Hi all,
A few months ago, I purchased a 2005 Mustang GT with a 5 speed manual transmission.
It had just clicked over 100k, but is in amazing condition and drives like a new car.
However, since no long term maintenance had been done (just regular oil changes) I did a full tuneup.
The first thing I did was full fluid changes in the engine, transmission and differential.

I then took a trip to visit my parents and show off the new ride, which is about 250 miles round trip at 80mph. My fuel computer said I was getting around 24 - 25mpg, which I thought was great.
I'd also been averaging about 16 - 17mpg around town.

Fast forward a few weeks, and I did the next part of the tuneup, which was new spark plugs, coil packs, and oxygen sensors.
These are what I went with:

Spark Plugs: https://www.americanmuscle.com/ford-...gs-0507gt.html
Coil Packs: https://www.summitracing.com/parts/MSD-82438
O2 sensors:
Upstream: https://www.summitracing.com/parts/bch-15716
Downstream: https://www.summitracing.com/parts/bch-15717

Everything installed nicely (aside from the driver's side upstream o2 being a pain to get to), the engine runs really nice, power feels good, etc.

However, I went on another round trip to go visit my parents. Same drive, same interstate, same speed, same weather, etc. 250 mile round trip. I averaged around 19 - 21mpg. That's about a 15% drop in economy, which is really significant.
I am also averaging around 13mpg around town.


There's no check engine light, nothing seems out of order, etc. except the huge gas mileage drop.
As far as I know, these style of spark plugs cannot be gapped, correct?
I don't know where to start. Any guidance?

Much appreciated.

 

First, this:



I'm pretty sure it's 250 miles no matter how fast yer goin', dude.

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Ok, ok, enough playtime. Howdy! So there's this particular video I saw recently that might be of relevance...

So with that, maybe you swap out the MSDs for the original COPs. I hope you didn't toss 'em... they're really good coils. I know that in 186K miles, I'm still running the originals, no issues, no MPG issues from 'em... And I'm getting the numbers you were, still.

Really, you kinda went *juuust* a little overkill on that tuneup. The O2s were probably fine, they don't wear out, they just work until they don't, and then you get a pretty specific code. But it's done, that's fine... I think... Mostly, you don't really wanna swap things on these cars unless there's a problem with it, sensors wise.

You also said you have a manual. Clutch may be done at 100K. Although all the tests will indicate the clutch is fine, the MPG loss is exactly what I had happening for quite the while before I finally swapped my clutch out at 160K. I thought it was just age. Nope. Clutch had been done for about 40K miles, just wouldn't fail the tests. I hope not, of course, but 100K... 120K (or 280K on the odo) is the mark I'm going to swap mine out next time to save the gas. Just something to think about. Perhaps your newfound funtime machine was a bit less than enthused about being stomped about by its new owner... If you do, just buy a flywheel, easier to do than get it resurfaced, and not that much difference price wise, I found.

Other things to check is the MAF and throttle body needing cleaning. Injector cleaning maybe. Intake cleaning maybe. Fuel filter replacement possibly. Air filter, you didn't say that... If those weren't done yet. If/when you do the throttle/MAF cleaning, be sure to relearn the computer. I'd do it after the intake/injector cleanings too.

Probably too much info/diatribe. Sorry. Hope it helps anyway! Let us know either way! More info is always good info...

 

Did you measure/calculate the MPG the old fashioned way? In my experience the computer is fairly accurate, but I have seen reports that it can be way off, so before you get too worried about it, check it by doing your own math.

When you're up around 80mph, small changes in speed can make a difference -- if you were actually going a little faster and/or driving a little more aggressively (to test the results of your tune-up) that can make a difference too

After that, I might suspect the coils -- have seen some posts that the OEM ones are about as good as they get, MSD can have some issues

 

Heh, yeah.... I can see how I worded that wrong.

 

???

So, I came back to see if I had any responses to the long post I had made a few days ago and it looks like it's missing? It was here.
How bizarre.

Oh well, I'll rewrite it later and try to repost my response.

 

Just so you know... I didn't see any post. Perhaps you mistakenly didn't hit 'send' or something? Very odd it'd eat the post, unless you put in a link that the system said 'nuh-uh', but I wouldn't know if the forum has anything like that at all. Very weird to hear.

Please repost it, would like to know what you had to say.