O2 sensor bad??
Thread Starter
Staging Lane
Joined: Dec 2003
Posts: 70
Likes: 0
From: virginia
O2 sensor bad?? Hey guys. I have a question about my wife's '88 ws6 but I didn't know where to post it. I really need some help with it. My wife complained that her car wasn't running right but as usual I couldn't understand what she was trying to say. So I hooked up the scan tool and started checking things out. There were no codes and everything seemed to be okay until I got to the O2 sensor. It was jumping up and down anywhere from 50 millivolts to 980 millivolts to 300,to 100,700,etc. So I turned the car off and restarted it. I learned that when it restarts the car is in open loop, sensor reads 700 steadily and the car idles fine. After about 30 to 40 seconds the car goes into closed loop, O2 goes crazy and there is a slight miss. So I took it for a ride and at cruising speed it had the same crazy O2 problems but under hard acceleration the O2 read a steady 810 MV. The entire time I drove it the PCM stayed in closed loop. I was thinking that it might need the O2 sensor replaced but if that was the case then why would it function fine in open loop or at WOT. Somebody please help. Sorry for the novel but I really need some input. thanks
11 Second Club
Joined: Jul 2002
Posts: 1,426
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From: Beaver Falls, PA
there is more than one O2 sensor. are you getting the same readings on both? it is pretty normal for them to jump around while idling and part throttle crusing, that is just the way closed loop works. it is the effect of the PCM trying to keep the engine running at a stoich A/F ratio. when you reach full (WOT) throttle, the PCM goes into open loop and runs off the PE tables, thus your steady readings as the PCM isn't using the O2's anymore. i'd look elsewhere for your problem.
Thread Starter
Staging Lane
Joined: Dec 2003
Posts: 70
Likes: 0
From: virginia
Yes, this is an '88. I understand that the readings from the O2 sensor will vary somewhat because the PCM is trying to keep the A/F stoich. I just wouldn't expect the PCM to drastically add or subtract fuel, I would expect it to try to keep the A/F ratio as close to 14.6 to 1 as possible, not cause the engine to run extremely rich or lean. From what I know the optimal reading should be around 700MV, less than 500MV is considered lean and over 900MV rich. I would think the PCM would try and maintain the reading as close to 700MV as possible. Please correct me if I am wrong and please keep the comments coming. TIA
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Thread Starter
Staging Lane
Joined: Dec 2003
Posts: 70
Likes: 0
From: virginia
Bomax, I plan on testing the O2, thanks for the link. Shouldn't the PCM try to keep the reading as close to optimal as possible? This thing is jumping randomly all over the place. If the test shows that the O2 is OK then what do you think could cause the miss only in closed loop? When in open loop you can sit a cup of coffee on the intake and barely see any ripples. In the past year I have did a full tune up, pulled heads to clean them up and replaced the valve seals,compression test showed 160PSI on all cylinders +/- 5Psi, and vacuum at idle was a steady 20 Hg. The car rus fairly well for what it is, just puzzled at why it only misses in closed loop. Any other suggestions?
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Staging Lane
Joined: Dec 2003
Posts: 70
Likes: 0
From: virginia
Thanks Bomax, I will test it in the morning. Could that be the problem, that the O2 sensor is dirty? I have only had the car for 2 years, paid 1000.00 for it, but the previous owners didn't do any kind of maintenance on it whatsoever. I have put alot of time into replacing everything. From what I can remember the O2 has a shield over the tip with holes in it. Maybe it is horribly dirty restricting flow to the sensors tip. Will let you know how it goes. Thanks for the help.
Thread Starter
Staging Lane
Joined: Dec 2003
Posts: 70
Likes: 0
From: virginia
Bomax, the O2 sensor has a single wire. I am assuming that I should connect 1 lead to the wire and the other lead to the body of the O2 sensor. This is what I am going to try. Will check back later. Thanks again for your help.
Thread Starter
Staging Lane
Joined: Dec 2003
Posts: 70
Likes: 0
From: virginia
Pulled the O2 yesterday morning, had a crack down the side. picked up a new one from the parts store, cured 95% of the idle issues. The miss is almost non-existent. I haven't bothered testing the old O2 yet because of the crack. Maybe I will one day when I'm bored. Thanks for helping out Bomax, checked out your site, good info.
