2000 Sierra, 5.3L, 103K miles. Having an ongoing issue, driving me NUTS. Any tips appreciated. I replaced the fuel pump module with a Delphi unit due to a bad sending unit this past March and also replaced the intake/valley pan gaskets/knock sensors and harness, and also valve cover gaskets in April....... A few months ago, I started experiencing some light to moderate pinging on acceleration. Within a day or so, the pinging was gone. Perhaps some bad fuel or something. But I decided to run some sea foam through it anyways just in case. I had done this before with no ill effects. I also cleaned the MAF sensor with the proper cleaner at that time...... Within a few days, the truck started behaving a bit strangely. Seemed a bit sluggish on acceleration, especially when warming up. Idle seemed maybe a little low. Shifting characteristics also changed. It seemed slow to downshift when going around a corner and was just acting wierd in general. Also seemed like the truck was slowing down faster when coasting. Thinking I had reinstalled the MAF wrong and/or neglected to tighten something down, I removed it, cleaned it again, and reset the computer. No change. Replaced fuel filter. No change. With the hesitation and crappy shifting, I was POSITIVE I had screwed the MAF up somehow when I had cleaned it........so I bought a new one. No change of course. Thinking about the seafoam, I pulled all the plugs (AC Delco Iridiums), which only had 30K miles on them. They looked alright. Beginning to fall into the trap of throwing parts at it and after some more research, I decided to replace the front O2's, which were original anyways. No change. Bought new plugs. No change. :grrrrrr: At that point, I stopped the parts changing madness before it got out of hand and haven't done anything since then (couple months now). I also checked the fuel pressure. Was within spec for the bleed down test. Running at idle, it was at just over 50 IIRC, but the needle bounced around/vibrated when I would rev it up. Not sure if that matters or not. No fuel visible in the pressure regulator hose but I think I could smell a very slight trace of fuel odor when sniffing around the hose port. Again, not sure if that matters. It's totally driveable, never misfires or runs rough, no codes, fuel mileage unchanged. It's just a PIG. Certainly not the peppy truck I was used to. With the very cold weather recently, it feels like an old carb engine with a sticky choke when I drive it away in the morning. Colder weather definetlly makes it worse. I highly doubt its the intake gaskets I replaced. I've replaced intake gaskets on various vehicles over the years and never screwed it up. Feels like I'm hauling something all the time. I also notice a very slight hissing or whistle sound under moderate to heavy acceleration. I thought vacuum leak at first but it actually sounds more like air being forced OUT somewhere rather than in. Pehaps I partially clogged the cats when I did the seafoam? IDK, all these symptoms could be a total coincidence and not related to the seafoam.