djmark7 Posted January 10, 2020 Share Posted January 10, 2020 hi akll, i have a 96 legacy just replaced the 2.5 motor cel is on cat code with the old motor was getting 23-25 mpg with same code, the new miotor im getting 16-17 mpg, any suggestions? Quote Link to comment Share on other sites More sharing options...
el_freddo Posted January 10, 2020 Share Posted January 10, 2020 What’s the mileage on each engine? Have you done a compression check on the replacement engine? Why was the engine replaced? Cheers Bennie Quote Link to comment Share on other sites More sharing options...
djmark7 Posted January 10, 2020 Author Share Posted January 10, 2020 (edited) 1st engine blew hg lost oil pressure it ran, made a lot of noise, 171000 on replacement Thank You Bennie Edited January 10, 2020 by djmark7 Quote Link to comment Share on other sites More sharing options...
1 Lucky Texan Posted January 11, 2020 Share Posted January 11, 2020 possible misfiring from non-oem plugs/wires possible bad knock sensor possible bad ECT sensor double check all ground connections. Quote Link to comment Share on other sites More sharing options...
djmark7 Posted January 11, 2020 Author Share Posted January 11, 2020 CEL is for tha cat, no other codes popped up Quote Link to comment Share on other sites More sharing options...
1 Lucky Texan Posted January 11, 2020 Share Posted January 11, 2020 (edited) P0420 can be caused by other issues, some of which may not throw a code. OF COURSE an older car could need a cat. just that, vacuum leaks, exhaust leaks can also throw that code. many folks have had cracked/bad knock sensors that didn't throw a code. Plus, regular misfiring could explain poor mileage AND the excess unburned fuel could have destroyed the cat. the rear O2 sensor is what 'monitors' the cat, it or its wiring may be bad. Cats are pricey, never change one based ONLY on the 420 code, there are other tests that can help pin-down the problem. Edited January 11, 2020 by 1 Lucky Texan Quote Link to comment Share on other sites More sharing options...
djmark7 Posted January 13, 2020 Author Share Posted January 13, 2020 Thanks, Tex Quote Link to comment Share on other sites More sharing options...
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