Mileage...something wierd...help

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Sep 10, 2007 | 06:51 PM

offroad21

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Mileage...something wierd...help

I have a 1994 with 351 and 97K on the clock. There is a 190 mile run I make each month on the freeway and I use the cruise control at 65. I was getting 17 mpg. Truck has never had a tune up, but regular oil and other fluids. So I put in new regular autolite plugs gapped at .55, New Taylor 8.8 wires, MSD coil -Cap and rotor, K&N FIPK, and had an exhaust shop remove the 2 cats and install a flowmaster 'Y", 3" Magnasound high flow cat, 3" magnasound muffler, and 3" tale pipe. And the compression is very good. No other changes. The problem is now on that same run TWICE, I have only gotten 14 MPG. Anybody have any ideas as to what is going on hear??????
Thanks

Sep 10, 2007 | 07:41 PM

cfrives3

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From: shawnee KS

I can't tell you why, but the same thing happened to my dad on a mazda he bought for running around and for the mileage. He did a complete tune up, and the mileage dropped about 3 mpg. We have never figured this out either, so I would like to second this question, Why would mileage drop with a tune up?!?!?

Sep 10, 2007 | 08:10 PM

godblessmud

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as with all mileage related issues, consistencey is a very key point, how was traffic on the days in question? how was the weather, the tempature? are you sure you wernt just driving it harder cause it was feeling so peppy after the tune?

id make the run a few more times and compare runs from the days most similar to the conditions that were present during the initial 17 mpg run. try that before assuming something is "wrong". you didnt mess w/ the timing during the tune did you?

Alot of times people think they are losing/gaining mileage after a change is made to their vehicle, when in reality they arnt considering the many other factors that affect mileage.

Sep 10, 2007 | 11:00 PM

Kemicalburns

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From: Bend,OR

you gapped the plugs and running a better coil but did you bump the timing as well to 12*btdc with spout connector removed? did the exhaust shop move your o2 sensor from its original location? is it still hooked up?

Sep 10, 2007 | 11:19 PM

offroad21

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O2 Sensor is the same in the same place. Timing was not changed from 12 degrees and I checked that. Conditions were the same ( or as close to as possiable) actually one of the before runs was the hottest ambiant temp. I am erally stumped by this one. I just tried re-setting the computer...we'll see tomorrow.

Russ

Sep 10, 2007 | 11:49 PM

aurgathor

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From: Lynnwood, WA

I don't know what made the change, but aside from the exhaust changes (unless you threw them away) you could probably put the old stuff back to see if that makes any difference.

The golden rule is that you change only thing at a time, that way, it's easy to know what and how changed mpg.

Sep 11, 2007 | 08:11 AM

Kemicalburns

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From: Bend,OR

i would pull codes, just to see if anything is trippen but not setting of the CEL. was there a head wind perhaps on the last few trips. and when you checked the timing you did that with the engine warmed up and the spout disconnected right.

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Sep 11, 2007 | 08:31 AM

Conanski

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Joined: Jun 2006

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From: Ottawa, Ontario

Quote:

Originally Posted by offroad21

K&N FIPK

That exhaust is system is very close to what I have and I saw an increase in milage with mine, so I'm leaning towards the FIPK as the problem. Is this a mass air vehicle? If so, and the K&N is an open element filter then it's definitely responsible for the milage drop. If this is a SD vehicle and it's an open element filter then it's very likely to be the cause. If the filter is enclosed and drawing air from outside the engine compartment then it's not the problem.

The issue with the open element filters is they change the stock induction from an actual "cold air" system to one that draws in hot underhood air. Hot air is less dense and won't build as much compression so TQ output falls. The open element also exposes the mass air meter to fan wash, and that will totally mess up the measurements it makes. Dyno tuning has proven that fan wash hurts both power and milage.