ok i filled the tank up at the station, the meter said it took a little over 9 gallons (how many gallons is the tank btw?). reset the trip meter, and 80 miles later im at the same mark on the fuel gauge. fill it up again, 9 gallons again. so i burned up 9 gallons of gas in 80 miles, thats roughly 9mpg. i let the car warm up for about 10minutes in the morning every morning, sit in traffic maybe 30 minutes a day if not more. is there something seriously wrong here or just because of a lot of idling that im getting such bad mileage. i just got a nice exhaust system installed last week with free flowing cat, the old cat was clogged. might the better flowing exhaust get me better mileage? still got to wait a few more days till i can tell. also does changing the o2 sensor really help? thats all i keep hearing about. o2 for our cars is around 20 dollars right? also my digital egr valve has a small leak, would that be affecting my mileage too? i admit i drive it pretty hard but i should be getting at least a couple more miles to the gallon right?

 

Idle time really kills mpg. The exact ammount our tanks hold is 15.5 gallons, I made a post on the history board to find out. I run all highway and am getting 26.1 mpg but I spend maybe 8 minutes of idle time.

 

Idle time does almost nothing for your car. Think of it. Of ALL possible running 'modes' idleing uses the absolutely least gas. Besides, the ecu likes to even lean out the mixture at idle.

At 9mpg, you are burning way too much fuel. The car should be running horrid, if not just a little rough.

First, replace your O2 sensor. big one right there. like $25.

Next, replace Cap, Rotor, Plugs, and Wires. Make sure you are burning all the gas in the cylinder! Maybe even a new MSD coil, if you want to splurge a bit.

Check your ignition module. Be sure your timing is set correctly, and how many miles are on the engine? Might need a new timing belt while you are at it.

For reference, I got 24.4mpg on my 2.8/T5 w/ 3.42s running like a madman all over the place, and stop and go.

Just search for TomP's Major Tuneup. Do all that, and gas mileage will come back. Also, yes, the freer flowing exhaust will increase your volumetric efficiency (VE) and increase mileage as a result.

 

just got done with a tuneup a few weeks ago. new spark plugs, wires, dist cap, rotor, and oil change. bought a new fuel filter but havent installed it yet. i guess i'll also pickup an o2 sensor. i dont know how to check the timing, but i have access to a timing light. if someone has a guide to follow for a 3.1 i'll give it a check.

the car has ~73100 miles, but i dont know if thats accurate. seems a little low for a 1990. odometer seems to be ticking along fine but i dunno. it seems to run fine to me, but it is backfiring when i rev it even a little bit in neutral, i cant get past like 2k and letting off the gas without popping.

 

then definitely get a new O2 sensor.

and if you do 40 minutes of idling per day, figure that time (conservatively) as roughly 10-20mph, if you want to get a better estimate of what 'city' mileage should actually be on your car. 40 minutes each way per day of idle is a lot of wasted gas.

basically, to judge how the car is running, you estimate how much actual driving you'd use that fuel wasted at idle on.

 

Or better yet take a highway trip from one gas station "fill up there" and go to another and fill up, maybe 50 or so miles away or something like that. That way you can accurately figure your mpg better.

 

You know you are VERY lucky I'm in a good mood! I was about half-way done typing up what I'm about to re-type now, and I get a freakin **** IM which somehow loaded up a **** site without me clicking....so I close the site and guess what? No Thirdgen! AHH!

Ok...timing..
Anyone PLEASE PLEASE correct me if I'm wrong...it's been a little while since I owned the 'maro and I'm starting to forget things, and I don't want poor Tek here to accidentally set his car to 25 degrees of timing because of me!

How to check AND adjust base timing:
Find your balancer timing plate. See the degrees? Figure out what you want to set it at. Stock is 10 degrees. Looking for more umph? Do 12. You MIGHT need to run mid-grade gas with this, but most don't. Mark this degree line with white-out, nailpolish, etc. Mark it neatly if you want your timing to be more accurate! Find a notch in your balancer itself. It's got a few.. Mark the notch as well. Start the car. Let it warm up nice and good, and make sure it gets to normal idle. Turn it off. Now, look on your passenger side in the engine bay..by the fender, there is a single wire connector right there..that's the EST (Electronic Spark Timing). Disconnect it. Get in the back of the distributor and find the hold-down tab. I forget exactly where it usually is..but loosen it..just enough to be able to move the cap. It takes two people to do this by the way. One of you hold hold the cap in place..don't wiggle it or move it AT ALL! I assume you have an inductive timing light....so the big plastic piece, clip that around the wire going to your #1 cylinder (passenger side, closest forward). Connect the black to the negative, and the red to the positive on your battery (duh... lol) Someone else, start the car. Aim the gun and pull the trigger. BANG. You should see the mark on the balancer somewhere around that tab. Chances are, it probably won't be lined up quite right with the mark you made on your tab. While one person has the light, the other VERY SLOWLY rotate the distributor cap until you get the two dashes to line up together. Clockwise advances timing, counterclockwise retards it. Once they match up, tighten the hold-down back down and turn the car off. Re-connect the EST and be happy..it's done. Remember..the EST plays with the timing, so don't check or adjust your timing with that connector in..it's gonna read probably around 25 degrees instead of 10 or so!

***...I'm never typing this ever again!

ALSO...with what I know of the EGR valve, it's probably very possible that your EGR problem is causing the backfiring.

 

thanks for that post nixon. hopefully i'll get around to checking the timing this week. where is the general area the timing plate is located? u put the mark at which u want the timing on the plate itself? also, what are the notches your talking about? sorry if i seem like a complete moron, but i am

 

Timing plate is right above the harmonic balancer, in the back...I forget whether it's on the driver or passenger side..

Thats right..mark the timing you WANT on the timing plate itself, and then mark a balancer notch, and line the two up. The notches, look at your harmonic balancer..get under the car if you need to, and look around on it..there should be lines cut in it straight forward. Probably 3 or 4 of them all the way around. Those are the notches.

 

hmm, i think i know what is causing my bad mpg. i noticed today my tripmeter got stuck on 83 miles Odometer didnt look like was spinning either. i knew something was wrong with my odometer from the start because it had 72.5k and it was a 1990. is tripmeter repair possible? i think it gets stuck on 83miles everytime, i'll know for sure next time after i reset it.

assuming the tripmeter is accruate up until 83 miles (probably not, i'll keep a sharper eye on it tho), i think i can figure out my real MPG by getting gas as soon as it hits 83 and seeing how much it takes to fill it up. my tank is 15.5 gallons right?

 

.

 

or, every time it hits 80 you can reset it, so you can drive 160 or 200 miles before getting gas to get a better result. you should be able to recall the number of times you've reset, it should be about once per half tank of gas...

or just reset it all the time, and base the calculation on the real odometer...

 

well, it appears both the odometer and trip meter are completely dead now. i reset it but it stays at zero. the odometer doesnt move either.

 

haha...join the club...today i was cruzin around and noticed my odometer was still on 49502..lol trip odometer never worked..so now i have no way of figuring my horrible mileage...boo hoo...lol

 

I have to fix my tripodometer again I think my rubber band fix broke on me. I've noticed on many of the tripodometeres the gears dont stay in contact with the other gear that drives them so I put a rubber band on the trip gear holder or what ever its called and it held it tight against that gear and worked perfect. 1 tenth of a mile on the speedo was 1 tenth of a mile on the trip. I'm probally gonna fix it this weekend if the weather is bad and I cant work on my shop.