Rough idle cold p0171 p0174 p0300

TYPE GENDY · January 15, 2014

Hi,

I'm having trouble with my truck idling very rough in the mornings. The colder it is, the rougher it runs. Once i get driving and it warms up the idle smooths out, but not based solely on the water temps. It does run and drive fine the whole time however. I pulled the three codes for lean conditions (banks 1/2) and random misfire. It was doing this sporadically last winter as well, but not as bad as this year, and not during the warmer weather. I checked around visually for vacuum leaks but couldn't find anything or hear any hissing noises. I have a code reader and i checked the maf sensor reading as i thought that was the issue, but it was spot on with the engine off in accordance with local weather station data....

Any suggestions please?

vkjohnson · January 15, 2014

I had a p0300 last month and it ended up being bad spark plug in cylinder #1. Bad plug at 65,000 miles. How old are your plugs? Could be a possibility at least.

I'll add that I had the same rough idle as well, but didn't have it long enough to see how temps changed it. Your other 2 codes probably show the real cause though.

TYPE GENDY · January 18, 2014

Hmm, i didnt think spark plugs, i should check them. Thanks! Mine are about ~5yrs / 50k miles.

I pulled the maf sensor and looked at it, and it wasn't dirty at all, and looked fully intact. I'm sure that doesnt tell the full story, but based on the looks and that it is reading correctly with the engine off, idk...

I found a really good write up for this condition: http://troubleshootmyvehicle.com/gm/4.3L-5.0L-5.7L/how-to-test-a-lean-condition-1

My LTFTRM1/2 values were > 10% and my o2 sensors were oscilating between .2 and .85 (iirc). I couldnt spray around with carb cleaner at this point because the truck was all warmed up, will try that next too

Edit: Also a new fuel pump was installed last March so that should rule out fuel pressure issues

bossier62guy · January 18, 2014

Check your fuel pressure....my money is on low fuel pressure or a vacuum leak. What brand of fuel pump did you install last time around?

I just helped the neighbor change a fuel pump on a Tahoe beause of the same codes....the codes had been present for a long time before the pump finally just quit.

Scottorado · January 24, 2014

I just had a P300, flashing CEL, and rough idle out of the blue. New plugs and wires cleared it right up.

It is a 2001 5.3 with only 75K on it, but after 13 years, the wires just got too old I guess.

TYPE GENDY · February 18, 2014

I just had a P300, flashing CEL, and rough idle out of the blue. New plugs and wires cleared it right up.

It is a 2001 5.3 with only 75K on it, but after 13 years, the wires just got too old I guess.

Scottorado - rough idle, how did the truck drive otherwise? What brand wires did u go with?

I'm still chasing this issue, and the most perplexing part is that it will idle so rough and even stall, yet it drives just fine...??

Scottorado · February 18, 2014

It went from from running fine one day to shaking and sputtering the next time I started it a week later. You could drive it, but there was a lot of heistation below 1500 RPM.

Your truck looks about the same age, so if you have stock wires they are probably shot. I used Bosch wires from Autozone ($62), and AC Delco 41-962 plugs ($7). I would have got the AC wires, but they were special order and I wanted them then.

The only comment on the Bosch wires is I get some ignition noise on AM radio with weak stations.

Good luck.

TYPE GENDY · February 21, 2014

Cool, thanks! I put new ac delco plugs / wires in tonight. I'll see if that does the trick, i'm really hoping. I wont know for a few days though, becuase its gonna be 30/40s here all weekend. Only does it when its like less than 20 degrees.

efritz · February 21, 2014

Very likely the intake manifold gaskets. There is tons of info out there about this as it is a very common issue. It only happens when its cold out, and you get bank 1 and 2 lean codes and sometimes a random misfire code. There will be no other obvious systems besides poor idle. Even if you think its gone, the next time the temp drops, it will be back, trust me I've changed out a dozen or more of them with 100% success. The only way to really tell is to spray some starting fluid around the top of the intake cathedrals when its idling poorly, it will usually change RPM. The reason it drives fine otherwise is the amount of air taken in vs the amount leaked in is much smaller when under load. You seem like you are competent mechanically, so this will be well within your range of abilities to replace (should also do knock sensors while you're in there). Takes about 3 hours and a lot of paper towels and carb cleaner to clean before assembly. I could give step by step if interested.

TYPE GENDY · February 21, 2014

Cool, thanks efritz. I did see that listed often as a usual suspect during my research, i'll keep it in mind. Although i wouldn't want to do the job until it warms up, at which point my issue will probably be hibernating until next year, compromising my ambition lol. Thanks!

TYPE GENDY · February 25, 2014

I put new ac delco plugs / wires in tonight. I'll see if that does the trick, i'm really hoping. I wont know for a few days though, becuase its gonna be 30/40s here all weekend. Only does it when its like less than 20 degrees.

Unfortunately that didnt make a difference

Snow White · February 25, 2014

Mine was the throttle position sensor, but it acts the exact same way if you have intake manifold gaskets going or a fuel pump dying... the fuel pump is the easiest to eliminate by checking pressure.