Rough Idling and Acceleration Problems

tollelegetollelege Member Posts: 10
edited March 2014 in GMC
Just a couple of days ago, my 91 Sierra 5.0 liter (60K miles) developed a problem. It has two components which are undoubtedly related. First, idling is a problem. It will almost die, then resume normal speed. This cycling continues, especially when the engine is just started. It appears to improve, but not go away, once the engine warms up. The second is that acceleration appears to be poor, at least poorer than normal. This hesitation continues even with a warm engine. It might not be terribly noticeable to someone who has not operated the vehicle. The truck is still driveable, but certainly not as smooth as a few days ago. The things I have changed include: new PCV, new spark plug wires, new distributor and rotor, and new fuel filter. I added a can of engine cleaner thinking that I might have a problem with dirty injectors. I've only used a few gallons of gas since addition of the cleaner so it may not have had time to do its work. The timing checks OK. This may or may not have any relation to the problem, but my wife ran over a curb on the driver's side just before the problem was first noted. She didn't think anything scraped bottom but she did comment that it was quite a jolt. Any ideas about what to try next?


  • zueslewiszueslewis Member Posts: 2,353
    they have a bunch to do with idle and acceleration.
  • jgmilbergjgmilberg Member Posts: 872
    Check your PCM connection, on your truck it is not a bolt on connection, so make sure the connector is tight and secure. The PCM is behind the glove box, and you have to remove the glove box to get at it. A super hard sharp jolt could knock the connector loose.

    Another thing to look at is the EGR valve, O2 sensors, and ignition module under the distributor cap.
  • tollelegetollelege Member Posts: 10
    Well, I have followed some of the posted suggestions but to no avail. I replaced the oxygen sensor and EGR valve. I have not replaced the ignition module since the timing shows to be OK. Maybe that's not relevant and I should replace it as well. Opinions???

    The engine seems to be running lean. I did get a check engine light after changing the oxygen sensor. A probe of the computer said that the oxygen sensor had picked up a "lean" situation. That, combined with the poor power and erratic idling probably suggests that I am not getting a rich enough mixture. The shop manual suggests checking for vacuum leaks. I'll check that tonight but haven't seen any obvious problems with broken or disconnected lines. Are there locations that are particularly problematic for leaks? The shop manual says I should see 7-9 " vacuum at the EGR. Is that a reasonable system vacuum???

    The idling and acceleration improves as the engine is run, but still is far short of the performance I had a week ago before the problem surfaced. Any ideas????????

    Thanks in advance.
  • jgmilbergjgmilberg Member Posts: 872
    Check your fuel pressure, if it is low that would explain the lean condition. If that is good, then vacuum leaks would be next, start at the throttle body, check all the vacuum lines to where they go for cracks and breaks. If that's all good check the throttle position sensor, for proper setting and resistance values. If everything else fails check the injectors out. They make a kit for checking that, but it is expensive and you may want to just take it in for that test.

    The ignition module is responsible for firing the plugs, and you will show proper timing but the timing will check fine with the gun. If you have had problems with hot starts and no starts along with these other problems swap that out. Some stores can test that unit if you remove it from the truck and take it to them.

    I didn't see anything about you changing the plugs. That might be another thing to change if you haven't already. Make sure your exhaust is clear too, a clogged cat. conv. can give some of the symptoms if the guts are all chunked up inside the shell.

    If you can't figure it out don't hesitate to take it in to a shop/dealer to have it checked out. A lot of the time the cost of the diagnosis is less that throwing parts at it. I have done it several times and it really saved some time, money, and frustration.
  • swschradswschrad Member Posts: 2,171
    the most expensive way to fix anything, in fact, unless fortune smiles on you early. proper diagnosis always saves money... unless you go in for a computer scan immediately after filling your car, and the $$$ light comes on right away. that issue is almost always a loose gas cap or one with an internal issue.
  • tollelegetollelege Member Posts: 10
    To bring this discussion to an end, I finally took the advice of several in the Town Hall and let a repair facility deal with my problem. The source was the fuel pump. It was only supplying about 8 psi which accounts for the erratic idling and poor acceleration. Too bad the pump was located in the fuel tank or I could have saved some money. But $500 later, I'm back on the road and a little wiser about some of the issues that impact driveability. I wish my shop manual had provided better diagnostics to assess the fuel pressure. I would probably have still let a repair facility perform the work, but could have avoided their diagnostics charge. Thanks to all those who offered advice.
  • swschradswschrad Member Posts: 2,171
    there could be enough wear on the fuel pressure regulator that getting a big shot of gas again from a new pump puts IT over the edge as well.
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