I was getting rather poor gas mileage with my 88GTA L98 that was in bad need of a tune-up, averaging around 12-13mpg even on the Highway. I had a bad O2 sensor that was causing a lean condition error code with the exhaust smelling very rich. I did the tune-up this weekend with Bosch O2 and fuel filter, new pcv valve, MSD coil, rotor, and wires, and rapidfire sparkplugs. The error code is gone, but mileage doesn't seem to have improved much. I haven't run through a full tank yet and calculated exact numbers, but judging from the fuel guage and current trip mileage I'm still looking at about mid to low teens, and that's with about 70-75% highway miles. I just checked the timing tonight and it was at around 0* (with the esc wire disconnected) I'm going to set it tomorrow (don't have any tools with me tonight), but could that account for some of the poor mileage? What do you recommend setting it at as a base for now (Won't have time to play around with it until this weekend, just need to set it somewhere to get me through the rest of the week. I've got a G-tech to test it with this weekend (no place to run 1/4 mile times until the strips open, but there are some areas I can relatively safely run 0-60 times and get horsepower calculations) Also, before the tune-up I was getting around 195hp on the G-tech. Will I see a significant improvement with the tune-up and appropriate timing?

Thanks in advance for any help given.

ps. The car is mostly stock. It's got a TB airfoil, K&N airfilter, and MAF screens removed (Still has the fins, though I plan on removing those this spring with my newly acquired dremel, as well as plenum porting when I buy some SLP runners and edlebrock base) And a flowmaster 3" from the cat back (I suspect the cat may be gutted from the loudness of the exhaust, but I don't know for sure. I'll be checking that when I get some headers. If it is, I'll replace with a hi-flo aftermarket cat)

 

6 degrees advanced.

 

yeah you might after the tune up depending on how old the wires, rotor, cap, plugs, oil, was. if it was really old and your timing is off, it might give you a few extra ponies, but set the timing at around 6 advanced. you should notice a difference in fuel economy.

 

There could be a lot of things causing the problem you say you are having. Doing the tune up should have eliminated a lot of the possible casuses. You didn't mention how long it was since your last tune up though, so yes, you could have found your problem already. Now as for the timing. Make sure you disconnect the ECM before you start the car. The stock timing was set at 6* advanced as many people have mentioned above. Many people with some mods tend to run a little more advanced say 8 to 10, but 6 should be a good place for you to start. The hp numbers you quote sound much too low for that car also. The stock motor should have been producing around 245 and with the small mods you have done, including the most recent from the tune up, you might have picked up 10 - 20 more. Good luck.

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87 GTA 350 L98
K&N filters, beefed up TH-700r4, Accel Manifold, Ported Plenum and Runners, Fuel Press. Regulator 49.5psi, air foil, modified MAF, SLP cold air box, Hypertech Thermomaster chip w/160 thermostat, MSD 6AL and Blaster 3, B&M shift kit & megashifter, Moser Ford 9in Rear with Richmond 3.50 and locker, Edlebrock TES headers and Cat Back,Hotchkis setup in rear, Poly suspension and Koni in the front, relocated MAT, bypassed TB coolant, 300 lbs lighter

Fastest ET's
Stock-0-60 6.8, 1/4 15.0 @ 94
Best-0-60 4.7, 1/4 12.8 @ 112
Norm-0-60 5.2, 1/4 13.2 @ 108