Reader Question: Deleting Old Cats?

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Here’s the latest reader question, along with my reply! 

Doug asks: Do you know anything about whether gutting or cutting out catalytic converters would effect fuel mixture and ignition? Suppose I had a beater 2003 Land Rover Disco with clogging cats. Not really worth the replacement cost but it’s an unstoppable beast otherwise and I’d like to keep it in the rotation. It has O2 sensors fore and aft and I wonder if it’s going to try to compensate somehow. Disclaimer: Of course, this would be very much against the law and I would never consider actually doing it. I’m just looking for background information for the novel I’m “writing.”

My reply: Once upon a long time ago, removing the cats – with a “test pipe,” you could actually buy them at auto parts stores – was well worth doing for the performance and mileage improvement obtained thereby. But that was because these old style cats – especially the “pellet” cats GM used back in the mid-late 1970s and into the early ’80s – were horrendously restrictive and also because the engines were designed decades before catalytic converters were even conceived of.

My 1976 Trans-Am, for instance, runs immeasurably better with a free-flowing 2.5 dual exhaust system and no cats than it did when it was choked by a 2.25 inch single exhaust feeding into one converter.

The 455 under its hood dates back to the mid 1950s; this ancient carbureted engine was never meant to be catalyzed.

But modern cars with engines designed for cats are a different story – and so are modern cats, almost all of which are the lattice/honeycomb type that are not hugely restrictive and (in V8 applications) are usually two – so as to not plug up the exhaust. The exhaust is a system now and integrated with the engine’s other systems, including the fuel system, via 02 sensors. There is also a certain amount of backpressure built into the system on purpose. If you remove the cats, the fuel system will likely go into open loop mode and the affect of the change in backpressure may actually reduce mileage/performance.

You can get around the “check engine” (OBD) light coming on via installing just the right resistor in the 02 harness but the engine may still not run better and may run worse.

If it were me, I’d get a set of high flow cats and mufflers, which will improve the vehicle’s performance and mileage. I’d seriously consider tube headers while about it, too.

. . .

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  1. Worst part about the damn cats on a pickup/4wd is the damn fire danger. Can’t use them in the summer for what they are intended for.

  2. When it runs and it’s got its snow shoes on, the hero of my novel calls it 6000 pounds of Get the Fuck Outta My Way.
    The hero of my novel might work on it but the hero of my novel would rather not spend on it.
    So, I guess the hero of my novel will just hack em out. “Can’t break broken” is what the hero of my novel’s Dad always said.
    Thanks for the tips.

    Lucas the Prince of Darkness

    • Most likely true, however, not in all cases. 1999 VW Beetle with AEG 2.0L gasser, for example. Had one, and I chased wacky fuel trim issues with VCDS for months. Found and cured vacuum leaks, replaced MAF, front O2 sensor, etc., all except rear O2 sensor. COULDN’T be that one, I thought, that one is only for OBD2 snitching on cat efficiency!

      Still had intermittent fuel trim lean issues. So, after some deep diving online, I found the real VW service docs for the AEG’s EFI that described what the ECU was using the front and rear O2 sensors for. It turns out the rear in this particular application is used as a gross cross-check on the front, in addition to a cat efficiency check for OBD2 purposes. Ran live data with VCDS over multiple test runs and confirmed the problem.

      So, a sluggish (not completely dead, just slow responding) rear O2 sensor caused the ECU to think the front O2 sensor was not working right, and it started biasing the info from the front O2 sensor, causing the lean fuel trim error message. Replaced the rear, no more fuel trim errors. Sadly, with OBD2 and “modern” EFI, one must find the right info on the exact application to know what in the devil the designer programmed the blasted thing to do.

        • Unfortunately, I suspect more than just VW’s apfelwein-and-schnapps-befuddled engineers are doing this same nutty thing. I can tell you MoPar does not, at least not with the couple of EFI MoPars I’ve dealt with recently. More reason for the Right to Repair movement, and that whole thread here earlier on alternate EFI options. Opaque software is a very bad thing, especially in a motorcar.

  3. Downstream sensors dreaded p0420/1 can be defeated with an 02 spacer or 2 spark plug nonfoulers stacked to remove the sensor from the exhaust stream.


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