Myth: Perturbed Feed Gas

Pete Mechanic Newark, Delaware Posted   Latest   Edited  

This is one of my favorite myths. Moreover it's a great technical myth which is perfect to banter about on this site..

It was inaccurately taught for many years that manufacturers intentionally drove the exhaust rich and lean and rich and lean. Those who taught the myth believed a perturbed feed gas made the catalyst work better. -some instructors still teach this incorrectly-

The truth is that a stable feed gas is the the ideal feed gas. Perturbed feed gasses are simply a result of inefficiencies in the internal combustion engine and all manufacturers have always worked towards making the catalyst feed gas as stable as possible for production vehicles.

What makes this myth particularly interesting is that diagnostic approaches based around the myth actually were effective in helping techs repair cars. In fact learning the truth generally has little to no impact on diagnostic strategy. I learned the truth back in the mid 2000s from Randy Bernklau and Jim Kemper. 

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Randy Analyst
Denver, Colorado
Randy
 

Be careful, keep your statement in proper context. What you say is true of pure feedback non wideband systems. Many modern wideband systems do perturb the feedgas but for different reasons than was taught 20 years ago. Some will perturb over 15 seconds rich/lean and as much as 2%. Since you put my name in there I felt obligated to clarify. 😀

+8 Ð Bounty Awarded
Rudy Technician
Montebello, California
Rudy
 

What is a pure feedback non wideband system? Do you mean a system with "conventional" O2 sensors? Can you site a couple of examples and reasoning as to why manufactures now intentionally pertub feed gas?

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Pete Mechanic
Newark, Delaware
Pete
 

Hey Rudy, Yeah with conventional O2s. Hopefully Randy will chime back in but I'll speculate that they might use perturbed feed gas to assess catalyst function.

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Randy Analyst
Denver, Colorado
Randy
 

I’m not talking about intrusive monitoring, I’m talking for fuel Control. It’s a feedforward design with a secondary feedback component but does not use the wideband for feedback in the normal sense. We first taught about this system in 2009 when Mike Mccarthy covered it in our 16 hour fuel trim class. I know we taught it at least twice since then in Baltimore. 😀 You should try and diagnose

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Scott Manager
Claremont, California
Scott
 

Please do tell more... I’d love to see a case study some day. When these fail IM240, how far over the pass/fail threshold are they?

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Randy Analyst
Denver, Colorado
Randy
 

The ones I have seen come in around 15 to 20 gpm CO and our cutpoint is 15. Cert level for those we’re 3.4 gpm, if I remember correctly so they are 5 times cert. Where they usually bite a technician in the butt is they suspend monitors and can do so for a very long time and never set a DTC. As far as a case study, I’ll think about it. Jim and I have semi retired from training and I have been

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Scott Manager
Claremont, California
Scott
   

Thanks Randy, so the 1.5x cert = MIL rule isn’t working in these cases? I remember hearing an engineer state once “we do everything we can legally to keep the MIL off”. I understand and appreciate the time, energy and effort that goes into creating/delivering content and want to personally thank you for all that you’ve done for the industry. As for the myth’s, yes unlearning in order to

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Randy Analyst
Denver, Colorado
Randy
 

Re MIL rule, it’s not necessarily just that, it gets down to software and sub routines. When software makes a request and the answer never comes, it just keeps waiting and waiting and waiting. In the meantime, they set a DTC for HVAC fan speed (B10E9) that turns on the MIL and causes an IM failure. Suffice it to say I don’t see our IM240 going away anytime soon. I‘ll make you a deal, if someone

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Scott Manager
Claremont, California
Scott
 

Deal! 🤯

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Pete Mechanic
Newark, Delaware
Pete
 

Ohhh, I remember now, at least vaguely; the back of the napkin discussion at Champions or some such place with Howard Pitkow and Guido, about VW AF sensors?? I think I forced it out of my brain beings I don't work on Euro and it hurt to think about it..... :o)

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Randy Analyst
Denver, Colorado
Randy
 

Ford has been using it since about 2010. 😀

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Pete Mechanic
Newark, Delaware
Pete
 

Randy! dude you're killing me! LOL Just when I though I knew the rules I gotta learn it again, is that what you're saying???? Always appreciate you setting me straight BTW. :-)

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Jim Curriculum Developer
Frederick, Maryland
Jim
 

Ah, the Jim and Randy show. The way you two team teach is really good in my opinion. I so miss those days in Baltimore and the post class discussions. The solid information and learning opportunities were greatly appreciated. Hope you are doing well.

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Brin Diagnostician
Melbourne, Florida
Brin
 

Oh, I wish we could make that happen again!

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