Low idle rpm and dipping volt meter

gmc
1500

#1

Went to start the truck on Saturday and had a dead battery. Not too surprised because I only drive it once a week or so. Charged it up and Sunday it fired right up. Then it started to lose rpm down to 400 then back up to normal but continued to dog up an down in that range. Noticed the volt meter dipping down and figured the alternator was going out so pulled it out and took it to get tested Monday. Kid came back out and said it was fine. I’m going to get it tested somewhere else today to confirm its good before putting back in but was wondering if it is good where should I look next?


#2

sounds like you’re jumping the gun. I would concern myself with fixing the idle problem first off. What year GMC 1500? What engine?


#3

2004 5.3 with about 85k on it.


#4

Your engine’s computer will adjust over time as certain conditions or parts degrade. It has ability to span a fairly wide range. Then, when your battery dies abruptly and the computer loses power, those adjustments are lost and it defaults back to the factory settings. Now it doesn’t run so good. Sometimes, it can re-learn and go back to where it ran decently but often, the change is too far off.

I’d be looking at a TB or IAC inspection/cleaning along with any normal maintenance items like spark plugs that may be coming due.


#5

Had the alternator checked at a different shop and of course it confirms that its still good. To get to the belt on the alt you’ve got to remove the tube from the air intake so at the store picked up MAF and throttle body cleaner. Figured while its all off might as well. Hadn’t thought about the computer going back to defaults though. Good point. I’ll have to give it a chance to recalibrate after I clean things up. Should have picked up an air filter while I was there.


#6

How long did you charge it?
It takes at least 8 hours to fill a dead lead-acid battery.
There are no shortcuts.
I suspect the battery isn’t fully charged.


#7

Had Sunday off so back to the truck. Problem was solved by cleaning the mass air flow sensor. I did jump the gun by going straight to the alternator but it was my first suspicion. I did change the air filter as well. There were mud dauber nests all over the bottom. Looks like I need to pay more attention to regular maintenance to avoid this happening again.