Might want to replace the lines and then connect a vacuum pump through a T while running and see what the actual vacuum readings are. This can help narrow down issues.

 

My lines are all good, I tested them separately using a vacuum gauge with a T, hooking one side of the T to one end of each line, plugging the other end of that line, and sucking on a line on the other side of the T. Once I did this, I checked that each component could hold a vacuum in a similar way. The valve on the back of the engine was the only part that could not hold a vacuum, like the diaphragm inside was bad. Today I plugged up the end of the line that runs to the valve on the back of the engine, and was rewarded with a smoother running engine. I may just run with the line plugged up, as it is cheaper than a new valve, and we only have visual emission inspection for vehicles that old. I doubt anyone will even know to check that valve, anyway. If they do, I will just move the blockage so it cannot be seen.

 

Only downside I know of is that that truck will run rich as the system detects a false lean condition if any part of the AIR system is not functioning. I believe my truck has those issues as well but I've found it to be manageable so far as mine is only an occasional driver/hauler.

 

Looks like I may be putting a new one on. I drive my truck almost everyday, so if there is a chance of this increasing fuel economy, even if only a tiny amount, it will probably be worthwhile for me to do. It is already terrible enough with the 4 speed that's in it.

 

I found a book I forgot I had on vehicle emissions (describes the Ford emissions pretty good) and after reading it was thinking about why it would run rich. When there is no vacuum, does it direct the air toward the cat or the exhaust manifold? If it directs it to the manifold, then I can see that it would run rich because the oxygen sensor would say it is running lean. On the other hand, if it is directed toward the catalytic converter, it shouldn't really have an affect on A/F ratio. If anything, maybe I'll just put in an elbow to direct air only to the cat and somehow plug up the pipe to the exhaust manifold.

 

A lot depends on how the computer reads each condition and what it does as a result.

 

According to my book, the air is only directed to the exhaust manifold when in open loop (doesn't look at O2 input) and is used to heat the O2 sensor more quickly. My O2 sensor is a 3 wire, so it should have a heater built into it. If I permanently direct the air to the cat, then it shouldn't mess with the O2 sensor. As far as I know, there is nothing to monitor where the air is directed. And since I already blocked off the vacuum line, the TAD (I think that's the one) should get no readings of any kind that the valve is no longer there. The only reason I could think why it would possibly still fail the KOER test is if the computer directs the air to the exhaust manifold while in closed loop (O2 sensor would be reading) to see if the system is putting out any air, which would make the O2 sensor give the reading of a lean mixture. That is the only way I can think of that the computer may be able to tell the valve isn't right.

 

And in case you are wondering what book I am reading, here's what it is. I think I got it at a library book sale. It does a decent job of explaining the operation of the Thermactor system that my truck uses, in my opinion. I have found it to be helpful in figuring out what each part does & when and how the parts interact with each other.

 

I have same engine but a 95, the thermactor systems should be the same. I installed new heads that did not have the smog ports. I removed both thermactor valves and rerouted the air pump lines to go directly to the convertor. Left vacuum lines and sensors in place and capped them off where they would normally attach to the valves.

In your case just unhook the vacuum lines from the valves and plug. Truck will run fine and I think like you said all airflow will go to the convertor and that won't hurt anything. The air going to heads helps with emissions at start up if I remember correctly. Doing this won't help or hurt mpg at all.

 

From what I read, the air going to the heads burns the unburned fuel from the heads before the O2 sensor, which is supposed to help the O2 sensor heat up quicker. Like I said before, my emissions book helped to understand how the Thermactor system works.