Removed Catalytic Questions

Thinking about a 1994 F-150 5.0 but the owner has removed the cats. That’s not a problem where I live. The intent would be a keeper vs. a flip. However I don’t do or know much when it comes to cars that have cats and computers so I have question(s).

What happens with the O2 sensors etc on or near the cats?

Would I have to deal with a constant check engine light?

Does removing the cats have other unintended consequences I may have to deal with?

Are there other questions or situations I don’t know of that I don’t know?

Thanks in advance

I don't think you'll have a single issue.  1994 only uses O2 sensors before the catalyst, so not having a cat should be something the computer has no idea about.

I had a 94 Ranger (Mazda B4000) that I gutted the cats and it just sounded a little louder.  No CEL

Yep if the car doesn't have O2 sensors after the cat, the only downside of removing it is increased pollution.

(If you really want to put it back, good aftermarket units are available from companies like Random Tech and Magnaflow)

To add a little more general info:

If the car was a '96 or later, you would definitely have an error light because the OBD-II (On Board Diagnostic) system would determine the cats were not working properly. That's literally what it's there for, to monitor the health of the emissions-related systems. But since a '94 doesn't have that, it's not an issue.

One consequence you might have to deal with is that a catless car may be more difficult to sell, as you won't be able to sell to some regions or states. Not a big deal if you sell it by parking it at the end of the driveway with a sign on it, might be bigger if it's the sort of car someone will buy from across the country.

Also, it's gonna smell bad :)

Keith Tanner said:

To add a little more general info:

If the car was a '96 or later, you would definitely have an error light because the OBD-II (On Board Diagnostic) system would determine the cats were not working properly. That's literally what it's there for, to monitor the health of the emissions-related systems. But since a '94 doesn't have that, it's not an issue.

One consequence you might have to deal with is that a catless car may be more difficult to sell, as you won't be able to sell to some regions or states. Not a big deal if you sell it by parking it at the end of the driveway with a sign on it, might be bigger if it's the sort of car someone will buy from across the country.

Also, it's gonna smell bad :)

Seconded.  And for that last reason alone, I'd probably at least slap a cheap high flow in there to knock the smell down in traffic. 

Yeah, everytime you pull up to a traffic light you'll probably get a good dose of uncatted exhaust.  I did this in one car and after getting terrible headaches when driving I put the cats back. 

Keith nailed what I was going to say. Look at your local market, if you don't think it's going to be something you can get rid of in less than a week, or you don't want to keep it, I'd pass. 

Also look into historic/vintage registration for your car. My 94 Miata is catless, with vintage tags, so the yearly inspection is safety-only. 

On many cars, putting a low restriction CAT on means you get a light unless you also install a spacer that moves the lower sensor further away from the flow.

wspohn said:

On many cars, putting a low restriction CAT on means you get a light unless you also install a spacer that moves the lower sensor further away from the flow.

Fortunately that won't be the case here with the OBD1 system.

If it were me, I would add a catalyst.  After working with emissions for most of the past 30 years, I'm kind of surprised how sensitive I am to it.  We had some test cells that seem to have leaks....

wspohn said:

On many cars, putting a low restriction CAT on means you get a light unless you also install a spacer that moves the lower sensor further away from the flow.

That's not low restriction, that's low effectiveness. It's too small if you have to try to fool the sensor. The reason it's low restriction is either because the core is more open (which means less surface area, and thus less catalyst is exposed) or it's an expensive metallic substrate. If it's the former, you need a physically larger cat to manage the same level of scrubbing.

Aside from the things mentioned above, one consequence of removing the cat is that the exhaust is less restrictive.  This a good thing if you're looking for power, but changing the VE of the engine like that needs to be matched by changes in the fueling.  Most fuel injection systems have sensors to detect the air flow and enough authority to change the fuel flow, but some of the early ones were based on a strategy called "speed/density" that effectively bakes the VE curve of the engine into the fuel map.  Changing the VE requires changing to map, which is something that wasn't easy/possible to do on those early ECUs.

I know Ford used speed/density on Mustangs in the late 80s, but by 94 they were mass airflow.  I don't know how that impacts F-150s.  So it's theoretically possible that without cats the engine might be running lean.  Then again if it was really a problem and the current owner did it years ago, it would probably have already blown up by now.

Lots of input re “the smell”. Are you referring to the sulfur smell? My impression regarding the sulfur smell is/was it came from a cat that was going bad for some reason. Is that not correct?

In reply to L5wolvesf :

No, it's the tang of HC for me. I'm pretty sure that's what it is.

L5wolvesf said:

Lots of input re “the smell”. Are you referring to the sulfur smell? My impression regarding the sulfur smell is/was it came from a cat that was going bad for some reason. Is that not correct?

IIRC the sulfur smell comes from a cat that loads up with hydrogen sulfide when running rich for a long period of time, then burns it off once it returns to a more normal rich/lean cycle. 

Keith Tanner said:

In reply to L5wolvesf :

No, it's the tang of HC for me. I'm pretty sure that's what it is.

Pretty sure it's NOx as HC and CO should be low around stoich/idle. I also agree just throw some cats on there to take the edge off and be a good steward of the planet if you plan to road drive it.

On some vehicles, removing the converter will cause the EGR to not work properly, not enough backpressure in the exhaust, and it will set codes.

I know this is the case on pre OBD-II Chevy Vortecs.  Fords (in)famously used a differential pressure sensor, which may help or hurt, here.

I put about 60k on a 95 F150 with the cats removed and a 3" single exhaust (stock exit) with a 40 series flowmaster.  Never noticed the smell, was definitely a bit louder.  Didnt make any more power, didnt really make any less either.  

Paul_VR6 (Forum Supporter) said:

Keith Tanner said:

In reply to L5wolvesf :

No, it's the tang of HC for me. I'm pretty sure that's what it is.

Pretty sure it's NOx as HC and CO should be low around stoich/idle. I also agree just throw some cats on there to take the edge off and be a good steward of the planet if you plan to road drive it.

There is significant HC and CO wo a catalyst. 

I definitely notice the smell, oddly it's worse for cars that have them but have been taken off.  I wouldn't take one off a car that came with one or buy a car that someone else had, but if they never had them then that's fine.

codrus (Forum Supporter) said:

Aside from the things mentioned above, one consequence of removing the cat is that the exhaust is less restrictive.  This a good thing if you're looking for power, but changing the VE of the engine like that needs to be matched by changes in the fueling.  Most fuel injection systems have sensors to detect the air flow and enough authority to change the fuel flow, but some of the early ones were based on a strategy called "speed/density" that effectively bakes the VE curve of the engine into the fuel map.  Changing the VE requires changing to map, which is something that wasn't easy/possible to do on those early ECUs.

I know Ford used speed/density on Mustangs in the late 80s, but by 94 they were mass airflow.  I don't know how that impacts F-150s.  So it's theoretically possible that without cats the engine might be running lean.  Then again if it was really a problem and the current owner did it years ago, it would probably have already blown up by now.

 

By 94 with the honeycomb catalysts (and the relatively low engine outputs) the ECU is perfectly within it's parameters if you gut the cat.  The gutted cat on my 94 Ranger never gave a code or skipped a beat.  The ECU and injectors were more than capable of adjusting for anything.

And yes, by 94, the 5.0L were MAF as well.

L5wolvesf said:

Lots of input re “the smell”. Are you referring to the sulfur smell? My impression regarding the sulfur smell is/was it came from a cat that was going bad for some reason. Is that not correct?

Referring to the lack of sulfur smell.  Running without a cat can make it smell like an old 60s muscle car running rich.

Paul_VR6 (Forum Supporter) said:

Keith Tanner said:

In reply to L5wolvesf :

No, it's the tang of HC for me. I'm pretty sure that's what it is.

Pretty sure it's NOx as HC and CO should be low around stoich/idle. I also agree just throw some cats on there to take the edge off and be a good steward of the planet if you plan to road drive it.

Yes, it happens during rich operation, like WOT.  But modern emissions-controlled vehicles rarely operate at stoich.  They're close to it at cruise (13s) but pretty rich at WOT (in the 11s).  This is to keep cylinder temps down and reduce NOx.  The whole thing is a trade-off for manufacturers.  In order to get them to conform to max allowed outputs of the three main parameters, they end up pushing the HC (running a bit rich) to mitigate NOx.  That's why so many of the commonly available plug-in tuners can get you 10-15 hp by leaning out the fuel map and adding a little timing advance - things that OEMs can't do without risking failing the NOx part of the testing process.

Curtis73 (Forum Supporter) said:

In order to get them to conform to max allowed outputs of the three main parameters, they end up pushing the HC (running a bit rich) to mitigate NOx. 

IIRC this is also because it's a lot easier to build a cat that will break down CO and HCs than it is to deal with lots of NOx.  It's also part of why diesels pretty much all need DEF to meet current standards.