I have a 2001 F150 SuperCrew (I need the extra car seat room for all the
kids) with a 5.4L v8. It now has about 68Kmiles. Over the recent XMas
break, I replaced the plugs with MotorCraft double-platinum, same type &
number as the original stock plugs. Over the past few months and about 6K
miles, I have experienced about a 2 MPG decrease in gas mileage, which makes
no sense to me. I have no noticeable decrease in performance. I have kept
records of every gas receipt (and every other receipt) for this vehicle, and
graph them periodically in Excel. (yeah, SWMBO says I'm really really AR - I
already know this...) Also, I do and have been purchasing gas at the same
place for years - Texaco then Shell 93 octane) As far as I can tell, there
are no vacuum leaks, and I have a new air filter, PCV valve, etc. The
engine is completely stock.
Most driving is in town, and I was getting about 13.57 MPG. Now, I'm
getting 11.48 MPG (smaller sample obviously, but a clear deviation in the
pattern as compared to the past several years)
Anyone care to speculate as to what might be going on here, and what might
be causing my sudden decrease in gas mileage? I suspect this is somehow
related to changing the plugs, but the replacements are the same as the
originals and there is no "miss" or anything else I can notice/find.. I
would appreciate any suggestions as to what/how to look for my lost gas
Does the manual recommend premium for your truck? If not, changing to
regular may increase your mileage a bit, in addition to being less expensive
to buy. Premium fuel is diluted with additives that make it more resistant
to detonation, and it actually has a lower BTU content than regular. Most
modern engines have knock sensors that will retard the timing if detonation
occurs, and premium may allow the engine to run a little more advance, which
is good. It may even help offset the lower energy contained per gallon of
premium. But never by enough to compensate for it entirely. The only
reason to run premium is for an engine with effective compression so high it
absolutely requires it. And the manual should clearly spell it out.
Before you go changing parts, connecting your truck to a good scan tool
might give some clue to the problem. It could be an O2 sensor. Or
something as simple as a bad cooling system thermostat. Tires aired up to
at least the manufacturers recommendation? Brakes dragging?
Tires replaced last August, and slightly over inflated by about 3-5 PSI; I
like the "feel" better. Brakes OK, and not dragging.
The manual says use regular, but I usually use premium because I was under
the impression that the additives keep the engine cleaner and provide more
HP. That could be completely wrong though...
O2 sensor sounds like the most likely culprit, except I thought those either
worked or they didn't, and there little or no "sortof works" ground. Is my
assumption also incorrect, or am I thinking of the "mass air flow" sensor?
Also, the decrease in mileage corresponded almost exactly with changing the
plugs, which seems like too much of a coincidence. I'll pull one of the
plugs and see what it looks like - I expect it should look pretty good.
So, bad cooling system thermostat stuck closed would generally cause
overheating; stuck open would generally cause timing advanced too far, yes?.
I haven't noticed either of these symptoms, nor any temp gauge readings
outside of their normal range.
If I had managed to slightly crack one of the plugs during installation,
could this produce the decreased gas mileage symptoms without a noticeable
miss? Would a scan tool be able to detect this?
Thanks for the advice.
===O2s can get "lazy", and not set a code. 68K its time...You only need to
change the front one(s). Speaking of Mass Air Flow Sensors, you should clean
yours, a dirty one can cause ping, which will kill the performance and
retard the timing.
Stuck open would cause it to run a little rich and probably a little retard
====It has to be a pretty big miss to set a code. If you had a problem with
the plugs you should be able to feel it. A slight miss can cause the engine
to run rich too, the computer sees the extra O2 as a lean condition and
makes up for it. Did you gap the plugs?
I pulled the plugs today, one at a time, and noticed that one of them (front
cylinder on passenger side) was noticeably covered with a hard brownish
deposit - the kind I would normally attribute to "additives", and can be
flaked off with something hard like a small pocket knife. I replaced this
plug (for free from my local parts dudes - thanks Van's).
I don't know if this indicates a problem with the plug itself, or with the
plug wire or coil. A tank or two of gas should confirm if this is "fixed"
or not. I may be asking for more help in a week or so (or hopefully not :-)
Also, thanks (to everyone) for the well-founded advice on 87 vs 93 octane on
a stock engine.
On a related topic - If I don't plan to "drive like a maniac" (i.e.. current
performance is satisfactory for me), does it make more sense (economically)
to purchase 97 octane gas and continue with a stock engine, or a buy a
93-octane chip like the Diablo SuperChip and continue to buy 93 octane?
================Dave, just leave the thing alone. Get it back to stock running right. There
is nothing you can do cheaply to get better milage. It's just the way it
Seeing how you went to Van's, I assume you are in Austin, Tx.
I agree with all. Leave it stock.
Somebody tell me whether I'm right or wrong. It's been to long since I've
read a plug. Lignt tan would be coolant/water?
Regular gas has all the detergents required to keep injectors clean.
Chevron is generally recognized as a good brand, but others are fine.
O2 sensors can get "lazy", where they do not toggle back and forth between
lean and rich like they should, yet still not be bad enough to trip the
check engine light.
Agreed. I guess there is no chance a vacuum hose to a fuel pressure
regulator or something accidentally got disconnected during the plug change?
Stuck close would cause overheating and should become rapidly apparent. But
stuck open or partially open would fool the electronics into thinking the
engine needed a richer mixture, similar to running too much choke on a
carbureted engine, especially if the truck is used for short trips.
Scan tools typically cannot detect misfire, but there are more deluxe
testers that can. But scan tools are great at reading O2 voltage and
coolant temp in real time. A misfiring cylinder can often be felt under
load. Plugs can be cracked during installation, or even be broken before
you take delivery of them by being dropped, etc. Also, the spark plug
cable itself can be damaged during the removal from the old plug.
Yup...abslutely no HP gains unless the engine requires is (and retards
timing w/o), or you're running a modified engine with higher
compresssion/forced induction/or advanced timing (chipped).
No real difference in keeping the engine cleaner with reputable 87
octane...if you want, Techron seems to be one of the better OTC fuel
treatments, and you could run that every couple of months and save
more than enough money to pay for it.
Like the other gentleman said, the additives in 93 octane decreases
the overall amount of energy avalible per gallon. In some brands,
espicially in some cars, there is a very noticable MPG drop, and some
HP drop (visible on dyno).
While we're on the topic, ALL of the products that purport to boost
the octane rating of gas are a total waste of money. The treat rates
of, say that 104+ stuff, needed to actually change the octane rating
of 10 gallons of gas by 2 octane is something like 5 bottles. (The
advertise their increase in points WHICH IS ACTUALLY decimal points).
Run race gas if you need high octane for a modified engine...add large
quanitities of toulene...but don't buy any of the octane boosters.
Spark Plug Gap!!! The new plugs are gapped to close, a wider gap, up to
a point, will not only give more power but more miles per gallon. Add
another .020 to the gap and you will get your MPG back. A U grove or
Split type plug is also good for about half a tic in MPG. Also make sure
your air filter is clean as well as the throttle body and plate.
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