PO171 and PO174, 2012 6.0l

[sdeeter19555]

I've searched, nothing hits my symptoms exactly.

 

Truck has 208k, fuel pump replaced as PM around 150k, exhaust bolts and gaskets around 190k, replaced O2 sensors and spark plugs around then also.  I bought this truck new...

 

I had my alcohol in fuel (AIF) go wonky on a trip pulling the camper a few years ago, started reading 20% plus AIF even running non-alcohol gas.  Running e85 was not possible, it would almost not start.  So I assumed the O2 sensors were going bad since they are used to interpret AIF and replaced all four with GM units.  No change.  A cold start on gasoline was typically bulky and stumbling for about 15 seconds and lopey.  I also had a low rpm bucking or surging, but the truck has always had a touch of that since new but this is definitely worse but tolerable. 

 

Fast forward to last week, I had the inspection garage run the codes (which I already knew were 171/174 (but no CEL, ever).  He check some parameters quickly and the fuel trim and alcohol were all out of whack.  He reset that and WOW, nice normal cold starts, truck ran and drove like new, but it (174) would trigger a CEL (171 pops a soft code shortly after reset, 174 may or may not pop a CEL a while later).  No other codes are present.  

 

A few days later running nothing but e10 93 octane gasoline, and my AIF is back up at 30% and won't come down (it fluctuates slightly, but is around 30%).  My cold start stumble and low rpm surging are also back.  I took a trip on Wednesday to Annapolis and netted 16 and 17mpg, the truck runs good overall.  Still have 171 and 174 as soft codes.

 

Anything particular to look at?

[sdeeter19555]

Ok, they smoked it...no vacuum leaks. 

 

They ran some tests and watched the results on their reader/scanner.  Everything lead them back to a MAF sensor.  There was NO CODE present for the MAF that ever popped.

 

They replaced the MAF and the codes have stayed away for 75 miles now.  Before the PO171 code would manifest within a minute and the PO174 would come on after a few miles.

 

So for now, it looks like the problem with the codes may be solved.  Hopefully that also solves the AIF problem...time will tell.

[davester]

2 hours ago, sdeeter19555 said:

Ok, they smoked it...no vacuum leaks. 

 

They ran some tests and watched the results on their reader/scanner.  Everything lead them back to a MAF sensor.  There was NO CODE present for the MAF that ever popped.

 

They replaced the MAF and the codes have stayed away for 75 miles now.  Before the PO171 code would manifest within a minute and the PO174 would come on after a few miles.

 

So for now, it looks like the problem with the codes may be solved.  Hopefully that also solves the AIF problem...time will tell.

If you took it to a shop, that shouldn't have surprised them.  The FSM for my '12 3500, it says that code can happen due to the MAF, w/o setting a MAF error.  I'd copy/paste the exact text, but the stupid mitchell website doesn't allow copying text.