SMaster

Just wanted to share a little because I’ve been given so much on this forum.

Tale of P0171 & P0174

Wife came home from a trip in the Vette and said, “check engine light is on.” I asked her what she was doing when it came on and she said just cruising down the highway. I go out and check the codes on the DIC and found the two codes above. I cleared them and came back in the house and logged onto the CF and started searching.

I hooked up HPTuner and went for a ride. The LTFT’s were pegged at 25! Soon, the CEL came back on. I reset the fuel trims and the CEL and soon it was back to 25 and setting the codes again.

Just happens a week before I went for about a 30 minute cruise while scanning and everything was running perfect so I had a recent scan to compare to. Fuel trims were –3 to +7 or so.

First, checked for vacuum leaks and found nothing.

Second, took the K&N off and checked for leaks and cleaned the MAF. Checked the couplings, etc. Found nothing wrong.

Third, checked the fuel pressure. 55# exactly. No problem there.

Was getting ready to put it on the lift and check the O2 sensors when she walks in and says, “hey, I bought gas at some little station in the middle of nowhere, could that cause a problem?” It was a Citgo station in Albion IL. I asked her what kind it was and she said it was the “good stuff.” She said it did say something about a blend.

So…, I hook up a line to the fuel rail, jumpered the relay and pump some out. You couldn’t even smell gasoline in this stuff! It was almost totally clear! After pumping about 14 gallons out I went and filled it up with Shell 93, hooked up the scanner and went for a ride.

Everything was back to normal.

So, the weird gas caused the P0171 and P0174 and I just wanted to share that. I know the problem would have fixed itself when I ran the stuff out but some people could have spent a small fortune at the dealer or a repair shop before figuring out it was the gas.

Milan

I bet you anything that after half a tank of gas the CEL comes back on.

I had this problem all the time after doing an intake and cat back. Finally took it to get it tuned after doing my LT's and it was starving for fuel in the mid range.

After the tune I have not seen this problem reoccur using all types of gas.

The only real fix is to get a tune, I promise it will just come back if you clear it or think a tank of gas caused the problem.

I used to think it was gas too, then I realized I was being extremely naive.

SMaster

I had the car hooked to my scanner/laptop and watched the LTFT go from being pegged at 25 back to normal from just changing the gas. I'm not naive, nor inexperienced in building, or tuning fuel injected engines.

Thanks for your input though.

BlackZ06

From your description sounds like it might have been a tank of E85 .... that would explain the extremely lean condition .... and the lack of gasoline "smell". Good thing you cleared it out ..... our PCM can't possibly inject enough to prevent a lean condition .... I believe you need about a 40 percent larger charge for E85 versus gasoline.

BlackZ06

Here's how to test to see if there is alcohol in the fuel .....

I'll use a liter as a size ... you can use any volume ..... the point of the test is that alcohol dissolves in water ..... which will separate out the alcohol into the water.

Use a clear (glass) container.

Put 900 ml of "fuel" into container.
Put 100 ml of water into container.

Cap container and shake for 10 to 15 seconds ... uncap to relieve pressure .... cap and shake again for 10 to 15 seconds.

Allow the mix to sit for about 5 minutes.

At the bottom of the container should be 100 ml of water ..... if there is now more "water" .... say 150 ml ..... then there is alcohol in the fuel .... in this example the fuel is about 5 percent alcohol.

If it is E85 you may have to try a 50/50 mix of fuel and water to provide enough water to absorb the large amount of alcohol in the fuel.

SMaster

I don't have any of it left to test. I put it in my Blazer and Venture van. I'm sure it was some kind of blend. It was only slightly colored.

I just wanted to share the experience so someone else doesn't spend a bunch of money when there can be other reasons for setting those two codes besides all the ones I read here.

Milan

Cool.

When it comes back on after a few days of driving the joke will be on you.

BlackZ06

The only joke is you .....

Checking for contaminated fuel is step 4 in the diagnostics for the P0171 and P0174.

SMaster said he hooked up his laptop with HPTuner on it (used by tuners .... duh ....) and monitored his Long Term Fuel Trim. He noticed that it was pegged at 25 percent and would quickly return to that when he would zero it out. Once SMaster put 93 octane in the car, his LTFT returned to its normal range. Since the "trigger" for the two DTC he was getting is a LTFT over 23 percent ... and he is back to a range of -3 to +7 ..... problem is SOLVED.

rustyguns

agbrownmc

A few years late on the discusion, but I just got these two codes P0171 and P0174.
I am in the military and coincedentally and unfortunately require to purchase gas on base from time to time. As a Marine we aren't allowed off base in our working uniform, so if you are low on gas, you have to do it on base. For one thing, for the life of me cant figure it out, the gas is more expensive, secondly, I track my fuel consumption and efficiency like someone with OCD, and gas on base is 75-100% of the time WORST on Fuel economy. That is why I try not to buy the damn stuff.
Back to the Codes:
I just put in a tank of the garbage gov't gas, that they charge gov't employs more money to buy than regular gas in town, and got the 2 DTC's mentioned above. I saw many opinions on the Forum about these DTC's, but since I read this post: http://forums.corvetteforum.com/c5-t...and-p0174.html
It sure does sound likely that this is the problem, sh*tty gas !
Now i just have to figure out how to dump it.

DIYdreamcar

Funny, mine just dropped those codes the other day right after I got done changing tranny and diff fluids. I was REALLY low on gas that day too. Hasn't come back after resetting and making sure that only Chevron gets in the tank, I wonder...

Thanks for making me think


-Ed

Does it ever make you wonder what happened to respecting your elders?? I would have thought that in the mostly uninhabited state of Idaho they would still teach children that lesson