86 Corvette MPG troubleshooting
Thread Starter
Racer
Joined: Aug 2009
Posts: 496
Likes: 4
From: Cordova TN
86 Corvette MPG troubleshooting
I've searched all over and thought I'd just start a new thread.
86 stock Corvette. We just purchased it in August and have put nearly 1500 miles on it and we are averaging around 13mpg. From experience with these engines I know they should do better. The car runs/idles fine and had plenty of acceleration.
Just because it was new to us with 92k miles I changed out the o2 sensor, fuel filter, air filter, plugs/wires and cleaned the MAF + throttle body when we bought it. No real change on the MPG since it was getting 12 or so when we got it so it bumped up 1mpg.
I have done a pressure test and got 43 before turning the engine and once cranked it held 38. When I shut it off it held at about 40 and fell to 32 after 15 minutes.
The car is showing no codes but the last time I cranked it up in the garage in the 5 minutes it took me to walk around and open the garage door it had put some black soot(not a lot but some) onto the door.
I have purchased some bosch IIIs and my question is uwith all of this could the injectors be the problem? They look to be the original style.
Thanks for any info. Just w]looking for some reassurance before I tear down the TPI system on a car that is running fine except for the MPG.
86 stock Corvette. We just purchased it in August and have put nearly 1500 miles on it and we are averaging around 13mpg. From experience with these engines I know they should do better. The car runs/idles fine and had plenty of acceleration.
Just because it was new to us with 92k miles I changed out the o2 sensor, fuel filter, air filter, plugs/wires and cleaned the MAF + throttle body when we bought it. No real change on the MPG since it was getting 12 or so when we got it so it bumped up 1mpg.
I have done a pressure test and got 43 before turning the engine and once cranked it held 38. When I shut it off it held at about 40 and fell to 32 after 15 minutes.
The car is showing no codes but the last time I cranked it up in the garage in the 5 minutes it took me to walk around and open the garage door it had put some black soot(not a lot but some) onto the door.
I have purchased some bosch IIIs and my question is uwith all of this could the injectors be the problem? They look to be the original style.
Thanks for any info. Just w]looking for some reassurance before I tear down the TPI system on a car that is running fine except for the MPG.
Last edited by megtom2; Nov 14, 2009 at 10:12 AM.
Burning Brakes
Joined: May 2007
Posts: 965
Likes: 13
From: turn 12 at Road America or TX
At a constant 70 mph on a flat road you should see 24-27 mpg. If you see more than that, you probably have one or more low resistance injectors which confuses the ECM. Use a GPS unit to see if the speedometer and odometer are correct because someone may have changed the rear end ratio without changing the tranny VSS gears. And there are host of other things that could be wrong with that many miles on the car ...
Thread Starter
Racer
Joined: Aug 2009
Posts: 496
Likes: 4
From: Cordova TN
Mostly city and going by the dash. It can't be too wrong since I have checked the odometer to double check it.
Maybe I'll keep the injectors in the seal for now.
I'll pull out our GPS to check the speed/odo meter.
Maybe I'll keep the injectors in the seal for now.
I'll pull out our GPS to check the speed/odo meter.
Le Mans Master
Joined: Jan 2003
Posts: 5,858
Likes: 7
From: Santa Maria CA
Apperently that's why they added reset buttons. The readout tends to stick and become inaccurate faily often, specially after entering city traffic after a highway drive and vise versa. Have been hitting the reset button?
Corvette Stories
The Best of Corvette for Corvette Enthusiasts
10 Reasons the C6 Z06 is Still A Performance Benchmark After 20 Years
Joe Kucinski
How Much Horsepower Every Corvette Engine "LOST" in 1972
Joe Kucinski
Top 10 DOs and DON'Ts for Protecting Your Convertible Top!
Michael S. Palmer
Top 10 Most Explosive Corvettes Ever Made: Power-to-Weight Ratio Ranked!
Joe Kucinski
150 hp to 1,250 hp: Every Corvette Generation Compared by the Specs That Matter
Joe Kucinski
8 Coolest Corvette Pace Cars (and Replicas) of All Time
Verdad Gallardo
Top 10 Corvette Engines RANKED by Peak Torque (70+ Years of Muscle!)
Joe Kucinski
Corvette ZR1X Will Be Pacing the Indy 500, And Could Probably Race, Too!
Verdad Gallardo
Top 10 Corvettes Coming to Mecum Indy 2026!
Brett Foote
Safety Car
Joined: Jun 2006
Posts: 3,629
Likes: 28
From: FL
If the mileage does turn out to be accurate one item worth checking that I haven't seen mentioned would be the coolant sensor. It can be out of range and still not set a code, if it is reading cooler than actual it will impact fuel economy.
If you have a scan tool compare the coolant temp to IAT after sitting overnight, the two should be very close. That is not a perfect test but will often reveal a problem.
If you have a scan tool compare the coolant temp to IAT after sitting overnight, the two should be very close. That is not a perfect test but will often reveal a problem.
Safety Car
Joined: Jul 2008
Posts: 3,966
Likes: 4
From: Made In America
St. Jude Donor '10-'11
If the mileage does turn out to be accurate one item worth checking that I haven't seen mentioned would be the coolant sensor. It can be out of range and still not set a code, if it is reading cooler than actual it will impact fuel economy.
If you have a scan tool compare the coolant temp to IAT after sitting overnight, the two should be very close. That is not a perfect test but will often reveal a problem.
If you have a scan tool compare the coolant temp to IAT after sitting overnight, the two should be very close. That is not a perfect test but will often reveal a problem.
Welp found my problem. The coolant light keeps coming on but the radiator and resovoir are always full.
