cdk 4219 Posted March 9, 2019 Report Share Posted March 9, 2019 Getting abysmal fuel mileage in a 2012 Silverado 2500hd 6.0 flexfuel truck(8mpg) and am getting repeated 0171 and 0174 system too lean codes. Thought it may be from the exhaust manifold leaks, changed those and still same codes. Checked for vacuum leaks with brake cleaner, no change in idle. Truck doesn’t run rough, but seems to take a long time to find idle after throttle is depressed ( hit throttle to pull into garage, let off throttle, truck is still at same rpm) cleaned MAF no change. Evap purge valve stuck ? Bad MAF? Where to look? Quote Link to comment Share on other sites More sharing options...
STEVE-O Posted March 9, 2019 Report Share Posted March 9, 2019 Sounds like there is a vac leak somewhere. Smoke test the system to be sure Quote Link to comment Share on other sites More sharing options...
zeitgeist57 Posted March 9, 2019 Report Share Posted March 9, 2019 Does it have an aftermarket intake? Is it running any sort of tune? Quote Link to comment Share on other sites More sharing options...
Cordell Posted March 9, 2019 Report Share Posted March 9, 2019 Have you been running E85? It sounds like it needs it’s calculated ethanol percentage reset. Quote Link to comment Share on other sites More sharing options...
Geeesammy Posted March 9, 2019 Report Share Posted March 9, 2019 Cordell knows more than me, but you can't always be reliant on idle change to show a vac leak. Just today I had a Tahoe that had a vacuum leak that would only present itself in scan tool data and idle would not change. Just food for thought. Quote Link to comment Share on other sites More sharing options...
craig71188 Posted March 9, 2019 Report Share Posted March 9, 2019 A bad MAF is a possibility - need a scan tool to look at fuel trim, unplug MAF and see if it changes I understand there is also a "muffler" on the air snorkel (intake piping AFTER the MAF), these can come loose/crack and allow unmetered air in (vacuum leak) Quote Link to comment Share on other sites More sharing options...
cdk 4219 Posted March 9, 2019 Author Report Share Posted March 9, 2019 Yes I thought vacuum leak, time to get a smoke machine, as the brake cleaner may not be the best. It’s all stock, and I just got the truck 1200 miles or about 150 gallons ago😳. I have run e85, but maybe the prior owner did. Quote Link to comment Share on other sites More sharing options...
Cordell Posted March 9, 2019 Report Share Posted March 9, 2019 (edited) Yes I thought vacuum leak, time to get a smoke machine, as the brake cleaner may not be the best. It’s all stock, and I just got the truck 1200 miles or about 150 gallons ago��. I have run e85, but maybe the prior owner did. So does it have E85 in it now? If it does I’d bet money that this is your issue. E85 Flex Fuel Description E85 compatible vehicles no longer use an alcohol sensor to determine and adjust for the alcohol content of the fuel in the tank. Instead, the vehicle calculates the alcohol content of the fuel through measured adjustments. The ethanol calculation occurs with the engine running after a refueling event has been detected via a measured change in the fuel level sender output. The virtual flex fuel sensor (V-FFS) algorithm temporarily closes the canister purge valve for a few seconds and monitors information from the closed loop fuel trim system to calculate the ethanol content. This logic executes several times until the ethanol calculation is deemed to be stable. This may take several minutes under low fuel flow conditions such as idle, or a shorter time during higher fuel flow, off-idle conditions. Air-fuel ratios and the corresponding ethanol percentage are updated following each purge-off sequence. The fuel alcohol content percentage value can be read on a scan tool. When an E85 compatible vehicle is built, an ECM or PCM replaced, or if the learned alcohol content has been reset with a scan tool the fuel system will need to contain ASTM gasoline with 10 percent or less ethanol content. A minimum of 11 liters (3 gallons) must be put in the tank in order for the vehicle to recognize a re-fueling event. It is not necessary to turn the ignition OFF in order to have the re-fueling event recognized, however local safety regulations should be followed. After the re-fueling event, the system registers the amount of fuel that was added, relative to the amount that was in the tank. Reading fuel trim and O2 sensor activity, the system determines if the fuel added was either ASTM Gasoline or ASTM E85. Based on that determination, the system adjusts to the expected alcohol mix in the fuel tank, and then the fuel trim and O2 sensor activity fine tunes the adjustments. The system must remain in closed loop in order for this adjustment to occur. Numerous short trips after switching from gasoline to E85, or E85 to gasoline, can result in driveability symptoms due to the inability of the system to adjust for fuel composition by not attaining closed loop So basically run the truck empty, fill it with regualr gasoline, then drive it for an extended period of time without shutting it off. Before you possibly waste a bunch of time let me hook my tech 2 to the truck and check the flex fuel calculation. If that’s your problem you’ll chase your tail and never fix a godamn thing. It’ll take 5 minutes and if that’s not it you can go right on down this diagnosing road you’re on. I’m not working today, but I’m at Dan Tobin 5 days a week. Edited March 9, 2019 by Cordell Quote Link to comment Share on other sites More sharing options...
xlr8tn Posted March 9, 2019 Report Share Posted March 9, 2019 Might want to check fuel pressure while you're at it. GM=fuel pump at some point Quote Link to comment Share on other sites More sharing options...
cdk 4219 Posted March 9, 2019 Author Report Share Posted March 9, 2019 So does it have E85 in it now? If it does I’d bet money that this is your issue. So basically run the truck empty, fill it with regualr gasoline, then drive it for an extended period of time without shutting it off. Before you possibly waste a bunch of time let me hook my tech 2 to the truck and check the flex fuel calculation. If that’s your problem you’ll chase your tail and never fix a godamn thing. It’ll take 5 minutes and if that’s not it you can go right on down this diagnosing road you’re on. I’m not working today, but I’m at Dan Tobin 5 days a week. No sorry it hasn’t had e85 in 1200 miles since I’ve owned it. I only put 87 in it and drive it around most of the time empty. Quote Link to comment Share on other sites More sharing options...
aussiek2000 Posted March 11, 2019 Report Share Posted March 11, 2019 #1 cause on those truck is people forgetting to hook the PCV pipe back up from the passenger valve cover to the intake tube. Have to lift up on engine cover to see it Quote Link to comment Share on other sites More sharing options...
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