# Really bad gas mileage problem



## mtlmark (Oct 17, 2010)

I got my GTO back in October used, and since then I have not been able to get more than 12 mpg's in the city, 16 hwy, no matter how I drive it. The previous owner had a K&N CAI and Corsa Exhaust installed. Since I bought it, I have:
1. Changed the oil to synthetic
2. changed the plugs and wires
Niether had any affect on the mileage. The car seems to be running really rich, the tailpipe is pretty black and discolored. The car does not hesitate or idle funny. Recently I removed the Cold air intake to install 2 new belts, and when I did I cleaned the Throttle body (with throttle body cleaner) and MAF sensor (by spraying only with MAF cleaner). This seem to make the mileage worse, down to about 10.5. I am thinking it is a Vacuum leak, MAF sensor, or oxygen sensor. The car is not throwing any codes, and I took it to the dealer for an alignment, and had them scan it for codes, and they told me the tune was stock as well, but I am wondering if that is true, becuase the skip shift is gone, and the light is not on for it. Any suggestions?


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## jpalamar (Jul 22, 2008)

You can disable the skip shift with a plug in adapter from SLP thats $25. So many things can make the car not run great. Its really hard to guess over the net.


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## mtlmark (Oct 17, 2010)

It's not the plug, I checked under the car. As far as guessing over the net, the forum is here for me to get ideas from others that might have had the same problem.....I tried to provide enough info to take some of the guessing out of it.


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## Mike_V (Mar 10, 2006)

So how does the car run, besides MPG; you notice anything at all? I can't imagine any mod the previous owner could have done that would result in that MPG, so something has to be wrong with it. You'd just think it would run like crap. Have you confirmed your speedo is right with a GPS?

I would not trust that it's a stock tune. I'd probably spend the cash to take it to a tuner to know what's going on. Are the CATs still in place?


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## pocketmaster (Feb 26, 2010)

What did your plugs look like when you changed them?? Were they worn or show signs of running rich ??


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## pocketmaster (Feb 26, 2010)

Mike V has a good suggestion about getting it re-tuned. There are many variables that can be verified with a good knowledgable tuner.


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## mtlmark (Oct 17, 2010)

Yeah, the tune is where I am leaning as well. The car runs good, no stalling, idling issues. The performance shop here where I live chrges 500 for a dyno tune, I talk to the guy and he said he would check it for vacuum leaks and make sure the o2 senlooked good, so I think that is where I am headed. I will keep you posted, thanks for all the good advice.


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## silversport (Mar 23, 2007)

if you had an automatic and drove short trips I might have said you are about normal but you have an manual shift...your mileage sounds low (mostly your highway)...the tune should tell if your plugs didn't...good luck...
Bill


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## mtlmark (Oct 17, 2010)

Alright guys, update time:

I never did take it to get it tuned. I went this last weekend and got a new battery because the old one was on it's last legs. I figured I would clean the TB/intake filter and MAF sensor at the same time I istalled the battery. When I was cleaning them, I noticed a loose wire and plug on the drivers side of the engine by the intake, I figured it was to the old stock air box, didnt think much of it. Finished cleaning and hooked the battery back up (the new battery), put the intake back together, and low and behold, the CEL came on. I took it to Autozone, had them scan it, 

COOLANT TEMP SENSOR

that is what the loose plug went to, it had broken off (luckily at the sensor, so the plug and harness were still good. Installed the new sensor. Apparently without this sensor, the car was in open loop mode the whole time, because it couldnt figure out that the car was to operating temp. Now I am getting 15 city, around 23 highway....NICE!!!!!!


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## ZEIKE524 (Dec 28, 2010)

That almost sounds too good to be true. Yeah it would effect ur milage but that much? crazy


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## BobS (Jul 28, 2005)

That's amazing. But why didn't the car throw the code long ago? 
The new battery shouldn't have anything to do with it, right?


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## BatmanGTO (Jun 18, 2011)

I just ran into this problem on my truck a few weeks ago and spent 2 weeks trying all kinds of stuff (old 95). The plugs were solid black and the truck was flodding itself out, it could hardly burn enough fuel to even run. Turns out it was the CTS, I found by accident the wire going to is was broken at the clip. It probably thought the truck was super hot.


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## mtlmark (Oct 17, 2010)

Yeah mine was broke as well but on the sensor side. The car not only gets better mileage, it runs better as well. If I would have took it to the tuner, they probably would have figured it out, the software would have told them it was in open loop mode. After reading about it, it makes perfect sense. I don't know how I didn't notice the broken clip when I changed the plugs a couple of months back.

As far as the code, the only thing I can think of is the previous owner erased it. By unplugging the battery, it might have reset the computer. The guy at Autozone said the code may have cycled off after a while, I am not sure how that works.....


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## dude515 (Mar 30, 2011)

with the coolant temp sensor out the system (and someone correct me if im wrong) sets a default temp of someting like -20 F. causing the ecu to dump in fuel to compensate for the supposed 'colder air' and to try to bring the motor up to operating temperature. and im supprised it didnt effect your gas mileage even more than you stated. i have seen cars with the same problem that push out clouds of black smoke because there is so much unburnt fuel running through the exhaust. if the car has been running like that since october i would have the cats checked out.


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## Poncho Dan (Jun 30, 2009)

BatmanGTO said:


> I just ran into this problem on my truck a few weeks ago and spent 2 weeks trying all kinds of stuff (old 95). The plugs were solid black and the truck was flodding itself out, it could hardly burn enough fuel to even run. Turns out it was the CTS, I found by accident the wire going to is was broken at the clip. It probably thought the truck was super hot.


Sweet, you got 'er figured out. I remember that one... a real head scratcher. :cheers


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## BatmanGTO (Jun 18, 2011)

lol, yeah. I forgot to update that thread. My GTO was arriving and I was so concentrated on that I just plain forgot.

About the black smoke, yeah, the truck was smoking like crazy off and on, which would explain why it would sometimes run, I guess when the connector was just right it was working.


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