Hey M. Yount, figured out crappy mpg!!!

[SmockDoiley]

So I had to drive a few hours today so I decided to fill up the tank and start the trip meter. First off, let me say that the car is totally stock, has only 16k on the factory H-pipe, and brand new O2 sensors. Also, let me say that the rear tires are 235s and when driving through an odometer check on the highway my odometer showed 1.1 miles for the highways 1 mile. From my calculations this means the odometer is punching 66 miles per 60 I do and so on. With that said, after my trip which was mostly highway driving and maybe 15-20 city driving, I now have 230 miles on the trip meter and probably another 20-30 miles left in the tank at least.
So I took the 230+20 left, got 250 and divided by 1.1 to get real mileage and got 227 and then I found that the fuel capacity is 15.4 so I divided that into the 227 and got 14.76 mpg. Tell me if this all sounds good to you. The mileage isn't the best since it was all highway driving and the car is all stock. I cant smell any gas, and there aint no Check Engine light. This obviously is better than what my wife was complaining about, but I think it should be better since everything under the hood is unmolested and really clean. I dont want to hear anything about O2 or cats since its all fresh or brand new. Car runs great except a little sluggish at a light. Powerband under 1500 rpms feels like the car has been driving in Florida at noon in the summer, you know when the car just feels slow to react. It has no idle surge and it fires right up, but I can feel the idle in the steering wheel. I guess if I can squeeze another 30 miles out of the tank then Ive got a 15-miler but I think it should be better.

 

[eric88gt]

Estimating how much gas is left, and how many miles you think you can go will give not give you a very accurate number IMHO. I've run my car to the point where I thought I was on fumes, and don't remember ever putting in the full 15.4 gallons, maybe 14.5, but never over 15. So if you estimate that you can go another 20-30 miles, but in reality could go another 50-60, your mileage estimate could be off by a couple of mpg. What you should really do for a more accurate mileage check, would be to get the car completely full, drive the car for a while, then when you need to fill up again, fill it up at the same place you did the last time. Now figure out your actual mileage traveled, and figure out what kind of mileage you're getting.

With that said, I do agree that your mileage should be better than that on a stock 5.0 in good running condition. A couple of things I would look at would be the ECT, and ACT sensors. If either one is not operating correctly you would probably have poor gas mileage and the car may seem a little sluggish. I had a similar problem and replaced the ACT sensor and the car picked up about 3-5 mpg and it also fixed the laziness the car seemed to exhibit at low speed part throttle acceleration.

 

[HISSIN50]

What you should really do for a more accurate mileage check, would be to get the car completely full, drive the car for a while, then when you need to fill up again, fill it up at the same place you did the last time. Now figure out your actual mileage traveled, and figure out what kind of mileage you're getting.

With that said, I do agree that your mileage should be better than that on a stock 5.0 in good running condition. A couple of things I would look at would be the ECT, and ACT sensors. If either one is not operating correctly you would probably have poor gas mileage and the car may seem a little sluggish. I had a similar problem and replaced the ACT sensor and the car picked up about 3-5 mpg and it also fixed the laziness the car seemed to exhibit at low speed part throttle acceleration.

Smock, i know you have been fighting this for some time. so i will leave my two cents out of it.
that said (here we go), i really agree with Eric. my first thought was about milage computation and method. then first thought on causes was ECT and ACT (the 'choke' on our EFI's).

i cant remember if you pulled codes for poop and grins before......i know not having a CEL is kind of a pain....

also, even if you are not getting a code, the above sensors can be bad. e.g., the sensor is telling the puter that the motor is cold (and richening things up), even if you are running at 190*. but the sensor is reading within spec (no code), but it is not reading correctly for the situation.

i need to check the values myself (if all else fails, i believe Probst has the values posted for each sensor [at different temps]) - i have the numbers somewhere - lemme know if you need them (im sure someone has a link or the numbers, but in case - and if you are interested). FWIW, my 88 GT vert (aka the pig) gets 15MPG in town. but my size 14's are heavy.

 

[jrichker]

As requested...

Pin 25 ACT signal in. at 50 degrees F it should be 3.5 volts. It is a good number if the ACT is mounted in the inlet airbox. If it is mounted in the lower intake manifold, the voltage readings will be lower because of the heat transfer.

Pin 7 ECT signal in. at 176 degrees F it should be .80 volts

The ACT & ECT have the same thermistor, so the table values are the same.

Here's the table: The values are +/- 15%, so don't get worried if your readings vary a little bit.

Voltages are measured across the two connector pins of the sensor with the sensor connected.
68 degrees F = 3.02 v
86 degrees F = 2.62 v
104 degrees F = 2.16 v
122 degrees F = 1.72 v
140 degrees F = 1.35 v
158 degrees F = 1.04 v
176 degrees F = .80 v
194 degrees F = .61

Ohms measures at the computer with the computer disconnected, or at the sensor with the sensor disconnected.
50 degrees F = 58.75 K ohms
68 degrees F = 37.30 K ohms
86 degrees F = 27.27 K ohms
104 degrees F = 16.15 K ohms
122 degrees F = 10.97 K ohms
140 degrees F = 7.60 K ohms
158 degrees F = 5.37 K ohms
176 degrees F = 3.84 K ohms
194 degrees F = 2.80 K ohms

 

[powershift351]

I get 16 hwy and 11 city with my set-up, you should get much better on a stock car

 

[freakazoid]

To give you an idea of mileage extremes; I get 22 to 24 mpg mostly highway miles.

' 87, 5 speed, headers, catback, K&N filter, msd ignition.

My mileage might be an extreme case, you could use it as a benchmark.

One thing I did do when I first purchased the car 5 years ago was replaced ALL the sensors. That made a big difference with mpg.

Good luck, gas is getting expensive !!!

 

[ECU5.0]

To give you an idea of mileage extremes; I get 22 to 24 mpg mostly highway miles.

' 87, 5 speed, headers, catback, K&N filter, msd ignition.

My mileage might be an extreme case, you could use it as a benchmark.

One thing I did do when I first purchased the car 5 years ago was replaced ALL the sensors. That made a big difference with mpg.

Good luck, gas is getting expensive !!!

how much did all those sensors cost you? im probably gonna do taht this summer just because the car has 195k on it and probably none of them have been replaced. i get about 19 mpg on the highway

 

[bmorrisj]

freakazoid & ECU5.0.... im with you guys, my 87 is getting around 20mpg hiway miles...i got o2 sensors whith my o/r xpipe, i think i should replace those sensors mentioned above because i know i can squeeze out more....

 

[SmockDoiley]

Obviously I know the ideal thing is to run the tank down several times and fill it back up and figure how much gas was used but this method still got me in the ballpark which is good enough to determine. I know this car, and I know how much farther it will go, and when I finally do get gas if there is a discrepency I will correct it, but for now the mileage stands pretty close. Even if the car does go another 20-30 more miles, its still low at 16 something a gallon. So a 95 EFI car that gets 14.5-16.5 mpg on the highway is horrible. Also, I previously posted that I was following my wife on the highway and when she went to pass someone the exhaust had 2 puffs of black smoke, one from each pipe and one right after the other so not at the same time. Black is gas, but that was before I changed the pipe and 02s. I dont know if its still doing it or if what I did would even fix that. Where are the ECT and ACT sensors located at on the car? Thanks.

 

[bmorrisj]

my car would puff black smoke and leave char marks on the ground when i start it up... apparently a blown head gasket hosed my left bank o2 sensor.... but now i have no cats...new o2 sensors and no smoke at all what-so-ever...

 

[Michael Yount]

The tire size/odometer error explains some of it - but the mileage is still low in my opinion. Are you driving it pretty easy and trying to get good gas mileage? If you're really horsing (no pun) it around for 230 miles, then your mileage isn't all that bad. But 15-16 on the highway just cruising around the speed limits is too low I believe.

Why wouldn't you just reset the odo, and when you go to fill it up, record the amount it took to fill it, and divide that number into .9X the odo reading? I don't understand all the estimating when an actual number is easily available.....is it me? Do that each time you fill up and over 4 or 5 tanks you'll get a pretty good average mpg.

Have you pulled codes?

 

[SmockDoiley]

I set the trip meter to zero when I filled it up. This isnt my car and nor do I trust my wife to do this since she refuses to. In her world its ok to just complain about it and thats all. This was a chance I had since I was taking it on a trip. When I came home the car was just on the red empty line and had 230 which after the wrong tire calculation came out to 209 miles and then adding another 20-30 miles for what was left in the tank. Exact?no, very close?yes. And as I guessed, before I could fill the car back up or use up the rest of the gas, my friggin wife took the car out and put "like 10 bucks in". There goes that!!!!! So for now, this is all we have to go on, and I wish people would stop getting hung up on the calculation method when they should be helping to solve the problem. As for driving, the car was on cruise control at 73-75 mph.

 

[willys1]

Yeah,thats real bad mileage I have 410's,and get 21 on the road Give it a total tune up!! Check your timing,and change your fuel filter Good Luck

 

[Michael Yount]

"I wish people would stop getting hung up on the calculation method when they should be helping to solve the problem"

C'mon Smock - we know each other well enough by now you can call me out by name. LOL

We really are trying to help solve the problem. Have you pulled the codes? Several people have asked, but we haven't gotten an answer from you yet (or I missed it).

By the way, why didn't you just pull it into a station on the way back home and refill the tank?

 

[HISSIN50]

Michael, i think it is me too (i often sound like a broken record).

i may have missed it, but pulling codes and checking some values on a couple of sensors (thanks for the numbers, Joe. BTW, how do you do that?) might really tell something. and that would be my advice on how to further diagnose/solve the problem. sorry, but im stupid and have to pick my nose while probing sensors to figure stuff out (i lack the vast experience of M.Y, Jrichker and you other pro's to be able to tell you something definitive).
the puter cars are more of a headache and complicated to diagnose, but that same headache-maker (the puter holding codes) helps us to diagnose problem areas, IMHO. you can remind me of those sentiments the next time im in here ranting about how i hate EFI and miss carbs.
good luck Smock - we all want to see you get this whipped!

 

[SmockDoiley]

I havent pulled codes yet because every parts store I go refuses to touch it, and I dont have a tool myself. I'd go buy one but Im dumping money into my 85 and my 460 project.

 

[HISSIN50]

as Jrichker so often posts, you can do it with a paperclip (no money out of pocket). oops, you dont have a CEL - you can use a test light in lieu of a CEL to flash the codes.