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Sudden HUGE decreases in MPG, 14.5 to 7.5

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Old 03-19-2013, 09:22 PM   #1
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Sudden HUGE decreases in MPG, 14.5 to 7.5

I filled up two days ago and everything seemed fine, then when I was coming home from work monday I realized I had already gone through a 1/4 tank, this morning I dropped down to almost 1/2 on my way to work, about 13 miles.

My first thought was that someone was stealing gas, but I stayed at my girls place last night and was at mine before that, also I watched it go down all the way to work. I've been getting mid to high 14 mpg since I've owned the truck, about a year. Now itís around the 7.5-8mpgs so a HUGE decrease.


It's not the tires or winter gas, there has been little fluctuation since I put the tires on and the onset of winter gas, if it was either of those it would have happened long ago, I have done nothing to the truck in the last month or two. I have in the last six months changed the motor oil, diff oil, greased the joints, new air filter, cleaned the MAF and throttle body and replaced the spark plugs with ones form the dealer. I have no codes or pending codes. Not towing anything, nothing has changed in the last month or few days.

Any ideas? I've found a few suggestions around the web but mostly people saying it's winter gas or large aggressive tires, which is not the case since it hasn't been an issue for quite some time and suddenly dropped to almost half.

Thanks.
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Old 03-19-2013, 09:24 PM   #2
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clogged cat
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Old 03-19-2013, 09:26 PM   #3
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Isn't 14 still pretty bad? I have consistently been getting 19-21 on my 4x4 1st gen?

Could be vacuum leak? Fuel filter?
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Old 03-19-2013, 09:36 PM   #4
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Quote:
Originally Posted by RiverBoyTRD View Post
clogged cat
How would I check? have to take it off?

Quote:
Originally Posted by TimmyB View Post
Isn't 14 still pretty bad? I have consistently been getting 19-21 on my 4x4 1st gen?

Could be vacuum leak? Fuel filter?
It's bad but not very in a double cab, at least from what I've seen in the firs gen forum.

Wish I was getting 20

I was planning on changing the fuel filter in the near future, I doubt it has been changed. I would think if there was a vacuum leak a coed would show though?
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Old 03-19-2013, 09:40 PM   #5
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Quote:
Originally Posted by RiverBoyTRD View Post
clogged cat
^This. Same thing happened to my truck, but i was closer to 5-6 mpg.

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Originally Posted by nagorb View Post
How would I check? have to take it off?
Backpressure test. Basically you remove the front o2 sensor and replace it with a backpressure test gauge. If the cats are clogged, bp will be high.
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Old 03-19-2013, 09:45 PM   #6
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Did you gap the plugs? Don't know much at all about the 1st gens. My plugs were close from the dealer but not right on, one was actually almost touching. Idk? Wouldn't explain the crappy mileage all of a sudden. Good luck .

Brett
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Old 03-19-2013, 09:46 PM   #7
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Quote:
Originally Posted by jberry813 View Post
^This. Same thing happened to my truck, but i was closer to 5-6 mpg.



Backpressure test. Basically you remove the front o2 sensor and replace it with a backpressure test gauge. If the cats are clogged, bp will be high.
Did your rig feel like it was under power also?

Brett
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Old 03-19-2013, 09:46 PM   #8
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Quote:
Originally Posted by jberry813 View Post
^This. Same thing happened to my truck, but i was closer to 5-6 mpg.



Backpressure test. Basically you remove the front o2 sensor and replace it with a backpressure test gauge. If the cats are clogged, bp will be high.
I'll have to give it a try, I'll either pick one up or use my buddies. Hope it's that simple and not something all fucked.
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Old 03-19-2013, 09:48 PM   #9
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Quote:
Originally Posted by Raceline08 View Post
Did you gap the plugs? Don't know much at all about the 1st gens. My plugs were close from the dealer but not right on, one was actually almost touching. Idk? Wouldn't explain the crappy mileage all of a sudden. Good luck .

Brett
I did, they were fine though.

Quote:
Originally Posted by Raceline08 View Post
Did your rig feel like it was under power also?

Brett
Mine doesn't at all seems to drive just the same.
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Old 03-20-2013, 06:57 AM   #10
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Quote:
Originally Posted by jberry813 View Post
^This. Same thing happened to my truck, but i was closer to 5-6 mpg.



Backpressure test. Basically you remove the front o2 sensor and replace it with a backpressure test gauge. If the cats are clogged, bp will be high.
I agree with this! DANG I remember that thread. We were chasing all sorts of potential possilble causes with that truck, if it is the one I remember.
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Old 03-20-2013, 09:07 AM   #11
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Quote:
Originally Posted by BamaToy1997 View Post
I agree with this! DANG I remember that thread. We were chasing all sorts of potential possilble causes with that truck, if it is the one I remember.
Yessir! That was the thread. A month endeavor to find this:

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Old 03-20-2013, 09:28 AM   #12
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^^^ clogged cat is top suspect

if cat is not clogged,
if tailpipe has black soot it is running too rich, and could be ecu malfunction
or a/f sensor malfunction but not throwing a code
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Old 03-20-2013, 09:41 AM   #13
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if not a clogged cat - leaking fuel line?
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Old 03-20-2013, 01:08 PM   #14
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Low tire pressures (obvious),
dragging brakes (should cause friction heat),
bad sensor causing ECM to increase fuel (many sensors to check)
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Old 03-20-2013, 02:22 PM   #15
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on my 99 5vzfe

when my a/f was on the fritz not yet blowing a code
and at the same time my TPS was about to go bad (also not throwing a code)

my tailpipe was sooty...replaced a/f when it finally shat p1135 at me and
fixed tps and blam, clean pipe again
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Old 03-28-2013, 08:02 PM   #16
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Sorry for the lack of update been busy at work.

So my MPGs went back up to 13.5ish, still low but not nearly as bad. I figured my buddie would have something to test the back pressure, dude usually does. Rather then buying one I figure I could just take the cat off and run through a tank. Still haven't had time to do it though, not sure how loud it will be, I was thinking of fabbing up a through pipe.
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Old 03-28-2013, 08:09 PM   #17
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Quote:
Originally Posted by nagorb View Post
Sorry for the lack of update been busy at work.

So my MPGs went back up to 13.5ish, still low but not nearly as bad. I figured my buddie would have something to test the back pressure, dude usually does. Rather then buying one I figure I could just take the cat off and run through a tank. Still haven't had time to do it though, not sure how loud it will be, I was thinking of fabbing up a through pipe.
Another way to check your cat is by heat. Get a laser temp gauge. If the cat is hotter than the rest of your exhaust than its bad. They will glow cherry red sometimes when they are clogged. But a laser temp would tell you if there is a big difference.
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Old 03-28-2013, 08:13 PM   #18
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Quote:
Originally Posted by Millertime187 View Post
Another way to check your cat is by heat. Get a laser temp gauge. If the cat is hotter than the rest of your exhaust than its bad. They will glow cherry red sometimes when they are clogged. But a laser temp would tell you if there is a big difference.
Thanks! I've got a few of them at work, I'll check tomorrow.
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Old 03-30-2013, 07:52 PM   #19
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Quote:
Originally Posted by Millertime187 View Post
Another way to check your cat is by heat. Get a laser temp gauge. If the cat is hotter than the rest of your exhaust than its bad. They will glow cherry red sometimes when they are clogged. But a laser temp would tell you if there is a big difference.
I checked it yesterday, I left the numbers at the shop but it was hotter before the cat, the cat and pipe after were cooler. I think it was 450 before the cat and 350 at the cat and after.
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Old 03-30-2013, 09:36 PM   #20
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Bad mpg

This sounds like a cat issue to me too.
A good indication is really poor mpg and loss of power.
The power loss symptom is sometimes hard to determine.
If you have a light truck with good hp it may not show up easily.
One way to determine real power loss is to load the truck up, and hit a long steep grade. If u are depressing the gas and downshifting way too much, this is strike one.
The easy no high tech tool test before pulling cat is to remove the o2 sensor before the cat, tie it up, and hit the hill again. If u run right up the hill, strike 2.
Final test, pull the exhaust and look at the inside! It usually shows broken or chunked up pieces. Strike 3.
Time to post on TW for a used cat, hit the junkyard, or buy a new high proformance cat.
Good luck.
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