Missing MPGs...

delta.tango · Jan 14, 2016

Long time listener here...

Approx. a year ago, I was getting around 300 miles on a full tank; however, my mpgs have steadily dropped over the last several months so that right now I'm barely getting 200 miles on a tank. I haven't changed gear ratios, tires, or driving patterns, and I started noticing the drop with the summer blend. The truck runs great, I haven't noticed any drop in performance...just the faster movement of the gas hand.

My truck: 1990 4.0 (.030 ob, 410 cam, headers), 4x4, m50d 5-speed, 4.10 gears, 33" tires

The truck stays well maintained. I've recently put in new plugs, wires, fuel pressure reg, injectors, coil pack, o2, tps, pcv, I swapped the MAF with another with no effect. No vacuum or exhaust leaks.

I cant find any fuel leaks while it's parked...

If anyone has any ideas they're willing to share, I would be greatly appreciative...one day gas will be expensive again and I'd like to be prepared :icon_thumby:

SenorNoob · Jan 23, 2016

You'll need a MPG number before anyone can help much. From your described setup I'd expect 17-18 MPG anything better would be dependent on your driving style.

adsm08 · Jan 23, 2016

Miles per tank is a worthless number because a "tank" is not a standardized unit. You can't be sure you are putting the same amount in every time until after you fill up, so MPT is not a useable metric. You need to have two standard units, so miles and gallons, to make a good metric.

Also, if you try using MPT you have to consider that the fuel level sensor isn't particularly precise, and may not be accurately reflecting the actual amount of fuel left.

What you need to do is divide the gallons put into the tank by the miles driven since the last fill up.

delta.tango · Jan 30, 2016

adsm08 said:
Miles per tank is a worthless number because a "tank" is not a standardized unit. You can't be sure you are putting the same amount in every time until after you fill up, so MPT is not a useable metric. You need to have two standard units, so miles and gallons, to make a good metric.

Also, if you try using MPT you have to consider that the fuel level sensor isn't particularly precise, and may not be accurately reflecting the actual amount of fuel left.

What you need to do is divide the gallons put into the tank by the miles driven since the last fill up.

Very well...

MPG less than a year ago = 17-18 MPG

MPG today = 10-11 MPG

I reset my odometer each time I fill up...today I got 10 MPG from a full tank; a whopping 175ish miles

Like i said, nothing has changed that I am aware of since I was getting 17-18 MPG and it is steadily getting worse

SenorNoob · Feb 1, 2016

From your description, I'd suspect a lack of maintenance. Not necessarily a total lack, but maybe one or two very worn sensors...

RonD · Feb 1, 2016

First thing I would do is check the spark plug tips, very light brown is what is expected, darker brown means running rich.
What did the old plugs look like?
If plugs look good then fuel is leaking out somewhere, it isn't burning in the engine.

O2 sensors are rated for 100k miles, as they age they tend to cause engine to run richer and richer over time, this is a "fail safe" because lean will melt pistons

So if in doubt about age of O2 sensors change them, you may only have 1 on a '90.
My '94 4.0l has 2

Restricted exhaust flow will lower MPG without rich burn, just less power so more gas pedal is needed, and it happens slowly so hard to tell it is happening.
A $25 vacuum gauge can tell you general engine condition and if exhaust is restricted, good read here: http://www.gregsengine.com/using-a-vacuum-gauge.html

Also what does temp gauge show, above 1/3 and just below 1/2 is what you want after warm up, cooler running engine uses more fuel.
Thermostat should be a 192degF, people often use 180degF thinking cooler is better, in this case it is NOT, above 190degF means more efficient fuel use and cleaner oil, that is from S.A.E., the "oil guys".

I run a can of Seafoam in the gas tank once a year, seems to keep injector tips clean, I notice my idle is better after this, so better spray from tips, no drips

, probably should do it more often but then I wouldn't notice the difference, and I am cheap, lol.
One of the benefits of fuel injection is that you are not coating the inside of the intake with fuel vapor like you did with a carb, but dripping fuel does waste some on the intake vs a spray

I envy your earlier MPG, I am loaded with canopy and tools so get 13/14mpg around town, 16mpg on the highway, longer trips, lol.

jeremysdad · Feb 1, 2016

Back flush your heater core/replace. It is the replacement for the 'water pump bypass hose' from SBF days.

No flow=cold motor.

Free to try, yielded great results for me.

How's your heat? Mine was lukewarm yesterday, today it's roasting turkeys.

delta.tango · Feb 2, 2016

RonD said:
First thing I would do is check the spark plug tips, very light brown is what is expected, darker brown means running rich.
What did the old plugs look like?
If plugs look good then fuel is leaking out somewhere, it isn't burning in the engine.

I've actually replaced the plugs twice since I started to notice the drop in mileage...both times they looked great.

I'm almost convinced it's a leak, but I can't seem to catch it in the act.

RonD said:
O2 sensors are rated for 100k miles, as they age they tend to cause engine to run richer and richer over time, this is a "fail safe" because lean will melt pistons

So if in doubt about age of O2 sensors change them, you may only have 1 on a '90.
My '94 4.0l has 2

I replaced the O2 sensor a couple months ago...no change

RonD said:
Restricted exhaust flow will lower MPG without rich burn, just less power so more gas pedal is needed, and it happens slowly so hard to tell it is happening.
A $25 vacuum gauge can tell you general engine condition and if exhaust is restricted, good read here: http://www.gregsengine.com/using-a-vacuum-gauge.html

I pulled vacuum readings today; no issues.

RonD said:
Also what does temp gauge show, above 1/3 and just below 1/2 is what you want after warm up, cooler running engine uses more fuel.
Thermostat should be a 192degF, people often use 180degF thinking cooler is better, in this case it is NOT, above 190degF means more efficient fuel use and cleaner oil, that is from S.A.E., the "oil guys".

After several weeks of fluctuation, the 192 deg tstat stays put at 192

RonD said:
I envy your earlier MPG, I am loaded with canopy and tools so get 13/14mpg around town, 16mpg on the highway, longer trips, lol.

You and me both!

delta.tango · Feb 2, 2016

SenorNoob said:
From your description, I'd suspect a lack of maintenance. Not necessarily a total lack, but maybe one or two very worn sensors...

The only ones left that haven't recently been replaced (with motorcraft parts) are the IAC and the crank position sensor. The new IAC will be here tomorrow...I don't expect it to fix the problem, but it's time for a new one anyway. I will report back with any significant changes as a result of the replacement.

On a side note...does anyone have any tips for finding a gas leak?

SenorNoob · Feb 2, 2016

Go by your nearest box store and ask for a few of their shipping dividers. Lay them out on the ground and pull the truck up onto them. Let it idle there for a few minutes. Any leak should be pretty easy to spot that way.

Sent from my MotoE2(4G-LTE) using Tapatalk

AllanD · Mar 15, 2016

Often a loss of mileage complaint is related to a wonky ECT sensor.

ECT=Engine coolant temp, IE the sensor that tells the Computer the engine temp (NOT the Gauge sensor)

Because the Computer runs things richer when it thinks the engine is cold...

The big problem with electronic engine controls is "Almost right" sensor outputs...

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