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Discussion Starter · #1 · (Edited)
Had an 02 pegged, figured wouldnt hurt to replace 260k sensors, especially as the old engine burned oil, they were ashed up some, not bad. Ordered new bosch's from rock auto that were the top sellers. Issue was a bare wire, fixed wire, 02s seemed to be working as good or better than ever. Replaced MAF at same time. After 100-150 miles CEL came on, tbh, i thought it was a trans sensor that lit it, i hit the OD button right as it started to **** into OD and it made it goof for a sec, i assumed it threw a slip code or something so i wasnt in a hurry to check codes between working lots of hours on nights and the weather generally sucking. I was wrong to say the least. 15 codes all related to 02s. At least half are an equal share of all 4 sensors regarding heat circuit, thought i got a picture of the specific message / codes, guess i didnt. 2 or 3 were regarding bank 2 lean. STFT1 -.8% LTFT1 14.8% STFT2 3.1% LTFT2 14.5% These seem to be pretty consistent, at fully warmed up idle (not sure if helpful honestly). There is also short term fuel trims listed for all 4 sensors. Pre cats match the short term above, post cats are both stuck at 99.2%, i have no idea if thats good or bad. The 02s are cycling between .085 and .805v roughly once ever 1.5- 2 seconds, all 4, again, i have no idea what to make of that.

Would like to point out that i never EVER had the CEL come on since owning this thing for the last 55k, and it always ran fine and got appreciable fuel mileage (21-23). Here lately its been getting 18-19, it hasnt been idling much regarding warm ups so it cant be attributed to that. Does seem to have a bit of an inconsistant idle, cackles more on decel than i remember, and sometimes almost seems like a miss if youre partly out of the throttle. Could this be because its a PI engine with a SCP chip? Does the computer just not like these sensors? I cant really get the car back on the lift in the foreseeable future to swap the old 02s back in to rule it out. Hoping you guys can shed some light, I appreciate your time and patience.

 

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The only thing I can add is: it's normal for the o2 voltage to cycle up and down while cruising and hold steady at around .8v at WOT.

Al
 

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Bosch sensors for these cars do not work as well as the NTK ones; they were the oem. I went thru a few of the bosch ones, and they were especially sensetive to contaminants.
 

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Discussion Starter · #4 ·
I was super careful with the sensors, didnt even touch them below the threads, they already had anti sieze on them. So should i focus on putting the originals back in to rule out the new sensors?
 

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I'd look elsewhere for the problem, personally. the way they are switching, I think they're ok.
Bad o2's go to a voltage and stay there.The reading the computer uses is an average.
 

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Discussion Starter · #6 ·
Why the 02 heater codes as well as never having the issue before hand, but that would just rule out coincidence. I have no idea where to look next. Have checked for exhaust leaks, injectors seem to be working great.
 

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The STFTs represent the commanded lambda to achieve perfect fuel burn, so if they're staying around 1 then the O2 sensors are doing their job, at least, getting the mixture dialed in. If you have a wideband O2 to do a reality check to ensure it's actually running at 14.08:1 then you can rule out the O2s being bad.

If the original MAF was acting up and was off when the car was tuned, and the new MAF sensor is within spec, that could explain it. It's also possibly a vacuum leak, though all the heater circuit codes is puzzling. The front O2s run through the engine harness and the rear O2s run through the transmission harness so a common harness issue is less likely. I would not be surprised if the HEGO fuse is blown, or possibly the MOFSETs in the PCM are acting up?
 

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The heater codes were bad o2 sensors on Lazarus. One of the rear ones shorted it took them all out.
 

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Discussion Starter · #9 ·
Car wasnt "tuned" per say, just has a chip with a canned tune on it. As far as STFT staying near 1, need some context, is -.8 and -3.1 "near" 1? Replaced the MAF because the old one didnt seem to be doing much according to the scantool, .1-.3v iirc, new one does the same thing so i dont think anything is really different there. As far as the sensors cycling, id read that theyre supposed to cycle every 15-20 seconds not every 1-2 seconds, but my take away was that cycling is more or less good. As far as looking elsewhere, care to elaborate? Computer dealy says o2s, i look at o2s, o2s seem to be working... dont have the slightest idea what to do next besides regret doing a tune up.
 

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Check for vacuum leaks.
 

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What MAF sensor and injectors was the canned tune written for?

So to give some context, lambda is the term used to describe the mixture independent of the fuel type. Lambda 1 is always perfect combustion and can represent 14.64:1 with E0 gasoline, 14.08:1 E10 gasoline, 9.76:1 for E85 etc. To convert to AFR from lambda you multiply the stoich AFR by the lambda: E10 lambda .85 (a good lambda for WOT) is 11.97:1 AFR; lambda less than 1 is a rich mixture and over 1 is a lean mixture.

STFT is showing you the PCM's commanded lambda. In closed loop it indicates the lambda commanded to get an actual lambda of 1 as reported by the O2s. So if you see an STFT of .90 (or -10 in some software) it means there the PCM is having to enrich the mixture to get to an actual lambda of 1.0 - e.g. a lean condition.

Where it gets complicated is with adaptive fueling. The PCM keeps track of higher-than-normal STFTs (beyond 3-4% either way) and stores that correction factor as LTFT. This correction factor is applied to injector duty cycle - so an LTFT is opposite of STFT. An LTFT of 10% means 10% more fuel than what the lambda or STFT shows. In a tune this can be turned off to only need to look at STFT to see whether you're running rich or lean. That information ultimately gets used to dial in the airflow characteristics of whatever MAF sensor is on the car.

Anyway...

The PCM switches the mixture from slightly lean of lambda 1 to slightly rich of lambda 1 during closed loop cruise. In a perfect scenario it would go from about .98 to 1.02 but as I said before, ±4% is typically acceptable. The switching cycle is only a few seconds, nowhere near 15-20 seconds. As long as you're seeing that switching between rich and lean, it means the PCM has achieved closed loop "perfect" fueling and is maintaining it. A voltage of about .45 on a narrow band O2 sensor is lambda 1. The PCM is adjusts the STFT looking for the O2 voltages to go lean (~.20 volts) then rich (~.6 volts) and repeats that cycle.

Your O2 voltages seem OK for the most part, though the switching period is a little irregular.

Your LTFTs indicate about 15% additional fueling is required at idle. Either the MAF sensor is not correct in the tune, is acting up (not likely given your troubleshooting), or there is a vacuum leak. It's possible all 4 O2s are junk, but I don't consider that the first thing I'd run to and check. Look at your HEGO fuse under the steering wheel. If that's good do some testing of the harness for shorts on the heater circuits.
 

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I'm having some similar issues with my 97lx with an explorer swap and pi intake. Just replaced upstream o2s today and new plugs, seems to have helped but not fixed. I couldn't tell you how old the o2s where 304k on Odom so I didn't feel bad replacing them. But I also have a code for vent canister but don't know why. I'm thinking I may a have a vacuum issue in that area I get vacuum coming into that area but it doesn't make it to the charcoal canister so gonna do some digging into that issue when I have the time. Anyone know a good process in testing that function?
 

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Discussion Starter · #13 ·
What MAF sensor and injectors was the canned tune written for?

Your O2 voltages seem OK for the most part, though the switching period is a little irregular.

Your LTFTs indicate about 15% additional fueling is required at idle. Either the MAF sensor is not correct in the tune, is acting up (not likely given your troubleshooting), or there is a vacuum leak. It's possible all 4 O2s are junk, but I don't consider that the first thing I'd run to and check. Look at your HEGO fuse under the steering wheel. If that's good do some testing of the harness for shorts on the heater circuits.
Thank you very much for the in depth info. Stock 96 MAFS and 19lb/hr injectors are what the tune was written for. I will find what the IACV is running at. Been all over this thing recently looking for vacuum leaks, i really dont feel like it would be that, but it did recently get much cooler. I will put the old MAFS back in and see what (if) it changes. I didnt "do" anything when i swapped it in, maybe i was supposed to. Only pulled the old one because the car had a K&N on it when i got it and i know that can screw with em, its a little dirty looking, nothing crazy.
 
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