BAD MPG PART 25

okay guys so i’m back
-changed spark plugs (ngk platinum)
-cleaned maf
-cleaned throttle body
-mostly fixed exhaust leaks (very small ones now, not enough to throw a code)
-did front breaks and calipers (i think maybe they were sticking a bit i just changed them last weekend i think i see a slight mpg improvement)
———— i’m thinking maybe the o2 is going bad because i’m still getting high LTFT on both sides like +14.7% that means i’m running rich no? looking for ideas to test to see if the o2 are bad or not. what should voltage be and what not? i had po420 codes when i had leaks everywhere but now i have 0 codes but gas mileage still pretty bad i get about 400km full tank

 

High positive fuel trims indicate the PCM is adding fuel to compensate for a lean (not enough fuel) condition, it mostly uses feedback from the AFR (upstream) sensors but the O2 (downstream) sensors have some input as well. Any exhaust leaks upstream or near the AFR or O2 sensors will affect their readings.


Are your fuel trims pretty consistent across all RPM/Load ranges?

How many miles on the truck?

 

240’000km and no the fuel trims fluctuate a lot, when holding the pedal steady though it would be a pretty consistent number though but high numbers in the positive, all i have now is a very small leak at the upper cat flanges, i don’t think that would be enough to throw off the fuel trims this much though??

 

It may affect the downstream sensors but I wouldn't think would affect the upstreams.

150k miles it could be sensors getting a bit skewed but they will typically set codes.

Are the trims higher at idle than they are driving?

 

no i think trims are normal at idle i’m 90% sure it’s just when gassing and sitting at rpm’s like 2000 or 2500 they’ll be high

 

did my O2's and FPR
OTT states stock ECM is overfueled. Which I believe. Truck smelled like gas idling. Doesn't seem to anymore after tune.

 

i do not smell any gas while in my truck, what’s FPR and OTT? also did you change all 4 o2 sensors, did it help your mpg?

 

I mean they aren't super high but they are elevated, typically +/-10% is considered normal. Anything over 25% will trip a P0171/P0174.

Are you running stock wheels/tires/suspension?

 

yup stock everything and i don’t think any vacuum leaks i checked all lines by spraying them. possibly injectors clogged or fuel filter/fuel pump i’m not sure i been trying to figure out this low mileage all year.
i think i did have those codes when my exhaust was fully leaking though

 

If you had a vacuum leak fuel trims would be elevated at idle and improve as RPM/Load increases.

Certainly wouldn't hurt to check fuel pressure.

 

I mean, if a car has a vacuum line leak
ECM senses lean
ECM responds by dumping more fuel

I don't see how that can happen if the pump itself goes bad and becomes unable to deliver proper fuel
but who knows, maybe. Running lean supposedly = bad MPG.

Can a pump go bad now? Yes. There is no serviceable filter.
Can injectors benefit from a service? Yes. The hard part is downing the truck for turnaround time. Could be a week or two.

 

just trying to narrow down what the issue can be thanks for the advice man

 

99% of all fuel related issues result in a rich mixture. This is on purpose to protect the engine.

A weak fuel pump would drive the O2 sensors lean, the pcm will duty-cycle the injectors more and it will still likely end up rich (pos trims) even with a weak pump. The pcm's main answer to every weird situation is to simply over-fuel. The only control it has is to open the injectors more

OP how "small" are these remaining exhaust leaks? Can you see black soot?

 

How are you logging your trims? I see long term bank 2 hit 14% often under certain conditions without issues. If short term trims are not getting hit hard consistently I don’t believe it’s an issue.

 

yes i see a few little black pinholes with black around them

 

i’m not sure i can check tomorrow, what’s the difference between long term and short i though mainly the long term one matters

 

Really gotta deal with that before moving on. A diluted oxygen sensor is right at the top of the list for poor mileage

 

I’m not an expert, I understand it as long term is the trend, short term is the immediate. Under load is what matters. You don’t need to be wide open throttle, but check your long term and short term under load and see what the readings are. The reason I say this is because I am tuned and run e25. I monitor trims for this reason. I see bank 2 fluctuate a lot and seem to read high but bank 1 stays much lower. Also, short term don’t max out. What the heck the ECU is exactly doing I don’t know, but I find the reading to be misleading if only concentrating on LT bank 2. Obviously, a vacuum or exhaust leak can cause issues too.

 

okay thank you man i’ll check the trims again tomorrow and come back here

 

i was thinking muffler cement on them, i got everything welded already and guess what, still i find these leaks, is there actually any way to properly fix the small holes?