MPG- bad?

[clok1966]

I have a 87 with the 4+3 OD manual. Had the car a couple years now and it drovein town 99% of the time. When i bought the car i drove it home (about 100 miles on 70MPH speed limit) I was shocked when i was seeing 26-28 MPG instant, seen 29 even. This was summer, hot day and no wind on flat road. Now putting around town at 30mph i see 28-30 all the time (on long steady stretchs). But my avg is 13.9 (yes i do get on it once and awhile). I had a bad thermostat (found out this winter when i planned a long trip home to parents with it.. couldnt keep it above 110 degrees on road, so no OD, much less heat!) ran great in summer, never overheated or anything.. guess it was a tired old thermostat. Replaced it a few days ago and flushed the coolent. Took car out for first drives this spring (still only 50's here at best). Took it out on road and was getting 18-22 at 75. Now i swear i was in the 25+ range before at 75. No matter what i did i couldnt get past 22, normally was 20ish.

Friend says O2 Sensor may be bad (he thinks it smells rich when he is behind me in town). Old school mechanic (as in i havent done to much on new cars) the O2 the one in the flex pipe loooking thing on passnger side up top? Almost looks a bit like a spark plug?

And one last question.. other then actually getting to um, with aluminuim heads on the vette any problems with plugs? As in stripped threads and the like? I had some old (think K-CAR) chryslers i helped friends with and sad as it is, i can say almost every singel one i ever worked on had a stripped plug in the aluminuim heads.

 

[boomdriver]

as far as the MPG, I also used to get near 30 when I tried really hard.....but thats down to 22-23 at best. part of the reason is the wonder world of blended fuels (alcohols) that produce less energy so it takes more to yield the same results. See Mr. Gore regarding that silly crap... Less energy produced, more product used = more polution on most planets. Moving on.....

The O2 sensor is in the exhaust pipe driver side near the oil filter. The sensor that you see on top is the EGR switch. You might see a slight improvement in MPG with a tune up and new O2 sensor, but alot of it is simply the cheap fuels these days.


The aluminum heads are pretty solid as long as you follow the basic rules. Never change plugs in a hot head, (who would want to) and always start new plugs by hand and not with a wrench. This way you can feel the threads and not strip one out. It does happen.
And always warm up the motor before stressing it. The heads absolutely HATE old antifreeze or too much water, so stay on top of that.

 

[ex-x-fire]

Also winter blend gas isn't the greatest for best mpg. Clean the maf , & check fuel pressure, see if the pressure drops off when the engine is off. This will give you an idea of the condition of your injectors & regulator.

 

[clok1966]

Well I got one of the O2 sensors (didnt know there where 2). Any help on where they are located, just so i can save some time. I got the BEFORE the cat one. I am assuming its under the car? problebly in fron of the cat (hence the before the cat- name). Since i may ave to replace both, any ideas on locations.

87 is the year.

 

[John Robinson]

With your location and outside temperature your car will get poorer gas mileage. The purpose of the thermostat is to keep the engine operating temperature at a pre determined level for best mileage etc.

What does that mean well the colder it is outside the more fuel it takes to maintain the engine temperature needed for efficient emissions and fuel use.

So for that reason I would recommend that no one should replace the thermostat with a lower opening one. If the engine is not up to the temperature the ECM is looking for then it will keep adding fuel to try to get the engine up to the temperature it is programmed for. Basically the ECM is in chock mode all the time.

 

[TedC]

I've replaced plugs on both my 1988 and 89 with aluminum heads. No issue with stripped threads. When istalling, used torque wrench on the easy ones to get a feel for how tight they should be, then hand tighten the tough ones. Used anti-seize on the new plugs. I've had good luck with stock and rapid fires plugs and wires (all AC Delcos).

I think your friend gave you the best clue. It is running rich. Do you have a GM service manual? I would check the coolant sensor (resistance needs to be correct or it confuses the ECM) that feeds the ECM (not the one that feeds the dash), fuel pressure (leak down test), injectors (resistance). I replaced the 02 sensor on my used vettes during their first tune-up procedure. You will need a special socket available at auto parts stores that has a slot for the wire.

 

[boomdriver]

Clok1966,
87 only had 1 o2 sensor.
Later yrs have 2 and even 3, but the 87 only has a single o2 sensor in the driver side exhaust a few inches below the collector under the manifold. Look near the oil filter.

 

[clok1966]

Clok1966,
87 only had 1 o2 sensor.
Later yrs have 2 and even 3, but the 87 only has a single o2 sensor in the driver side exhaust a few inches below the collector under the manifold. Look near the oil filter.

thanks , just the post i was looking for.

For everybody else, thanks too. Will replace it and see how it improves, weather has wnet form mid 60's to 40's and rain so I may not get to it for a bit, will post on any change.

Several of the responces, again thanks...but...

Gas , do you think its changed that much in 2 years? I got the car 2 years ago this july.

Winter blend, this i know, wonder when summer blend starts.

Thermostat, hadnt thouhgt about the temp driving the computer to run richer, makes sense. New 195 is in it now, but till i replace the O2 sensor I'm not checking it. just found out i have a new $$$$ problem, looks like my power brake boster is dieing

 

[vetteboy86]

When is the last time you reset your avg mpg? I am thinking that I read somewhere that a better estimate is to reset your avg. and allow the computer to calculate over a few miles, over watching the instant. So cruising along the highway take the avg over a few miles. Same would go for city driving.

 

[clok1966]

I have no doubt the Instant is a bit wonky. My newer cars all have the instant, but its more of a "average instant". My van and caddy both when slowly accelerating up to 75 seem to have "timed" readings. As you slowly get up to 75, even at 75 it will take a minute or two of steady speed before I will see 28. The vette seems to show "instant" as its called. MPG will slowly climb and once i hit 75 and am steady it will normally show my MPG right away, all the other cars must have some computer stuff figureing it out.

I reset everything (battery unhook) a few days ago (my new brake problem, needed to reset the light in dash, wouldnt go off). Going to be a bit till i can research more as the weather isnt nice here and I have to work outside as my garage is way to cramped.