Help!! Bad MPG
Help!! Bad MPG I have 98 z28 m6 with 120k on the clock. The car had been getting 28 mpg on the hwy, went down to 24. So I changed plugs(tr55) and wires(msd), but my mileage stayed the same. The mileage has now gone down to 21mpg on the hwy. What should I check next!! This is the first ls1 I have owned. I have changed to ported GMMG exhaust manifolds, but that did not change my mileage
Tech Resident
Joined: Jan 2006
Posts: 5,117
Likes: 3
From: Philly
Originally Posted by Cory1970
I do have a de-screened mas air.
i'd guess oxygen sensors. after that, bad catalytic converter. after that, bad spark plugs.
honestly, the more i think about it, the more i think bad catalytic converter. that'll kill your mpg and power due to increased back pressure, especially at cruise. go behind your car and smell your exhaust. see if there's a hint of rotten eggs - a sulfur smell. that means your cat or cats are shot.
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Get someone with hptuners or efi live to see how fast your o2's are switching, one of them is probably slow. They can be slow and still not throw any codes. I'm pretty sure I have a lazy one myself...and I'm too lazy to replace it. 
I am having a similiar problem, I went down to 21city to 15city.
Only really difference is the A/C has been on. Does it make that big of a difference. I can't imagine so.been 100 plus degrees here lately.
May not be bad for some but for a penguin like me it sucks.
Only really difference is the A/C has been on. Does it make that big of a difference. I can't imagine so.been 100 plus degrees here lately.
May not be bad for some but for a penguin like me it sucks.
All I know is mine has gone down as well since this heat has been so bad. My AC is on all the time cause its 100 degrees outside. I would imagine it does make some kind of difference
Tech Resident
Joined: Jan 2006
Posts: 5,117
Likes: 3
From: Philly
Originally Posted by Cory1970
I have looked at the cats, they do not look cloged.
the plugs are new
the plugs are new
even so, the core could be compressed in the center but still look okay by peaking in it.
Originally Posted by ChocoTaco369
umm...how can you tell if the cats are clogged by looking at them? unless you saw the inside of them, which would involve cutting them off and rewelding them...
even so, the core could be compressed in the center but still look okay by peaking in it.
even so, the core could be compressed in the center but still look okay by peaking in it.