I’ve got an 88 corvette that right now is averaging about 10.5 mpg. The car is completely stock except for a muffler delete. I’ve just been driving around town as it’s my daily. It rarely touches the highway. Just did spark plus and wires and fuel and air filters. What can I do to raise that number.

 

O2 sensors.

 

I have a 1988 C4 Coupe with the 700R4 Automatic in it and a 3.07 rear end. I have done very little to my Corvette other than a new Cat Back Chambered exhaust system and a High Flowing aftermarket catalytic converter. According to my GTECH pro I gained just shy of 14 hp after the swap.

On the interstates going 60 mph my C4 will see 30 mpg. It has always been able to get good mileage. The first thing to do is be sure that your engine and the EFI equipment are working properly. My transmission was overhauled so it is not slipping at all.

Like fellow member Nomake Wan suggested you might start with your Oxygen sensor. The oxygen sensor is designed to last for 24 months or 50 k miles in OBD1 vehicles. In an OBD 2 systems they go twice as far.

When the O2 decides to fail it will do so on the "rich" side as to not hurt the engine. The O2 oscillates between .1 and .9 very quickly, as they age the signal gets narrower like .3 and .7 and so on. The other bad habit of old O2's is that they lag behind with the correct data. If the Corvette is left running rich for a long time the catalytic converter can get plugged and that makes the car run hotter and slower

Being a big believer in B&G 44K Fuel Injection system cleaner I would also put a bottle of this in a new tank of gasoline to clean things up a bit. B&G 44K is not cheap but it does work remarkably well in all the cars I have used it in.

Check your tire pressure, I tried mine around town at 32-33 psi but for long highway rides I will bump it up to 35 psi, harder tires get better traction.

You must be a favorite around town with no mufflers on your Corvette. I have chambered pipes on both Corvettes we own. The 1968 C3 Convertible with it's High Compression engine and the noise out the back gets a bit loud with chambered pipes. It is almost as loud as side pipes but the noise flows out behind me.

On our Corvettes we have the Cold Start Injector and they have been known to go bad like any other injector. Try unplugging the power to the 9th injector and see if there is any difference.

The three items that are critical to your EFI system working properly are as follows:
Oxygen Sensor: This device is used to measure the oxygen in the exhaust stream to control the Air/Fuel ratio. The O2 is a cheap and easily replaceable sensor and will make a difference in both performance and economy.

Coolant Temperature Sensor: This sensor measures the temperature of the coolant. It is what gives you the right amount of fuel to start in cold weather. It is critical that this sensor be working accurately. If reading too high you engine would be harder to start when cool. If reading too low it will keep the extra fuel enrichment on since it thinks it is colder than it really is. This would be like a choke that is sticking on and it too will plug your catalytic converter quickly if left unattended.

Mass Air Flow Sensor: This sensor will measure the volume of fuel being ingested by the engine. The MAF system has two relays that make it work properly, one for power (On and Off) and one for the Burn Off Function. The burn off is critical to keeping the data from the MAF accurate. Also be sure there is NO air leaking into the section of hose between the MAF and the throttle body.

We are here to help you in any way we can. You might want to be sure the Fuel Pressure regulator is working okay. The oxygenated gasoline will destroy the diaphragm in the regulator. Pull vacuum hose from regulator and smell for gasoline. If gas fumes are present that indicates a bad Diaphragm in the FPR.

 

Take your '88 onto the highway and set cruise at 60-65mph, you should see ~30mpg on flat terrain if you have an A4 with original 2.59 gears out back.
This is a much better MPG gauge than around town driving, IMO.

My '88 shows 30-31mpg under the above conditions.

 

The O2 sensor makes the largest difference to your mileage. Typically they fail slowly and don't set a code until they're really dead. If the car is running well otherwise and there is no issue with the fuel system, that is probably the culprit.

Drivers side Y-pipe, just below the exhaust manifold. There is a special tool to get them out but a regular wrench can do it. Might wanna hit it with penetrating oil because they dont like coming out.

 

So I went ahead and replaced my 02 with an AC Delco one from oreillys for like $20 and I saw a jump but now what I was hoping for. The car now averages 11.5-12 mpg around town and not much better on the highway. Anything else that could be robbing me of mileage

 

My Factory Service Manual has a section for Poor Fuel Economy although it's for a '90 . Some of the preliminary checks ask if there's any codes, check grounds, vacuum hoses for splits, kinks, proper connections, vacuum leaks at throttle body, intake manifold and wiring for proper connections.

Followed by engine coolant level, always open thermostat or wrong heat range. Ignition timing adjusted to spec, proper TCC operation, exhaust restriction i.e. clogged cat, and compression.

 

As was said by someone else earlier, your coolant temp sensor is another likely (and cheap) culprit. If it's reading too cold all the time, then the ECM will compensate by adding way more fuel than necessary.

Definitely get an FSM. They're only $20.

 

Verify ignition timing set to 6 or 8 BTDC, and if sluggishly accelerating, a cat could be plugged up. Sometimes best to look at those in the dark at night to see if they glow red. A bad vac leak would have it stumbling.

A scanner would tell you if you were stuck in open loop where the CTS is used to meter the fuel.

 

You can check the FPR by pulling the vacuum hose of and checking for fuel in the line. A leaking one will make it run rich and idle poorly .... this is what a bad FPR diaphram looks like. I've had to change mine twice in the years I've owned my 1986.
BAD FPR

 

I think for the moment I am going to replace the coolant temp sensor as the car was throwing a code. Don’t remember exactly which one I think it was code 17 which was the coolant sensor but the code stopped so I wasn’t worried. I’ll replace it probably during the weekend.

 

Test before you change... https://www.corvetteforum.com/forums...post1581310844

 

Yep. Though if he has an intermittent code, the wiring could be suspect as well.

Sounds like a bunch of fun.

 

I’ll take a look at it this weekend and see what I need to do.