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For the past couple weeks I've been consistently having a weird issue with my 2000 Sierra 1500 4.8L when I first start it up.

The engine will start and idle fine, but when I give it gas the engine doesn't always rev up. I can mash the throttle down all the way and not much happens, and then suddenly it "kicks in" and the RPMs go up and it accelerates.

This is the typical behavior I've been noticing:

I'll start up truck, back out of driveway (I don't think I accelerate much to back up since driveway is sloped). I put the truck in drive, and push the gas, and it'll accelerate. But then I'll push it more to get up to speed (only 30km/h in our little suburb road) and nothing happens. So I'll push the accelerator more, and nothing happens. And then a few seconds later it finally kicks in and the RPMs go up and the truck accelerates. This can happen a couple times while I'm leaving our complex, but it seems after that the truck drives totally fine. No hesitation, responds to throttle properly.

I decided to do more troubleshooting today so when I got the truck and started it, I put it into neutral and decided to just rev it up.

I observed the same basic behavior. It would rev up a little, but then the RPMs would go up a bit but stay very low (I stupidly didn't look exactly at what but def below 2000 I think, maybe even just 1000) even when mashing the gas all the way. And then if I backed off the throttle a bit something would make a "clunk" noise, and the truck would vibrate slightly, and suddenly the engine would go up to 4000+ rpm like it should.

I was able to do this 3 times and then something must have warmed up and I stopped being able to do it. Speaking of, this behavior only happens if the truck has been sitting a bit. If I turn it on and off after it's been running, the behavior doesn't occur.

I think you have to wait at least an hour or so (I haven't time it exactly).

No check engine light comes on but I checked the codes anyway. I got one in the GMC OEM module, but it seems unrelated:

  • P1638 - Not Passed SC Generator F-Terminal Circuit

Any thoughts would be appreciated!

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  • Welcome to Motor Vehicle Maintenance & Repair! I'd check the Throttle Position Sensor and see what it says. You should be able to use your scanner if it has live data to see the TPS data (% of throttle or some such). Just turn the truck to the "On" position, plug in the scanner, go to live data and look for the TPS %. If the TPS doesn't run across the range smoothly while you are doing this, this is more than likely the cause.
    – Pᴀᴜʟsᴛᴇʀ2 ♦
    Aug 10, 2018 at 21:12
  • Thanks. I'm not sure my current scanner has that but I have a Bluetooth one on the way that should work with the torque pro app and I believe that has that capability.
    – Matt
    Aug 11, 2018 at 4:18
  • I believe the Torque Pro app does as well. With an ELM327 reader, it's pretty slick.
    – Pᴀᴜʟsᴛᴇʀ2 ♦
    Aug 11, 2018 at 11:34

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