Myth: Perturbed Feed Gas

Pete from Newark Mechanic Posted   Latest   Edited  
Discussion
Driveability

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 from Denver

 

Analyst
 

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. 😀

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Rudy from Montebello

 

Technician
 

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 from Newark

 

Mechanic
 

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 from Denver

 

Analyst
 

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 these when there are no dtcs and it fails IM240. It’s a bi*****ch!

it first appeared around 2004 or so on some German PZEV cars and has shown up on more and more vehicles. Well, phone is about dead and lunch hour is over, the rest can wait until the next steak dinner.......

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Scott from Claremont

 

Manager
 

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 from Denver

 

Analyst
 

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 enjoying not having to build classes so my case study stuff is weak right now. Most of my time is spent on software and communication issues on the OBD side which is why I am enjoying Bob’s articles and can’t wait for when he tackles K-line. 😀

The other thing is, and relates to your myth series, is trying to unlearn myths so people can learn a new concept. That takes as much time as anything and doesn’t work well via internet. I’m not sure I’m up to the task right now so maybe some other fuel trim trainer can jump in and present a case study on the topic. They understand it, don’t they?

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Scott from Claremont

   

Manager
   

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 relearn is a tough activity and it appears that the ensuing dialogue around the myths I was hoping for is happening! 🤓 There‘s definitely lots of unlearning that needs to go around for sure!

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Randy from Denver

 

Analyst
 

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 else makes a thread on the 6 different types of rear O2 fuel control strategies, I’ll follow up with something on feed forward fuel control.

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Scott from Claremont

 

Manager
 

Deal! 🤯

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Pete from Newark

 

Mechanic
 

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 from Denver

 

Analyst
 

Ford has been using it since about 2010. 😀

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Pete from Newark

 

Mechanic
 

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 from Frederick

 

Curriculum Developer
 

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 from Melbourne

 

Diagnostician
 

Oh, I wish we could make that happen again!

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