Better fuel trims at idle.

I'm having better fuel trims at idle than cruising.

Idle are +0.0 to +0.8 on both banks

Cruise speed 2000 rpms negative 4.5 to negative 5.5 in both banks constant.

Vehicle has become a gas hog!

Any ideas?

Juanchos wrote: I'm having better fuel trims at idle than cruising.

Idle are +0.0 to +0.8 on both banks

Cruise speed 2000 rpms negative 4.5 to negative 5.5 in both banks constant.

Vehicle has become a gas hog!

Any ideas?

Rule of thumb is add the long term and short term fuel trims for each bank eg STF1+LTF1 and STF2+LTF2 only if either is above 10% is there much cause for concern.
However if your fuel consumption has increased dramatically it is worth investigating. It could be anything and there is no way for us to help without more data.

Andy is right your fuel trim numbers are what we would describe as perfect . However it is possible to use more fuel than you should and your fuel trim be normal for reference what is the year make and model of vehicle you are working on and what mpg are you getting (if possible calculate it from gallons used and miles traveled per tank) and what driving type do you do (city % hwy% ) and we can assist you further

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Thanks for taking the time and answer my questions, I've always been an automobile enthusiast and discovered ScannerDanner in youtube I love the way he discusses case studies and I've learned a lot about fuel injection!

My vehicle is a 2006 Mitsubishi Montero normally these vehicles don't have great fuel economy but lately i'm seeing 11mpg to 13mpg normally it was over 15. 67k miles on the clock and my driving is 50% city 50% highway.

I love the this vehicle and will like to keep it as top notch as possible.

New spark plugs (properly gap), wires, air filter PCV, Cleaned the EGR and put 2 cans of BG44k to a full tank 23 gallons.

MIL is off and no stored codes. I'm suspecting I have a cracked exhaust manifold before the upstrean O2, I will very and fix and report back.

I think an exhaust leak before the upstrean O2 qualifies for a fuel issue, do you agree?

Fuel trims got back to normal, guess what??? Switched to different brand of fuel with 93 octane and now fuel trims are almost all the time or near 0% LT and ST.

any ideas? alcohol amount maybe?

I tend to doubt an Ethanol content issue, as using fuel with excessive Ethanol content on a non-Flex vehicle results in positive fuel trims, not negative. The only time you get negative trims is on a vehicle that has over-estimated the Ethanol content of the fuel in the tank.

What's the ambient temperature in your area? Colder temperatures mean a denser air charge, and therefore more fuel is added. If you were getting 15 MPG when it was pretty nice out, and 11-13 when it's cold, then that could definitely be a factor.

Typically the temperature in my area a is mid 80's!

I did the test again and trims get better with the 93 octane from a different brand!
The other fuel that I'm seen immediately negative trims has 91 octanes, I doubt that 2 octanes will do that much difference that's why I'm thinking alcohol content.

By the way the car is due for timing belt replacement, do you think that will make a difference?

I doubt it at 68k if you had 150k on first belt of say possibly to cause fueling issues, does this have a maf sensor? They have a lot of control over fuel and I didn't see it answered but yes manifold leaks cam cause fuel issues as well . Also what type of plug was used? Copper, platinum ,iridium?

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Is a MAF sensor vehicle, Mitsubishi montero 2006 3.8 v6. Spark plugs were replaced with OEM iridium. Also wires and coils all new OEM. I'm begining to think of MAF issues as well, I cleaned it with CRC cleaner but no positive effect was noted.

Sorry for late reply take a look at your maf readings at idle with a warm engine we expect to see approx 4 grams per second give or take and on a wide open throttle run in first and second gear we want to see over 130 grams


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