Question : EXTREMELY poor fuel economy on 94 Grand Am 3100

[Karol]

Hello,

I have recently purchased ‘94 Grand Am that had been sitting still for at least three years (some 66k on it). It drives again now, but there is still one issue haunting me. Initially, there was a serious problem with the way it was driving (lack of power, hesitation on acceleration, rough idle, extremely high fuel consumption), but replacing the O2 sensor seemed to be a remedy. However after installing new sensor fuel consumption is still extremely high (around 7 mpg city driving, around 120 miles per tank!). Other than that car behaves almost normally (has the power and acceleration, idles pretty good). Also, car DOES NOT throw any CES light, or does this very occasionally.
Is it possible that since the car was sitting for so long and had accumulated a lot of moisture the wiring of CTS sensor has such a high resistance making ECU think it is very cold? Sensor itself screwed? Will this alone could explain such miserable fuel economy?
Any other ideas what might be a cause of this?

NOTE: in-dash gauge never seem to get above second mark (around 150 F), but it is hard to be believed since the speedo and fuel level gauge show unreliable data (speedo is by average 20 miles ahead of real speed).

[SappySE107]

Something is seriously wrong if the gauges are that far off. A scan tool would be ideal to see if its the gauges or the ECM. As for the mileage, aside from the ECM, it could need a new air filter, fuel filter, plugs, wires. Not surprising the O2 sensor made a significant difference.

[Karol]

Well I just kind of left the obvious things out of the message So to clarify things: all the filters along the plugs and wires were changed on the car. I am intending to scan the codes, but since this vehicle is a rarity over here, I would have to wait until a proper cable ships from US. Can someone recommend a good software to scan codes?

[Superdave]

I would replace the coolant temp sensor, if the car isn't going into closed loop that would explain your poor economy.


you'll also have to clean the o2 sensor until you get it sorted out. I'd use regular carb cleaner.


Good Luck!

[IsaacHayes]

An ALDL cable will work. You'll need TunerPro RT software, and there should be a definition on the site if not, use the .ads one I have in :

http://isaachayes.60degreev6.com/aldlcable/

You can also make the cable. I did the more complex setup:

http://isaachayes.60degreev6.com/aldlcable/circuit.gif

Or you can do a 2 transistor setup which is simpler:

http://cable.invisibill.net/

[60dgrzbelow0]

Karol...

Welcome to www.60dgreev6.com ...

Without knowing the length and breadth of your automotive knowledge... May I hazard the following suggestion?

Since you are trying to discover so many unknowns in finding and fixing the problem(s), I was thinking that the below listed links might cover many of the myriad things that can go wrong; some of which we might miss in mentioning, The VP (Video Poster) (Dave from Dave's Farm) has a whole library of useful videos covering many other areas where engine problems arise. If nothing else... they are entertaining and might even reveal something that none of us has yet to think of mentioning...


Automotive Mythbusters: Tune Ups (Part One)

http://www.youtube.com/watch?v=_lREH1C4Y4w

Automotive Mythbusters: Tune Ups (Part Two)

http://www.youtube.com/watch?v=6jH4GmaITUU

For the full series of "Automotive Mythbusters"...use this link:

http://www.youtube.com/results?search_query=automotive+mythbusters&search_type=&aq=f

[pocket-rocket]

Dave's farm is awesome. I've been watching his videos for some time now. Here is something I would never have the nerve to do, even though I know it works:

How to weld a gas tank with gas in it...

http://www.youtube.com/watch?v=tD4KiQEnxwQ

[LeftVentricle]

I'd be looking for a fuel leak somewhere if it runs fine. My old Suburban was getting something absurd like five miles per gallon, and it turned out to be a split fuel line going into the carb.

[90v65speed]

I would also check the brakes to make sure none are dragging. That will kill your fuel mileage.
Brent

[talon2swords]

Maybe check the TPS. Should be a way to check the voltage without buying another one.

[Karol]

Hello guys…thanks so much for your replies. I have changed a thermostat on this GA…mileage improved a lot, but it is still poor (averages some 12 MPG now in mixed driving conditions – mostly city though). However, it now signals clearly that something is wrong with its brains (bad shifting, kind of has a mind on its own as far as throttle is concerned). I finally got a cable done (thanks IsaacHayes) so will try to hook it up to a computer on Sunday. I guess I will need help on this as well . Anyway, I will keep you posted so others with similar problems will have an easier life if something like this ever happens.

[Karol]

OK…just one more thing off a topic just a bit If any of you can advise how to improve this 3100 performance wise. I am looking to bump a horsepower to some 200 hp…looking not to use turbo or anything like this. I guess this engine has a lot of potential power to extract

[bob442]

if there is old gummed up gas, you wont have enough fuel pressure, and it will cause all sorts of problems. you need to clear out your tank, flush the injectors, change your fuel filter. i would better reccomend checking you FP before you do that though. the gauges on those clusters go wonko all the time, i had to change the cluster on my 96, and i've header of others doign it all the time.

[bob442]

for your performance needs, do the hybrid swap, search it.

[74Spit]

I would check that the engine an ECU grounds are clean an tightly connected.
A poor ground connection can cause all of the issues you have listed.