New cats & O2 sensors --> lower MPG ? why?

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97tbird

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1997 Tbird v8 with 127k miles.
my car had the CEL for "efficiency below threshold" for cat(s) for the last 2 yrs.

Car always had a bit of a pinging since about 90k miles - goes away with premium gas, so have been using that (car was not driven very often..till now)
don't know if pinging is due to cats or something else..(carbon??)

2 weeks ago, replaced all 3 cats and front o2 sensors - and started driving the car on hwy

and now I am getting about 22 mpg driving at 75-80 mph on the interstate (!!!), a/c not on, cruise control on, and this is still on premium. The trip is 95% at the above speeds, rest is slower/city driving betw. 35-55 mph mixed.

how can this be? I think I got at least 24-25 mpg BEFORE the new cats; the best mpg i got before cats went bad was about 27 mpg. I'm really BUMMED with the results after getting NEW cats...
frown.gif

otherwise car drives and feels better with new cats.

Here's what I am gonna try:
-I am going to reset the pcm (idk if they did that after replacing the cats)
-gonna run Amsoil PI in tank
-cleaning MAF sensor as I type (it's drying)
-gonna try some REGULAR gas and see what happens; if it pings, and/or if the mpg gets better.
(is it possible that premium was ok when cats were bad, and now it's "expecting" regular gas to run "normal" again? adjusting fuel curves, etc??)

I will be driving the car 200 miles roundtrip 5 days/week this month, so I can check the mpg pretty often...

more info:
-plugs/wires ate only 35k miles old (platinum plugs are ok for about 75k)
-tire pressures are all ok
-air filter is about 10k miles old.
-CEL not on, never came back after replacing the cats
-no other changes other than the new cats/O2 sensors

any ideas why the mpg went DOWN? any suggestions to what else I can try to remedy this?

Thanks a lot I.A.
 
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Your o2 sensors might have been reading too lean, and you used the premium to account for that. Try going back to regular.

Maybe the new cats don't flow as well as the other ones?

(Also, the cat below efficiency warning only means that the downstream o2 sensor isn't able to see a difference in the oxygen level compared to the first o2. That can mean the upstream o2 is wrong, or the cat is bad, or the downstream o2 is bad. Or, if you were running lean, there's no more fuel left to react in the cat, and it can't do it's job.)

Clear the computer memory and run through a tank or two of fuel and see what happens then.
 
OK lets be realistic. You have a 12 year old V8 Ford T-bird. You're going 80MPH and your disappointed about getting 22mpg.
The car was never really meant to be effecient. The motor isn't meant to be ran on premium, therefore the gas is not being fully utilized in the combustion chamber.
Slow down, run 87 and you'll see acceptable numbers.
 
Thanks a lot for the feedback, guys.

Well, I have to be at my internship at 8 am most days, and i drive 1.5 hrs, if I go any slower than 75 mph, it's gonna be a real pita to get there on time, but I see what you mean...

I am going to use 87 octane this week (might be mixed for a day or 2), and check mileage again, with pcm reset, clean MAF and perhaps top tier gas....will report back soon.
 
Driving at higher speeds has more effect than we usually realize.
You will lose several miles per gallon, not just fractions.
 
OK;
tank was 1/2 empty - filled up with some regular gas, so now it has a mix of 87 and 93... (had to get exxon
frown.gif
I will get some Shell tomorrow, as I use half a tank everyday)

MAF cleaned, and reset the pcm

will see how it goes, and will check if I can notice any pinging with the mix...
 
Is it possible that you are getting *excellent* throttle response right now? If so, it is likely that the mixture is slightly rich but ECU will adjust it after you have driven it for a while.

I recall when I replaced MAF on my Maxima, the car drove like it was on steroids but the mileage went down immediately after wards. Similar principle applies when you change the O2 sensors. Also not all ECU will reset the long term fuel trim maps. I know Maxima does NOT but Odyssey does.

- Vikas
 
Update: ran the 1st tank with mostly 87 octane gas (had some 93 in it...)
got 23.84 MPG (up from 20.23 mpg from last time)
Did not notice any pinging (yay!) ... yet.
Exxon was mainly the brand
speeds: 95% miles around 75-80 mph (cruise on); rest around 40 mph average.

... so I am hopefully this will get better with time...

Next plan: to get Shell 87 gas, and add Amosil PI (which is on its way).

Speeds and trip will be the same; will report back.

(I might be able to report mpg again with some Shell 87 gas before I get the Amsoil PI.)

I might also try switching cars with wifey and drive her Elantra on some days, to see how much of an effect lower speeds would have on bird's mpg (wifey drives around 60 mph to work, 80 miles round trip every day)
 
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What brand of plugs in in the car?

If you go to Tccoa.com you will find the thunderbird club of america.

They might recommend a little colder plug to fight the detonation, that helps a little bit.

A copper core motorcraft plug/autolite is what works best for these motors.

Changing plugs is a 5 minute affair, no need for long life plugs.
 
Plugs are Motorcraft Platinums; recommended type; (about 30k miles on them I think)
wires are also Motorcraft (same mileage)

I have been member at tccoa.com since 2002 (user ID: otrbird); Thanks :)

Can you run colder plugs without changing anything? (I thought you had to change pcm programming or something like that...)
Might try Motorcraft copper plugs next time.

** why don't the Platinum plugs "work"?
can you explain what's better about copper plugs in this car? (except cost) **
Thanks.
 
I've had several Tbirds, including a supercharged 4.6L one that made 330 horsepower..

One step colder copper motorcraft plug will help with any detonation in your stock motor car.

Copper is a better conductor than platnium, the car will run a little better but the plug won't last as long. I used to change them once a year.

The plugs are about a dollar a piece, look on tccoa for the part numbers, I used to have them memorized but I forget them now.

In my supercharged car, I would run two heat ranges colder than stock.

Also what is the rear end gear ratio in that car? I had a 95 thunderbird with a stock motor, and 3.08 gear ratio. That car used to be good for 29mpg on the highway every time. That was with a dyno tuned EEC, and 93 octane.

My supercharged car had 373 gears in it, that used to be good for 15-19mpg..

When the car was stock with 327 gears, it would be good for 25mpg on a stock tune.

We also used to replace that 3rd cat converter with a Y pipe. If you look at the flowmaster catback system for this car it actually includes a Y pipe to ditch that 3rd cat.

Originally Posted By: 97tbird
Plugs are Motorcraft Platinums; recommended type; (about 30k miles on them I think)
wires are also Motorcraft (same mileage)

I have been member at tccoa.com since 2002 (user ID: otrbird); Thanks :)

Can you run colder plugs without changing anything? (I thought you had to change pcm programming or something like that...)
Might try Motorcraft copper plugs next time.

** why don't the Platinum plugs "work"?
can you explain what's better about copper plugs in this car? (except cost) **
Thanks.
 
I suspect fuel blend has been now switched to the winter one. My last tank average dropped from 31+ to 26+. I am in New England though.

- Vikas
 
JustinH - Copper core spark plugs may have steel, platinum, or other material for the tips.
Since all street cars use resistor plugs, conductivity is a moot question. Who cares how they achieve the recommended resistance?
 
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