I know what the a.i.r tube from the pump does for the stock catalytic converter. But what does the pump do for the engine? I know its there for emissions, but what is the actual function? Before any one jumps the gun here, I'm not planning on removing it. I'm just trying to settle a curiosity. Thanks.

 

There are different diverter valves that are vacuum controlled by solenoids. One of the 2 are called the TAB (thermactor air bypass) either sends the air from the smog pump out into the atmosphere; or onward to the TAD valve. The TAD (thermactor air diverter) valve either send the air into the cat or into the crossover tube (into the motor). Click the rubber vacuum system link in my sig for more info.

 

Thanks for spelling out the the components and what they do but what is the purpose of the air from the TAD going into the motor? Does it assist the A/F ratio during certain rpms/ accelerator input? Is it some kind of vacuum regulation? Also, since I no longer have the factory cats and have hi flow obd2 style cats (no a.i.r. tube required). I would assume that plugging the tube will give me undesired results since the TAD will switch between cat tube and motor. Am I even close in my thought process here?

 

I'm not sure what the impact would be if you capped it off, if anything I'd let it vent to the atmosphere. The solenoids are vacuum controlled, but thats pretty much the only time vacuum comes into play. It does lean out the mix at certain rpms, when the ECC needs it to.

 

Under certain engine conditions the fresh air is pumped into the exhaust ports. The PURPOSE is to mix with the spent gases from the combustion promote continued burning (yes, it still continues to burn when fresh air is introduced) of certain hydrocarbons in the exhaust before they reach atmosphere. It does make for a slightly cleaner exhaust, especially on a manual transmission vehicle.

On SOME vehicles, the pump alternately supplies fresh air to the cat, cooling it slightly.
Hope this helps.

 

With that said. With running the obd2 style cats, would the extra air going through the exhaust ports have any adverse effects or would it help these cats do a better job? The way I understand it is that the obd2 cats dont require any air injection for the catalyst to do its job properly. Am I running the risk of prematurely burning out my cats. I know its a lot of questions but It's the only way I can learn and understand. Thanks again guys!

 

The secondary air injection into the exhaust ports extends the life of the cats. There are conditions during which the EFI system intentionally adds excess fuel for driveability issues. When that happens, it does not all burn in the combustion chamber and the secondary air is activated so that the additional fuel is burned up in the manifold. It has to get burned somewhere and if it's not in the exhaust manifold it will be in the cats. Like everything else in the world, the less work the cats have to do, the longer they'll last

 

The air pump does NOTHING for the engine - it's not an engine managment system; it's strictly an emissions system. And cooling is the opposite of what the system does. The upstream air (going to the heads) pre-burns some of the fuel coming out of the exhaust ports, and helps generate enough heat to get the cats working. The downstream air (going to the cats) provides Oxygen & Nitrogen for them to work.

Modern cats begin working at lower temperatures, require less heat to achieve working temperature (less mass), and store Oxygen, so no air pump is needed. If true modern cats are installed on an old vehicle, the air pump becomes redundant, but removing it is still illegal. For more info, read these captions:

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What would you think he should do with the air going to the cat when he has the newer style cat? Just have that specific hose vent to the atmosphere as well?

 

Venting is all that he can do with it

 

Run it into the cat. Extra air won't hurt it, and that's what the law says to do: keep all the factory systems working the way they were built to. Most replacement cats come with a nipple to connect air, even if they don't require it. They just include a blanking cap. Since this thread is about a '90, excess air won't affect anything since there are no downstream HEGOs.

 

Gotcha, I didn't know that there was an air inlet on the cat still. Thanks for the info, as always!

 

Well I suppose i could run the tube into my converters but I think it would be more work than what its worth. First of all neither cat has a tube inlet. I purposely ordered them this way because of the info I already knew at the time ( obd2 style cats don't require secondary air) and because I wanted the best flow as possible to go with my Flowtech LT'S, X pipe, and Spintech. And running one converter to suffice the a.i.r. tube just wasn't going to cut it. I just had no idea that the same thing was happening upstream in the exhaust ports. Thanks to all of you who helped me to understand this. I greatly appreciate it.

Edit: regarding the legality of having the tube connected and maintaining the emission system as designed, I was unaware of this until now. I guess my understanding was this, as long as I have converters and the pump in " working" order than I am ok. If my a.i.r tube not being connected is against this I may need to look into something to correct it. Idk, maybe run it into at least one side of the exhaust for appearance purposes?

 

Call your local emissions testing facility &/or search the web for the EPA/DOT requirements. Remember that you have to comply with BOTH.

The change in exhaust flow due to the nipple &/or air pump flow is about as significant as the difference in drag on a cruise ship due to barnacles - it's certainly there, but neither the passengers nor crew can possibly feel or appreciate it. If you were working on a professional race car, it would matter. But you're working on a 20-year-old pickup with an engine designed ~40 years ago.

 

We don't have emissions testing here. I hate to confront Steve, but he's **** about this kinda stuff and my attitude is "do what you want, it's your truck" - it's not like you are running without cats and I'd bet that even with the A.I.R. system completely removed, you could still pass a full IM-240 tailpipe test anywhere.

Also to Steve - I don't see ANYWHERE where we can't discuss this kind of stuff in these forums. If you want to take me to task on this, have a moderator contact me if I'm out of line. No disrespect intended.