Surging while cruising.

[TheGreatRambino]

Hey guys, I bought a 2010 (1500 4.7 160k) early last year and about 4-5 months ago noticed that, while I was cruising steadily and with no throttle change, the rpm would fluctuate about 50 rpms. Speed changes a little as well. I have no CEL, but I did get a p0104 code a couple weeks ago. I managed to clear that code by cleaning some dirt off of the rubber gasket that mates the plastic air box to the throttle body. Other than that the truck has had the standard 4.7 tick on cold starts but has no other issues or symptoms. Gets about 17mpg on the interstate. Thank for any help. I can take a video tomorrow if necessary.

 

[Elevated 2013]

Have yo done any modifications before this started to happen? It Sounds like a possible vacuum leak then. Especially if the air filter box being cleaned helped a little. With the engine idling spray carb cleaner around the engine gaskets and vacuum lines and see if the engine surges or tires to die.

 

[TheGreatRambino]

Thanks for the reply. I agree it seems like a vacuum leak to me as well. I tried that first with starting fluid but it wont hurt to try again. I didn't clean the air filter box. It was the gasket between the throttle body and the empty plastic box on top of the intake manifold. It had a fair amount of dirt in it. I replaced that gasket with a new mopar part on friday. I did clean the throttle body which made it idle a little higher but during all of this there's been no obvious change to the surging. Truck is stock.

 

[Elevated 2013]

Do you have an idle issues, like rough or surging there? I’m surprised cleaning the throttle body raised the idle speed as it should have lowered it and smoothed it out.

 

[TheGreatRambino]

No idle issues. Very smooth. It raised the idle maybe 50 rpm, could have been where the dirt came from to be honest as I found that later, could be that I didn't get it in there quite right and caused the p0104 code myself. It was a couple months between the cleaning and the code so I could have easily accumulated the dirt then. Who knows. Crazier things have happened.

 

[Elevated 2013]

When you say cruising, are you using cruise control or just keeping a steady speed? If you’re using cruise control it almost has to be a vacuum leak.

 

[TheGreatRambino]

Both actually, I hadn't considered that...

 

[Elevated 2013]

When was the last time you replaced your PCV valve?? Might want to try that and also spray the line from it to the throttle body. The only difference between cruise control and cruising is vacuum control.