Really Crappy MPG

**F-Body Man** · 09-03-2007, 07:18 AM

Hey all, I have been tring to figure out why my WS6 is getting such crappy gas mileage. I get around 18 - 19 MPG on the highway/interstate. I have an (M6) also. I haven't used N2O since I bought the car so it's not because I am hot doggin it. I just want the great MPG these normally get for being mostly stock. Hell my old 97 Z (M6) Got 26-28 MPG before the new race motor I put in it. And I still get about the same gas mileage as the WS6 right now with it. So I know there is some kind of problem.

Here are the mods I have installed that are power making.
SLP Lid
K&N Filter
T-Body Coolant By-pass
FRA Mod
Gutted Cats (Gas Mileage didnt change after this)

The Mod that was on the car when I bought it.
NOS Direct Port Nitrous (w/ all the safety stuff, etc.)

I looked at the injectors to if they after market, but they look stock. Is there a way I can tell to make sure they are stock? Also if they all Not would that cause my crappy MPG? Any Help would be great.

**sreve** · 09-03-2007, 10:13 AM

did you put in sims or tune out the rear O2s? check eng light? some active codes will trigger the use of low octane timing tables. speaking of timing is that nos pulling any timing? check the fuel pressure and a properly working regulator. with a scanner look at the fuel trims,iat,ect.

**blue02Z** · 09-03-2007, 10:50 AM

also it prolly wont hurt to change your plugs wires and fuel filter. you might wanna put some fuel treatment in there. from my experience marvel mystery oil works well as it also lubes the fuel pump. change your fluids and one thing people overlook is that the inside of the tb also gets clogged with carbon deposits. go get some tb and intake cleaner and spray it into the tb while the engine is running. it cleans it all out.
how many miles are on it

**JonB** · 09-03-2007, 07:53 PM

Originally Posted by blue02Z

also it prolly wont hurt to change your plugs wires and fuel filter. you might wanna put some fuel treatment in there. from my experience marvel mystery oil works well as it also lubes the fuel pump. change your fluids and one thing people overlook is that the inside of the tb also gets clogged with carbon deposits. go get some tb and intake cleaner and spray it into the tb while the engine is running. it cleans it all out.
how many miles are on it

All of the above would be a good starting point, a good tune up might be all it needs.

**F-Body Man** · 09-03-2007, 09:21 PM

I have already cleaned the T-body, used Fuel System treatment, changed the fuel filter, and it has Denso plugs. The NOS system isn't doing anything right now on the car. The WS6 has a Stock tune for the moment. The motor only has 40,000 miles on it.

I am going to be ordering New O2 sensors, MSD Wires, and NGK TR-55 plugs next. I hope that fixes the Fuel Issue. I know I don't think it's my O2s since the car run great and strong. But I am running super rich.

**bluefields88** · 09-03-2007, 11:54 PM

how would the O2 sensors mess with the MPG, and how can I tell if mine need replacing, and where are they. haha. I get crappy mileage too, no matter how I drive. going 75 on the freeway I get like, 20 mpg.

**F-Body Man** · 09-04-2007, 03:44 AM

Originally Posted by bluefields88

how would the O2 sensors mess with the MPG, and how can I tell if mine need replacing, and where are they. haha. I get crappy mileage too, no matter how I drive. going 75 on the freeway I get like, 20 mpg.

The O2 Sensors tell the computer how much fuel it need to add or subtract to the motor. If they arent working proper, you can get crappy gas mileage, the car can run like crap, etc. The Primary O2s are located nearest to the Manifolds or Headers (if you have them). The Second set are located behind the Cats toward the rear of the y-pipe. Hope that helps yah.

**F-Body Man** · 09-07-2007, 06:13 AM

TTT about the injectors question? How tell if the Fuel injectors are after market if they look stock? Also I would assume that if the Injectors are bigger then stock, that could be one of the causes of my bad gas mileage?

**MYTFASTSS** · 09-29-2007, 01:40 AM

Any Update??? I'm curious as to what the issue was. My 98 SS M6 got around 20-21MPG in the city and achieved a stunning 29.5MPG from San Jose to Los Angles (mind you this is Calif gas which is some of the shittiest in the country).

My Current 02 SS M6 seems to be getting really bad gas Mileage. Haven't really tested it yet, but from what the gauge is reading, half tank would be around 115 miles. It should be getting 145-165 miles @ the Half marker. I don't have a CEL. I,ve done all the basics except the Oxygen sensors (O2's). Plugs/fuel filter/gas treatment/clean air filter/ Cleaned TB/. I do have 3.73's but this should not effect MPG that much!!!!

any help, and hopefully an update from OP

**bluefields88** · 09-29-2007, 01:42 AM

I get 14 mpg with my 323s...... and that's a mix of city and freeway. :/

**F-Body Man** · 10-13-2007, 06:33 AM

I get about 18 to 19 MPG with most of that driving on the highway. I am waiting on the LS6 PCV upgrade I bought to get here. I am burning alot of oil, and I know that is one thing given me bad gas mileage. I got new NTK O2 Sensors waiting to go in once that PCV upgrade gets here. I also have NGK TR55, and MSD Wires coming to. I hope they get here soon. I want to at least elminate those if that is not the issue.

But as I have asked a few times now, Is there anyway way to tell what what size injectors I have if they look stock? Because I have a NOS Direct port system installed with a 150 shot and wasnt sure if the injectors was upgraded or not. The system was installed already when I bought the car a year ago.

**Stricken1** · 10-14-2007, 03:27 PM

I get 12-13 around town but got 26 MPG when I went to ATL for the G11 meet. M6.

**ultra_badass** · 10-14-2007, 03:30 PM

18-21 in town and about 24-30 on the highway

**sunsetorangess** · 10-14-2007, 04:48 PM

Your Two rear 02 sensors Dont do anything but tell your computer that your cats are working. You said your cats where gutted, if thats the case then you should be throwing a ses light, if not then you either have sims or your car has been tunned. What color are the tops of your injectors? If he was only running 150 shot, sounds like your injectors would be stock. How do you know the car has not been tunned? You said you were burning oil, i would do a compression check on each cylinder to see if there are any iffy numbers, could show you the problem. Plugs and wires will help ya out a bit but doesnt sound like it will solve the issue. Faulty or "lazy" front 02's will send the computer into closed loop mode, which will pull some timing and pretty much allow you to limp the car around until you can fix whatever is wrong with it. Its just safe parameters gm has come up with.
J

**Rivi** · 10-15-2007, 04:41 AM

1st thing i would check is the engine getting up to proper temps and getting into closed loop operation.What thermosat are you using?If its a 160 the fuel mileage will suffer and if you are never getting into closed loop the 02's are doing basicly nothing.

**Mecinoid** · 10-22-2007, 01:51 AM

Sounds like washed out rings and lousy compression to me. The previous owner probably used the NOS a bit much. Normal as NOS is addictive. I'd like to see the update as well.
Subscribing......

**jag42420** · 10-23-2007, 09:16 AM

+1 on the compression check. I suspect your rings/pistons are worn due to excessive NOS usage. That would cause both oil consumption and the loss of mileage. Check your spark plugs for oil, I'd be willing to bet they are not clean. How many miles on the car?

**b.lee** · 10-23-2007, 10:51 AM

my old z28 auto ls1 got 25-28 mpg highway w/ 3.23 gear
my new z28 auto ls1 gets 23 city and about the same on the highway w/ 3.73 gear

the higher the gear, obviously, it will lower the mpg.