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Rough Idling and Acceleration Problems
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?
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Comments
Another thing to look at is the EGR valve, O2 sensors, and ignition module under the distributor cap.
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.
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.