for a while (over 10 minutes) and at same time truck acts like there is a massive load (i.e. you can feel the motor drop a notch , if a i had a tach i would swear it loses 150 rpm or so)

last night the damn thing was having a real issue idling when the gas tank was down to around a 1/4 (i usually never let it get that low) it wanted to die at the lights, filled up and continued on to dinner with the wife, when we came out an hour later it started up and drove home but still seems to have a slight miss and the alt gauge stayed @14 , never rising above like it normally does here in the winter after you first start it.

things i have done involving the mileage issue, took to dealer and had their best truck mech. ( a friend to boot) look at it, could find nothing, i have NO check engine codes that are setting, i have replaced temp sensor in the intake, cehcked knock sensor but it is the only sensor i have not replaced, egr and solenoid, tps, MAP, and new tbi baseplate gasket, also i have not run the timing back up since i did an gm intake cleaner thru the vacuuum port on the tbi, truck seems to run fine albeit @ 2 degrees retarded!!! for the most part the truck runs good its just irritating on the gas but i am wondering if this new idle issue and the alt bounce are trying to tell me something (fuel pump going causing delivery issues??? my gm truck mechanic did put the gauge on and it was running about 12-14 psi. at idle in the shop and we are stumped. any ideas out there lmk. thanks,

When you did your tune up, did you happen to clean the engine with water?

I'm wondering if there is a connection somewhere that corroded. Did you put dielectric grease on the connections?

-ACES

You may have an O2 sensor shorted....unplug them (if there is more than one) and try it again. I should add, dont run the engine but for a min or two...because, if the sensors are of the heated type, they may become damaged if they collect moisture. Basically, if it starts and you have they same symptoms, shut if off and look else where.

tune-up back in august when mileage started to drop off, yes all wires got grease, engine has not gotten a hose job, forgot to mention yes truck recieved new 0-2 at the same time as the MAP, the cleaner i referred to was GM's top engine cleaner thinking maybe there was a carbon build-up issue in the intake causing the ping at TDC. the alt. still is doing the funky voltage dive, it seems that once the battery is charged and the truck is idling it just drops down to around 11.5 or 12 on the scale for a second or so then jumps back to 14-14.5 but the thing that makes me curious is that the motor seems to drop noticably rpm wise at the same time, would a bad fuel pump cause a draw like that? Or could it be bad connections in the fuel pump wiring as gm has had issues on most of the trucks, also could a fluky fuel pump cause a drop in mileage and or timing issues even though the inline test showed 12-14 psi?

When you set the timing did you disconnect the brown/white wire to put the truck into base timing or "limp in mode"? Also if the knock sensor is out of wack it will retard your timing. When was the last time fuel filter was changed?
Check battery connections and connections on back of alt, making sure they are clean and tight aswell as all grounds.

3 things come to mind, and they are expensive too. (fairly)
Fuel filter (not very probable IMO)
Bad dist
bad fuel pump.