[Warrior102]

What everyone else said and check the air box for a rodent nest/clogged filter.

[ember1205]

Your car will add or subtract fuel based on a number of different sensors and readings. Here are a number of things to look at if your LTFT levels are heavily negative -while driving- (the engine needs to be under some load for this to be accurate):

- MAF Sensor: Dirty or broken sensor can ultimately result in too much fuel being added in the first place
- O2 Sensors: Bad sensors could be mis-reading the fuel levels in the exhaust gas as being higher than they really are
- Fuel in the oil: Any excess fuel that migrates into the engine oil will ultimately result in more fuel in the combustion chamber than expected

Some things to try:

- Check / change the oil: If you can smell gas on the oil dipstick, this is a pretty clear giveaway. If, after changing the oil, the issue goes away for a while, then you have to go looking for leaks elsewhere (likely the high pressure fuel pump)
- Clean the MAF Sensor and test it
- Replace the PCV valve

These are easy and comparatively cheap things to do that won't hurt the care in any way and may help troubleshoot the issue.

[Sunflower]

Originally Posted by ember1205

Your car will add or subtract fuel based on a number of different sensors and readings. Here are a number of things to look at if your LTFT levels are heavily negative -while driving- (the engine needs to be under some load for this to be accurate):

- MAF Sensor: Dirty or broken sensor can ultimately result in too much fuel being added in the first place
- O2 Sensors: Bad sensors could be mis-reading the fuel levels in the exhaust gas as being higher than they really are
- Fuel in the oil: Any excess fuel that migrates into the engine oil will ultimately result in more fuel in the combustion chamber than expected

Some things to try:

- Check / change the oil: If you can smell gas on the oil dipstick, this is a pretty clear giveaway. If, after changing the oil, the issue goes away for a while, then you have to go looking for leaks elsewhere (likely the high pressure fuel pump)
- Clean the MAF Sensor and test it
- Replace the PCV valve

These are easy and comparatively cheap things to do that won't hurt the care in any way and may help troubleshoot the issue.

I change oil every 3k mobil1 530. Have since new. I sent a sample to blackstone and they said it had no fuel in oil and was the cleanest wearing LT1 they’ve seen. HOWEVER, when I change the oil the LTFT goes down for a while. I guess pcv and maf are next, and the other first fix stuff. Really hoping it’s not fuel system stuff ��

[KingLT1]

Running pure fuel stabilizer probably fubard the O2 sensors. Try some fresh premium fuel and go drive it a bit and see if the fuel trims normalize. If not you might need new o2's or maf might need cleaned as previously mentioned. I doubt it's anything too serious.

[ember1205]

Originally Posted by Sunflower

I change oil every 3k mobil1 530. Have since new. I sent a sample to blackstone and they said it had no fuel in oil and was the cleanest wearing LT1 they’ve seen. HOWEVER, when I change the oil the LTFT goes down for a while. I guess pcv and maf are next, and the other first fix stuff. Really hoping it’s not fuel system stuff ��

What's your definition of "an almost empty tank"? When the needle reads "E", you commonly still have 1-2 gallons of usable fuel left in the tank (with a bit of additional fuel that the pump won't pull).

How much stabilizer did actually put in the tank and what was it that you put in there?

How are you measuring the LTFT levels? The car needs to be being driven with a light load on the engine. Basically, put it out on the roads and set the cruise control and drive it for 20 minutes or so. The readings at idle are not going to be as reliable as ones while it's being driven.

Why did you start looking into this? Has the car thrown any codes? If not, does it show codes are pending? If there aren't any codes for it, then you either don't actually have a problem -or- you're seeing things that aren't bad enough (yet) to cause a code.

My truck threw a Check Engine light last fall. I had driven it, almost all highway, for about 2.5 hours. Parked. About an hour or so later, I started it back up and drove it about 2 hours on the highway back toward home. Parked the truck again. When I started it back up, the CEL was on and it surprised me because the truck seemed to be running completely normal.

A day or two later, the CEL went off. Then a couple of months later, it came back on and I started to look into things. I bought a cheap OBD-II tool and use the Track Addict app on my Android phone to see the codes, clear them, and continue troubleshooting. After a variety of test drives, tinkering, and some research that showed that the high-pressure fuel pump in my specific truck's engine has a known design flaw allowing it to leak fuel into the engine, I made the decision that the pump needing replacing and ordered a new one online. My mechanic was hesitant to install it for me because he likes to be the one that does the troubleshooting to make a recommendation like this, but I showed him the Track Addict recordings of a couple of test drives and relayed to him all that I found. He did a quick bit of looking himself to determine that my truck did have the problematic pump and said he'd do the install for me since everything I gave him was pretty reasonable data to work with.

It's been a few months now and there's a marked difference in the LTFT readings, better fuel economy, and no codes. For reference, my LTFT readings would drop to around -25 idling and the high (negative) teens while driving. Those numbers now are about half the value they were before.

Cleaning and testing the MAF are generally easy to do. Track Addict can help you identify MAF issues as well by watching the readings it's picking up and comparing that to what you can interpret they should be based on air pressure and such.

Replacing a PCV valve is easy and cheap, but is also seldom the problem (my mechanic said he would have suggested I start with that if he had been diagnosing because he would have to me to "just go do it" myself and it's cheap).

The HPFP is a more costly endeavor to take on, but it can be somewhat contained from an expense standpoint if you're like me and have a private mechanic that will install parts you bring to them. I think the total cost to me for my truck, using an OEM pump, fuel "pipe", and manifold seals was about $600-$650. And honestly, if I had a garage where I could tear the truck apart and work on it, I would have done it myself as the job is a bit time-consuming but not very hard to do.

[Tschw]

Just cap off the pcv to the manifold and see if the fuel trims fall into place. Easy way to rule out fuel in the crankcase. Grams per second on the maf should be close to the engine displacement

[ember1205]

Originally Posted by Tschw

Just cap off the pcv to the manifold and see if the fuel trims fall into place. Easy way to rule out fuel in the crankcase. Grams per second on the maf should be close to the engine displacement

The flow rate through MAF is going to vary depending on engine speed and load. At idle, it's likely to draw at least 6-6.5 grams per second, possibly a bit higher. Give that LTFT values aren't as reliable at idle with no load, this may or may not actually provide concrete data.

It's typically more useful to plot the flow rate along with engine speed as they should roughly correlate with each other providing the amount of load on the engine doesn't change drastically in the process.

Plotting the flow rate with the TPS is also useful as these two should correlate very closely.

A bad MAF Sensor often shows up with a plot line that doesn't closely line up with engine speed and or TPS changes.