Worse mpg on a fresh tuneup; wtf?

90_Red_LX · Feb 14, 2006

Recently, as in about a month ago I gave my car a tuneup. New plugs/unused wires that I bought a while ago, FRPP 9mm wires/cap/rotor/pcv valve/pcv grommet/pcv mesh filter that goes with the grommet/and a fuel filter. My car used to knock arount 175 miles with a 1/4 tank left, and around 200 when I fill up. Right now with a 1/4 tank left it reads 120miles, and my first fillup was around a 170 or so miles. What could the deal be? I'm thinking about pulling some plugs and seeing if they look rich. The car drives like it used to otherwise, and my driving habits haven't changed. The only thing I did during my first fillup was I may have given it a couple extra WOT runs to see if I could detect a power difference. I assumed that may have been the case the first time so I didn't mention it. The car is warmed up about 5-10 minutes in the morning, and I didn't notice a change in gas mileage before the tuneup so I don't understand? What could it be?

jrichker · Feb 14, 2006

Dump the codes and see what the computer says is wrong…

Here's the link to dump the computer codes with only a jumper wire or paper clip and the check engine light, or test light or voltmeter. I’ve used it for years, and it works great.

See http://www.troublecodes.net/Ford/
OR
See http://www.mustangworks.com/articles/electronics/eec-iv_codes.html

IF your car is an 86-88 stang, you'll have to use the test lamp or voltmeter method. There is no functional check engine light on the 86-88's except possibly the Cali Mass Air cars.

Codes have different answers if the engine is running from the answers that it has when the engine isn't running. It helps a lot to know if you had the engine running when you ran the test.

Trouble codes are either 2 digit or 3 digit, there are no cars that use both 2 digit codes and 3 digit codes.

For those who are intimidated by all the wires & connections, see http://www.actron.com/product_detail.php?pid=16153 for what a typical hand scanner looks like. Normal retail price is about $30 or so at AutoZone or Walmart.

Or for a nicer scanner see http://www.midwayautosupply.com/detailedproductdescription.asp?3829 – It has a 3 digit LCD display so that you don’t have to count flashes or beeps.. Cost is $35.

90_Red_LX · Feb 14, 2006

Wouldn't it trip the check engine light if a trouble code came on? The only other person to drive it other than a mechanic to align my front end up was my dad, and he would definitely notice an engine light trip on; same for me. I'll see if I can dump the codes sometime and see what they say.

HISSIN50 · Feb 14, 2006

The puter can store codes without triggering the CEL, especially on something relatively minor that comes and goes.

Good luck.

sunil6784 · Feb 14, 2006

did you gap your plugs ?

JBeezy53 · Feb 14, 2006

What plugs did you use?

Roland69 · Feb 15, 2006

did you drive it harder to see if your tune-up helped. My MPG is always different

vristang · Feb 15, 2006

I can't think of anything other than the above plug questions as a potential cause. Unless something was inadvertently unplugged while doing the tune up.

It does need to be said however that the method of checking mpg by the fuel level gauge is far from accurate.
If you want to use mpg as an indicator of anything you will need to keep better records than this.
A simple excel spreadsheet can keep track of miles per fillup, gallons per fillup, and provide an accurate measure of MPG.

302RollinHard · Feb 15, 2006

Also it depends on whether you get your gas from the same place eg Chevron, Shell, etc. every time. Different refining methods yield different densities of gasoline and hence more or less gas mileage. Also, maybe your area is changing to winter blend gas. That also worsens gas mileage? My 2 cents.

90_Red_LX · Feb 15, 2006

When I look at the fuel gauge and my trip meter I can't calculate the fuel mileage, but based on the fact I've been driving the car for a couple years I know what the mileage usually is at certain point on the gauge. I understand what your talking about though, a more efficient monitoring system. The only problem is that it's noticeable without the use of logging it. The car used to get an average of 17mpg. 18-20 when I used the highway more often.

The plugs were gapped at .054 I think? It was inbetween the factory specs. My dad gapped the plugs and he's pretty experienced at it.

I fill up with exxon gas at the same station every time.

I did play with the car a little bit, but nothing much out of the ordinary, but somethings not right. 15mpg on a 2.73 geared car tells me something isn't working right. I'll see if I can pull the codes, didn't get a chance to tonight.

HISSIN50 · Feb 15, 2006

15 MPG in the city with getting on it can burn some fuel. Some would argue that the 2.73's are hurting you since you might have to stand on it longer and harder to accelerate at a given rate (where a geared car would require less throttle input due to the mechanical advantage). Now if this is all highway driving in 5th with 2.73's, something might be wrong.

vristang · Feb 15, 2006

90_Red_LX said:
When I look at the fuel gauge and my trip meter I can't calculate the fuel mileage, but based on the fact I've been driving the car for a couple years I know what the mileage usually is at certain point on the gauge. I understand what your talking about though, a more efficient monitoring system. The only problem is that it's noticeable without the use of logging it. The car used to get an average of 17mpg. 18-20 when I used the highway more often. .

I see what you are saying. In my experience though there is too much variation in what the gauge reads.

I have filled up thinking I would have a very high mpg, only to find out that the calculation put me in my normal range. vice versa is true as well.

I can get over 20mpg on the highway, and I average around 17-18mpg. Every once in a while I get down into the 15 range. Possible causes include; throttle enthusiasm, variations in fuel mixture (even if filling up at the same station), and deviating from my standard driving cycle.

My point is just that without some concrete numbers to work with, it is hard to use MPG as an indicator of engine health.

As far as solutions to your concern...
A vac leak or bad wire connection would be my first guesses. Sometimes the plug wires can be tough to get properly seated on the plug. dielectric grease should be used.
While working in the engine bay it is fairly easy to knock loose a vac line, but even easier to crack a deteriorated line.

Hope it helps
jason

90_Red_LX · Feb 16, 2006

Would it be possible my plug wires aren't any good? I had them laying around for a year or two, not sure which. They weren't used and were kept in their box though?

Most of my driving is in the city, so is it possible it could vary a lot? I'm going to fill up tonight so I can see what my numbers are on this tank. She still runs as strong as when I first got her, but the past couple oil changes I'm having to add a quart aroudn 2500-2800 miles. I change anywhere from just before 3000 miles to as soon as I get a chance after 3000miles. This probably has nothing to do with my fuel mileage, but I added it just in case.

vristang · Feb 16, 2006

90_Red_LX said:
Would it be possible my plug wires aren't any good? I had them laying around for a year or two, not sure which. They weren't used and were kept in their box though?

Most of my driving is in the city, so is it possible it could vary a lot? I'm going to fill up tonight so I can see what my numbers are on this tank. She still runs as strong as when I first got her, but the past couple oil changes I'm having to add a quart aroudn 2500-2800 miles. I change anywhere from just before 3000 miles to as soon as I get a chance after 3000miles. This probably has nothing to do with my fuel mileage, but I added it just in case.

I think city driving could vary some, I suppose.

Are you dripping oil from a gasket or burning it?

Zero_chance · Feb 16, 2006

City driving alone would be the biggest factor because traffic is never the same.

ninety15.0 · Feb 16, 2006

I'll be honest with you, i got better milage in my car with 3.73's than i did with 2.73's around town. Also it only take a couple WOT rips through the gears to mess up your milage. Fill the car to the top....take it out for a cruise on the highway for like 100 miles go right to the gas station fill it to the top again and do the math. see exactly where you are at and compare it to "normal"

352Ford2 · Feb 16, 2006

some times new parts are bad, checkall your plugs, wires etc, to see if you got some bad parts

I have also had issue with spark eco if I have really low resistance ignition system (MSD wires +non resistor plugs)see if your plugs are resistor plugs, but those FMS wires usaly have pretty high resistance.

90_Red_LX · Feb 18, 2006

Ok guys I pulled the codes. No codes were stored on the engine off test. On the engine running test, 94 showed up which is:

94 AIR system inoperative - Air Injection

I have it tied off to my o/r pipe and I've had the pipe on it for over a year now. I think it may have flashed a 54 also, but I'm not sure.

I'll also add that my pipe is tied off, but it should still be operative, as it can pull air (just not exhaust) back since it's not plugged off or anything. It's been like this for a while with no errors so I dunno. Another code 94 list came up with this:

94 (r): Thermactor air injection system inoperative, bank #2. Suspect AIR pump, diverter valve, solenoids, harness.

Michael Yount · Feb 18, 2006

I'll add that city driving has SO MANY variables it's very difficult to make any back to back comparisons that are useful. And getting on it, even a little, can make a BIG difference in these engine's mileage. They can get decent mileage for their performance -- but when you exercise the motor, especially around town, mileage is gonna plummet fast. And 'warming up the car for 5-10 minutes' is absolutely a gas mileage killer. It wastes fuel, and the car warms up much quicker while being driven.

Do a test with a longer drive on the highway. A 2.73 geared car running well ought to knock down low to mid 20's on the highway. If it will, and if it's running ok (no CEL's or unexpected codes) then your gas mileage is doing the best it can. Improving it around town will be an issue of you improving your fuel conservation driving skills. No warm up, no 'getting on it', low rpm shifts, etc.

By the way, you need to actually calculate your mileage -- I don't understand why people insist on simply listing how many miles between fill ups. Reset your trip odometer to zero - or note the mileage reading when you fill it up each time. Note the # of gallons it took to fill it up. Divide the first by the second and you have your mileage. Keep records of that over a few months, and you'll have a good idea of what your car averages.

90_Red_LX · Feb 18, 2006

I calculated my miles per gallon, it was 14.582 when I filled it up the other night. I was just stating that the trip meter read a lot lower than it used to.

Worse mpg on a fresh tuneup; wtf?

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