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WTH would cause this kind of AFR fluctuation?

  • Thread starter Thread starter FastDriver
  • Start date Start date May 14, 2019

FastDriver

I was uncomfortably high & wearing a helmet
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#1
  • May 14, 2019
  • #1
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?

 

FastDriver

I was uncomfortably high & wearing a helmet
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#2
  • May 14, 2019
  • #2
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

I was uncomfortably high & wearing a helmet
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#3
  • May 14, 2019
  • #3
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

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#4
  • May 14, 2019
  • #4
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

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#5
  • May 15, 2019
  • #5
Not surprising given the size of the injectors @ 120 lbs/hr.
 

a91what

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#6
  • May 15, 2019
  • #6
What are you using as a replacement? My LU47 may be too small for the 393.....
 

FastDriver

I was uncomfortably high & wearing a helmet
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#7
  • May 15, 2019
  • #7
TrickFlow 120 lbs injectors that cost $500 on their site are really rebranded Delphi part# 621061. The cheapest I've found are here:

https://polmaxracing.com/index.php/product/8-new-genuine-lucas-delphi-120lb-1260cc-fuel-injector-e85-low-resistance-bosch-rc/

While the website looks less than stellar, I called the American number and spoke to the guy who knows his stuff. So, I'm giving it a try. I'll let you know how it works out.
 

FastDriver

I was uncomfortably high & wearing a helmet
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May 19, 2019
#8
  • May 19, 2019
  • #8
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.

 

a91what

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#9
  • May 19, 2019
  • #9
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

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#10
  • May 19, 2019
  • #10
I don't mind having an extra set of injectors, since both of my cars use the same kind.
 

FastDriver

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#11
  • May 19, 2019
  • #11
Much better!



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.
 

a91what

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#12
  • May 19, 2019
  • #12
Have the new ones seen ethonal? If not pull em out
 

FastDriver

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#13
  • May 19, 2019
  • #13
No. Just gasoline. Why do you ask? The old ones haven't seen ethanol in a year and a half.
 

a91what

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#14
  • May 19, 2019
  • #14
Leaving ethonal in them can gum them up, I was going to suggest flushing them if you had.

I have seen a WB fail that way but not to that extreme.
 

FastDriver

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#15
  • May 19, 2019
  • #15
What should I flush injectors with?
 

a91what

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#16
  • May 19, 2019
  • #16
I use brake clean, the non chlorinated stuff. You can hold the injector open with a 9v battery.
 
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Noobz347

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#17
  • May 19, 2019
  • #17
Glad you found it. Only other thought I had was about turbulence across the MAF sensor at that RPM.
 

FastDriver

I was uncomfortably high & wearing a helmet
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May 19, 2019
#18
  • May 19, 2019
  • #18
Reasonable thought, but no MAF on this one.

a91what said:
I use brake clean, the non chlorinated stuff. You can hold the injector open with a 9v battery.
Click to expand...

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.
 
Last edited: May 19, 2019
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