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Old 06-21-2020, 01:24 AM   #1
Join Date: Jul 2019
Location: Asheville, NC
Posts: 42
Year: 1988
Coachwork: GMC
Idle Surge Part II- The Saga Continues

Hey all, had a bit of an idle surge on my bus since I got it. Surges about 50-100 RPM, probably a little less but itís very audible. Was going to just adjust the buffer screw and reset the idle but I just noticed the ammeter is the surging in synchronization with the engine.

That points to it possibly being an electrical issue, so now Iím thinking mechanical adjustments might not help with this issue.

Though, I probably still will perform the aforementioned repairs. Because it is possible itís just taking more power because of something mechanical.

Anyone dealt with this before? Was it an electrical issue or mechanical?

Gonna look into it tomorrow. What components should I scrutinize first?

Thinking I might adjust the idle first and see where that gets me.

Short video attached. Hopefully youíll be able to hear it.

Many thanks for any insights!

silvermachine is offline   Reply With Quote
Old 06-22-2020, 11:18 AM   #2
Bus Geek
Join Date: May 2014
Location: West Ohio
Posts: 3,334
Year: 1984
Coachwork: Bluebird
Chassis: International 1753
Engine: 6.9 International
Rated Cap: 65
First off, you need to put your engine in your profile or at least in the post, so other members know what you're working with. I get that your bus is an 88 gmc, but there were several engine options available in gmc buses in 1988. Help us help you.

Second off, that gauge isn't an ammeter but a voltmeter. Why it's fluctuating with the idle is anyone's guess. Stock gauges like that aren't really noted for their accuracy, so take a handheld voltmeter to the batteries and see if the fluctuation is actually there. If the voltage is actually fluctuating at idle, it could be a bad alternator or voltage regulator, or it could be an electric load kicking on and off causing the alternator and engine rpm to fluctuate.
My build: The Silver Bullet
Booyah45828 is offline   Reply With Quote
Old 06-23-2020, 02:56 AM   #3
Bus Geek
Join Date: Sep 2015
Posts: 3,853
Year: 2002
Coachwork: Thomas Built Bus
Chassis: Freightliner FS65
Engine: Caterpillar 3126E Diesel
Rated Cap: 71 Passenger- 30,000 lbs.
You also might like to tell us what speed the engine is idling at in the video.

Are your batteries fully charged?

I could see the voltage output from the alternator being a little low if the engine speed is low, so the voltage would follow the engine speed to some degree. Does the voltage keep steady at a steady engine speed faster then idle?

Of course, this does not explain the surging.
Native is offline   Reply With Quote

bluebird, bus, gmc, idle, surge

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