50 miles on 1/2 tank of gas

[NooStang]

Cross posting

'87 5spd 5.0...guessing at 2.73 stock gears based on performance.

Doesn't seem to be running rich (no bad smells or black smoke) and I can't find a fuel leak. Anyone have this kind of fuel economy? Did all gaskets on the plenum and the plenum-intake gasket. Idle is mostly smooth and has a nice set or freshly cleaned/tested 19lb injectors. New IAC valve. Set TPS to .98v. Smoke tested and found a tiny bit of smoke from the EGR valve, but when I sprayed carb cleaner around EGR theres is no change at all to the idle. Did the idle reset three times and cannot get idle lower than 900rpm.

Runs and sounds like a stock cam.

Has slight intermittent miss at 900rpm.

Pulled KOEO codes and got nothing unusual.

Replaced all of the vacuum hoses when I did the gaskets.

Maybe intake gasket?

Fuel gauge seems to be working as I had it on 1/8 tank when I got the car about a month ago. When I filled the tank it took about 30 minutes to finally reach full, but it seems to be accurate.


Did have an issue where it died at a stoplight, but restarted fine. Shortly thereafter(1 mile) it would intermittently not respond to changes in the accelerator pedal. Seemed to be OK when I kept the RPMs over 2000. This was after being in city traffic for about 30 minutes. Car temp was stable in the lower 1/3 of temp gauge during the drive.

 

[General karthief]

Miles per gallon measured over several tanks, average them out, more accurate idea of actual average MPG. Run as much fuel out as you dare each time.
This should show you how accurate the gauge is too.

 

[Mstng93SSP]

It's hard to say as you don't really know how many gallons of gas you have used in the 50 miles. You can't base it on the gas gauge. Who knows how accurate rhe gauge is.

 

[Noobz347]

Look for any extra switches they might be laying around



View: https://www.youtube.com/watch?v=-tv6Bd_4KVY

 

[General karthief]

Look for any extra switches they might be laying around



View: https://www.youtube.com/watch?v=-tv6Bd_4KVY

I bet the MPGs on that Goat suck

 

[Mustang5L5]

I probably use 1/4 tank of gas every time i take my car out for a 5-10 mile cruise. Driving style plays a huge role, and I certainly don't drive my car for fuel economy.


BTW, EGR tends to often leak. This portion of the engine system is not under vacuum so it's not a vac leak. It's actually under positive pressure as engine gases are routed to the egr valve to be pumped into the intake. Any minor leak here doesn't effect performance. Of course a major leak is pushing exhaust gas into your engine bay.

 

[FastDriver]

Seems like somethings wrong to me. You can put just over 16gal of fuel in the 15.5-rated gallon tank. So, you can get a good idea of how accurate your gauge is. I'm usually just hitting the top white gas gauge line at 70-90 miles.

So, you have atock 2.73s and your speedo is accurate?

I've never done worse than about 16mpg for a tank in any car I can remember. Lately, in my H/C/I daily with a decent blend of highway and city, I'm getting about 20. It has gearing similar to a 2.73 car.

 

[NooStang]

Thanks for all the replies

I refilled with 7.95 gallons of fuel. Calculated out to 8.025mpg. With idle time maybe 11mpg. I am not super easy on gas pedal, which is part of the problem.

I did swap out the 21/22# leaky injectors for a set of rebuilt 19s about 45 miles into that tank of fuel (64.2 miles total travel on trip). Also installed a ford ignition module.

Verified speedo is correct with GPS. Sure seems like 2.73 to me.

Certainly something wrong, but I would expect at 8-12mpg to have burning eyes standing behind the car, but it just doesn't smell rich or have black exhaust smoke. If this has a 15.5 gallon tank then it had 1/2 tank of fuel. Must be the gauge or sender messed up. After burning the 7.95 gallons of fuel the gauge read 1/8th.

I need to pull and check plugs soon.

 

[limp]

I used to get 6mpg with this car back in High School.... The fuel gauge moved like the tach did.....

 

[NooStang]

I used to get 6mpg with this car back in High School.... The fuel gauge moved like the tach did.....

My buddies 455 '68 Firebird got the same.

 

[limp]

My buddies 455 '68 Firebird got the same.

It sucked.... If I was picking up a date on the other side of town, I took my Dads car to pick her up and came back and got my car....

 

[FastDriver]

Did you disconnect the spout when you set timing, and what did you set base timing to?

 

[NooStang]

Set to 10 with spout connector removed. Will run this tank down and see what I get this time. Will try to ease up on the pedal a little.

 

[90sickfox]

21# to 19# injectors. Did you change the mass airflow sensor ? Burning that much fuel is a lot for a mustang. Could be O2 sensors, or a ton of other things. You need to pull the codes from computer.

 

[NooStang]

Small sample of only 47.2 miles, but fuel economy was 11.19mpg (4.216 gallon). Driving was largely sub 50mph.
MAP/SD car.
KOER: 91 and 33 (full operating temp).

 

[FastDriver]

I've got to wonder if you're running on all eight cylinders. I would start with doing a complete tune-up. Plugs, wires, distributor cap and rotor, fuel filter, check timing. Maybe use a piston stop to verify timing.

Your codes suggest you need to replace your coolant temperature sensor, and your O2 sensors. Those are definitely problems that would cause poor gas mileage.

Do that stuff first, and if you're still having trouble, do the tune-up.

 

[NooStang]

Code 21 has disappeared (I wiggled the ECT sensor wires), but 91 and 33 persist. 91 is the right o2 or bank unspecified?

Code is "R" according to Innova. How is the "key off" while "engine running"?

 

[KRUISR]

When you set the base idle with the IAC and spout disconnected, what do you set the idle to? You should be able to get it down to 550-650 rpm.

 

[NooStang]

I set to 650, but it idles closer to 750*. After driving for several days I unplugged IAC (made by Standard) to try and set again, but it would not idle - just died. Out of frustration of setting numerous times (countless at this point) I just plugged the IAC back in and counted my blessing at 750. *750rpm assuming the tach is correct.

Was reading older threads. If I swap 02 sensors around will the 91 turn into 41?

Which '87 Ford service manual has the trouble shooting procedures for the codes, anyone know?

 

[General karthief]

Two things; 47 miles will not give you an accurate MPG, needs to go at least two, but better with multiple tanks of fuel. And lug'n around at low rpms don't work either.
Put new O2 sensors in. Clear the computer.
Oh, and you can't just randomly adjust the idle speed, there is a specific procedure.