92 f150 exhaust question

hey all, had to replace my entire exhaust system on my 92' f150 (inline 6). the old exhaust had an openning for the o2 in the front and another opening for a line from what appears to be some emissions parts
(unsure what it is) right before the cat (it was a welded piece that attached to a rubber line about 1" thick that run to the front of the engine near the top). i bought a replacement exhaust system from advanced auto that bolts in and hangs correctly but does not have this extra openning for this line. anyone familiar with this engine can let me know what the purpose of this line does (if i trace it up, it looks like parts of it recircs the exhaust back to the engine and some escapes to the exhaust). i'm having difficulty understanding why the new system does not support this line and if it's really necessary. i bought the car used in DC but wonder if this was an original california car with some extra emissions required for that state (am currently in DC).
thanks.
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Sounds like the tube is for downstream air from the thermactor system. When the motor is at operating temp, this air (from the "smog" pump) is delivered to the exhaust manifolds but, when the cat is cold, the air is pumped to the converer to help bring it to temp quicker.
When we get into some of these aftermarket parts, especially for older vehicles, it can become a "one size fits all... or nearly all" thing. It could also be the result of somone failing to read the footnotes in the parts catalogue.
HTH

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Shoe Salesman wrote:

Nope Shoe, Jim has it right. The air is diverted to the cats during warmup to help light off the cats quicker, then diverted to the manifolds to help burn off unburned HCs before the exhaust reaches the cats. For a long time 3\4 ton and up trucks were exempt from some emissions regs.(It was that way in the 70s, through at least much of the 80s and possibly very early 90s). It's likely that his pipe was intended for an F-250 or better vehicle without the Thermactor system. I've seen the 300CID\4.9L in F-250s many times, not sure about heavier duty trucks. (Thus the melding of aftermarket part numbers)
Rott... Take care of that motor in your truck and you will own it for quite a while. The hose you are referring to was probably steel down to the cat originally. They tend to rust off just above the converter. Someone probably "repaired" it with hose.(hopefully silicone). To do it "right" you would have to have someone weld a nipple into it to attach a replacement steel pipe to (Pipe is about $30). You could do the nipple and existing hose also, but the hose usually burns off at the cat in a short while.
If you're in an area that doesn't do emissions testing, you could remove the Thermactor pump and related plumbing, but on a 92 it would probably cause codes to be set in the ECM and the EEC_IV system to default into "limp" mode. (Mr Warman can likely elaborate on that if it's true, I don't remember).
On a personal note, it's a shame that Ford had to stop producing the 300\4.9. I'm told that they couldn't make it meet emissions in later years. That motor was a real workhorse. Given a choice between a 300 and a 302 (4.9 and 5.0L) in a pickup truck, I would take the 300 hands down. (And I'm a big fan of V-8 Ford motors). You could just about drag your house down the street for 200K miles, give the motor a valve job, and drag it another 200K, with an F-150 equipped with a 300cid\4.9L six and a 3 speed manual trans. That's stretching it a bit I know, but it conveys the reliability and torque of that motor. They were tough little suckers with lots of torque that would run for miles and miles..
Hope this helps, Tom Adkins
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hi all, thanks for the response.
the replacement cat does have a nipple that is sealed but is significantly smaller then the metal tube from the thermactor (tom, it did indeed rusted off as you said)-- like 1/4" to 1". would this nipple suffice if i were able to find a converter or is it too small?
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hey all, seems like some tangental conversation has ensued. i think i found my answer and just wanted to share.
the newer dynomax direct fit style exhaust system comes with a pre-capped nipple off the catalytic converter. i believe i can purchase a universal air tube kit to tie the old style tube/thermactor to this new style (smaller) nipple. if the weather holds up tomorrow i will give this approach a shot and report back.
thanks again.
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Welcome to UseNet.... Not often that threads can stay on topic as they occur on what is possibly one of the purest forms of roundtable discussion available. Ofen, the sidebars are more interesting and gather much more information than the original post.

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Ok, so, he said its a straight 6 right? So its got 1 manifold right? Its got an O2 sensor in that manifold (or just below it). How can you pump air into the manifold (when its warmed up) and not give the O2 false info?
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By programming the PCM to ignore O2 sensor signals under conditions when air is being injected upstream.
Notice that in Jim and Tom's replies the absence of the word 'always.'
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So its downstream when cold then diverted to atmosphere when it reaches norm operating temp, exept under certain conditions? What are those conditions?
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Your question was; "How can you pump air into the manifold (when its warmed up) and not give the O2 false info?"
I gave you the answer. The rest has already been covered.
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No, I just asked what those conditions were. What was covered was when warmed up, it goes to the manifold. Then you said not "always". What I want to know is the "not always" part when its warm. When during closed loop does the ECM not look at the O2 (when is it injecting air in the manifold in closed loop) like you said? If you don't know just say so, I'm just trying to learn about the system here....
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Please refrain from misquoting me. I did not say "not always." I said that Jim and Tom's replies did not contain the word "always." If what was covered (by your admission) was when warmed up, what more needs to be said?

Who said anything about "closed loop?" Do you understand what "closed loop" means? If you did, you wouldn't have made that statement.

Forget the "system," work on the basics first. a) closed loop versus open loop b) cold operation versus warm operation c) purpose of the oxygen sensor
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You know what aarcuba, I'm thinking you don't know how it works and your just trying to sound like you do. I know very well the "basics" you listed above. My question was: Its got an O2 sensor in that manifold (or just below it). How can you pump air into the manifold (when its warmed up) and not give the O2 false info? You replied, "By programming the PCM to ignore O2 sensor signals under conditions when air is being injected upstream." Again, my question was simple....what are those conditions? Let me give you a few examples so you won't be confused:
1. On decel 2. At WOT. 3. At cruise. 4. At no time when the vehicle is in closed loop will the air be diverted to the manifold, I was wrong, it diverts it to atmosphere. 5. At no time when the vehicle is in closed loop, I was wrong, it diverts downstream. 6. I have no idea, all I know is the basics. 7. I will come up with a smart ass answer to try and look smart like: When it ignores the O2 it's not *really* in closed loop. Or, you just said "warmed up" not closed loop. 8. I will turn this into an argument about closed loop instead, so people don't know that all I know is the basics.
I'm thinking the real answer is either #4 and/or 5 AND 6. But you will come up with something like 6, 7, or 8.
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It should be obvious that pumping air into the exhaust manifold during decel would cause afterburn up to and including the point where the muffler is split open.

O2 is ignored during WOT, so yes, it could be done then.

If that's what the engineers choose.

By definition, if it's in closed loop the O2 isn't being ignored.

By definition, if it's in closed loop the O2 isn't being ignored.

Read a book then.

That would be a fact. The system can NOT ignore the O2 -and- be in closed loop. If you have a problem with this, then you don't understand closed loop, you don't understand the basics.

They are two entirely different things, just like an engine can be not warmed up and in closed loop.

Feel free.

Well, if you're smart enough to claim that I don't know how it works, why are you asking such basic questions and more importantly, why are your questions seasoned with misunderstanding?
I think the answer you're looking for is; any time there is excess CO or HC in the exhaust that would not be scrubbed by the catalytic convertor due to any number of factors such as, WOT, a cooled off cat, insufficient oxygen to the cat to support oxidation, during a hot restart when fuel vapors may be high, etc.
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lol Never mind.......please.
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Back before EFI and fedback carbs, we didn't really have to think much about it...... under decelleration, the air would be diverted away from the exhaust to prevent backfires. As far as vehicles presently being manufactured, I don't know of any with a A.I.R. type system.... though many have electrical secondary air pumps to supply air to the cats to "light them off".
This is a long drawn out way of saying that I am unsure of the operating strategy of the system..... I will say that the pumps were sized for the application.... I will say that many times I have felt and/or heard diverted air coming from the fenderwell mounted muffler.... without giving it a second thought simply because the system was working. Remember that these old systems were, by and large, speed density systems..... The PCM knew the engine displacement, it knew the throttle opening, it knew barometric pressure and intake manifold vacuum as well as engine rpm - more than enough info to establish a reasonable fuel mixture. Taken one step further, the engineers knew the size of the pump and the ratio of the pulleys... (this is assumption and home grown rationalization).... based on this, the engineers know how much air is being commanded where... and adjust programming to suit.
With the level that todays technology has attained..... all I can say is that some of the mandatory courses I take are enough to give you a really good immitation of a migraine.
To indicate how prehistoric our early attempts at feedback fuel control were..... my 1985 F150 4X4 had the EFI 302..... Thermactor pump installed (even in Canada) With a single O2 sensor planted at the back of one exhaust manifold. Chevrolet did the same with their pick ups.... Some had air pumps, some didn't but they all had one single O2 sensor at the rear of one manifold. Dodge muddied the waters even more.... on their pick ups, they went with a "pulsed air" system..... There was no air pump but, instead, there were one way check valves (identical to the antibackfire valves in the GM and Ford systems)that allowed atmospheric pressure into the exhaust .... remembering that each positive pressure wave in the exhaust is followed by a negative pressure wave. The Japs used this system for many years with good success.
Now... if anyone noticed, I always refer to pick ups of all sorts and sizes.... I live in a very small (but growing) town in the middle of the great Canadian arboreal forest..... I don't get to work on cars very often (which is a good thing... I'm fat enough and old enough that you need a stick of butter and a shoehorn to get me in and out of them). As far as early emissions controls, Canada was a redheaded cousin.... in most cases, we lagged behind the US about 3 years. This made for a repair manual nightmare as we searched for anything pertinent to the vehicle we were working on. In other cases, we never did see some of the stuff that was made (and failed) in the states.
I realize that this is a pretty long dissertation... but, hey.... It's Christmas and we can only watch "Miracle On 34th Streeet" so many times....
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and smog testing cars in Ca -the king of emission controls state. Its been a long time since I've worked on a 90 or so vintage Ford p/u s but I do recall the downstream pipe. During a smog check you were not required to test the AIR system, you just had to make sure it was all there and not obviously modified. If something was wrong it would probably fail the tailpipe part of the test anyway. I also remember Chevys that would/or should only pump air to the manifold before it was warmed up and in closed loop. It wasn't uncommon for them to pump air upsteam and cause the car to fail because of high CO. I think some V8s had the O2 on one bank and the AIR manifold on the other(IIRC) to eliminate that problem I guess. Like you said, looks like the "smog pump" is gone. :)
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Ford's air pump strategy is not the same as what GM does.
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