I know it's not something a Corvette owner should worry about but I noticed when driving my C5 that it barely gets 15 MPG, and this is mostly highway and I'm being light with the gas pedal. Now the reason I worry is cause this car sat for 5 years. Previous owner had damaged it to nearly junk yard worthy status, but I had restored it myself. Just put insurance on it 3 weeks ago.

It has a K&N filter that was oiled 1 year ago. I believe the spark plugs are original but like I said it sat but it has been started nearly weekly. Next week I'll be putting in AC Delco Iridium Spark Plugs. I was thinking of running Seafoam as well. Half through the intake manifold and the other half in the gas tank.

Any of these seem like good ideas? Anything else I should try doing?

 

Nothing wrong with any of that that you're going to do. Any codes on the DIC?

I'd say something is definitely up because my 383 heads/cam/intake c5 is getting pretty much the same (average).

 

If its been sitting that long, maybe the injectors are stuck and keep dumping fuel. I'd try changing those out next.

 

Theres NO reason why you shouldn't get 28-30 MPG highway. My highly modified LS6 (ported heads, nasty cam, ported FAST 90/90 mm Throttle Body, fully loaded with two people just got 26mpg on my trip from CT to GA. and back. We were NOT going slow either.

Some things to provide and some things to check:

Mileage?
Modifications? (original tune??)
Tires? OEM?? Tire pressures??? (28-30 PSI)
Are your Brakes dragging
Have you ever cleaned your MAF?
CATS. Wonder if there working and not damaged/clogged?

Do you have ANY DTCs?? Read and post. Then CLEAR ALL and take it for a drive and before you turn the engine off, read and post all the DTCs.

I would look at your O2 Sensors. They along with your MAF allow the engine to run efficiently.

Clean the MAF with MAF spray and reinstall making sure that you completely seal the ducting between the MAF and Throttle body. While you have all that apart, I would also clean the throttle blade.

Use an inferred laser thermometer and see what temp your cats are during full temp operation. Compare inlet/CAT and outlet temps

Last but not least, purchase a Scanner that has LIVE SENSOR Data. I have an ACTRON CP9180 and I can observe O2 sensor data, Long Term Fuel Trims etc...... Long term fuel trims can show you how the engine is running.

If you need new spark plug wires I highly recommend the GM Performance Red Wires. There $65. and the work excellent. The Iridium plugs are also recommended and work great.

Before I modified my 02 ZO6, I could get 29-30 if I tried. Normal just highway driving having fun, I would get 27-28.

Bill

 

Mileage is 85,000. Nothing really except K&N filter and new stainless steel mufflers. It was rear ended. Brakes are working fine, even hand brake works. Had to grease the hand brake cable to fix it. I haven't cleaned the MAF. Front tires are run flats with rears as regular tires, and they all have ~35 PSI. As for the cats it's entirely up in the air. I have to get my hands on a inferred laser thermometer.
I checked a year ago but yea I need to check again.

I polished the throttle body a year ago. That should be really clean.

I use Torque on my Android phone with a bluetooth OBDII adapter. I think it should be able to show live data.

 

Got the DiC codes and here's the list.

C1214 Sol Valve Relay Contact or Coil CKT Open

B2282 Battery #1 Fault
B2284 Battery #2 Fault
B2286 +5V Reference Fault
U1064 Loss of Communications with BCM

B2605 Seat Front Vertical Position Sensor Failure
B2860

All codes are cleared for now.

 

These DTCs point towards a LOW VOLTAGE issue. If they come back you have some troubleshooting to do.

B2282 Battery #1 Fault
B2284 Battery #2 Fault
B2286 +5V Reference Fault
U1064 Loss of Communications with BCM


Have your battery fully tested. If its good, look towards a dirty ignition switch.

The 1214 DTCs means you have a bad relay inside the EBTCM. You can send it to one of the ABS Fix it people or you can fix it your self.

Bill

 

That sounds very familiar. It's a lot like what you do with a Jeep Grand Cherokee.

 

The battery drained a few times during the winter. Maybe that's the cause?

 

Took it for a test drive down the road and the only code that came back was B2605.

 

Do you ever smell gas? With that kind of mpg and the car running okay points to a small leak.

 

my expedition el does better (17mpg hwy)

something must be wrong some placce my 03 zo6 gets 26.5mpg hwy cruising 75+ mph and 18.5mpg city and I have a heavy foot

I'd clean the injectors regardless since it sat so long, youtube it something you can do your self I did it to my boat a few years ago worked out well

 

Have you had an alignment done recently? Or possibly the tires could be causing drag? Knowing the car has been rear ended could cause concern as the car could literally be driving sideways on the road if it was hit hard enough. Especially if the rear cradle is shifted at all. I would start there. You would notice the mileage issue on the open road more so than low speeds on city streets. Think of is as the rear is fighting the front half of the car for direction on the road.

 

I'm going to change the plugs today and run seafoam through the motor and take it for a long drive. I'll report back.

 

It's going to be a while before it's running again. Broke one of the wires while removing the plugs and it probably needed a new set anyway. Funny thing is I wasn't really trying to pull that wire out. Ordered a set of MSD wires, cause everything local is junk and overpriced to boot. Until I get the new set of wires I can't even start it.

BTW here's photos of what was in the engine. Don't look like anything original, and I have no idea who makes the wires. Probably all junk anyway. I think the plugs were burning fine, but I can't ever really tell.

 

WOW! Those gaps look grand canyon wide! Give them a measure and see what they were.

 

You have some eye cause I couldn't see that. But according to my gap tool it's .060 on all plugs. The new AcDelco plugs came .040 and I left them alone cause I was told that's proper. I think the originals are suppose to be .035? BTW the old plus are Denso.

 

Attention: Please disregard the Set Gap Per Vehicle Specification statement on the side of an ACOelco(R) Iridium Spark Plug Box. The statement will be removed from the boxes in the future.
Supercede:

This bulletin is being revised to add the Chevrolet SSR and the 2005 Pontiac GTO. Please discard Corporate Bulletin Number 03-06-04-060A (Section 06 - Engine/Propulsion System).

^ A new spark plug has been released for use in the above vehicles. The new spark plug has an Iridium tip instead of the current Platinum tip. Due to the different tip design the gap of the spark plug has also changed.
^ The new spark plug P/N 12571164 with AC Delco P/N 41-985 is gapped to 1.01 mm (0.040 in) when the spark plug is made.
Notice: The spark plug gap is set during manufacturing and should not be changed or damage to the spark plug may result. Any new spark plug found to not be properly gapped should not be used.

The above TSB includes 97-04 corvette ls1 and ls6 along with many other gm vehicles. The wires look O.E..

 

Got the new MSD wires and installed them then I ran Seafoam through the engine. I haven't had a chance to see what the MPG is yet but new problems arouse. It was raining and I gave it a bit of gas and then ABS lights started to appear. This time C1234 and C1243 came up. I took off the left rear wheel to examine the wheel speed sensor harness since a lot of people claim that's usually the problem. Bent the pins a bit and threw some dielectric grease. I used my multimeter and the sensor does produce a signal when I turn the wheel. Got in the car and floored the gas pedal a few times and the codes came back but eventually stopped. The only codes that keep coming back are B2605 and B2860, and I can't clear out B2605. Though C1243 could be because it's not been used for a while. After 5 years of sitting I wasn't flooring the car around until it's worn in properly.

 

Took it for a long drive today and finally got to see the results of my effort. This time I got a good 20MPG average with about 90% highway and 10% local from driving 48.8 miles. So the plugs wires and Seafoam must have helped. Also the Seafoam is still in the remaining gasoline so it's not completely ran through the engine. I was very careful with the peddle but I would occasionally hit 70mph.

Just for comparison my 04 Lexus es330 does about 19 mpg but that may need a new air filter. Again using the cars gauge.