MPG Drastic drop

**FMBL** · 10-24-2010, 05:21 AM

98 z28 no mods. Used to always average around 20-22mpg. A few weeks ago it dropped to 16 then to 14 and is now averaging around 12.

I have already changed the fuel filter, spark plugs and plug wires.

It runs great just not sure why there would be such a drastic drop.

**dav-02ws6-m6** · 10-24-2010, 07:33 AM

the first thing that pops to my head is o2 sensors but without any codes it hard to tell.

**bryceslu** · 10-24-2010, 04:03 PM

Originally Posted by dav-02ws6-m6

the first thing that pops to my head is o2 sensors but without any codes it hard to tell.

x2 on 02's

**Y2KPewterSS** · 10-24-2010, 04:09 PM

I wonder if its not the 02s as well. Something else to check, would be your tire pressure. It wouldn't be the cause of your drastic drop, but could be contributing somewhat.

**Smittro** · 10-24-2010, 04:14 PM

Originally Posted by dav-02ws6-m6

the first thing that pops to my head is o2 sensors but without any codes it hard to tell.

Originally Posted by bryceslu

x2 on 02's

Originally Posted by Y2KPewterSS

I wonder if its not the 02s as well. Something else to check, would be your tire pressure. It wouldn't be the cause of your drastic drop, but could be contributing somewhat.

Dis.. And if you're mileage is getting up there and still on stock Decarbons.. Tho I don't see that effecting it so drasticly as the 02's possibly being due for replacement.. Sounds like over enrichment maybe happening.. imho

**Tyler_LS1** · 10-24-2010, 04:32 PM

like they said, check the o2s and replace em. thats where i would start.

how many miles on the car?

**RyanJM** · 10-24-2010, 04:47 PM

+1 for the 02's also might wanna clean the maf that could lose you one.or 2 mph if.it's a little dirty. While you have it apart I'd clean the throttle body as well.

**Y2KPewterSS** · 10-24-2010, 05:06 PM

Clean the MAF with MAF cleaner also.

**FMBL** · 10-24-2010, 06:25 PM

The car has 180,000 miles but runs great. I have already cleand the MAF with MAF cleaner when I did the plugs wires and fuel filter. I had it inspected after that. No codes. I would think that if it was running too rich it would have a hard time passing emmisions with the DFW metroplex inspections. Tire pressure is good. I will change the o2's and see if there is any improvement. I am sure it is due for them any way.

**shady milkman** · 10-24-2010, 06:57 PM

Originally Posted by FMBL

The car has 180,000 miles but runs great. I have already cleand the MAF with MAF cleaner when I did the plugs wires and fuel filter. I had it inspected after that. No codes. I would think that if it was running too rich it would have a hard time passing emmisions with the DFW metroplex inspections. Tire pressure is good. I will change the o2's and see if there is any improvement. I am sure it is due for them any way.

cleaning is only so much. With that many miles it is time for a new maf and o2 sensors for sure. i bet you will see a great improvement

**Smittro** · 10-25-2010, 02:56 AM

Originally Posted by FMBL

The car has 180,000 miles but runs great. I have already cleand the MAF with MAF cleaner when I did the plugs wires and fuel filter. I had it inspected after that. No codes. I would think that if it was running too rich it would have a hard time passing emmisions with the DFW metroplex inspections. Tire pressure is good. I will change the o2's and see if there is any improvement. I am sure it is due for them any way.

Originally Posted by shady milkman

cleaning is only so much. With that many miles it is time for a new maf and o2 sensors for sure. i bet you will see a great improvement

I think 80k is the "norm" for thinking about changing the 02's iirc.. MAF is constantly subjected to finite particles (even more so with a K&N among other things it causes), like slowly being sand blasted.. Could be the issue but I'm still leaning with the others on the 02's..

**mammoth713** · 10-27-2010, 09:44 PM

dont wear boots while driving

**FMBL** · 10-28-2010, 01:31 PM

OK. We moved to our current house during the summer. Looking back I would say that the drop started around the time school started. Most of the driving my wife does is very short distance right now most every trip is less than 2 miles. I filled up Monday night been driving back and forth to work and we had to go out of town today. About a 140 miles round trip. Averaged 23.8 mpg.

I short distance is not good for a vehicle but I did not think it would make that big of a difference.

**jakez28** · 10-31-2010, 06:03 PM

Ls1s absolutely hate short distance...just like any other v8

**themurphishman** · 11-01-2010, 05:33 AM

any decrease in power? may be time for a compression check, although they're not a quick 5 minute job on the LS1's.

**FMBL** · 11-01-2010, 10:30 AM

No loss of power. Runs great after the tune up. I know it's not good to drive one short distances. But I still think a 10mpg frop is crazy.

**Firebirdjones** · 11-01-2010, 11:52 AM

Originally Posted by FMBL

No loss of power. Runs great after the tune up. I know it's not good to drive one short distances. But I still think a 10mpg frop is crazy.

10 mpg to me sounds a just a tad low even for short trips, sure you don't have a hole in the tank?

That's pretty much all I've been doing with mine lately, taking my son to and from school which is a short 5 miles up the road driving through 25 mph zones all the way. I just filled it up today and checked mileage, 20.23 mpg. That's an auto with 3.23's and a 3400 converter.

**MrMasterCraft** · 11-01-2010, 06:49 PM

Originally Posted by FMBL

I short distance is not good for a vehicle but I did not think it would make that big of a difference.

It's not the distance, but not allowing all the fluids to reach operating temperature, that's detrimental to a vehicle in general.

I know that with LTs, different Y with hi-flo cats, and a Borla muffler, I can squeeze 25.5mpg out of my Y2K Z. That's starting in second gear (A4), shifting light-footed at no more than 1600RPMs, and keeping my highway speed below 70mph at all times, with 2.73s. This is with the car throwing 4 O2 codes. Still have to have them replaced

If I roll strictly around town, I'm looking at about 180miles to the tank. And I usually fill up at 1/4 tank or slightly below (which we all know is well below a true 1/4 tank in these cars). I'd say I put between 11 and 12 gallons when I fill up almost every time... sooo... 12 gallons on 180 miles is around 15mpg. I like to hold gears around town though, hate using my brakes.

Couple questions I'm asking is, doing these calculations and figures, have you been the one driving the car the entire time as it's been decreasing? Any difference in holding gears (engine breaking and such)? My girl is a pretty cautious driver, and too scared to do anything stupid in my car. Yet I would never trust a gas mileage calculation by her. But that's just me

**Firebirdjones** · 11-01-2010, 07:06 PM

Originally Posted by MrMasterCraft

I know that with LTs, different Y with hi-flo cats, and a Borla muffler, I can squeeze 25.5mpg out of my Y2K Z. That's starting in second gear (A4), shifting light-footed at no more than 1600RPMs, and keeping my highway speed below 70mph at all times, with 2.73s. This is with the car throwing 4 O2 codes. Still have to have them replaced

If I roll strictly around town, I'm looking at about 180miles to the tank. And I usually fill up at 1/4 tank or slightly below (which we all know is well below a true 1/4 tank in these cars). I'd say I put between 11 and 12 gallons when I fill up almost every time... sooo... 12 gallons on 180 miles is around 15mpg. I like to hold gears around town though, hate using my brakes.

You're trying harder than I am. I set cruise on 75 mph and get 25-26 mpg with mine on the highway, using 3.23 gears. Around town with a loose converter I get 20, I just leave it in overdrive all the time, but around town I'm lucky to go fast enough to get the converter to lock (38 mph).

You're getting 15 in town??

**MrMasterCraft** · 11-01-2010, 07:16 PM

Originally Posted by Firebirdjones

You're trying harder than I am. I set cruise on 75 mph and get 25-26 mpg with mine on the highway, using 3.23 gears. Around town with a loose converter I get 20, I just leave it in overdrive all the time, but around town I'm lucky to go fast enough to get the converter to lock (38 mph).

You're getting 15 in town??

Thats probably the norm I would imagine, if you've got all O2s functioning properly. None of mine are. Hell I think I even got a knock sensor code last time I pulled up my codes on the scanner

If I'm cruising around in town, I won't shift until 2400RPMs atleast, and that's if I know I can cruise into the next gear, and not immediately hit the brakes. If I enter the 55mph zone, I won't shift into OD unless I know I can hit the cruise without being interrupted. And this is only on the weekends that I drive around town, never during weekday trips.

And sometimes when I'm cruising between 2k-2200rpms on the pewter car's setup, I like to lay into it, let off, lay into it, let off, because of the drone I get. The white car with the loudmouth is between 1800-2k.

I love doing it