Okay team, before any suggestions are made I have thoroughly gone through my FSM to try to diagnose this problem. For those of you that have followed any of my posts you will know that I have performed extensive work to this vehicle. All new suspension brakes bushings Etc. The only thing I have not done is rebuild the motor or rebuild a transmission. The vehicle seems to run fine, has a decent amount of power for an engine that has 145,000 miles on it. However, my mileage is horrible. At least I think it is in comparison to what other people are saying they get out of a full tank of gas. I have checked and I do not have any brakes dragging, I have replaced the cap rotor wires and plugs, Bosch injectors, and still according to calculations I am only getting about 10 miles per gallon. I calculated this based upon how much fuel was put in upon fill up rather than relying on the dash gauge. Everything within the fuel system is brand new with the exception of the fuel pressure regulator. I have tested the fuel pressure and it is within specs however, I do believe I am losing more pressure then I should be as the car sits overnight. It takes numerous cranks before the vehicle will start so I am aware that my fuel pressure regulator is most likely bad. My question is though if the fuel pressure regulator is bad is that enough to cause my fuel mileage to be that horrible? I have seen other post where people say that they are getting 300 miles to a tank of gas or more. My driving is mainly local no Highway although it is not stop and go heavy City traffic. I am just at a total loss as to why my fuel mileage is so bad. I love driving this car especially after all the work I've done to it but the poor gas mileage makes me question how enjoyable it really is. I know my motor is probably in need of being rebuilt possibly if even just to freshen it up but I'm not sure if that could be a cause of poor gas mileage. I'm at a loss, please chime in.

 

o2 sensor ??

 

That is brand new as well. I should also mention that I deleted the EGR and smog pump as well. Have not had the prom flashed, not sure if that could be the issue.

 

Is your parking brake fully releasing?
Don't fool yourself into thinking new parts ensure good parts.

Why if the pressure is in spec? The regulator won't cause fuel loss unless the diaphragm tears and you would have fuel getting into your Vac lines. Additionally if you have high fuel pressure the ECM should be compensating by leaning out the BLMs since more fuel is flowing past the injectors than expected.

II would suggest seeing what your BLMs are and pulling the plugs to confirm if you are running rich or not. At least to isolate if it is an issue causing too much fuel to be in the combustion chamber or if it is something else causing high load. Also, how is your catalytic converters? Those getting plugged up can also drag economy down. Maybe you don't have a brake dragging when they are cool, but have you checked for close to even heat on all 4 rotors after a drive?

 

I have pulled the plugs and they look normal. As for the converters I thought about that I wanted to pull them and Hollow them out but the previous owner had an exhaust system put on and everything is welded together. I can pull each side of the exhaust independently but everything is welded from the converter back. Not sure how hard it would be to hollow them out with them welded to the rest of the exhaust.

 

Some are DOA. Do you know how long it sat on the shelf?
It would be good to see the actual voltage and cross counts to confirm it is working properly. Here again, the plugs would tell the story. If the 02 was reading incorrectly and telling the ECM you are lean, you BLMs will spike and you will run rich. Should be able to see this on the plugs. While the 02 may not be the only cause, it is a potential cause for the mixture to be off.

 

Do they get equally hot? If one is getting much hotter than the other after a short drive, you might be able to determine how much of an effort you are willing to put into it.

If your plugs are fine, then you are probably not running rich. You probably have a load issue. Something could be dragging, a bad bearing, a bad accessory, etc.

 

WHAT KIND OF DRIVING?

 

Would have to assume he is not always going WOT, but that sure would explain it.

 

That's what I was thinking, given his attention to maintenance. It's possible to baby these and get good mileage....but it's also quite possible to enjoy 'em and get 10, 12...in local driving. Every stop light, stop sign, on ramp and turn is a drag race!

 

I would also check your Coolant Temperature Sensor along with your Oxygen sensor and the Catalytic Converter.

The first is the easy one, find your CTS and measure what it is sending to the computer. If it is reading low it will turn on the cold enrichment and that will cause a Corvette to use more fuel IF it thinks it is cooler than it really is. It is like having a Choke stuck in the "on" position while driving. I would clean the connectors to the CTS and for the O2 just to be sure they are reading correctly. I use something called DeOxIt to remove the corrosion from connectors, cleanliness is important for accurate readings.

The O2 wires should be checked verify that the O2 is not having issues. I have seen faulty O2's that send a "full rich" mode to protect your engine from damage. That would clearly make your engine use a lot of fuel.

The Catalytic Converter could get plugged up even while sitting. To see if you have a plugged Catalytic simply remove the Oxygen sensor and start the engine, if you get some air leakage that is normal, if you get a lot of air coming out then you might have a plugged converter. A car running Overly rich would destroy the catalytic converter pretty quickly. Any oil burning in your engine will also shorten the life of the O2 and the catalytic converter.

Is the car warming up normally and then making a smooth transition to Closed Loop operation? Does the exhaust smell the same in "Open Loop" Mode as it does in "Closed Loop"? The reason I ask this is that the O2 is not used in "Open Loop" and becomes involved when transitioned to "Closed Loop". If the smell of the exhaust gets richer in Closed Loop then it is likely the Oxygen sensor.

I bought four O2's for one of my family vehicles and three worked and one was bad from the manufacturer. I buy all my O2's from a company that sells lots of them so the units in their inventory are fairly "Fresh". Storing new O2's is not a good practice unless they are very hard to get. Bad O2's do happen and they are very hard to measure the performance of a Oxygen sensor without a good scanner or an oscilloscope. When Your Corvette was built the O2 sensors were only designed to be used for a maximum of 24 months or 50k miles. There are folks who swear that they work for decades but that is not completely true. As the Oxygen sensor ages it's signal narrows and then it gets slower which cause the car to be unsure what to do and will default to a richer setting to prevent any damage from running too lean. They can be a PIA to replace at times but keep a fresh O2 in your Corvette and you will enjoy better performance and economy!!

I am not a believer in removing the EGR systems from the L98 engine series as they really do perform better with the EGR system operating. YOU MUST get a new chip made for your Corvette including the EGR removal/delete as you will never get the car to run properly without the new program deleting the EGR functions. The way your Corvette is now you will have higher cylinder temperatures on the roads and develop detonation when it gets hot. The new chip is critical when you delete the EGR. Running the old software that incorporates the EGR will not help your engine run it's very best. After you install a new chip with the EGR delete built in your car should operate a lot better!

My 1988 C4 has an automatic transmission with a 3.07 rear end and with everything functioning properly I was able to break into the 30 mpg range on the highways. That was with everything working on a C4 with 100K on it with the AC and CD turned on while cruising. I have a C3 that gets single digits in town so I don't want two Corvettes that get terrible mileage. The C4 is a cruising Corvette used around town.

 

Did this just start happening? Like within the last two months?

 

It's only been on the road for about two months now since I completed it. This is about the 4th tank of gas I have put through it. I'm only getting about 200 miles to a tank of gas. I drive it normally with the exception of an occasional quick takeoff.

 

I feel like the fuel suppliers are giving us all crap gas. The fuel mileage in my vette and truck (tunes) have gone to **** since corona. It’s like they’re battling lower demand And cheaper prices with shittier fuel.