I have a 99 F-150 4X4 with the 5.4. Cats are getting clogged up and I am thinking of just cutting them off and welding in a pipe. My concern though is with the O2 sensors and what that may or may not do to the readings for fuel mixture as a result. Has anyone ever successfully done this?

I really dont want to have to replace the cats as it is going to be very expensive to do so. Any help or advice with this would be GREATLY appreciated.

 

If your cats are getting clogged up, your truck needs a tune up. Only poorly running vehicles get clogged cats. Clogged cats typically indicates a rich condition, excessive oil consumption, coolant entering exhaust, misfires, or extreme lean condition. Often plugged fuel filters or failing fuel pumps are diagnosed as clogged cats.

Removing cats is not a straightforward thing. Yes the truck will still run ok, but your CEL will be on constantly. The problem with that is that if a new problem occurs that would set the CEL and requires your attention, you won't know about it. The seriousness of this depends a lot on you. You have to ask yourself do you want a nice truck, do you want the option of selling it later on, can you live with breakdowns out of town, etc. If you can live with that, or if the truck is basically going to be used a ranch beater, then by all means just live with the light or put some tape over it.

The biggest maintenance issues that often turn into cat problems is the ignition system and the O2 sensors. Spark plugs should be at least inspected if not replaced, every 80,000 - 100,000 miles. Some may recommend them more often that that. If you don't get proper ignition, you don't get proper combustion. This wastes fuel, reduces power, and damages the catalytic converters.

O2 sensors are maintenance items. The recommended replacement interval is 100,000 miles. If the O2 sensors start to degrade, they often give no warning signs. Codes are only set when the sensors start to fail, but their accuracy can be compromised long before that point. The sensors voltage drops as they degrade, and the computer cannot tell the difference between a degraded sensor and a lean condition. The computer adds fuel to compensate, which cause reduced fuel economy, and excessive fuel in the exhaust. The same conditions that damage cats and cause them to clog also tend to foul up O2 sensors.

Other recommended maintenance is cleaning the throttle body and intake, and the fuel system. Gunk in the intake really does affect how the engine runs, and one or more partially clogged injectors results in some cylinders running lean while others run rich.

To answer your question in greater detail. If you remove your cats, the computer will detect that they are missing and will set codes accordingly. If you are in an emissions area, most inspection stations will fail you automatically if your CEL is lit. As mentioned earlier, if a new problem arrises, you will not know about it since the light is always on anyway.

To make the light go away with no cats can be a little tricky. You can try removing the rear sensors from the exhaust stream somewhat to dampen their readings and make them less sensitive, but this method often does not work, or can cause O2 sensor codes. This method really is a shot in the dark. Electronic simulators have similar issues. The only proven way to keep the light off is to run a tuner. The tuners let you change all sort of engine parameters, and that includes the sensitivity of the catalyst monitors. You just disable emissions monitoring, and the CEL won't be a problem, yet legitimate engine problems will still trigger codes. This approach is the most expensive, but it always works, can be disabled with ease, and with proper tuning, can increase both performance and fuel economy.

I am not convinced your cats are needing replacement, I would pursue all the proper tune up work and see if that doesn't clear up any issues you are having. If the tune up work doesn't help, then pursue whatever you decide to do with the cats.

 

Thank you very much. Looks like i will not be cutting them off.

I replaced two of the O2 sensors 5 months ago and it removed the check engine light for about 2 weeks and then it returned. I wasnt driving the truck that much then though. I just located two more O2 sensors on the other cat on the drivers side that I did not know the truck had. I had located the passenger cat, replaced the sensors up and downstream and didnt realize there was another cat with sensors.

Im wondering if this is part of the problem as they dont look like they have been changed and the truck has high miles on it.

Also the truck starts up and runs great, but once it has warmed up and then I turn it off and then have to turn it back on with the motor warmed up, it cuts out and acts like it is running on two or three cylinders for about 10 seconds and then smooths right out and no more issues. Is that the cats, or could that be the 02 sensors recalibrating?

Also when on the few times I have opened up the truck, or floor it, it dogs out like it has a governor on it. I had thought this was clogged cats. Maybe not?

 

Does not sound like clogged cats, sound more like leaking injectors. Bad cats would run poorly all the time, will increasing restriction as load and RPM increase. You can run some tests on the fuel system, see its running pressures, and test to see if it looses pressure after it is turned off. Had a van that acted exactly that way, had some other mechanics pulling their hair out, but most of them were not doing proper diagnostics either.

 

I did secefully replace cat even don't weld then clamp with band clamps. Work perfect. Not willing to write search over my name if you interest. Clogged cat throw code for sure.

 

I Sawzall them out, beat out the ceramic with a rod, and MIG weld the shells back in place. I remove the exhaust system to do this since I want 360-degree weld penetration.Stock pipe is heavy and less likely to leak.

Scratching three alignment marks at each cut ensures welding them back correctly.

I ignore CELs, but I'm located where that's not a problem. Runs fine, no problem. If it ceases to run fine, I'll care enough to read the codes.