1,200 Mile Road Trip - P0171 / P0174 / P0300 Engine Codes

[mals]

So I just got home from a 1,200 mile road trip towing our travel trailer.

About 100 miles into the trip, the check engine light comes on. I pull over at the next exit and pull the codes. I have a P0171 - System Too Lean (Bank 1) and P0174 - System Too Lean (Bank 2). I check to make sure that all of the intake looks good and that my AirAid MIT didn't come loose at either end. I clear the codes and drive another 1000 miles without any lights or codes.

This morning on our last leg of the trip, again about 100 miles from home the check engine light comes on again. I assume it's going to be the same codes, but I pull over at the next exit and pull the codes. This time it is a P0300 - Random/Multiple Cylinder Misfire Detected. I hadn't noticed any change in the engine before the CEL came on. Again, I cleared it and drove the rest of the way home and the P0300 did not return.

Are these codes something to be concerned about at this point? Are they indicators of something that I can take care of now before they develop into something larger?

Any pointers, tips, or experiences would be greatly appreciated.

-mals

 

[cmc76]

without knowing the milage on your Yukon. first guess is intake gaskets. wont kill anything, mine are bad and the parts are sitting in my kitchen still to do

 

[mals]

without knowing the milage on your Yukon. first guess is intake gaskets. wont kill anything, mine are bad and the parts are sitting in my kitchen still to do

Ah, sorry. It has 162k on it now, but I have only owned it for the last 7,000 miles. I have no service history before that, but it seems generally well taken care of for a 12 year old vehicle.

 

[cmc76]

Id say given the milage its a strong possibility. You can googlehow to test for gasket leaks. It doesnt appear a horrendous job. Ive just been lazy to do mine.

 

[BlueCollarTahoe]

when i got those same codes around 93k, maybe 96k, i ran through the whole list and had a shop do comepltete diagnostics. turned out to be the 02 sensors going out. changed those and good as new.

 

[mals]

So over the past few months these three codes have popped up occasionally. Generally after 1-2 hours of highway driving. Sometimes it's just one of the lean condition codes, other times just the random misfire. Driving from MA to NJ after Christmas about an hour in I got the P0300 code, pulled over to check the code and cleared it. Drove another 4 hours and no codes came up for the rest of the ride.

Looking at common symptoms that come up for all the codes, as @BlueCollarTahoe found, a faulty O2 sensor can trip them. Is it possible to have a bad/failing 02 sensor without ever throwing an 02 error code? I can get all four Bosch 02 sensors from Rock Auto for $150. Mine appear to be original, so I guess there is no harm in swapping them out after 165k. Is there any reason to spend the extra for AC Delco sensors on our vehicles?

If not, I guess my next step would be to pull the intake and replace the gasket (and knock sensors while I am in there) since that seems to be the next most likely cause given the age and mileage. I just assume that a bad intake gasket would throw the codes more consistently then I am seeing.

 

[swathdiver]

It's worth investigating but I think that the upstream O2 sensors are for the engine and the lower ones measure the health of the cats. I'm suggesting that you may only need to replace two O2 sensors and not 4.

 

[mals]

It's worth investigating but I think that the upstream O2 sensors are for the engine and the lower ones measure the health of the cats. I'm suggesting that you may only need to replace two O2 sensors and not 4.

Thanks for the info, but the downstream ones are $10 less each then the upstream ones. If I'm going to be under there fighting them out of the exhaust I'd rather just lose one day to the project than a second one down the line. Also, I think the conventional wisdom with 02 sensors is if one fails early replace just the one. If you are replacing them due to age/mileage you replace them both.

 

[iamdub]

Have the spark plugs ever been replaced?

Maybe they're worn and you're intermittently getting a weak spark/misfire under load. This would normally cause a rich condition in addition to throwing the misfire code. The PCM may lean out the mixture to compensate but since the misfire condition is temporary, then the exhaust is now actually just lean, so you get the lean codes.

Or, it's been running with a weak ignition for so long that the O2 sensors are sooted up and they're not accurately reading the exhaust. They're only "seeing" a small amount of exhaust and therefore are reporting the exhaust as lean.

Or, the ignition system is fine and your fuel supply is weak. It may be sufficient for normal driving duties, but inadequate under load when more fuel is needed. Since it seems to happen after so much drive time, maybe the pump is getting hot and weaker, leading to lower volume and/or pressure. Insufficient fuel supply can and usually does cause the symptoms you're experiencing.

Just check a few simple things (ignition components condition, fuel pressure, etc,) before throwing a bunch of parts at it.

 

[iamdub]

It's worth investigating but I think that the upstream O2 sensors are for the engine and the lower ones measure the health of the cats. I'm suggesting that you may only need to replace two O2 sensors and not 4.

This is true. The "downstream" or "rear" O2s only monitor the cats and have no effect on fuel trims. The "upstream" or "front" O2s monitor the air to fuel ratio of the exhaust.