Low O2's

[Razor]

Helping a friend, his car reads low 700's for O2's at WOT.

Computer swapped.. same, New AC Delco AFS20 O2.. same.. New Fuel pump(340) new filter.. same..

Chip swapped for a higher fuel version. There has been no knock, and the EGT's at 25 PSI are high 1400's low 1500's.

Fuel pressure changes will change EGT readings. higher FP will result in lower EGT's.. O2's still crashing.

Everything looks normal cept the O2's read approx 80 points lower on the scanmaster II.

Anyone ever encountered such a thing. I mean like 680-710 for O2's and the EGT will be low 1500's. On my car 680-710 for o2's i'm going into meltdown....

The O2's never go into the 800's under any condition. I guess last thing to check is wiring harness??

Thoughts??

Thanks..

 

[ijames]

First, is he running leaded fuel? I have a friend who runs C16 basically all the time, and I've watched his car kill a brand new one-wire Denso sensor in 10 minutes. Put it in, start it up to check the cross counts and peak readings, looked ok, idled a minute, the max reading started to drop, hustle to the staging lanes, and before he made his first pass it was already so dead that blowing black smoke out the tailpipe the O2's never got over 720. He probably has enough lead in his exhaust system to kill a sensor after running three tanks of unleaded . Second, you can get low O2's when you are toooo rich, as well. Maybe try the first chip, turn the boost down a couple psi for luck, and drop the fuel pressure 3-4 psi (and keep a good eye on the knock!).

 

[Razor]

Carl.. good.. but no race gas in his car for the last 4 years.

And dropping FP will get the EGT's climbing and O2's crashing even further..

Told ya this is weird.

Thanks.. anything else ya can think off.. I'm even thinking could the headers be sucking air in through a crack cuasing this condition?

 

[Eric Stage I]

There is a ground specifically for the O2 sensor. I think its tan as I recall, in with that bunch of grounds on the back of the head.

Eric

 

[ijames]

Originally posted by Razor
And dropping FP will get the EGT's climbing and O2's crashing even further..

Have you actually done this on this car? I know it "should", but I've seen it go the other way if you were too rich and didn't realize it. Lean it out and the O2's came up, then go even leaner and they come back down.

An exhaust leak usually only matters at idle and light throttle, not wot.

 

[Razor]

Carl, no its definately reading wrong. Knowing this car, I'd bet on a ground issue. Not much else

Eric, does that tan colored ground go to the ECM? I mean from ECM to back of head?

I need to pull out my wiring diagram.. But sounds feasible.

I'll post back after I do some millivolt readings on the ecm grounding wires.

Julio

 

[Eric Stage I]

Yes, the tan O2 ground is pin D6 at the ECM.

Eric

 

[2QUIK6]

I had a strange O2 problem similar to this when I moved my O2 to the DP, I switched it back to the turbo uppipe and it worked fine again. And it was with a heated O2

 

[pacecarta]

razor , this sounds like my situation, o2s are low , low low and record eratic like a heart attack, egts were 1580-1600 no kr at 25psi , and to top it off my MAF was randomly spiking down from 255-132

going by SM readings i would see the lows O2 readings at times dip under 600mv since SM stores lowest with almost no KR except at the shift around 95mph

so a threw some fuel at it in the chip +10% and 02s did go up . after some top end blasts i found i had a creeping problem with the boost , start at 25 and then rise from there with no chance of puting the pedal to the floor . so i ported the W/G hole and while i was there did some poking around and found i had a crack in header(new crack) , repaired that and thought that was it, took another spin to 100 and saw some 820s on the SM start out and settle to 770s . still no kr now but egts went down to 1480 range and 02s were still recording some lows 680 range . and now i couldnt get the boost to go over 22psi foot to the floor

took to the track car spools like its pig rich and trap speeds showed its just not getting it done .
DS was timing out so i couldnt get any recordingd that night so all i had to go by was time slips and egts .
10% was too much fuel with stuttering at top of each shift and i couldnt get my boost only seeing 20-21psi
so i pulled out some fuel to +5% , spooling better but o2s are back down under 700 recorded on SM and egts at 1496 with a slight mph increas and boost now maxed to all i can get at 23psi

so that night behind me i clean the contacts again for the DS
i finally got it working . A reveiw of DSfile the o2s resemble a heart attack .fluctuating 40-60 mv , peaks at 760 low at 700 and my maf was spiking down randomly but often from 255 to 132 then right back to 255
swapped in the stock maf (took out the translator) and MAF readings steadied out

today armed with a HD actuator and the RJC controller im back at base fuel and it spools mad and egts are back at 1560-range and no KR even to 27psi but the o2s are still eratic nd low by anyones standards , (640s)

everything is new or less than 1 yr or 1000 miles and plugs and 02 are fresh
talked to eric and he mentioned the ground and it could be but havent had chance to do a thorough check of the harness with all the rain we've had .

as far as i know all my grounds are double grounded to ensure good contact

if i find the answer ill post it but for now im trying to ignore the O2 readings and see what the next run down the track yeilds which may not be till tues or wed night

 

[bruce]

EGTs are highest at Stoich., either side of it and they drop, if everything is correct.

On several of the aviation pages they discuss this.

 

[BoostKillsStres]

Bruce, I don't quite understand your post. Is the "EGTs are highest at Stoich" dependent on other things you didn't mention? My egt's are usually 750 at idle in C/L but on the highway, level ground, still C/L I usually see about 1150. I don't see 10 degrees of timing causing 400 degF diff to occur. Are idle egts in actuality higher but because of such limited airflow the headers bleed off more of the heat so we see numbers so much lower than highway driving?

I guess things would be quite a bit different if egt was measured right the port instead of right before the turbo?

 

[Bens87tr]

Bringing this post back. I talked to Razor today and he told me about this post (thanks for the time BTW, Julio). I'm running into the same thing. Boost today was at 24# and egt maxed at 1581. I'd like to bring that down a little still, but it's within reason. o2's between 680 and 725 for the most part. Brand spanking new Delco unit too. MSD 50's are at around 75% duty cycle so it's getting some decent fuel. PAC knob at the 12:00 position. But I digress...

So, where is this wire again and did anyone mess with it and see results? I'd sleep easier at night if the o2's were up.

 

[pacecarta]

the ground for the O2 is in the harness where it wraps around the a/c box near the passenger head . It is a a tan wire normally ended in a loop and attached with several other grounds to the back of the passenger head with a bolt , on mine i pulled it was already cut off near the back of the head and tied to a common ground that attaches everywhere in the engine compartment so i pulled from the harness and grounded it to the bolt under the ignition coil , along with the other grounds back there .
i checked my ground and it reads perfect to the sensor , (new delco and new denso) , also checked the purple feed wire from the sender to the ecm and it too was perfect (at least for continuity with car at rest) .
eric revised my chip (thanks again eric!!) and now my blms are in order (within -2/+3 OF 128 in all cells ) and my idle has much improved with the translator set on base setting , he also added more fuel (10%) and my O2s are up in the 800s at start and a little steadier but not stayng there as the run goes and still spikes down to 740s If i was just going by SM with what it records i would say my O2s are low low , directscan shows the o2s are fine at the beginning but theres still the spike downs about every second or so ( between spurts of alky maybe??)


i have welded the driver header but still cant help think its leaking once it gets hot hot (under boost hot)
im going to try another set of headers (pro welded) and if that doesnt cure it i'm throwing in a new engine harness.

 

[bruce]

Originally posted by BoostKillsStres
Bruce, I don't quite understand your post. Is the "EGTs are highest at Stoich" dependent on other things you didn't mention? My egt's are usually 750 at idle in C/L but on the highway, level ground, still C/L I usually see about 1150. I don't see 10 degrees of timing causing 400 degF diff to occur. Are idle egts in actuality higher but because of such limited airflow the headers bleed off more of the heat so we see numbers so much lower than highway driving?

I guess things would be quite a bit different if egt was measured right the port instead of right before the turbo?

Highest for a given condition. WOT is a much higher load, so if ran Stoich there, the EGT will be higher at the higher loads.

You might have a false read on what Stoich really is on your idle.
At low engine speeds, the self EGR'ing of the engine due to overlap makes a direct reading of AFR difficult. Not to mention there are long pauses between when the O2 *sees* the exhaust slugs of gases going by.

The *unofficial* standard for EGT measurement is 2" from the exhaust valve seat.

 

[BoostKillsStres]

Thanks Bruce,
BTW I'm seeing a pretty close to stoich reading at idle w/ the WB usually right at 14.5:1 w/ a pretty flat line of the WB log in DS. I poll the WB at 80x/sec and of course DS logs at 18f/s for whatever thats worth. My WB is in the same place as yours from what I remember, in the cat pipe.