WeSki Posted October 9, 2007 Share Posted October 9, 2007 About 3 months ago my city mileage dropped 5 MPG. My 92 LS400 has 177k miles and it runs great yet my mileage dropped from 19 MPG city to 14 MPG, the highway MPG stayed right around 24 MPG. A week later I noticed an engine miss at 1600 RPMs which is about 45 MPH. I can’t hear or feel the miss at any other RPM or speed nor in neutral and running the RPMs up and down. The LS has plenty of power and flat out gets with it if I put my foot into it. The plugs, wires, rotors, and caps were replaced at 113K miles. So with 60k plus miles on those parts I went ahead and changed the plugs rotors and caps to see if it would make a difference but it did not. I took a 300 mile trip this weekend and I got the lowest highway MPG I’ve ever had at 22 MPG. So once again I need your help. What kind of life should I get out of a wire set? What else can I look into? Thank you Quote Link to comment Share on other sites More sharing options...
pishta Posted October 9, 2007 Share Posted October 9, 2007 About 3 months ago my city mileage dropped 5 MPG. My 92 LS400 has 177k miles and it runs great yet my mileage dropped from 19 MPG city to 14 MPG, the highway MPG stayed right around 24 MPG. A week later I noticed an engine miss at 1600 RPMs which is about 45 MPH. I can’t hear or feel the miss at any other RPM or speed nor in neutral and running the RPMs up and down. The LS has plenty of power and flat out gets with it if I put my foot into it. The plugs, wires, rotors, and caps were replaced at 113K miles. So with 60k plus miles on those parts I went ahead and changed the plugs rotors and caps to see if it would make a difference but it did not. I took a 300 mile trip this weekend and I got the lowest highway MPG I’ve ever had at 22 MPG. So once again I need your help. What kind of life should I get out of a wire set? What else can I look into? Thank you My guess would be a failing coil. check both with a ohmeter. My wire set is original after 90k and it tested fine with a meter, still within specs and still flexible, no cracks can be seen. Quote Link to comment Share on other sites More sharing options...
Jerry Bransford Posted October 9, 2007 Share Posted October 9, 2007 I'd put my $$$ on a failed upstream (nearest the engine) 02 sensor which will cause the computer to switch the engine so it's running full-rich "open loop" mode. Quote Link to comment Share on other sites More sharing options...
ono8ono Posted October 9, 2007 Share Posted October 9, 2007 I'd put my $$$ on a failed upstream (nearest the engine) 02 sensor which will cause the computer to switch the engine so it's running full-rich "open loop" mode. my 94 LS only get 10 mpg with 85K miles on engine; How to determine the O2 sensor not working? Quote Link to comment Share on other sites More sharing options...
91LS400vgb Posted October 9, 2007 Share Posted October 9, 2007 My 91 LS O2 sensor was changed at 92k and the fuel economy improved dramatically. Some mechanics change them as preventive maintenance every 80K as they tend to get "lazy" with age, while others prefer to test them before replacing. Quote Link to comment Share on other sites More sharing options...
Jerry Bransford Posted October 10, 2007 Share Posted October 10, 2007 I'd put my $$$ on a failed upstream (nearest the engine) 02 sensor which will cause the computer to switch the engine so it's running full-rich "open loop" mode. my 94 LS only get 10 mpg with 85K miles on engine; How to determine the O2 sensor not working? It takes a diagnostic tool but at 85K miles, it's time to change out your "upstream" 02 sensors anyway. I'm a weekend gearhead but not on the Lexus so I don't know how many 02 sensors it has, probably two since it's a V8 and they're normally found just below the exhaust manifold... so there's likely two, one on each side of the engine just below the exhaust header. No need to worry about the downstream 02 sensor on the back side of the catalytic converter, it won't cause this kind of problem. Quote Link to comment Share on other sites More sharing options...
glenmore Posted October 10, 2007 Share Posted October 10, 2007 My 1991 LS400 has always had crap MPG. I may go ahead and change out the upstream sensors. There are the 2 excellent tutorials at lexls that covers these repairs. Best tip is to use the much cheaper generics that you splice yourself. glenmore 1990 LS400 2000 C280 1991 300CE Quote Link to comment Share on other sites More sharing options...
Jerry Bransford Posted October 10, 2007 Share Posted October 10, 2007 Best tip is to use the much cheaper generics that you splice yourself.glenmore 1990 LS400 2000 C280 1991 300CE What, you have two Mercedes Benzes and a Lexus and you worry about paying $10 more to get one that already has the right connector plug installed? I've been a wiring/electronics guy for 45 years that can splice wires in his sleep but I'll still go for a spliceless connection whenever I can. :) Quote Link to comment Share on other sites More sharing options...
paulo57509 Posted October 11, 2007 Share Posted October 11, 2007 About 3 months ago my city mileage dropped 5 MPG. My 92 LS400 has 177k miles and it runs great yet my mileage dropped from 19 MPG city to 14 MPG, the highway MPG stayed right around 24 MPG. A week later I noticed an engine miss at 1600 RPMs which is about 45 MPH. I can’t hear or feel the miss at any other RPM or speed nor in neutral and running the RPMs up and down. The LS has plenty of power and flat out gets with it if I put my foot into it. The plugs, wires, rotors, and caps were replaced at 113K miles. So with 60k plus miles on those parts I went ahead and changed the plugs rotors and caps to see if it would make a difference but it did not. I took a 300 mile trip this weekend and I got the lowest highway MPG I’ve ever had at 22 MPG. So once again I need your help. What kind of life should I get out of a wire set? What else can I look into? Thank you Dirty air filter? Have you been filling up at another station? Different brands of gasoline? Perhaps the fuel has had a change in formulation....more methanol? Quote Link to comment Share on other sites More sharing options...
glenmore Posted October 11, 2007 Share Posted October 11, 2007 What, you have two Mercedes Benzes :) That's why I have to save $$ wherever I can! I just changed the sensor on the 300CE, because I don't think it ever has been changed. The car has 146m and I bought it at 111m. A generic Bosch O2 sensor was $25 while a direct fit Bosch was $114. My C280 just had a CEL indicating O2 sensor. Generic was $80 and direct fit was $110. The cheapest direct fit for the Lexus (on ebay) is $62 and the generic (also ebay) is $25. You are right, no splices is always better. I always think of these repairs in terms of saving money so I can buy some nicer tools. :) glenmore 1990 LS400 1991 300CE 2000 C280 Quote Link to comment Share on other sites More sharing options...
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