WTH would cause this kind of AFR fluctuation?

FastDriver

My dad had a bra
SN Certified Technician
Sep 5, 2001
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Seaside, CA
Check out this 10 second data log. The

Peak to trough AFR is ~13 to 9.3. I think this may be the bottom of the scale for BS3's sensor. I checked all but plugs 7 & 8, and so far they looked good. I tried idling it in the driveway and pulling each injector's connector 1 at a time. It didn't make an immediate difference, but then it may be so saturated in fuel it'll take a bit to burn it off. I dunno.

If it were a fouled plug, I would expect to see that cylinder read lean, not rich. My only guess is an open injector. I honestly had no idea until I connected the computer for the first time in a while that this was even happening. At low engine speed, I could feel what felt like a miss, but it wasn't consistent, and I just figured that the engine didn't like load at low rpm.

Do you have any ideas?

627149
 
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FastDriver

My dad had a bra
SN Certified Technician
Sep 5, 2001
4,052
961
214
Seaside, CA
Is it possible this is an electrical issue or O2 sensor issue? I was wondering if I'd ever see these kind of pulses on the other side of the turbo.
 

FastDriver

My dad had a bra
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Sep 5, 2001
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I've got a set of injectors on order. Will update if it solves the problem. This is a nicer way to diagnose an injector issue than a damaged engine.
 

a91what

SendMeUrDataLog
Mod Dude
Apr 6, 2011
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agreed. I have seen this issue before and usually address it as either an injector or a missfire, but that is one heck of a dip in the AFR value.. a missfire will show as a lean spike not a dip
 

FastDriver

My dad had a bra
SN Certified Technician
Sep 5, 2001
4,052
961
214
Seaside, CA

FastDriver

My dad had a bra
SN Certified Technician
Sep 5, 2001
4,052
961
214
Seaside, CA
Well, this is disturbing. It wasn't an injector. I just replaced the whole set and am still getting this issue. Ignore the blue line at the top... just made a mistake copying the pic. Anyways, here's an interesting (to me) observation: Higher revs make no difference in the rate of the AFR fluctuation. I wondered if it might be a valve issue. So, I left the MAP on, but as you can see, nothing funny going on there.

I was so confident that in injector was going to be the issue. I guess that's what I get for thinking. Damn. I don't know what to do next. My best guess is that it's in the tune. I'm gonna have to go back to the drawing board, because this is unacceptable.

627447
 

a91what

SendMeUrDataLog
Mod Dude
Apr 6, 2011
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Hillsborough county
Can you pull all the enrichments up? If it's the tune then the PW should be spiking...

If you want to recoupe some on the injectors PM me, my 393 will be thirsty.
 

FastDriver

My dad had a bra
SN Certified Technician
Sep 5, 2001
4,052
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Seaside, CA
Much better!

627488


Bad O2 sensor. I looked around for something like that, before I ordered the injectors, but couldn't find any O2s that failed like that. The lack of change at various RPM gave it away. I'm debating whether I should put the old injectors back on to keep the new injectors fresh.
 

FastDriver

My dad had a bra
SN Certified Technician
Sep 5, 2001
4,052
961
214
Seaside, CA
Reasonable thought, but no MAF on this one.

I use brake clean, the non chlorinated stuff. You can hold the injector open with a 9v battery.
Good tips, here. Just knocked that out. Hopefully, the old injectors will still be good when I need them to diagnose another problem in the future.
 
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