TCCoA Forums banner

1 - 20 of 20 Posts

·
Registered
Joined
·
850 Posts
Discussion Starter #1
history:
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... :(
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.
 

·
Registered
Joined
·
28 Posts
well.... since you just started driving it again, I'm gonna throw out what I have found and I'm not sure it's been discussed or not (sorry gm, haven't searched for it ;) ) ... but around here ALL the gas stations have gone to the gasoline/ethanol mixin the past year. All of the sudden, I started gettin like 19 to 20 mpg (about 300-310 miles per tank). Well, I found ONE older, family owned station in town that still sells just straight gasoline and BOOM. I get 370-390 miles per tank 23-24. And this is a mainly city (well country) driving. Not much highway. A lot of the guys I'm around are claiming that the mix with the ethanol doesn't burn quite as hot as just straight gasoline. I never researched it any further, but there is NO doubt my car gets 3-4 more mpg on the straight gas. Oh yeah, I'm still running just regular, no plus or premium. and that's all i have to say about that :D
 

·
Super Moderator
Joined
·
11,961 Posts
yup....ethanol releases 26.8 kJ/kg, and regular gasoline releases 42.7 kJ/kg.....so a gallon of a mixture of the two will most definitely release less energy than straight gasoline...it could be responsible for the drop in fuel economy
 

·
Registered
Joined
·
1,100 Posts
+1 about ethanol! I live in NC too and in my area only a few stations carry the ethanol mix. I avoid them at all cost! I learned about the lower mileage while driving from NC to TX and back. I also stay away from premium or super unleaded. Without a tune those octanes do not help unless you are trying to prevent pinging.
 

·
Registered
Joined
·
850 Posts
Discussion Starter #6
yeah that's the only reason I use(d) permium (pinging)

Tonight i filled up half tank with regular, so now it has a permium/regular mix...will see if it still pings (1st time regular use after cat replacement)

so where can I get ethanol free gas? Shell?
(all i have near me is Exxon and BP)
 

·
Registered
Joined
·
28 Posts
Around here it's all according to the gas station, not if it's BP or Shell, etc. There's about 7 or 8 BP stations in town, but the one I get the ethanol free gas is a BP station. All the others are mixed gas/ethanol. Evidently it has to do with the station requesting it. I also get straight gas at another station in the town where I work, and it's a Phillips 66. Don't quite get as good mpg, but about 50-60 miles/tank more than the mix. If they have ethanol in the gas, I believe it's required by law that they have notices around the station.
 

·
High-Mileage 4.6L Thrasher
Joined
·
3,232 Posts
What brand o2 sensors did you install?
I've had repetitive problems with Bosch o2 sensors-when I replaced them with OEM NTK sensors,the problems went away.
JL
 

·
High-Mileage 4.6L Thrasher
Joined
·
3,232 Posts
but weren't Bosch the oem O2 sensor manufacturer for the Tbirds?
Not all of them. Most all were NTK,but a few were possibly Bosch. And then there's the difference in an OEM spec'd part vs the "one-size-fits-all" crap that they sell at the parts stores.
JL
 

·
Registered
Joined
·
1,573 Posts
I put denso O2's in EVERY car that I have that needs them(or is around the recommended replacement time). Never had a problem with them.
 

·
High-Mileage 4.6L Thrasher
Joined
·
3,232 Posts
I put denso O2's in EVERY car that I have that needs them(or is around the recommended replacement time). Never had a problem with them.
Denso typically makes a good o2 sensor, but after my fiasco with Bosch and their "equivalent replacement" sensors, I'll never use anything but OEM part number stamped sensors from now on. Spending 80+ hours chasing a moving target on idle tuning because of ****ty "equivalent replacement" sensors teaches you a harsh lesson.
JL
 

·
High-Mileage 4.6L Thrasher
Joined
·
3,232 Posts
so are Densos any good then cause I'm about to purchase 4 and densos are cheap, I just don't want them to actually be cheap.
Do they have the OEM part numbers on them?
If not,I wouldn't touch them.
JL
 

·
Super Moderator
Joined
·
11,961 Posts
OEM sensors on 97s are definitely NTK.....i remember removing the originals, and they definitely weren't bosch....i'd venture out on a limb that the other year MN12s were probably NTK as well

as cheap as denso's maybe, im positive you can get NTKs for the same, or perhaps a negligible fraction more
 

·
Banned
Joined
·
754 Posts
this is what I'm looking at on rockauto $30 each, NTKs are $41 each(Pre-cat, 2 left), Standard Motor Products are about $35 and AIRTEX are $51 each, marked with an OE number.

Densos are also pre AND post, others on RA are marked as pre and post.

I'm thinking I might grab the NTKs for pre and Densos for the post.

DENSO Part # 2344610
Wire Length 13.15; # of Wires 4; O.E. Quality Replacement Sensor; Rear
Wire Length 13.15; # of Wires 4; O.E. Quality Replacement Sensor; Front
 

·
Registered
Joined
·
850 Posts
Discussion Starter #20
UPDATE:

Update: ran the 1st "premium-free" 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 a bottle of Amosil PI to gas (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.)
 
1 - 20 of 20 Posts
Top