Drop in MPG

[rollinstone]

I put new Motorcraft platinums set to .054 on my 97X 5.0L and immediately got a 2-3 mpg increase on two tanks of gas (19.8 mpg). Couple of weeks later I received a set of Motorcraft 8mm plug wires thru Fordparts mailorder, installed them and mpg dropped to below what it was before I put the new plugs in (15.2 and 16.4 mpg ???). I didn't expect miracles, but thot I'd at least hold on to the 2-3 mpg increase.

I went back in and ensured all the wires were fully seated, same thing topside...still less then enviable mpg. A friend suggested checking the new wires' resistance with a multimeter...possible bad wire not carrying spark. But, she sure runs smooth...no hint of running on less then 8 cylinders. Anyone have any idea what's going on?

Kinda weird...I think I'm going to reinstall the old wires.


r.s.

 

[wired_af]

ya just put the old ones back on if you really did notice a drop in mpg

 

[twomedicine]

mpg drop

Same brand of fuel? Topping it off the same way as always?

Some fuels that contain the 10% ethanol aren't supossed to give you the same amt of mpg as straight gasoline.

Sounds strange...maybe give it a few more trys. Mine varies too but usually has an attributible cause//

good luck, 2med

 

[Alec]

By any chance did you regain some lost power and you're using the gas more?

 

[X~FACTOR]

Would getting 10mm wires do any good? Would that give extra power or just an overkill?

 

[2001ExpSport]

I haven't touched my truck, but for maintainence for more than a year and my mpg fluctuates between 12 and 15. To get an accurrate mpg number you need to average 3-5 tanks of fuel. You need to drive EXACTLY the same everytime.

For example:

198
207
255
223
227

average=222

average mpg on 17 gallons= 13

 

[rollinstone]

Thanks for the prompt replies! Actually, I have been religious about driving under the same conditions...speed, route, etc. And I always use the same gas, and don't stomp the accelerator...never have, never will.

Right after I post this I'm going to pull the wires one at a time and check the resistance. The old ones were hitting about 8 to 10 ohms. My understanding is that a good set of wires should come in under 1 ohm with 0.5 being optimal. I'll report the results for everyones' edification.

r.s.

 

[chadb]

Plug wires

You might check to make sure they are all on in the correct order even though I bet it would run horrible if they weren't. Another thing to consider is that maybe all don't have continuity to the spark plugs. Another thing to think about is maybe something else is going on. You might want to pull codes and see if everything is ok. I personally don't really see how properly functioning plug wires could drasticly effect your milage. Maybe while you were changing them something got bumped loose?

 

[rollinstone]

Checked the resistance of the wires last night and they each averaged about 7 ohms. I was told to expect the resistance to come in at about .05 to 1.0 ohms. The old set of wires came in around 8-10 ohms, so I'm not sure what I found out here.

Any ignition wizards out there?

r.s.

 

[Webster3]

What about the computer? Is it possible that the new plugs changed things enough that the computer needs time to adjust the other components to deal with the new plugs?

If that guess is good, taking the battery cables off for a little while to make the ECM reset may help?

Perhaps some of the mechanics in the group may know if that guess makes any sense.

Ed

 

[TheJMan]

I have the exact same problem on my 4.0. I was getting about 13 mpg with my old plugs and wires. I took out the factory plugs which had 79K miles and they had obviously way too much gap, so I replaced them with Bosch +4s. I also replaced the wires while I was at it. Now I get about 11.5 mpg (I do 95% city driving). I even disconnected the battery for 15 minutes with no change in performance or mileage.

I get no CEL, and the engine runs smooth as glass, but it just keeps guzzling gas.

 

[aldive]

Are you getting proper spark from the coil?

 

[aldive]

Did you check the spark from the coil to each plug?

 

[rollinstone]

spark from coil to plug

On older ignition systems it was a matter of simply touching the plug end of the wire to the engine block or piece of grounded frame and observing whether there was a spark or not...never tried this with an electronic ignition...same deal? Also, as you all well know, getting at those wires is a major production on most Xs.

Is there any way to test for spark without removing the wires one at a time...like attaching an inductive timing light to a wire and looking for the light to flash??

r.s.

 

[Nate1]

You are sure you wired 1<->1 right?

 

[BradE.]

Also, make sure the wires are not touching/crossing each other.

 

[rollinstone]

1<->1

I was very methodical replacing the wires...only did one at a time, that way ensuring I replaced 1 for 1, 2 for 2, etc.

Will look tonight to see if any are touching. There are several guide clips, but even so, it's tight in there and one or two may be crossing.

Keep those cards and letters coming in...you guys are the best!

 

[rollinstone]

wires checked

The saga continues:

Here's what I did last night...I hooked up my inductive timing light to each wire one at a time just to see if a spark is being delivered. Sure enough, the light flickered on each wire. What I noticed, however, is that wires 1, 2, 3, 4, 6, 8 all fired with a consistently steady flicker in concert with idle speed. Wires 5 and 7 exhibited a much faster flicker punctuated by occasional dead spots eek: The bottom line...those two were different than the others.

If there are in fact occasional dead spots showing up on the light, and if those are translated to the plug, then it would follow that plugs 5 and 7 are not firing all the time. Over a 300 mile tank of gas, that could cost efficiency. Sound reasonable?

I guess what I'm finding out is that the wires are OK and that the electronic ignition is not delivering a proper spark, right? What next? Is there a way to test the ignition upstream to see if there's a faulty coil or whatever an electronic ignition uses?

r.s.

 

[twomedicine]

crazy

crazy stuff...like they say on the financial news when things don't make sense at all : "go figure"

Maybe try your old wires again with same filling station, same fuel.

Or with your new wires intact still, try some other gas station's pumps. Maybe ruling out the weights and measures type fraud (you are seeing & paying for more gallons going in on the pump than for actual)

And if your wires were leaking you'd see elec. flashing around the wires when observed with the hood raised and on a dark night (I have had this and the mileage was horrible)

Interesting problem though

 

[aldive]

Test the coil.