Rough idle surging

LAHDUDE

MEMBER
1997 GMC SIERRA
5.7L
V8
196,000 MILES

I’m having an issue with the truck when sitting at an idle you can feel a slight shake that’s rougher than the rest of the idle. It will go around stop and run smooth then shake a little and continue fine it also surges slightly while accelerating. I’ve also noticed when pulling into a parking lot from driving for a minute when I slow down and hit the brakes the fan on the A/C will get softer and blow less hard. And last at night you can see every light in the truck flicker from headlights to dash lights even dome lights. Took to a local mechanic but they said it was nothing they didn’t seem like the brightest shop though more of ones to blow it over if it’s running okay.

5 Replies

JACOBANDNICKOLAS

MECHANIC
109,708 POSTS

Hi,

Based on your description, it sounds like a voltage issue. It could be a weak battery or the alternator may be going bad. I'm basing that on the fan slowing, the lights flickering, and so on.

What I suggest to try first is to check the alternator. It is really simple to do. All you need is a voltmeter or multimeter. Here is a link showing how to do it:

https://www.2carpros.com/articles/how-to-check-a-car-alternator

Make sure the battery terminals are clean and tight as well.

Let me know what you find or if you have other questions.

Take care,

Joe

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Saturday, August 14th, 2021 AT 10:30 PM

LAHDUDE

MEMBER
3 POSTS

Joe, I have recently replaced the alternator and got the batter tested shortly before that and it tested to still be good.

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Sunday, August 15th, 2021 AT 12:13 PM

JACOBANDNICKOLAS

MECHANIC
109,708 POSTS

Hi,

The only thing that leads me to believe it's electrical is when you say the lights flicker. The other things could be caused by a vacuum leak, low or high fuel pressure, and so on.

Have you checked to make sure the ground to the engine block from the battery is clean, tight, and free of corrosion?

If everything electrical is good, the flickering shouldn't happen. Commonly, that is a faulty voltage regulator, but that checked good. So, I would suggest checking grounds, connections at the battery, and so on.

If that all checks good, since it has a rough idle, checking for engine vacuum leaks is where I would start. It has a lot of miles on it, so it isn't uncommon for a hose to dry out, crack, or fall off.

Here is a link that shows how that is done:

https://www.2carpros.com/articles/how-to-use-an-engine-vacuum-gauge

Let me know what you find or if you have other questions.

Take care,

Joe

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Sunday, August 15th, 2021 AT 8:18 PM

LAHDUDE

MEMBER
3 POSTS

Is it possible I just got a bad alternator?

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Monday, August 16th, 2021 AT 10:25 AM

JACOBANDNICKOLAS

MECHANIC
109,708 POSTS

Hi:,

It is very possible. If the part is remanufactured, it isn't uncommon for them to fail. Did the lights flicker at all prior to replacing the alternator? I ask because the regulator is integral in the alt and when the new alternator came a new regulator.

It's a theory at this point. I have a test for you to try. You will need a voltmeter or multimeter. Follow this link and let me know the results:

https://www.2carpros.com/articles/how-to-check-a-car-alternator

Keep in mind, the alt may be charging, so focus on the voltage output to see if it varies.

Let me know what you find.

Joe