2004 6.0 Rough Idle and hard starts when cold

[smee907]

Hey guys, I recently picked up a 2004 GMC Savana Quigley 6.0 with about 183k miles. It has a couple issues that I was hoping other guys would have some good advice on.

Its hard to start cold and has to be kept running for a few minutes before it will idle on its own. After that, it has a low idle and can stall if you're steering in a parking lot and get off the throttle too suddenly. The last issue is that when you come to a stop like at a light or stop sign, the voltage will drop from around 14 to about 9 then take several seconds to climb back up to 14 again. There is no check engine light, and I can't pull any codes with my cheapo scanner. The service light comes on for a short time when it first gets started. It runs good aside from those things. It has plenty of power, and

 

I did some research and read up on other guys with the same and similar issues, but so far haven't found the cause of mine yet.

 

I cleaned the MAF with CRC MAF cleaner which was actually pretty clean. I noticed that 2 of 3 bolts were missing for holding it onto the air box, so I replaced them. Cleaning the MAF made no improvement.

 

I cleaned the throttle body which was also pretty clean. There was some buildup on the throttle body around the butterfly valve, so I cleaned everything off thoroughly with q-tips and CRC Throttle Body Cleaner. I had unhooked my battery before unhooking everything for the throttle body, and I thought everything was fixed after hooking it all back up because it started great and idled perfectly afterwards. I took it for a drive and the voltage stayed right at 14 and didn't drop with the stereo, headlights, front heater, and rear heater all running at the same time when I came to a stop.

 

I started it the next day before having to go somewhere and it had the same issue again. I figure the battery reset the problem until it was run and shut back off again. I unhooked the battery before having to leave about an hour later and it confirmed it. It started right up, and ran perfectly for my 35 mile drive without low idle or voltage drops.

 

My last initial thought was needing an intake manifold gasket, but I'm still not getting any codes, and I would imagine that it would still run bad even with the battery reset like that. I've heard of people having their ECM firmware updated and fixing their problems, but don't want to throw money at the dealer. I can have my intake manifold gasket done, but don't want to throw money at that either if its not the problem. I did have a guy recommend getting a coolant temp sensor because he had a similar issue on a Ford, and I think I remember reading that a couple guys with 6.0's had the same type of problem with theirs and fixed it with the coolant temp sensor as well.

 

I wanted to see if anybody on here had some good advice to give before I do anything else because the next things to throw at it won't be as cheap as the rest. Thanks, and sorry for the long post...

 

Tom

 

[Podor]

Is it throttle by wire or does it have an actual throttle cable?

[smee907]

It has throttle cables.

[Podor]

It sounds like the IAC motor is not controlling the idle. GM vehicles usually don't set a code for IAC issues. The Idle Air Control motor is a stepper motor that controls air flow through a port in the throttle that controls the idle. The port may be clogged, the motor may be defective or there could be a wiring issue. There are probably 2 torx screws holding it in on the side of the throttle body. There is a cone shaped plunger with a spring on the back side. The motor pulls the cone away from it's seat to raise the idle by creating a controlled vacuum leak. Make sure there are no deposits on the cone an make sure the port is clean. If it looks OK or doesn't fix the issue, most likely the motor itself is bad. Super easy to replace, just make sure to hold on to the screws.

[smee907]

Update: I just got a replacement IAC valve and installed it, so I'm going to monitor it for a few days.

[Shawn2021]

Is your car fixed?

[Shawn2021]

Mycar is fixed now , I had the same issue, I replaced with another used  throttle body , It's fixed now, the problem is Idle Air Control  ,this one is selonoid that controls idle,

if this selonid is not working properly , youu wont notice , because does not throw any engine code.

Edited September 5, 2021 by Shawn2021