Fuel Mileage went south over night

[LS1 TA]

I took a trip to San Antonio in the middle of November and the TA got about 30 mpg gallon the entire way there. I went from San Antonio to Dallas on the 18th to watch the cowboys beat the Redskins and the TA repeated its great mileage as it always does. I went back to Dallas from San Antonio on thanksgiving day and my mileage was crap approx. 24.5 mpg. On the way back to New Mexico it was the same 24.5 so I changed the fuel filter hoping that it would be a cheap fix. I went to El Paso TX for christmas this past weekend and the Mileage was still crap. My car is giving off no SES light or anything, so I assumed that it wasn't an O2 sensor any other electrical components.

[97LT1Z28]

There's a lot of other factors that can play into your fuel mileage such as wind speed/direction as well as traffic conditions (lots of cars & not able to hold steady speed or lots of passing, etc.).

There are also things that don't throw an SES light that can affect it too such as dirty MAF or slightly-fouled spark plugs.

Is it missing or running differently than it did a cuple of months ago?

[LS1 TA]

I just put new plugs in the car about 2 moths before I went to Texas as well as A new K&N air filter. The car is running like a champ as it always has. It has always gotten between 30 and 31 mpg on the Highway the entire time I have owned it, so to get 24.5 was a total shock to me.

[Injuneer]

Some possibilities.....

Is it possible there was too much oil on the new K&N, that might have coated the MAF sensor?

Does the fuel composition change in your area in the winter? In many states, they add components (oxygenates) to reduce pollution but they also reduce fuel economy (although not by almost 20%).

Is there any chance you used fuel that was contaminated with lead at some point in the trip?

Why would you feel that a dirty fuel filter would hurt your fuel mileage? If it was clogged enough to reduce fuel pressure, it might make it run leaner, and the long term fuel corrections would compensate?

Have you ever had is scanned, to look at the long term fuel corrections? A "false" lean condition (O2 sensors, O2 sensor wiring, exahuast leaks before the O2 sensors, misfires) will cause the PCM to elevate the long term fuel corrections, making it run rich. Is it possible you cracked one of the plugs when you were installing them, or that one of them came loose?

[LS1 TA]

Some possibilities.....

Is it possible there was too much oil on the new K&N, that might have coated the MAF sensor?

Does the fuel composition change in your area in the winter? In many states, they add components (oxygenates) to reduce pollution but they also reduce fuel economy (although not by almost 20%).

Is there any chance you used fuel that was contaminated with lead at some point in the trip?

Why would you feel that a dirty fuel filter would hurt your fuel mileage? If it was clogged enough to reduce fuel pressure, it might make it run leaner, and the long term fuel corrections would compensate?

Have you ever had is scanned, to look at the long term fuel corrections? A "false" lean condition (O2 sensors, O2 sensor wiring, exahuast leaks before the O2 sensors, misfires) will cause the PCM to elevate the long term fuel corrections, making it run rich. Is it possible you cracked one of the plugs when you were installing them, or that one of them came loose?

I guess those are all possibilities I can look in to. It just seemed weird to me that Nov 18th in the evening it got 30mpg and then Nov 22 in the AM it got 24.5 mpg traveling the same road with the same conditions with the same gas stations.

I also don't think it was the air filter because the entire trip to texas and the first trip from san antonio to dallas it did just fine. I will try and find some place to get it scanned I guess.

Could clogged catalytic converters screw with your mileage?

[Christopherrr]

Could clogged catalytic converters screw with your mileage?

Yes, it definately can...but you'll notice a drop in power as well as a different exhuast note.

[LS1 TA]

Yes, it definately can...but you'll notice a drop in power as well as a different exhuast note.

Nope, can't say I am having any of that. What about my air intake temp sensor telling the computer that it is way cooler than it actually is?

[Trash man]

ditch the k&n and get a paper could only help

[Injuneer]

Does the LS1 have the IAT sensor integrated into the MAF sensor (unlike the LT1, which has a separate sensor)? That would certainly screw things up by producing excessively high MAF sensor readings, but again, bad MAF readings (within reason) would cause an incorrect fueling, but the PCM would try and correct for it. The PCM also runs a rationality check on the MAF numbers, to see it they make sense. Its a course check, only identifying a problem if the MAF numbers differ from the predicted numbers by more than ~25%.

[LS1 TA]

Does the LS1 have the IAT sensor integrated into the MAF sensor (unlike the LT1, which has a separate sensor)? That would certainly screw things up by producing excessively high MAF sensor readings, but again, bad MAF readings (within reason) would cause an incorrect fueling, but the PCM would try and correct for it. The PCM also runs a rationality check on the MAF numbers, to see it they make sense. Its a course check, only identifying a problem if the MAF numbers differ from the predicted numbers by more than ~25%.

So I ran my car to work this morning with the IAT sensor disconnected and I didn't get a SES light.... Is this normal?

[Christopherrr]

So I ran my car to work this morning with the IAT sensor disconnected and I didn't get a SES light.... Is this normal?

I did once after cleaning my filter and forgot to plug it back...didn't get a SES light but it did run rough...then again, I only drove it for less than a mile.

[Joshu]

I did once after cleaning my filter and forgot to plug it back...didn't get a SES light but it did run rough...then again, I only drove it for less than a mile.

Same here, I left the IAT sensor unplugged by accident after cleaning my filter too. I ran it about a mile with out getting the SES.

Different question, would a clogged cat produce a higher pitched tone or a lower pitched tone?

[Joe 1320]

Some possibilities.....



Does the fuel composition change in your area in the winter? In many states, they add components (oxygenates) to reduce pollution but they also reduce fuel economy (although not by almost 20%).

That was my first thought. There could be many things in combination........ perhaps a combination of low air pressure in the tires, possible air/fuel mixture issues, possible spark issues...... could be lots of things, not just one small one.

[Joe 1320]

Different question, would a clogged cat produce a higher pitched tone or a lower pitched tone?

A clogged cat would cut down exhaust noise, sometimes it will make a hissing noise as exhaust is forced though.

[LS1 TA]

That was my first thought. There could be many things in combination........ perhaps a combination of low air pressure in the tires, possible air/fuel mixture issues, possible spark issues...... could be lots of things, not just one small one.

I think it is just one problem because it just dropped over the course of 3 evenings. The night of the 18th 31mpg the morning of th 22nd 24.5 mpg.