Piston Slap: The Oxygen Network

by Sajeev Mehta

Published: December 2nd, 2009

TTAC Commentator xyzzy writes:

One of the O2 sensors on my 1998 Lexus LS400 recently failed, throwing a P0161 OBDII code (at 175K miles). I’ve cleared the code and verified it comes back, so I don’t think it’s a transient failure. I haven’t yet replaced it, but I have noticed that my gas mileage, as reported on the trip computer, has improved significantly since the failure. Before it was 23-24 mpg, now I’m looking at 26. I know that one of the functions of the O2 sensor is to help regulate mixture, so I’m guessing the mixture is now leaner without one of the O2 sensors. I’ve noticed no discernable change in performance of the car. Assuming I can clear the error code before taking it in for OBDII emissions inspection, is there a downside to just leaving it and running aggressively lean (if that’s in fact what’s happening)?

Sajeev answers:

If your O2 sensors have 175k on their tickers, you’re asking the wrong questions. But the answer to all questions is the same: replace the O2 sensors. That’s all four of them, partner.

Running old O2 sensors is like running an out of tune carburetor. Some manufacturers recommend changing them after 50k for peak efficiency, but I’d recommend every 100-150k for any application. (i.e. When your ride goes in the shop or you are motivated to spend some quality time underneath it.)

Even if the sensors are “good,” you can’t clear an OBD-II error code and pass an inspection: do so and the service tech gets a message saying the vehicle isn’t ready to test. So they’ll take your money for the privilege, then promptly fail your car…and ask you to come back after a week of driving. And, again, the code shall magically resurface.

Or conversely, it behooves you to do the right thing. Bite the bullet, and do a tune up (O2 sensors at the bare minimum) on this car.

[Send your queries to mehta@ttac.com]

#Lexus #LS400 #PistonSlap

Sajeev Mehta

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5 of 21 comments

Japanese Buick on Dec 03, 2009

What would you do to make a muscle sedan out of it? I love mine just as it is, haven't done any mods. Effortless acceleration, good gas mileage for a 300hp V8 -- 22-25 depending. I've owned mine for 8 years and 133K miles and so far only two unscheduled repairs -- the O2 sensor described here and a $200 headlight switch repair last year. As part of your due diligence on these, check on the timing belt status. It needs to be replaced every 90K miles, so if you're looking at one with, say, 175K miles, it may be due soon. It's about an $800 job.

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- PrincipalDan on Dec 03, 2009
  
  I know Lexi are supposed to be "whisper quiet" but I wish more manufactures would go back to chains for timing, at least they lasted forever. Muscle sedan, well how bout headers? Cat back exaust? Computer chip? I'm not talking radical here, just a little more omp! It already has dual exuasts right? How bout Flowmaster mufflers for a little more growl. Then with the miles you have I'd start checking the suspension bushings for wear and think about an upgrade. I'd do all of this really slowly, I wouldn't just throw my checkbook at the car, but then if I had a car like that (living in Northwest NM like I do) it would be for tearing up the interstates and the long state highways, I'd have a second tiny little 4-banger manual transmission car for commuting. But to each his own, I'm a hot rodder at heart. (My dad's had a subscription to HOT ROD since he was 15 yrs old, and a boy has to read something while he's in the restroom, lol. It had quite an impression on me. There's a warning to all dads out there. Be careful what your children get ahold of if they're voracious readers.)
  
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- Japanese Buick on Dec 03, 2009
  
  After driving mine for almost exactly 8 years (bought it in early December 2001), my recommendation for the first modification, if you're looking to make it more sporty, is a sportier suspension and tires. I would not change mine, I love the effortless glide and wouldn't want to ruin that, and I hated it when I had so-called sportier (i.e., noiser) Falken tires and I couldn't wait to wear them out and get back to the Michelin MXVs (luckily with Falkens, it didn't take long, heh). But the car does get the leans in curves at speed and you can't take corners fast. Personally I wouldn't change mine to make it noisier, but better cornering would be nice. Not sure how much of a factor that would be in New Mexico though, if the roads are mainly straight.
  
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- PrincipalDan on Dec 03, 2009
  
  Thank you for your suggestions, it is on my short list of rides to consider after getting my education leadership masters degree this spring, right after I get a principals job and start collecting the bigger paychecks. I've been teaching for 10 years. I meant roads that are usually fairly empty of traffic once you get away from the population centers but then very hilly and curvy. Think of all the Road Runner cartoons you watched as a kid, just at a little higher altitude. (Gallup, NM 6,500 ft. above sea level, population 20,000; nearest city 137 miles.)
  
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Sajeev Mehta on Dec 03, 2009

Perhaps you all forgot about this Piston Slap: http://www.thetruthaboutcars.com/piston-slap-fumble/

You all should know I got yo' back.

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