I have an 02 5-spd Cavy with an Ecotec and bolt-ons... Just recently I've been noticing I'm getting horrible gas mileage. When I put my header on I made an extension for my 02 sensor and I think it might be going bad. The stock one wasn't long enough for the bung for the header so I added on to it. I think I might have to get a new one and see if that helps. I don't think that's the main problem though. Something else might have gone bad and could have caused the bad mileage. I just finished my exhaust and it's gotten worse. Mods don't help with mileage as we all already know. What else could be causing this? I drive nicely and get 200 miles to the tank. I use to get 300-320. This sucks!
This may not be a solution to your gas mileage but checking to see how dirty your Throttle body is plus the inside of the intake manifold for oil and black soot may be part of the problem. Clean both with a throttle body cleaner in a spray can. After that I would check your spark plugs. If they have black soot on them you are running too rich. If the spark plugs look bad replace with recommended GM plugs. Do you have a larger than stock intake on the throttle body? I bet you do but if you don't I know the header is asking for more air flow to help it work correctly.
As far as the electical header set up I couldn't really help you with that part.
I was looking at what you had done to your car and you have a good intake system but you have the worst plug ever made installed in your engine. Bosh is the worst one out there for any car as far as I am concerned. Get some Delco or Iridium plugs and see how much better the engine will run. When the engine runs good you will get better gas mileage if you are not flooring it all the time. I would still check the TB and manifold to clean that up. Best of luck!
like you said ..o2 sensor...its a very big player on the engine..change it and everything will be better
Jim, actually, I don't run the Bosch plugs anymore. I don't know why I got those in the first place... I've tried them all and they all are terrible. They shouldn't be put in our car. I went and bought some Autolite Platinum plugs and it seems to be running a little better. I still think the bad gas mileage is from something else... I bet it's the 02 sensor...
I'm willing to bet that the problem is your O2 sensor also. It probably has something to do with you trying to modify the Sensor to fit in the new header.
If its not directly in the stream of exhaust flow, it will not function correctly. So the modifications you did probably have something to do with it. Another possibility is that the sensor was damaged when your were working with it - but that will usually set off the Check Engine Light.
As the other mentioned, clean out your Throttle Body, run some Chevron Techron EFI System cleaner through your car, and try to fix whatever you did to the O2 sensor, even if it requires buying a new one
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