What kind of mpg numbers should I be getting from my engine? I BELIEVE it's the H.O. 305 from 84. The carb had the only casting number I could see and it checked out, and I saw no signs of it being tampered with ever. So anyway, I only get around 16 MPG... on the highway. I usually average around 140-170 miles on a tank, although I took a recent trip that was all highway and got about 250. I had a slight problem with some low rpm sputtering for about 5 minutes until it warmed up until I replaced the IAT. I still get VERY mild misfiring when cold but it's barely there. I also used the TBI induction from my old engine instead of the carb, could that be my problem? Not sure what my rear is, I know it isn't stock as it's posi and it will spin my relatively wide tires better than my buddy's LO3 on stock wheels. definitely not anything taller than 3.55 would be my guess judging by highway RPM. Any ideas/tips guys? I get into the pedal once in a while but i'm not a leadfoot.

 

you think the gears have been changed.. is the speedometer right? seems like everyone has a smartphone with a built in GPS these days, so check it against that..
what is the cruise rpm at 60mph in 3rd gear? this will tell you your final drive ratio (2500 would be a 2.5:1 final drive ratio, for example..) and you can plug that into any number of online or smartphone calculators along with your tire size to tell you what the gear ratio is..

my car has 3.42 gears and 215/65/15 tires... i can't remember the cruise rpm right off hand since i haven't had it out since last October, but it averages 23mpg in my daily driving duties.. it's just an L69 with vortec heads, tiny little cam, and a quadrajet carb with Hedman longtube headers and a single 3" glasspack with a turnout in front of the right rear wheel...

 

I'd have to hook up an aftermarket speedo to be exact but i'd say its right around the 2-3000 RPM range going by sound. I would say I have some kind of problem that is killing my mileage then. The only thing not stock is the open element air filter and the TBI induction. Maybe the timing could be off? I never reset the timing after the learn period, perhaps it is being adjusted incorrectly by the computer? I have a 91 and the ECM is looking for a different cam, I've always suspected that me be my sputtering problem. Not sure how aggressive OBDI is though, I've had no experience with it with previous vehicles.

Edit: Also, how far below E will the gauge go before you're out of gas? Is the "up-float" when you accelerate the true reading? I've been below E and have it still float a bit over the line and only fit about 12 galons or so even when at rest it is below E. If I've always had 3 gallons or so hiding in there that would make abit more sense as to why it was so low as that would equal around 20 or so MPG as opposed to 16...

 

When calculating mileage you should just look at how much fuel it takes to fill the tank back up again. That's a lot more accurate than trying to guess how much was left in the tank and what the capacity is supposed to be.

With the engine swap and configuration of the whole TBI and ECM setup, my first guess for the mileage issue would be a tune related problem possibly causing it to run rich.

 

So its a cobbled up, don't run correctly and has mismatched
parts your worried about lousy gas mileage ?????????

 

im getting about 15 in my 1986 trans am with a goodwrench 350

 

RUN a tank of ETHONOL free gas it. That is all I run in mine. It will make a difference as the ETOH in gas draws water into your gas. Try it if you can find it.

 

just use your GPS enabled smartphone to see if the speedometer is correct.. if you don't have one, find someone that does... then fill it, drive it, and fill it again... divide miles driven by the amount of gas that you have to put in to fill it up...
your mileage numbers won't mean **** until you get the speedometer dialed in..

 

ok just a little update here. I have solved just about all of my problems thanks to mainly people on this website. I had quite a cluster-f of bad things all at once when I made this originally. I have a fresh ECM (that was fried), fresh 02 sensor that is operating perfectly according to my dmm, and I shored up a vacuum leak or two. I'm STILL not going into closed loop. I'm buying a tachometer today to properly set my minimum air as I have it either so high that I can go 15 mph without touching the throttle, or so low that it idles roughly and almost dies when I touch an accessory at idle. My stock tach is always pegged so I have been trying it by ear. This also sent my already bad mpg down to about 7, so I'm doing it right today.

I'm still not hitting closed loop, and the only other thing I can think of, and this is a reach, is pinholes in my radiator. I know I have them, I have them sealed as best I can for now, but I do notice some boiling at much less than overheating temperature (gauge indicated). Now, is there any chance that maybe the cooling system pressure is off and giving false temp reading to the ECM and that is my final demon preventing closed loop? Every thing I have seen indicates the properly working 02 and CTS, minimum time driving, and minimum speed are all that dictate the switch. MY CTS works, if I sit too long without turning on my fan it will read hot, and it will react accordingly when the fan is on or I begin to move, although that is the only thing I haven't actually replaced. Could I be on to something with the radiator thing or do I maybe just have a lazy CTS?

 

Ok I think I got this figured out for the most part. I got an aftermarket tach and set the minimum air properly. it idles in gear just around 650 rpm. I still get a bit of a rough idle though when I have my fan on. The second I turn it off it smooths right out. I did some more searching and am starting to think that because I don't have it wired to a relay, and just straight to a switch, that it's just pulling more amps than normal. I know my alternator is good, I measured the voltage directly from it, not the battery with all accessories running and with it in gear and it was still right at 14.10v the whole time. I also jumped the aldl when driving and am in closed loop finally as well. I'm guessing it's perfectly stoich, the light stays on equally as long as off, although my black tail pipes tell another story... Guess I'll find out with this tank of gas and see how my mileage differs. I can't thank everyone on this site enough, I've learned so much and saved what was about to be a junkyard pile and brought it back to life. Thanks guys!

 

Just wondering when you had her tuned up last?

 

Almost exactly a year ago, when I swapped the engine in. Gave her the works, dizzy cap/rotor, plugs, wires, fluids, valve cover gaskets and exhaust manifold gaskets. Didn't initially have a problem until a few months ago when I was having ECM troubles. It's gotten better since I dialed in some of the base readings and I am in closed loop now and just put 13 gallons in the tank so I'll see about the mileage in about a week or so. I do remember having it timed 6*advanced before and it seemed very happy, but I bumped it back to 0 when I was getting the really rough idle before I figured out it was the ECM. Being that I have TBI induction on what is likely an l69, or possibly an lg4, I think I may need to do some more tuning to smooth the idle out.