So lately i've noticed my Voltage Gauge varying between 12 - 15 volts (approx.) Since I've owned my truck it has always stayed right around 14 volts. Thought this may be the cause of some of the other issues (see below) I'm having with my truck. I checked voltage with a fluke meter and saw as low as 10 volts and as high as 14.6 volts at idle and revved to 1500rpms. My truck seems to run great when voltage is in the 13.5 to 14.5 range, but when it drops below 13.5 is when it starts acting up.

I brought it to the shop and was told that my truck has a Voltage Control System and it is normal for voltage to vary from 10 volts to 16 volts. I could see it vary between 12 - 14 volts but not as low as 10 volts. I think either the Alternator or Voltage Control System has a problem. All of the problems lately started after my battery was disconnect while at a shop getting an alignment.

Issues with Truck:
Intermittent Rough Idle
Bad Fuel Mileage
Intermittent Hesitation at WOT Launch
Intermittent delay for truck to respond to WOT while rolling 20 - 25mph (Go Pedal on floor, approx 4 secs, then truck takes off like a whipped mule.)
No Codes
No Knock Retard
Everything Looks normal when data logging with HPT
Cleaned Throttle Body and MAF which didn't help
New NGK TR-5 Plugs

I sure hope they can find the problem because it is driving me crazy. If it isn't found soon, I'm gonna trade the b!tch in and get a '09 2wd Silverado CrewCab with 6.2L if I can find one or will settle for the 6.0L

 

10 volts is waaayyyyyy too low. It really shouldn't ever drop below about 12.5v or so, which is about where the battery should sit with the engine off. I would have your alternator and battery tested.

 

Had the alternator and battery checked today and was told all is good (never saw it go lower than 12.6v).

After spending a while with a scanner hooked up, the mechanic said he saw a random misfire on 7 and 8. He said the random misfire is intermittent. I guess that is why the service engine light isn't coming on? Being it is on 7 and 8 I'm thinking it is a bad intake gasket. Going to bring it back to the shop next week so they find the cause of the misfire.

 

I don't think it should read as low as 12.6v with the engine running. Was the guy who tested it the same one who said 10v was OK? You might have a bad alt/reg or some other issue.

 

Corey (blackgmc) was telling me pretty recently that the injector pulse tables are effected by voltage. And as the voltage drops the injectors have a tendency to cause it to run richer.

 

find another shop. voltage below battery voltage (12.6v) is not normal under any circumstances. that is on a properly working system.

 

I agree, so I called a friend of mine who is a mechanic at a GMC dealership. He said the voltage will vary from 12.6 - 14.6 due to the Voltage Control System (or whatever it is called). He said it is designed to back off the voltage when it isn't needed. Two things are supposed to prevent it from backing off the voltage and they are running with head lights on or in tow/haul mode. I ran with headlights on and in tow/haul mode today and still saw voltage drop down into the 12volt range. Since I've owned my truck the voltage has never varied this much.

I also noticed when the volts drop in the 12volt range is when it starts idle rough, idle also drops from around 650rpms to approx 500 - 550rpms. Also seems this is when the misfire on 7 and 8 occur. Mechanic seems to think the problem is either a bad intake gasket or the fuel injectors aren't spraying properly. I'm thinking the misfire is causing the low/rough idle which is making the alternator not charge properly. Then being the misfire is on 7 and 8, I'm thinking the intake gasket is bad in that area?

 

Hmmm...I got the same prob after I installed efans. Make sure all ur connections are tight and check all the grounds. Next try and replace the fuseable link from alt to batt connection with a 4 gg and mega fuse. Search Big 3 wire mod. Mine idles rough when it dips too, but its all related to efans. If u have efans check all the modules and connections on that too.

 

Thanks for the info....my truck came stock with e-fans. I'm going to check out the big 3 wire mod.

 

mine is doing the exact same thing but it does it about once a week? i dont have efans, its an 04.... batter/alternator check is good, i have a high output alternator on my 01 that im going to try tomorrow to see if thats the problem, since alot of people keep on telling me its the alternator.... the reason the truck idles rough is cause of the low voltage which will make your injectors not to spray correctly......