It's an 88 351w fuel inj. It has a sorta pipe that come out of the back of both heads, and meets upward in the middle with some type of element paper on it. Might be some sort of emissions/exhaust part? The Bronco's had all the emissions equip taken off(supposedly). Anyways, it's rusted through completly, could I just take it off and plug it with some bolts? Or should I replace it..?

 

It is the manifold for the head portion of the air injection system. Yes you can plug the holes in the back of the heads with bolts.

 

Interesting to hear that "all of the emissions equipment has been removed" when the air injection crossover pipe is still on the truck. At best, someone attempted to remove the emissions equipment. Did they convert to a different engine control system too? If not then the truck isn't running right and never will. By '88 the EEC-IV system was fully integrated and the removal of the smog pump and "some" of the emissions equipment merely keeps EVERYTHING from operating properly.

You CAN plug the holes in the heads but you will lose the air injection that flows through the crossover pipe and into the exhaust. (But if the smog pump is gone and the pipe is rusted through there isn't any air going in anyway so the truck still isn't running right and won't.) This will change the oxygen levels in the exhaust causing the oxygen sensor to return a "rich" condition to the computer. At this point, the computer will lean out the fuel/air mixture to the point that the spark plugs will be starved causing them to overheat and fail prematurely. The potential is also there for poor fuel mileage and performance not to mention that pesky "check engine" light could end up staring at you until you fix the problem. (This assumes it hasn't burned out or been removed.) Other than that, shoving a couple of bolts into the air injection ports should work just fine. However if you should decide to fix it properly, make sure you replace the check valve at the center point where the fresh air from the smog pump enters the crossover pipe and make sure the hoses from the smog pump are intact.

Oh, did I mention that if the crossover pipe is rusted through BELOW the check valve, hot exhaust is exiting into the engine bay and typically melts the main wiring harness where it runs across the leading edge of the cowl? More commonly, the check valve snaps in half at its midpoint leaving the exhaust to be directed straight up the remaining portion of the valve which not only melts the wiring as mentioned but will blister the paint at the rear of the hood as well eventually. But if the pipe is rusted through, its all just blowing out the rust holes.

 

The truck doesn't really have any performance issues, and it gets about 16mpg. But the spark plug thing is funny, I changed them twice and they were white, and the check engine light is always on. Soo what should I do? Replace the pipe or convert it to a different engine control system?

 

Not wanting to pull an old thread out of the grave but, I have an 86 351w 4 bbl and my thermatic (sp?) check valve rusted just where the rubber hose hooks up. I had an exhaust leak when idling but when the engines rpm's increased it would close the check valve and all was good. I decided to change the check valve and in the process broke the crossover pipe. I guess it was rusty also. The airpump has the belt taken off and in frozen up. I planned on trying to get it all running correctly but now I'm wondering if its worth to still try and fix. Or should I just remove the stuff. I know that greystreak is a big fan of leaving it on, but after searching I haven't found much info on how bad the truck will run without it. Being its a carburated engine. Will it run bad at all? I put about 4000 miles on this thing since 03 so its not like I drive it alot.

 

Remove the pipe and plug the holes in the heads, they should be 5/8" coarse thread. Even on an EFI motor removing the smog system has absolutely no(zero) effect on engine operation, but having parts of the system attached and leaking can produce an extreme lean condition, not sure it would do the same on a carbed motor but who knows.

 

Supposedly, there is no effect in removing it. However, how this is possible, I still have yet to have it explained in a manner that makes sense. The Thermactor System introduces fresh air into the exhaust system UPSTREAM of the O2 sensor. Er go, with the Thermactor plumbing to that point removed, the O2 sensor will NOT read this fresh air (translation = increase in oxygen level) when the computer is "expecting" to "see" it. This has the potential to at least trigger fault codes if not change performance due to fuel/air mixture not being optimized when the computer expects the fresh air input from the Thermactor system. I welcome an accurate explanation as to how this can have zero effect on things given that, at some point, the exhaust gasses flowing past the O2 sensor will NOT have the oxygen levels that the computer is "expecting to see". The computer is given control of the Thermactor system so it can control this air flow. Removing this portion of the system removes the computer's capacity to control it and optimize performance and emissions parameters.

I know it has been done and that few if any folks have seen what they consider adverse effects as a result. However, I refuse to believe that this has "zero effect".

 

That's just it, when the computer is in closed loop the thermactor system is not supposed to put any air in the exhaust upstream of the O2 sensor, it should go directly to the cats or simply be dumped to atmosphere. The only time the system should inject into the heads is on cold startup and before the O2 sensor comes into play.

 

Isn't the computer just controlling the ignition advance at this point?

 

Well no, it's always controlling the engine and that means altering injector pulswidth and dutycycle, changing the timing advance, altering the IAC valve based on input from the TPS to achieve the desired engine idle speed, etc.

 

Ok, if that is the case, why bother to put the air injection upstream of the O2 sensor at all? Your description of that part and function of the Thermactor system renders the injection upstream of the O2 sensor superfluous. What purpose does a little extra fresh air in the exhaust immediately downstream of the exhaust valves accomplish during a cold start?

 

But its carburated.

Whats the IAC valve? And I thought TPS was fuel injected also.

Sorry for being ignorant.

 

Yep, those are all EFI components. The Thermactor still provides the same function even with a carburetor. It injects fresh air into the exhaust to provide the necessary oxygen to make older catalytic converters function properly and to add the necessary oxygen to help ignite remaining pollutants in the exhaust. The additional heat only serves to increase the expansion rate of the exhaust gasses which never hurts in keeping them flowing towards the tailpipe. The added oxygen in the mix as the gasses help the unburned crud ignite and burn off. Catalytic converters use the air for the same purpose but they have the unique advantage of being able to achieve internal temperatures HIGHER than those of the combustion cycle to aid in the reduction of additional pollutants.

 

I have helped by-pass stuff before. And it's just a matter of finding the happy voltage for the sensor in question. It's all working from a baseline voltage. We found resistors to match the sweet spot for the sensor. It was a '96 so it was OBD-II. We did it because the fellow wanted the 5.8L and an E4OD in an early Bronco. It took a couple of weeks to figure it out. He bought a Baumann box to make the E4OD happy. I've got the numbers we came up with on the resistors wrote down somewhere. The rig ran like a scalded dog and never kicked a code.

 

That'd be great if the O2 sensor worked the same way the rest of the sensors in the EEC system does. The O2 sensor actually generates an electrical signal via the chemical reaction that takes place within it. There is no VREF supplied to the O2 sensor so the return signal is constantly changing based on the voltage generated by the chemical reaction taking place in the sensor. In this case, its damned near impossible to replicate.