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Terrible mileage after converter opened up (solved)


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Okay, Algore and company please leave the thread now and tune in to the most recent global warming propaganda station, as offensive stuff will follow. :D

 

If you have a problem with your cat converters clogging, and you end up gutting them (even temporarily to avoid engine damage), you may note some horrible gas mileage.

 

My truck... a 1996 K3500, 4 door, 8 foot bed, with 5.7 Vortec engine, 4L80e transmission.

 

My mileage went to 7.5 mph (yes, literally) after opening converters.  I had been getting 11 to 12 mpg.

 

Fuel was so rich it would fog you out sitting at the drive thru.  :o

 

I got lots of advice from various folks, up to and including that I needed to just "bite the bullet" and replace both cats and o2's or the truck would never get correct fuel mileage.

 

And the guy with the spinner in his ball cap of course points out that opening up the converters is ILLEGAL.  (he likes to put that in all caps)... And his name is Ogilvie, regardless of what his forum handle happens to be. 

 

But a GM tech who is a friend of mine told me to unplug the rear 2 oxygen sensors and see what happened.

 

That in fact worked *perfectly*...  my truck began to run like it used to, and mileage went back to 11 to 12 mph.

 

I do realize there is a way to use anti-fouling spark plug spacers to offset the 02's in the front (maybe?) to create a temperature difference, and get rid of the check engine light.  But my front 02's after 22 years don't want to budge.

 

I post this just in case it may help someone out there with the same problem.

 

Dan

 

 

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Hard to follow exactly what happened here, but you apparently hollowed out your catalytic converters and mileage went down and could smell raw fuel.  It seems unlikely that lack of catalytic converter(s) will cause your engine to run that rich.  Is it possible that you had a rich running condition that caused the converters to plug up?  

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Yeah, I do think the converters clogged due to rich running engine.  The truck has 260k on it.  Engine has 104k, but exhaust is original to truck.

 

Basically, leaving all 4 o2's connected with open converters causes the truck to stay in rich fuel mode all the time.  Unplugging the back 2 o2's (on the outlet of the hollowed converters) solved the problem of severely rich fuel mix.  I do still have the check engine light, with codes complaining about no data from 2 of the o2's... :)  But I can live with that. 

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