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  1. #1

    Default Gen IV 4.3 bucking/ cutting out

    I recently purchased an 89 GMC sierra. It is a 2wd 5 speed. It has 250k on the clock. I bought it for $500 because it has an exhaust leak and an issue with cutting out. I fixed the exhaust leak(manifold bolts walked themselves out). The truck also has new plugs, wires, cap and rotor. When I drove it home, If I lugged the engine even a little, it would buck like crazy until I got the rpms up. I figured that seeing as I didn't know how many hands have been on this thing, I'd start by checking the ignition timing. It was set at somewhere near 14 degrees. I set it to the factory recommended 0 degrees but I had my doubts. It still ran at 0 but it had no power. I then set it to 4 degrees and now it seems to run pretty good. I did have one issue going up a hill at 50mph in 4th gear. I shifted into 5th and it started bucking like crazy. I plan on changing the oil, filling the gas tank, cleaning the throttle body and flushing the coolant before the weekend. Is there something I'm overlooking? Is that bucking normal? I'm a total noob when it comes to tbi.

  2. #2
    Jr. Apprentice SkullMountainRider's Avatar
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    I'd start with the Fuel filter, then what you listed (oil, coolant, new fuel).

  3. #3

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    I'll do that. Is there anything I need to know baout cleaning a tbi throttle body or is is as simple as cleaning any other throttle body?

  4. #4
    Jr. Apprentice SkullMountainRider's Avatar
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    These are a bit tricky because of some pieces being ruined when disassembling the unit and need to be replaced, no matter what. If you have or have access to a Haynes or Chilton manual for your year, this will become a great asset to you. You can remove the fuel meter cover (top, holding in injectors), but be sure to NOT remove the four pressure regulator screws, in the fuel meter body. This part is under high pressure due to a heavy spring. Removing this could result in fuel leaks later on and the FM cover should be replaced. Another note is that any fuel line O-rings need to be replaced with new ones. The injectors carry filters as well, which could be dirty but, as I mentioned, is a very finicky system that if not done properly, could result in fuel leaks at the injectors. There is to much that can go wrong that i'd stick to the simplest fixes first. If needs be, put some injector cleaner in the tank. I'm curious as to how old the fuel in the tank is?

  5. #5

    Default

    Quote Originally Posted by SkullMountainRider View Post
    These are a bit tricky because of some pieces being ruined when disassembling the unit and need to be replaced, no matter what. If you have or have access to a Haynes or Chilton manual for your year, this will become a great asset to you. You can remove the fuel meter cover (top, holding in injectors), but be sure to NOT remove the four pressure regulator screws, in the fuel meter body. This part is under high pressure due to a heavy spring. Removing this could result in fuel leaks later on and the FM cover should be replaced. Another note is that any fuel line O-rings need to be replaced with new ones. The injectors carry filters as well, which could be dirty but, as I mentioned, is a very finicky system that if not done properly, could result in fuel leaks at the injectors. There is to much that can go wrong that i'd stick to the simplest fixes first. If needs be, put some injector cleaner in the tank. I'm curious as to how old the fuel in the tank is?
    Yeah, I think I'm gonna leave the throttle body alone. I took the air cleaner off and it looks relatively clean so I'm gonna call it good. You hit the nail on the head with the old gas. I filled up today with some 87 and a bottle of lucas injector cleaner and the difference is night and day. I thought it was running good before but boy was I wrong. I think I'm still gonna do a fuel filter for good measure but first I think I'm gonna flush the coolant and replace the tstat. I drove it to work today (37 miles one way) and I was a little scared that it was going to overheat at the traffic lights and when I was on the highway, it was struggling to even get to operating temp.

  6. #6
    Jr. Apprentice SkullMountainRider's Avatar
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    Good show! Let me know if the coolant, Tstat change helps. Note what the coolant looks like, You might have heavy calcium deposits in the rad. I've had a few plugged rads in my day.

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