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recently pick up a 2004 tahoe z71 as a cheap family truck and have been battling very bad mpg issues around 7 city and 11 highway. scanned the truck and found p0171 and p0174 so I checked my LTFT's and they were crazy high. at idle they are less than 5% but as soon as I give it gas it shoots up to around 15% and around 20% after 1500 rpm. the idles and drives fine with no reason to make me think it has any spark issues. I tried cleaning the MAF no change and I also unplugged it and the car started to stumble and ran worse than before. I checked for vacuum leaks around the intake manifold and didn't fine anything. when warm the car idles fine at around 550 rpm with no surging
this is the trims at idle and warmed up.
what's the easiest way to check for a clogged injector? to check the pump I should attach a pressure gage to the fuel rail?
This truck isn’t lifted is it? I had a 05 Tahoe that was lifted and got even worse gas mileage than my HCI Z28.
Yes pressure test at the fuel rail.
For the injectors you could use the old school trick with a wooden rod. Put it up to your ear while the truck is running and listen for clicking or injector cycling. It’s fairly easy to remove injectors to clean them up. But you would have to hook them up to a power source and run some carb cleaner thru them.
Can you check the voltage on your O2 sensors with that scanner? Should be switching back and forth, usually if it's reading lean like that they will be making less than 100 mV, sometimes all the way down to 20 mV.
You could have a vacuum leak or exhaust leak. Have you tested the EGR valve or any other emissions equipment (like the dumbass air pump)?
It might be your MAF, but unlikely if this is the only issue you are having. I would really be looking for air leaks into your exhaust.
Also, if it were an injector, it wouldn't likely show up on both banks like that. A bad "air pump" or EGR will though. So will ANY unmetered air past the MAF, intake vacuum leaks or exhaust leaks.
The more I think about it, the EGR or air pump could easily cause this condition, as well as your bad mileage. The air pumps GM put on some of these engines are used during warm-up typically, as I understand them, (since I don't have one on mine) to richen up your fuel by fooling the O2 to thinking it's lean. The pump is supposed to shut off under normal operation, but perhaps it's still on and causing the increase in airflow, which will cause the PCM to over-compensate fuel, just like a huge exhaust leak. The O2 sensor might as well be hanging out of the pipe and trying to read what comes out the hole. That would obviously cause your mileage to suck AND give you a false lean condition.
If you really were running around that lean, then your mileage would actually be better, right? One would think so at least.
The EGR, if it is opening and staying open could cause a similar issue. I'd try locating the air pump and disabling it first, if you have one. It will have plumbing going to the manifold. That and I would also remove the EGR and throw a plate on temporarily (or permanently lol) and see if your lean condition goes away. Obviously the PCM is causing this by dumping more fuel than needed into the engine.
Pull a spark plug and see if it's darker than midnight with a lot of carbon. That will tell the real story. I suspect that it would be since you are probably actually running extremely rich, which is why your mileage sucks.
1- the truck isn't lifted, it's stock except for an ebay intake.
2- I don't think I could check the voltage with the s anner but I do have a multimeter.
3-my muffler is rusted out right now didn't think it would be a problem. and I did notice some exhaust header bolts missing but i don't think it's leaking.
4- I don't think it's a vacuum leak cause the car idles smooth and fine plus my idle is steady at around 550 it doesn't bounce around
From those plugs it looks like you are actually running very lean. The white part under the electrode, under normal conditions would be light brown... That's optimal. Too white, like that, means lean condition. Too black indicates too rich, and the oil buildup on the edge is normal considering PCV dumps oil right in the intake.
So, fuel system has to be checked. If you have a pressure gauge, hook it to the rail and check for 58 PSI. Also, the MAF might be failing. It's hard to diagnose over the internet and without the proper scanner. Obviously your engine is not getting enough fuel.
I have the live data with the MAF info but my scanner is a bitch. I can't just use the USB to connect to the computer I have to get a memory card adapter and use that
That seems low. I usually use g/s, as in grams/second. I'll have to convert it on my hp tuners scanner to know for sure, but again, that's seems super low.
my scanner said the MAF at idle is 0.01 lb/s and 0.05 lb/s at 2500 rpm
Your MAF should be reading something like 30-50 lb/hr at idle. I'm thinking the MAF is shot, or signal wire is toast. Those numbers are rough estimates, because they vary based on a number of things, but typical airflow at idle for a V8 is about 5 g/s, which is 39.6 lb/hr (0.66 lb/sec). Could be more or less, hence the range, which again, is just estimated. My car is typically around 12 g/s at idle, so much more than 50 lb/hr.
You're going to want to fix that before driving this thing again. You're lucky you're not getting a lot of detonation or worse, pre-ignition. Could also be a signal wire too, like I said, because that is next to nothing airflow wise. Even a bad MAF would probably read more than that. Your plugs show the lean condition is in fact real, so it's good you caught this.
Ah, I see I forgot to divide by 60 again. My mistake. It was late lol. So maybe that's good too then... hmmmm. MAF I guess was too easy of a solution... you had to bring a real challenge I see. This is a strange one, if you're MAF is good.
I still think it could be the MAF though, since that's where the computer is getting its fuel calcs from. Then again you're not getting the 101 miscompare with VE.
What can your scan tool read? I would say at a minimum, MAF and your trims, but if you can also log Dynamic, MAP, O2 voltage, and EGR valve open/closed... that would be ideal. I know you're probably a bit limited though as to what you can scan. If you can log the airflow in grams/sec that would be easier for most of us to see. Or at least pounds/hr. The lbs/s is just too small of a number to see if there is a real problem, and you lose a lot of depth to the number, if .01 (.011 actual) lbs/s=5 g/s... etc.
Unless this has had a tune done on it at some point, I would still say this has got to be a mechanical issue, as in a sensor such as MAF or even MAP is no longer reading accurately. And other possibilities still include low fuel pressure, and bad EGR valve, which is what I would be checking first, since that is a pretty simple fix.
If the EGR is remaining open, exhaust gas will continue to recirculate into the intake, which is lean to begin with, and basically aggravate the lean condition while causing very bad mileage. It fits the symptoms, similar to a vacuum leak or exhaust leak. We know that your lean condition is real, so this is a good place to start troubleshooting.
If the MAF was shot, the thing will run like hammered dogshit and fall on it's face every time you stab the throttle, complete with popping, shuddering, stalling and hesitation etc... Not saying it's still not a possible factor, but if this is a recent issue, I'd be checking that EGR valve, and testing fuel pressure before I moved on to other sensors or tuning the MAF. A tune isn't going to help a bad EGR valve, unless you're deleting it from the tune after physically removing it. That eliminates the issue every time lol.
03-11-2018 | 07:27 AM
lm7 running lean only under load and bad mpg 
