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Discussion Starter #1 (Edited)
So the back story.

Truck handed down from father to son with high miles. Father ran about 40K miles with smog pump bypassed as it began making a horrible racket. Truck ran ok and no CEL but of course there were codes being thrown. Son takes over truck and decides to get it back right as from the factory. New smog pump and belt have it configured the way it was stock. Truck still runs good and gets 17-18 mpg. Have been driving it now with new smog pump for about 5K miles.

Did a check engine test and KOER came back with a code 311. Thermactor system not functioning. So I begin to troubleshoot.

1. Check vacuum first. All lines are ok and vacuum is present at TAD/TAB solenoids. Vacuum reservoir is holding vacuum. No problem there.

2. Check Air Bypass Valve near pass. header. It appears to be working. As per the Haynes manual it should vent air pump air after extended idle. It does this and vents through the canister muffler after ~15 seconds idle. Immediately stops venting to muffler when engine speed is raised above idle. All seems good here.

3. As per the Haynes manual, check the function of Air Diverter Valve. Seems to be a problem here. Haynes says that, on engine start. the TAD solenoid should pull vacuum on the Air Diverter Valve for between 100-150 seconds. Mine does pull vacuum but for only ~10 seconds. I thought, Hey! found the problem. Ordered a new TAD solenoid from Ford. $57 later and battery disconnected overnight to clear codes and the code 311 is still thrown and vacuum is still only maintained after start on the Air Diverter Valve for ~10-15 seconds.

4. Started to inspect the piping and hoses back there. Crossover pipe between heads look new. Air Diverter valve shows no rust from what I could see with a mirror. Nor do the two check valves.

So, some trouble shooting details:

1) Both metal pipes directly from the air pump get HOT after the engine has been running a while. Maybe this is normal as they are directly over the header. And the air bypass valve appears to be working properly. Just thought this was strange.
2)With engine warm, the crossover pipe on rear of heads is cool enough to grab hold of. Does this mean that I am constantly pumping air pump air through these directly into the heads?
3) Can someone explain the exact function of the TAD and TAB solenoids? I.e. when the TAD solenoid is pulling no vacuum on the TAD valve, where is the air going, head or exhaust pipe? What about with vacuum applied to the TAD valve?

The overall gist is that my TAD solenoid appears to never let vacuum through from vacuum canister to the TAD valve on back of engine. Again I checked all vacuum lines and they are good. In fact I even took the vacuum connector off the TAD solenoid and put it on crooked so the black feed line was on but the yellow line to the TAD valve was not hooked up. Started the truck and monitored the vacuum pull (or lack thereof) directly off the upper TAD solenoid vacuum port. Same result as previous.

Am I correct to interpret Haynes' check procedure literally or does it only apply with engine cold or under some other circumstances?

Finally, whats it going to hurt to keep running like this. The only code thrown is the 311 and as I say, I get pretty great gas mileage and it runs and idles well. Its just in the back of my mind now.

Hope I was thorough enough and appreciate any insight you can provide!

Edit: Forgot to mention. 95' 351W
 

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Discussion Starter #3
Hey thanks. Stumbled across that in my searches but it didn't really tell me the exact method that the TAD solenoid and diverter valve function. Is there a plunger in the valve that, with vacuum applied from the solenoid, raises and directs fresh air to the check valve in the crossover and with no vacuum lowers and directs fresh air to directly to the cat?

Thats what I don't fully comprehend. And I'm hoping someone more knowledgeable than me can explain why the TAD solenoid funnels vacuum to the diverter valve for the first 10 seconds after truck started and then shuts off so there is no longer vacuum going to the diverter valve. Anyone know the engine conditions that will trigger the TAD solenoid to supply vacuum to the diverter valve?

Thanks for the reference though!
 

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Discussion Starter #4 (Edited)
OK! So I did some google searching outside the forums and I think I may have answered one of my own questions. How to read the results of the test that Haynes suggested.

According to this guy on a mustang forum, fresh air is diverted to the heads when engine is cold and cat is cold. After warmup, fresh air is diverted directly to the cat downstream of the O2 sensor. So that makes sense. While in open loop and not relying on the O2 sensors it is no problem pumping additional air into the exhaust stream. Had this happen when the system is in closed loop the O2 sensors would see a whole bunch extra oxygen and read that as the engine running lean. Would likely lead to a forced rich condition by the computer.

And when the engine heats up and enters closed loop mode the diverter valve then redirects fresh air to the cats downstream of the O2 sensor(s) to help with the unburned stuff from the engine.

Sooo, my description of, on hot start, the TAD solenoid quickly cutting off vacuum to the diverter valve is starting to seem normal. Now I am waiting for the engine to completely cool down and will run outside and check the vacuum feed on a cold start to see if it appears to be directing to the heads.

Next issue might be, how exactly does the computer determine that a fault is present with the thermactor system? Does is monitor something with the TAD/TAB solenoids or maybe a trigger is pulled by the O2 sensor reading funny. I.e. air being dumped in above the sensors in closed loop mode? There is really not much of a return signal from this system!

With that said, how would the computer come to know that one of the check valves or diverter valve is bad? The only sensor downstream of these things is the O2 sensor(s). I guess it might see a lean condition if any of these things were leaking, but then why not throw a lambda sensor code at me to signal that it is lean. Would a code relating to the O2 sensor only manifest if the sensor itself was malfunctioning?

Also, saw this article referenced by miesk5 in another thread on FSB and it seems to indicate that a malfunctioning TAD solenoid could manifest itself as an overall rich condition from the O2 sensor seeing to much fresh air and richening the mixture. But that goes against the good gas mileage I see with this truck!

So my head is spinning now! Hope to hear your thoughts.


Update: So I started it cold and the TAD acted the same. Pulled vacuum on the diverter valve for about 15 seconds and then no vacuum. So the TAD is acting the same regardless of ECT. No amount of engine revving or waiting for it to run while cold could make the TAD start pulling vacuum on the diverter again. Don't know what to think.
 

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Discussion Starter #5
So one more update. Got to thinking about it and realized that my dad passed on to me a CD which I assume is the service manual but I never thought to use. Opened it up and looked at the diagnostic procedures for testing the A.I.R. system.

All the typical "check vacuum lines" and stuff checks I already knew were fine. Got to the step to check the function of the wiring going to the TAD and TAB solenoids. The service manual calls to put the ecm into Diagnostic Test Mode and test the voltage across the TAD and TAB solenoid connectors. The spec said you should see a slight voltage increase as you move the throttle plate while in this DTM mode. Tested the TAB solenoid and the voltage did in fact increase as I moved the throttle from the idle position up. The same was true for the TAD solenoid.

So this tells me that the TAD solenoid is functional as is and there seems to be no wiring problem.

I got out of the DTM mode, hooked connectors back up, and started the engine. Probed the back of the TAD and TAB solenoid connectors and found out that when the solenoid is engaged the voltage across the pins is 14 volts. When solenoid is not engaged the voltage is 12 volts. I was able to see this by probing the TAB solenoid. As expected, after extended idle the TAB solenoid would turn off and remove vacuum from air bypass valve which caused it to vent air pump air to the air pump muffler. Rev it up a little and the voltage jumps to 14 volts as the solenoid engages and the bypass valve now has vacuum and air is routed to the diverter valve.

So the thing here is that, for the first 15 seconds after startup the TAD solenoid is turned on and getting 14 volts. After ~15 seconds the solenoid must turn off and the voltage drops to 12 volts. I can actually feel it turn off by putting my finger on top the solenoid. I never can get it to turn back on.

Now for the tricky part. When the TAB solenoid is "on" and funneling vacuum to the bypass valve, the bypass valve lets air pump air to the diverter. Conversely, when the TAB solenoid is "off" air pump air is vented off through the air pump muffler. Sooo, if the bypass and diverter valves function at all the same, when the TAD solenoid is "off" (which it seems to always be), from the picture posted above by Seattle fresh air is being diverted directly to the cat downstream of the O2 sensor. This might explain why the truck does run pretty well and get good gas mileage. Kind of a restatement of previous musings but if it were putting it upstream of the O2 the computer might sense lean condition and richen it up resulting in worse gas mileage.

So where I'm at now is this: All vacuum lines are good and connected. Crossover tube and check valves feel and look (from mirror view) in good condition. TAB and bypass valve appear to work just fine. TAD valve does function properly. BUT it appears the computer is turning off the TAD solenoid shortly after startup and not turning it back on until next start. So now I think, what could be making the computer turn off the TAD solenoid regardless of ECT or TPS? Checked the resistance of the ECT sensor and it read 3.6 Kohms when the engine was warmed up. That seems about right. Didn't test it cold. Maybe it doesn't read right when cold. TPS reads nice and smooth from 1 at idle to 4.xx at full throttle.
 

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Satyr of the Midwest
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You may be looking at the DTC wrong. Just because the code says the system is inoperative doesn't mean that's necessarily so. All the DTC means is that the oxygen sensor signal did not change as predicted when air was routed upstream. So you could just as easily have carbon-clogged AIR injection ports, a funky oxygen sensor, exhaust leak(s), etc.
 

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yo,
Good work! As Sig advised there are other possible causes.
DTC 311, 312, 313 & 314; "...311 and 314 indicate the Secondary Air Injection system is inoperative. DTC 312 indicates that Secondary Air is misdirected. DTC 313 indicates that Secondary Air is not being bypassed when requested.
Possible causes: Visually inspect vacuum lines for disconnects in the AIR system. Visually inspect for proper vacuum line routing. Refer to VECI decal. Visually inspect Air Pump for broken or loose Air Pump Belt. Refer to Section 13A for adjustment/replacement..." READ MUCH MORE
Source: by Jim at http://www.justanswer.com/questions/sqj6-trouble-codes-fo-1995-ford-f150He shows da 1995 PCED On Board Diagnostics I A

Vehicle Emission Control Information (VECI) Decal; Contains Vacuum Diagram & Calibration Parts List for 88 & UP. On-Line for Free at Ford. Click "Quick Guides" in left panel; Scroll to & CLICK VECI Labels "Provides Vehicle Emission Control Information (VECI) and a related calibration parts list." Enter applicable info (need to know your Calibration number from your B-pillar sticker). Vacuum Diagram is the same as the one on the core support or hood or air filter cover. Suggest Right Clicking this Hot Link & Open in New Window
 

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Discussion Starter #8
A small update to this issue. From this thread, EGR problem? turns out my problem was a bad oxygen sensor. I had a ton of oil and crap on the outside of the sensor and it must've been affecting the way it performed. No codes were set for the O2 sensor but replacing it has fixed this problem of the computer thinking the AIR system is not functioning.
 

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yo KM!
Good!
btw, re; Therm Sys; I posted info from various well-regarded sources here recently @ http://fullsizebronco.com/forum/showthread.php?t=179186&page=6

Consider Ford Fuel Injection and Electronic Engine Control: How to Understand, Service and Modify, 1988-1993; & Ford Fuel Injection and Electronic Engine Control: How to Understand, Service, and Modify All Ford-Lincoln-Mercury Cars and Light Trucks, 1980 to 1987 by Charles Probst
Available at Barnes & Noble, AMAZON, E bay, etc.
 
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