OK,
some background first,, car is an 82 that I bought last spring with a little over 58k miles. Last summer I took it for its 1st real hiway drive, 160 miles round trip on the NJ Turnpike. Cruise set at 70, a/c on , got 22 mpg w/o so much as a tune up. Didnt do a thing to it till this spring when it started to bog down on acceleration. Did a complete tune-up, plugs, wires, cap, rotor, set tps etc. Still bogged. Replaced fuel pump with a '85, bumped fp to 14 or so, still bogged, checked timing still ng. After exhausting all I pulled a valve cover and discovered what eventually was a wiped cam lobe. Had cam replaced with as close to stock as possible comp cams grind. Car really perked up after that. I also pulled the CAT and replaced with a test pipe. So I'm expecting that since my power is up that my mpg might even be up a tad since the cam was probably going for a while? Well, yesterday took a 160 mile trip this time on the NY state thruway. Used the cruise both ways set at 70, going upstate no a/c, coming down I used the air. I got 16.5 mpg!! Thats a helluva drop. And after doing all that work... The car seems to run fine, only thnig I noticed was the cruise wouldnt hold a steady 70, seemd to bounce around, 73,, 67,, 70 etc. It usually sets about 3 mph higher I just havent gotten around to adjusting the screw on the side of the transducer yet. Also when I pulled the CAT there is an air tube that screws into the cat, I just tied that up to the frame for now, should that be plugged or capped? ANy ideas???/
ESU

 

This may not be attributed directly to your MPG problem but when was the last time the O2 sensor was changed? As they go bad your mileage will get worse and worse. Changing that might bump it up a bit even after you solve the main problem.

 

I would venture to say the O2 sensor has never been changed,,, but I'm not getting a CEL either,
ESU

 

i agree with db check your o2 sensor.surging up and down will affect gas mpg .the gas[regular or mid grade] that you use can also affect gas mpg .the air tube will not affect any thing if its not hooked up .mine is capped off .in the summer they blend the gas with more oxygen.you may have got a tank full that was not right .me, i would try again before i did any changes .p.s .i would have put in a bigger cam

 

OK,,, 3 questions,,

1- O2 sensor can be bad without a CEL?
2- location of O2 sensor, in the exhaust I assume?
3- Terry, what kind of mileage you gettin with your 82?
ESU

 

20 mpg or better .i think it could get better but, the right foot goes down hard alot .i have read that this is a problem on a lot of corvettes .

 

Some good suggestions here. I didn't think of the O2 sensor when we talked since those Crossfires are a different breed from mine. Forgot that one. I would try that change first although those O2 sensors can be pricey. PS-Agree you should have put a bigger cam in it when you made the change!

 

Can an O2 sensor be bad without a CEL though? Also where is it located? Cant seem to find my shop manual.
ESU

 

Yes O2 sensors can slowly fail, becoming less and less effective until they get so far gone they throw a code up. My Durango has two of them, and the mileage kept getting worse and worse. finally I ran codes, found nothing, changed the sensors, and the mileage jumped significantly.

I'm not sure where it's located on an '82, but it's not going to be far from the manifolds. You might have to jack up the car, and slide under to find it if no one else knows.

 

The O2 sensor is nothing more than a chemical battery. It uses O2, Heat and a catalyst to produce a voltage. Usually in the millivolt range. The amount of oxygen or the lack of Oxygen in the exhaust causes a reading that is used by the PCM to adjust the feedback carb.

The O2 readings are suppose to fluctuate from 250 Millivolts to 950 Millivolts. The O2 sensor produces a voltage compatible to the O2 content in the exhaust stream. The PCM then adjust the feed back carb to adjust for the rich or lean mixture.

When an O2 sensor goes bad, it will not correctly respond to the O2 content in the exhaust, It will sit at a voltage around 400-500Mv. which will cause the carb to run rich.

You can read the signal wire on the O2 sensor with a Digital voltmeter and actually monitor the O2 sensor operation to see if it is correctly responding to throttle position. If you blip the throttle the exhaust will go rich and the voltage will drop to around 250-300 Mv. If the engine coast down from an elevated RPM the mixture should go lean and the voltage should spike high 800-900 mv.

Over time and miles the O2 sensor gets coated with exhaust deposits or the catalyst wears out and it fails to respond correctly to the change of O2 readings in the exhaust. This failure to respond causes poor performance or poor fuel economy.

It is often difficult to determine if the O2 sensor or the feed back carb is bad. If you artificially richen the mixture by dumping fuel into the carb intake, you should see a rich reading.

Bill C

Bill C

 

I have 1982 vette i was thinking about changing the cam, what size cam do you think i should put in?

 

Bill...with the crossfire you have limited choice i'm afraid,Comp Cams do a few especially for the crossfire.

Click on the link below for their site and put crossfire into the search box to see what's available.

http://www.compcams.com/DefaultWide.asp

 

I agree with windup,, theres not much of a selection for crossfires. My cam went so I needed a new one and I didnt want anything too radical. Both crane and comp cams only offer 1 choice each for crossfires if I remember correctly. I used the one from comp cams it was a tad closer to spec than the crane and I also ordered it nitrited. It definitely perked things up a little. Also remember too radical a cam in a vette could adversely effect vacuum which could be a proble for cars that depend on vaccum a lot.
ESU

 

Changed the O2 sensor the other day and topped off the tank with gas from a different vendor. The sensor was only 21 bux from autozone (bosch). Tomorrow I have a 100 mile or so (all hiway) round trip to meet some buddies for our annual Sept 11 re-union so I'll see if there is any change in the mpg. Do I need to reset the computer at all due to the installation new sensor?
ESU

 

No need to re-set.

 

98 mile round trip,,, 4.3 gals of gas,,, 22.79 mpg,,, I feel way betterer.
Thx ESU

 

I'd recommend you install a low restriction cat converter and install a fresh O2 sensor. Without the cat converter, your injector calibration may be off. Also, if the psi is up on your injectors, could you be forcing more fuel through the system than using the stock psi? A potential rich condition?

Also, depending on the cam grind, you may have increased power at the cost of mpg... what is the lift/duration on your new cam?

I'd also check all vacuum hoses/systems on your car. If they are all original, you may be losing vacuum. I replaced all vacuum system hoses on my '80 and eliminated alot of gremlins in the process. Same goes for thermal sensors in the intake manifold that the vacuum hoses plumb into... either replace or remove them. I bought ALL vacuum related components from Dr Rebuild. Top notch parts and no regrets!

 

Ted,,

doesnt the computer adjust injector calibration?? Also I just installed a new o2 sensor 2 or 3 days ago. My mileage as of today is back up to the 22 range like when I 1st bought the car. Also if the o2 sensor is before the cat I dont see how the cat being in or out makes a difference? As for the cam here are the specs:

1987 1989 CHEVROLET 8 GOOD MID-RANGE AND TOP-END PERFORMANCE IN MONTE CARLO SS, CAMARO AND FIREBIRD WITH 305 HO, AND 350 11621-16 I 10751-16 O TRUCKS. (50 STATE LEGAL IN 81-87 CAR AND 81-92 TRUCK, 267-350 APPLICATIONS ONLY. C.A.R.B. E.O. D-19) BASIC RPM 1500-4500
Engine Size Configuration
305 C.I. V

Valve Setting: Intake .000 Exhaust .000 HOT

Lift: Intake @Cam 282 @Valve 423 All Lifts are based
on zero lash and theoretical rocker arm ratios.
Exhaust @ Cam 297 @Valve 446
Rocker Arm Ratio 1.5

Cam Timing: -- @--
Lift: Opens Closes ADV Duration
Intake -- -- 264 °
Exhaust -- -- 274 °

Spring Requirements: Triple Dual Outer Inner
Part Number 99848
Loads Closed 114 LBS @ 1.700 or 1 45/64
Open 297 LBS @ 1.280
Recommended RPM range with matching components
Minimum RPM 1200
Maximum RPM 4800
Valve Float 5400

Cam Timing: TAPPET @.050
Lift: Opens Closes Max Lift Duration
Intake (8) ATDC 32 ABDC 110 204 °
Exhaust 37 BBDC (3) BTDC 110 214 °

Vac was a steady 19 when I bought the car, was 18.5 when I checked it the other day. I checked it right out of the block and then at different points in the routing of the hoses and always stayed at 18.5. I also have used Dr Rebuild and found them to be great, I had a 79 that needed to be redone completely and their products were right on the money.
ESU

 

Outstanding! Enjoy the '82!

 

Great to hear.