Hemi idling rough

[Slowbro]

So first off, this is my first legitimate post on Ram Forum, I've been lurking a while but only just now have felt the need to post. Anyway, I have an 08 5.7 hemi 4x4 and as of today it's been acting really odd when I start it. Truck cranks fine and then the revs build to a little above what idle should be (so floating around and above 1k) and then fluctuate up and down. While this is happening the lights in the truck flicker gently. This is a new issue to the point that I drove the truck earlier in the day with no issue and came back to this new headache after leaving it sitting for a few hours. The only thing that has changed is the weather. It got cold. Like, 40 degrees F this morning down to around 10 degrees F with windchill tonight. I've had the truck since July so I haven't seen it experience truly cold weather until now. It's sitting at around 33k miles and is entirely stock. I've tried searching and have come up with everything from bad plugs, bad crankshaft position sensor to bad voltage regulators. I even found one guy who described the EXACT problems I'm having down to the cold weather, but the thread didn't yield any good solutions so I'm curious to see if I can get some more info on this!

 

[KGBIGCOUNTRY]

Plugs are every 30k on these trucks so thats a starting point. Are and codes showing up? Was it cranking over fine or did the truck start weak? The PCM can freak out alill on a weak batt.

 

[Slowbro]

Yeah I was thinking plugs would be a smart place to start since they're almost certainly the originals. The truck cranks fine which implies good voltage from the battery, but the fluttering headlights suggest otherwise. On top of that, no CEL or other dash codes.

 

[KGBIGCOUNTRY]

Would test the batt and also check to make sure the alt is charging it. For an 08 its not a ton of miles but those coulf be going south. To make sure there are no codes you can have it pulled to make 100% sure. Some codes will not throw the CEL but still store them.

 

[Slowbro]

I'll definitely put an obd reader on there to be sure it isn't something more serious. I'm really hoping it's just a low voltage issue though. It'll be interesting to see tomorrow how the truck performs when the weather creeps back into the ~45 degree F range.

 

[FlaglerMegacab4x4]

I would check the battery cold weather tends to finish off an already depleted battery I actually need to replace mine for this reason and like KG said check the alternator ensure its charging correctly and maintaining steady voltage

 

[Slowbro]

Well after testing it, the battery is in good condition but does need to be recharged. I guess the next move is to see why the battery got low to begin with. I'll go and put a multimeter on the terminals to see if the alternator is outputting properly.

 

[GotExhaust.com]

It could be something more serious, but I'll start with the simple stuff!

Low voltage in general can freak the ECM systems out, but the part of your post that stood out the most was the temp. drop.

1. The cold weather can obviously effect your battery voltage which opens up a Pandora's box.

2. If this is the first time it has gotten that cold, your cold start idle calibrations could just be off. The truck's engine management system stores information and learns what temp. air to expect upon startup. Temperature swings can be very evident at startup on late model V8s.

 

[Slowbro]

It's definitely the first time I've seen the truck in extremely cold weather, so it could be related to the start idle calibrations. It idled a little better today with only mild fluttering of the revs, but it's also back into the mid 30's temperature wise.

 

[KGBIGCOUNTRY]

Another thing to point out is could be a bad batch of gas. If its a station you always use its doubtful but can toss that into the ring.

 

[Slowbro]

I considered the bad gas theory, but the truck was filled over a week ago from a Hess station I frequent so it seems unlikely.

 

[KGBIGCOUNTRY]

10-4 just going threw the most likely issues.

 

[Slowbro]

Quick followup, I got a multimeter on my battery and came up with some interesting results. Truck off, battery is pushing around ~12.65v and with the truck on I'm getting around ~14.2v so the alternator doesn't seem to be the culprit. On top of that, the idling has evened out and I took the truck for a problem free test drive. I might have spoken too soon when I ruled out bad fuel!

 

[rjkfsm]

Quick followup, I got a multimeter on my battery and came up with some interesting results. Truck off, battery is pushing around ~12.65v and with the truck on I'm getting around ~14.2v so the alternator doesn't seem to be the culprit. On top of that, the idling has evened out and I took the truck for a problem free test drive. I might have spoken too soon when I ruled out bad fuel!

One thing I have learned from working in the fuel business is to never buy gas at a gas station where the tanker truck is filling up the storage tanks or even parked in the lot. The flow of new fuel stirs up a lot of dirt and water from the bottom of the tank (which never ever ever gets cleaned) and will end up in your gas tank.

RK

 

[Slowbro]

I believe it, the only reason I was skeptical is the truck is sitting at like half a tank so I was a little skeptical I could have bad fuel symptoms just now show up.

 

[hemihustlin]

Yes it is very important not to fuel when the tanks are being filled or just after they have been filled
Two other things ive heard/make sense so i belive: dont hold the trigger on the gas gun all the way. The less you are in a hurry the better. The faster you make it flow the less accurate the meter is. The slower you fuel the closer you will get to the number displayed.
Second, try to fuel in the morning or very late at night. I dont know about every where but here it says on the pump " volume corrected to 15 degrees celsius" if it has been 35 c since 8 in he morning by 5 at night the fuel in the tanks is most certainly above 15 c therefore skewing the volume readings.

Next, something i just thought of, we have winter gas and summer gas. The difference is the evap point, im not sure if that means there is more or less ethanol in winter gas but for sure it is a different formulation...
We are about to get it any day now