Looking for some suggestions for gas mileage that has recently (in last month) plummeted. I track my fuel economy both with a scanguage II and hand calculated at fill up. Lately, I'm getting 10-11 overall and my scanguage reads anywhere from 29-80 liters per 100km while driving 30-50 kmph. At initial startup, the guage reads 65-80 l/100km which converts to 2-3 mpg. I have been nursing the throttle, lots of coasting, doing my usual old man driving.

Truck is seems to be running well; no hesitation, not stumbling, idle seems normal. Other than the mileage, truck seems great. Can't figure out what might be wrong. Any suggestions? What values should I be tracking on the scanguage?

Edit: coolant temperature is normal. Heats up to consistant running temp of 190-ish.
Since November - Truck has fresh plugs, fresh pvc, cleaned MAF, cleaned egr, cleaned throttle body, brakes have been checked (not dragging), air filter seems clean, new upstream o2 sensors, transmission fluid change. Have had a few appearances of the po401 code, but not in over a month. Wondering if that is the dpfe as it seems to be original.

 

Several things.
As it gets colder the fuel mileage will go down.
Air density increases are detected by the intake air temp sensor as well as detected by the greater Ox content in the exhaust.
The OX sensors richen up fuel to maintain A/F ratio as set by the senors calibration.
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If you have a marginal 401 test failures, the EGR will not cut fuel back under light load conditions.
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Fuel may also be a part if the mix and if Ethonal has been increased for the winter months.
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Cold start fuel mix hangs on longer until the engine heats on each start..
Good luck.

 

Truck is still guzzling gas.

Bluegrass 7 - you mentioned that the egr might not cut back on fuel if marginal 401 codes are being read. Can you tell me more about this. My scanguage tells me when codes are set and I haven't seen one for a while. It also isn't showing any pending codes. Is there a way for me to tell if this is the case?

 

Ok, if there was any EGR code present, the computer suspends the EGR operation.
When this happens the fuel is not cut back during normal OD operation and fuel mileage would be affected.
If you have none of these codes the fuel is being cut back as it is suppoed to.
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During cold temperatures, the cold start is richer for a longer period of time until engine warmup.
Cold air is also heavier by volume and requires more fuel to maintain the correct air to fuel ratio as determined by the Ox sensors. This will cause a lowering of fuel milege.
The engine thermostat will have an effect on fuel mileage if it regulates to temps to far below about 195 degrees because the cylinder head temp sensor detects this and richens up the fuel some amount.
The air intake temp sensor also monitors intake air and could cause a small richening of fuel if out of tolerence.
Any combination of these could adversely affect fuel mileage.
These are all in the fuel control chain.
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Only other possabilities are plugged cats and leaking injectors, lazy Ox sensor operation, fuel pressure to high, gas leaking, gas quality, small intake air leak, low tire inflation and excess load being carried around.
Some owners use alcohol type gas additives such as water clearing agents that will affect fuel mileage as well from the extra oxygen released during combustion. This is detected by the OX sensors that goes back and richens up fuel even more trying to account for the extra oxygen..
About all I can offer for overall causes.
Good luck.

 

Thank you for your response.

There haven't been any egr codes for more than a month so, based on what you've indicated, I will rule that out as a likely source of fuel enrichment.

Monitoring the coolant temps with scanguage 2, the thermostat seems to be functioning normally. Truck warms up progressively to 202-ish then settles somewhere in the 190 range once the stat opens. Coolant level isn't changing and looks good.

IAT monitoring with scanguage, temps again seem relative to the external temperature.

Plugged cats - I can't tell if they are plugged or not. Truck has a lot of kilometers so I suppose it is possible. I have heard of some people having their exhaust manifolds glow from clogged cats. Haven't had this experience.

Leaking injectors. I'm not sure the best way to test for this.

The upstream o2 sensors are new in December so I hope they aren't lazy. Installed Bosch brand. The truck goes into closed loop within a few moments of startup.

I have a pressure tester so I can test the fuel pressure. I will do this this weekend.

I have not noticed any fuel leaks. I buy my fuel from 3 different gas stations, two of which are national chains, haven't noticed any variation that way so the fuel quality is probably okay.

Tire pressure is good. I have E-rated tires and run them at 55-60lbs and the truck is typically empty except for passengers and a fiberglass canopy. Canopy has been consistently on the truck since the first week I bought the truck.

Haven't tested for an intake air leak. Will have to try this soon.

I don't use any additives of any kind.

 

Okay, hooked up fuel pressure gauge to the schraeder valve. Pressure is reading 31-32 at idle.

Can't for the life of me find the pressure regulator to disconnect the vacuum hose to see if the pressure increases. Any help here is appreciated.

Turned off truck and left guage attached while checking internet for fpr location. Come back 5-10 min later and pressure has increased to 40??? Start up truck again and 32, shut it off and watch guage, slowly builds up to 40 again. Is this normal? I had assumed that it would hold then slowly bleed lower, not higher.
(edit: okay, poking around the net, sounds like this is normal, and may simulate the same scenario as unplugging vacuum from regulator. Would still like to find it though.)

I have a new fuel filter and will change it today. Just wanted to test pressure pre-filter change in case that was an issue.

Also, noticed that fan clutch seems to be engaged, more or less all the time, i can hear it running, shutdown for a few moments, then re-engage. I'd say its engaged 90% of the time. This is with the truck at idle in the driveway, scanguage showing temp around engine temp 166-168. Outside temp is 45-ish (6-8 celcius). Wondering if that is normal.

 

The regulator is on the driver side toward the rear on the fuel rail.
What you saw for pressure performance is fine. Leave the regulator alone.
Pressure rise is due to the under hood temperature rising for a short time after engine shutdown that causes fluid expansion/pressure rise.
Same thing happens to transmission fluid volume change. You only check that fluid level when hot.
Same thing happens to the cooling system.
No benifit in changing the regulator.
The fan operation as you saw it is normal.
As the clutch engages and disengages from radiator temp changes/ thermostat open and closing, you can hear the airflow change.
A way to look at the fan clutch operation is to hook up a timing light triggered off any cylinder. Shine the light on the fan blades and the strob flash will make them stand still then change speed as the clutch comes on and off.
I think you waisting time and money chasing things that don't exist.
Good luck.

 

Thank you for your response. I appreciate all of the shared knowledge and wisdom.

All of the parts I have replaced are maintenance and wear items, certainly justified given the mileage and unknown previous maintenance history. Things like plugs, fuel filters, pvc, brakes - these are all things I do to any used vehicle I purchase where the age / mileage / component condition seem to warrant it. The o2 sensors, egr check, maf cleaning, all related to a previous issue with 401 codes that seems to be resolved. The tranny fluid change has cured a delayed shift / refusal to shift issue that was present when the truck was purchased.

The drop in fuel mileage seems much more dramatic than any other truck I have owned for winter reasons. Under 10mpg doesn't compute for a gently driven v6 2wd truck. Honestly, that is what my v10 4x4 super duty did, and much more capably to boot.

I still plan to check for air intake leaks as was suggested earlier. Will probably try the propane method.

Any further suggestions are welcomed.

 

I would get hold of a Scanner and look at the long and short term fuel trim tables to see if they are out of line as to near the rich side.
Check the OX sensors for switching etc.
If there would be anything showing up, the scanner will show it.
Good luck.

 

Hmm. Took the truck for a drive after changing the fuel filter and changing the oil. Scanguage fuel economy average now shows 65 l/100km to a best value of 40 l/100km after a 5km drive. That works out to approximately 3.5-6 mpg.

Are you supposed to reset the computer (unplug the battery) after a filter change?

Looks like you're right and I need to hook up to a scanner.

 

An oil/filter change won't do anything for fuel mileage that fast.
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Rebooting the computer resets all the tables so you won't see any value, short term, doing a reboot because the fuel tables are "moving average" type tables, over time.
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Looks like your Scangage is taking you for a ride such that your reading too much into what it is showing you.
Only way to do average fuel mileage is to be exact and do accurate mileage vs refill the same way several times, then average the results.
Otherwise it's all wishfull thinking that causes you to look for issues that are not there.
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For example, I can get as high as 17.6 mpg on a short distance running average over 30-40 miles that is road dependent, but the real total distance running average can still be in the 14 to 15 range.
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Monitors can take the user for a ride in thinking/expectation.
Looking at instant mileage at any given moment, you can drive up that reading by throttle control.
If you watch the 'running' average' keeping the instant high will update the running average every so often in time as the software is written to do.
But the overall fuel used can still be quite a bit more after you consider cold starts, low speed running, shifting through the gears so often from stops, changes in road grade, changes in passenger loads, changes in ambient temps. etc.
Bottom line don't look for fuel milage that is impossable to get with a near 5000 lb truck.
Expect the best mileage in summer temps between 75 and 85F when A/C is not needed/used and engine control is at it's best for fuel mileage running in the 55-60 mph speed range..
Watching all this the last 9 years with my monitor has shown these 'expected' responses.
Most people never think about the differences between instant, average and real fuel mileage and put to much emphisis on the numbers they see from a monitor without realizing what's behind the way the monitors work and what they are responding too..
Good luck.

 

Just as an FYI, but if you have the defroster on continually, that will keep the A/C comprssor running to dehumidify the defroster air to defog the windshield quicker. If the defroster is not needed, turning it off will disengage the A/C compressor in cold weather.

 

Thanks for this tip. It is so humid here, the climate control is left in the defrost position pretty much full time in the colder months. I will make an effort to switch in once the windows are clear. Good suggestion!

 

What would anybody consider to average mileage on these trucks?
I have an 2001 f150 4x4 with a 5.4. Seems to get around 14 mpg no matter what. Is that a normal number?