Fuel economy issues...

[Tbird232ci]

Ok, so I feel like a total tool posting about fuel economy on an F-body forum. I know having an LT1, in more-or-less a sports car, with a handful of bolt ons, isn't the best idea of a fuel economic vehicle, but I feel I should be doing better.

The list of mods:
-Pacesetter LT's and ORY
-Hooker Aerochamber catback
-CAI and air foil
-1.6RR, LT4 springs, guideplates, studs and pushrods
-!EGR, !AIR, !EVAP
-O2 Extensions from ThunderRacing
-OTVC setup using Taylor wires
-NGK TR5's gapped to .035 (that was a boo boo, previous car required (.035)
-Bosch O2 sensors
-Trifecta Tune

The car has 132K miles on it, and a slipping clutch which doesn't help. I don't drive it very aggressively, but I have to fill up every 220-230 miles or so, and usually 80-100 of those miles are pure highway cruising, 6th gear at 70mph. I average 15-17mph depending on if I warm it up in the mornings. Cold here in MD. I also run nothing but 93octane in the car since the heads were milled a bit to true them up.

The only issues I know of is that it needs the hose going from the throttle body to the passanger side valve cover, it crumbled a bit at the end where it slips over the nipple on the throttle body. Other issue is that I have a vac T/Check valve for the HVAC broken. The car runs great, but I do have a bit of black smoke under acceleration.

Any insight would be great. I'm trying to get a new job, and might have to drive 80 miles a day for a little while. If you guys need any other information, let me know. Thanks a bunch!

[Z28pr0jekt]

Black smoke means you're running rich.

1. Have you datalogged it?
2. How old are your O2s?
3. Regap your plugs
4. How old is the fuel filter?
5. Fix that hose from TB to VC

[Tbird232ci]

I have not datalogged it. I do know the black smoke is a rich condition, which is the primary reason I brought it up. I also get a little oil smoke at WOT.

The oxygen sensors have maybe 3k miles on them.

The fuel filter is maybe a few hundred miles old.



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[Z28pr0jekt]

First things first then, regap the plugs to .050-.055 and get a new hose from VC to TB.

With boltons and 2.73s with my M6 I was getting 28-31mpg on the hwy and about 22 mixed driving.

I didn't drop down in the teens until after my cam.

But you can ALWAYS expect a drop in mpg with the cold weather; running the heat, defroster and the car taking longer to get to operating temp is all going to affect mpg.

How new is your air filter?

[BLK95-Z]

I thought cold air improved MPG? Ive been told cold air is more dense and takes less fuel to get the proper mixture, and hot air is just the oppisite. Is that not true?

[Z28pr0jekt]

I thought cold air improved MPG? Ive been told cold air is more dense and takes less fuel to get the proper mixture, and hot air is just the oppisite. Is that not true?
Cold air improves horsepower as long as the car is tuned for it.

When it's really cold out it takes a long time for the car to reach proper operating temp. When a car takes longer to reach these temps it is not utilizing fuel efficiently because it is staying in open loop which is inherently rich in comparison to the closed loop tune.

And when it is cold out it causes you to use things like the heat and defroster which draw on the power of the car and in turn decreases efficiency.

[Tbird232ci]

The vacuum leaks are on the top of my list, I just havent made any moves because of time and weather mostly. I will probably replace the plugs with the proper gap since they're cheap insurance.

Combined driving, in warmer weather I average 17mpg, in the winter it has been about 15.

As for my air filter, it's a K&N, been about 3K since it's been cleaned. May clean that and the MAF while I'm at it.

The car has 3.42's and an M6, but the way I baby the car on the highway would net me well into the 20's. My old, ragged out 86 Stang GT vert with an auto got me 29mpg on the highway cruising at 2200 rpms.

Also, I have read some bad reviews about Bosch O2 sensors. Is that likely a problem?



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[BLK95-Z]

Cold air improves horsepower as long as the car is tuned for it.

When it's really cold out it takes a long time for the car to reach proper operating temp. When a car takes longer to reach these temps it is not utilizing fuel efficiently because it is staying in open loop which is inherently rich in comparison to the closed loop tune.

And when it is cold out it causes you to use things like the heat and defroster which draw on the power of the car and in turn decreases efficiency.

I see. The open loop thing makes sense but why would using electrical things inside the car make any difference in fuel economy? Does the altanator get harder to turn under a load? I know the defrost runs the compresser but the heat is just a blower.

Sorry not not tryin to highjack Tbird, your MPG is pretty bad if your not flooring it a few times a day. I hope you get it straightened out

[Tbird232ci]

The higher the load on the alternator, the more power it takes to turn. My buddy rigged up a 65amp alternator to a 5hp lawnmower engine to build a makeshift welder. He struck an arc, and the alternator shut the engine off.

[CALL911]

Also check tire pressure. Many times this can be overlooked.

I think you mileage is slightly bad, but not horrible.

I average less than 10 MPG in my Formula in my sig. Granted its almost 80% intown driving right now.

Try a small road trip that is purely highway. You should see around 25 MPG or better with your setup. If it is still south of 20 MPG on the road trip, I would definalty keep looking.

[Tbird232ci]

Also check tire pressure. Many times this can be overlooked.

I think you mileage is slightly bad, but not horrible.

I average less than 10 MPG in my Formula in my sig. Granted its almost 80% intown driving right now.

Try a small road trip that is purely highway. You should see around 25 MPG or better with your setup. If it is still south of 20 MPG on the road trip, I would definalty keep looking.

I have kept up on my tire pressure, but that is a very good point. I keep em at about 40psi, mostly for the sidewall stiffness.

I haven't done a road trip, but I have done a fill up on an 80% highway tank, and it was about 18.5 mpg. I feel that with driving more than half of my tank worth on the highway should net me better than the 17mpg I get.

I do appreciate the input.

[Mike Hussy]

Have them first check your thermostat. When they fail, they are designed to fail open so as not to overheat your engine. But this also has the effect of having too much coolant passing though all the time.

You should probably have your air filter, spark plugs, and fuel filters changed since all of these can affect gas mileage.

Your mechanic may find other issues as well, but if you don't trust him, bring it somewhere else after and see what they find. If both mechanics agree, you probably have an accurate diagnosis.

[razor02097]

before modding I averaged around 20mpg city + highway. After long tubes, CAI and exhaust I average about 17mpg! It baffled me until I found out the reason....

my right foot :whistle:

[zooguy]

before modding I averaged around 20mpg city + highway. After long tubes, CAI and exhaust I average about 17mpg! It baffled me until I found out the reason....

my right foot :whistle:


same problem here but i droped from 17 city and 18-19 hwy to 14.3 city and 15.5 hwy once i got LT's headers but i also didnt put a y pipe on yet and the O2's are getting a false reading cause the are reading air flowing back in to the collectors

[Tbird232ci]

Have them first check your thermostat. When they fail, they are designed to fail open so as not to overheat your engine. But this also has the effect of having too much coolant passing though all the time.

You should probably have your air filter, spark plugs, and fuel filters changed since all of these can affect gas mileage.

Your mechanic may find other issues as well, but if you don't trust him, bring it somewhere else after and see what they find. If both mechanics agree, you probably have an accurate diagnosis.

I know this is an old thread, and I also know that this user has been banned, but man this post made me laugh. Thermostats typically fail closed, which is why cars do overhead when they fail. That's why you get the comment "check your thermostat" when you have overheating issues.

As far as the tune up suggestions, the first post covers everything suggested.

Next, having my mechanic finding issues. I giggled deep down inside, seeing as I AM a mechanic. Four ASE certifications, and one certification is in Engine Performance, essentially this type of thing. :heart:

Anyways, onto the real update:

Since this thread was started, I put new AcDelco oxygen sensors and gapped my spark plugs to .055. I also cleaned my throttle body, PCV valve, and MAF, which helped with drive-ability. The car runs significantly better, and is much more responsive. My fuel economy was up to 20mpg on average, with the occasional 21-22mpg weeks.

Since then, I put in a new fuel pump, due to a lean condition, and I installed under drive pulleys. I haven't been able to keep up on my fuel mileage because the fuel pump hanger O-ring fell in the tank and I had a leak. In a week or two, I can post another update if there is any interest.

[tarheelsoldier]

lucky yall... I have a full dyno tune, I have not datalogged it yet. but I am luck to get 15mpg. I think the 30# injectors I have may be the reason for this. Its been tuned for all of this but I am still running rich. so I don't know. But around 15mpg is what I get. M6 with 4.10s. Just FYI