Cat removal on 99 Expy
Tuned
Joined: May 2004
Posts: 334
Likes: 4
Originally Posted by yosemitesamiam
Where at man? What am I looking for?
Tommy
Senior User
Joined: Jun 2006
Posts: 285
Likes: 0
From: Glendora, CA
Originally Posted by tommyleea
You can hook it to manifold vacuum, and see if your exhaust is plugged up. If you increase RPM and stabilize, the vacuum will begin to drop off if the exhaust is plugged up. If a plugged exhaust is what you suspect. Cheers..
Tommy
Tommy
Elder User
Joined: Jan 2008
Posts: 605
Likes: 1
If the cats are missing or gutted you will set a Post Cat DTC. Something like insufficient activity or low switch rate. The amplitude of the waveform the computer "sees" at the Pre Cat O2s has to be reduced after your exhaust goes through your cats(which in your case are no longer there) and are then monitored by the Post Cat O2s. If the wave form is NOT reduced by a big enough margin(threshold is preset in the PCM) the computer says your cats aren't doing their job and a DTC will go set. If you gut your cats or remove them altogether you must delete the Post Cat O2 DTCs or use O2 Simms which is an RC circuit which outputs a sinewave that oscillates from 350mV to 750mV and back again. It just keeps doing this over and over again and the PCM is "fooled" into thinking the cats are working properly even though they may have been replaced with a straight piece of tubing or even gutted with a plumbers snake. I don't know if O2 Simms are available for the Ford high performance market, but they have been around for the GenIII GM LS series for years.
Senior User
Joined: Jun 2006
Posts: 285
Likes: 0
From: Glendora, CA
Originally Posted by eallanboggs
If the cats are missing or gutted you will set a Post Cat DTC. Something like insufficient activity or low switch rate. The amplitude of the waveform the computer "sees" at the Pre Cat O2s has to be reduced after your exhaust goes through your cats(which in your case are no longer there) and are then monitored by the Post Cat O2s. If the wave form is NOT reduced by a big enough margin(threshold is preset in the PCM) the computer says your cats aren't doing their job and a DTC will go set. If you gut your cats or remove them altogether you must delete the Post Cat O2 DTCs or use O2 Simms which is an RC circuit which outputs a sinewave that oscillates from 350mV to 750mV and back again. It just keeps doing this over and over again and the PCM is "fooled" into thinking the cats are working properly even though they may have been replaced with a straight piece of tubing or even gutted with a plumbers snake. I don't know if O2 Simms are available for the Ford high performance market, but they have been around for the GenIII GM LS series for years.
Sam
Thread
Thread Starter
Forum
Replies
Last Post
Pebcak
1997 - 2006 Expedition & Navigator
13
Jan 13, 2016 03:51 PM
19mfn96
1987 - 1996 F150 & Larger F-Series Trucks
3
Sep 14, 2010 10:15 AM
