Code 44 without a code 94

sln583

Dirt-Old 20+Year Member
Nov 13, 2004
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Quincy, MA
I have spent a couple of hours searching the forum on this and couldnt find a solution for this code on my 88 california 5.0



KOER test is giving me a code 44, Secondary air injection system inoperative (right side). Code 44 and 94 together usualy mean the Thermactor system isnt working properly, but a 44 on its own seems to mean the secondary air is being diverted just fine on the drivers side but not on the passenger side. My understanding is the thermactor air going to the back of the cylinder heads is not registering with the O2 sensor on the passenger side. I have tested the TAB and TADs and they are functioning properly and the smog pump is putting a good amount of air. I evan went so far as to pull off the pipe going to the back of the cylinder heads and blow compressed air through it and found it was not clogged.
At this point the only solution I can see to this code is that the passages in the cylinder head itself is clogged, preventing the thermactor air from entering the exhaust system. Am I missing something? Where exactly does the thermactor air go once it enters the back of the cylinder head and where does it exit into the exhaust? Has anyone ever had this problem before and any suggestions on how to fix it without removing the cylinder head from the engine?
I know it only diverts when the engine is cold but having any trouble code is bugging me.
The o2 sensors have less than 5000 miles and I had the stock heads rebuilt before this problem started.

I have found some great info on this forum, including the following explanation of how the secondary air system functions.

Here's how the EGR and Thermactor Air System (smog pump) work with the stock Mustang computer...

Some basic theory to clarify how the EGR works is in order…

The EGR shuts off at Wide Open Throttle (WOT), so it has minimal effect on performance. The addition of exhaust gas drops combustion temperature, increases gas mileage and reduces the tendency of the engine to ping. It can also reduce HC emissions by reducing fuel consumption. The primary result of EGR usage is a reduction in NOx emissions.

The EGR system has a vacuum source (line from the intake manifold) that goes to the EVR, computer operated electronic vacuum regulator. The EVR is located on the back of the passenger side shock strut tower. The computer uses RPM, Load. and some other factors to tell the EVR to pass vacuum to open the EGR valve. The EGR valve and the passages in the heads and intake manifold route exhaust gas to the EGR spacer (throttle body spacer). The EGR sensor tells the computer how far the EGR valve is open. Then computer adjusts the signal sent to the EVR to hold, increase or decrease the vacuum. The computer adds spark advance to compensate for the recirculated gases and the slower rate they burn at.


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Troubleshooting:
There should be no vacuum at the EGR valve when at idle. If there is, the EVR (electronic vacuum regulator) mounted on the backside of the passenger side wheelwell is suspect. Check the vacuum line plumbing to make sure the previous owner didn’t cross the vacuum lines.

Diagram courtesy of Tmoss & Stang&2birds. (the diagram says 88 GT, but the EGR part is the same for 86-93 Mustangs)
88Stang5.0Vacuum.gif


The EGR sensor is basically a variable resistor, like the volume control on a radio. One end is 5 volt VREF power from the computer (red/orange wire). One end is computer signal ground (black/white), and the middle wire (brown/lt green) is the signal output from the EGR sensor. It is designed to always have some small voltage output from it anytime the ignition switch is the Run position. That way the computer knows the sensor & the wiring is OK. No voltage on computer pin 27 (brown/lt green wire) and the computer thinks the sensor is bad or the wire is broken and sets code 31. The voltage output can range from approximately .6-.85 volt.


EGR test procedure courtesy of cjones

to check the EGR valve:
bring the engine to normal temp.
connect a vacuum pump to the EGR Valve
apply 5in vacuum to the valve.
if engine stumbled or died then EGR Valve and passage(there is a passageway through the heads and intake) are good.
if engine did NOT stumble or die then either the EGR Valve is bad and/or the passage is blocked.
if engine stumbled, connect vacuum gauge to the hose coming off of the EGR Valve
snap throttle to 2500 RPM (remember snap the throttle don't hold it there).
did the vacuum gauge show about 2-5 in vacuum?

if not, check for manifold vacuum at the EGR vacuum valve.
if you have manifold vacuum then connect vacuum gauge to the EGR valve side of the vacuum valve and snap throttle to 2500 RPM.
should read about 2-5 in vacuum


Late Model Restoration has the Ford Racing M-12071-N302 kit with the EGR valve & sensor along with the ACT & ECT sensors for $45. See http://www.latemodelrestoration.com/iwwida.pvx?;item?item_no=M12071N302 1&comp=LRS for more details


Thermactor Air System (smog pump) operation...

The Thermactor air pump (smog pump) supplies air to the heads or catalytic converters. This air helps break down the excess HC (hydrocarbons) and CO (carbon monoxide). The air supplied to the catalytic converters helps create the catalytic reaction that changes the HC & CO into CO2 and water vapor. Catalytic converters on 5.0 Mustangs are designed to use the extra air provided by the smog pump. Without the extra air, the catalytic converters will clog and fail.

The Thermactor air pump draws air from an inlet filter in the front of the pump. The smog pump puts air into the heads when the engine is cold and then into the catalytic converters when it is warm. The Thermactor control valves serve to direct the flow. The first valve, TAB (Thermactor Air Bypass) or AM1 valve) either dumps air to the atmosphere or passes it on to the second valve. The second valve, TAD (Thermactor Air Diverter valve or AM2 valve) directs it to the heads or the catalytic converters. Check valves located after the TAB & TAD solenoids prevent hot exhaust gases from damaging the control valves or pump in case of a backfire. The air serves to help consume any unburned hydrocarbons by supplying extra oxygen to the catalytic process. The computer tells the Thermactor Air System to open the dump valve at WOT (wide open throttle) minimizing engine drag. The dump valve reduces the parasitic drag caused by the smog pump to about 2-4 HP at WOT.

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Computer operation & control for the Thermactor Air System
Automobile computers use current sink technology. They do not source power to any relay, solenoid or actuator like the IAC, fuel pump relay, or fuel injectors. Instead the computer provides a ground path for the positive battery voltage to get back to the battery negative terminal. That flow of power from positive to negative is what provides the energy to make the IAC, fuel pump relay, or fuel injectors work. No ground provided by the computer, then the actuators and relays don't operate.

One side of the any relay/actuator/solenoid in the engine compartment will be connected to a red wire that has 12-14 volts anytime the ignition switch is in the run position. The other side will have 12-14 volts when the relay/actuator/solenoid isn't turned on. Once the computer turns on the clamp side, the voltage on the computer side of the wire will drop down to 1 volt or less.

In order to test the TAD/TAB solenoids, you need to ground the white/red wire on the TAB solenoid or the light green/black wire on the TAD solenoid.

For 94-95 cars: the colors are different. The White/Red wire (TAB control) is White/Orange (Pin 31 on the PCM). The Green/Black wire (TAD control) should be Brown (pin 34 at the PCM). Thanks to HISSIN50 for this tip.

To test the computer, you can use a test light across the TAB or TAD wiring connectors and dump the codes. When you dump the codes, the computer does a self test that toggles every relay/actuator/solenoid on and off. When this happens, the test light will flicker.
 
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The codes 44 & 94 are not side specific. The best information I have is that difference in the two codes is where they inject the air. Code 44 is the TAB or bypass valve and 94 is the TAD or diverter valve.

Codes 44 & 94 - AIR system inoperative - Air Injection. Check vacuum lines for leaks, & cracks.
Disconnect the big hose from smog pump: with the engine running you should feel air output. Reconnect
the smog pump hose & apply vacuum to the first vacuum controlled valve: Its purpose is to either dump
the pump's output to the atmosphere or pass it to the next valve. The next vacuum controlled valve
directs the air to either the cylinder heads when the engine is cold or to the catalytic converter when the
engine is warm. Disconnect the big hoses from the back side of the vacuum controlled valve and start the
engine. Apply vacuum to the valve and see if the airflow changes from one hose to the next.

The two electrical controlled vacuum valves mounted on the rear of the passenger side wheel well turn the
vacuum on & off under computer control. Check to see that both valves have +12 volts on the red wire.
Then ground the white/red wire and the first solenoid should open and pass vacuum. Do the same thing to
the light green/black wire on the second solenoid and it should open and pass vacuum.

Remember that the computer does not source power for any actuator or relay, but provides the ground
necessary to complete the circuit. That means one side of the circuit will always be hot, and the other side
will go to ground or below 1 volt as the computer switches on that circuit.


The computer provides the ground to complete the circuit to power the solenoid valve that turns the
vacuum on or off. The computer is located under the passenger side kick panel. Remove the kick panel &
the cover over the computer wiring connector pins. Check Pin 38 Solenoid valve #1 that provides vacuum
to the first Thermactor control valve for a switch from 12-14 volts to 1 volt or less. Do the same with pin
32 solenoid valve #2 that provides vacuum to the second Thermactor control valve. Starting the engine
with the computer jumpered to self test mode will cause all the actuators to toggle on and off. If after
doing this and you see no switching of the voltage on and off, you can start testing the wiring for shorts to
ground and broken wiring. An Ohm check to ground with the computer connector disconnected & the
solenoid valves disconnected should show open circuit between the pin 32 and ground and again on pin 38
and ground. In like manner, there should be less than 1 ohm between pin 32 and solenoid valve #2 and pin
38 & Solenoid valve #1.

If after checking the resistance of the wiring & you are sure that there are no wiring faults, start looking at the
solenoid valves. If you disconnect them, you can jumper power & ground to them to verify operation. Power &
ground supplied should turn on the vacuum flow, remove either one and the vacuum should stop flowing.

See the following website for some help from Tmoss (diagram designer) & Stang&2Birds (website host)

http://www.veryuseful.com/mustang/tech/engine/images/fuel-alt-links-ign-ac.gif

http://www.veryuseful.com/mustang/tech/engine/images/88-91eecPinout.gif


If you have a catalytic converter H pipe, you need to fix these codes. If you don't, then don't worry about them
 
The codes 44 & 94 are not side specific. The best information I have is that difference in the two codes is where they inject the air. Code 44 is the TAB or bypass valve and 94 is the TAD or diverter valve.

Jricker, your posts on this have been the most educational I have found and I would really apreciate your insight. I used your info,as well as the info from Charles Probsts book, Ford fuel injection and Electronic engine control, to arrive at my current theory that the passenger side head is blocked and not allowing air from the smog pump to reach the exhaust system on that side.
I just redid these tests today and doublechecked my info to make sure of the results before I disagree with you.

The codes 44 & 94 are not side specific. .[/

Probst lists code 44 as right side specific and code 94 as left side specific. I realize other code manuals do not, but I think his are accurate. He also lists code 41 and code 91 as left and right side specific. this had imortant implications when I experimented with the TAD valve.

The best information I have is that difference in the two codes is where they inject the air. Code 44 is the TAB or bypass valve and 94 is the TAD or diverter valve

I experimented with my vacum guage and found this not to be true. I dont think the solenoids for the TAB and TAD are responsible for setting the code 44 and 94. I think the O2 sensors set these codes based on whether or not the PCM sees a lean condition when thermactor air is diverted upstream to the heads and into the exhaust system during KOER. When air is diverted down stream into the catalytic converters, this happens after the 02 sensors and the PCM has no sensor to detect this has occured.

These are the experiments I ran to confirm this.

As I mentioned earlier Im getting a code 44 KOER with everything connected and working as it should. I confirmed the TAB/TAD solenoids function with a vacum guage. During KOER, the solenoids applied Vacum when they where supposed to, also eliminating the possibility of a vacum leak anywhere in the Thermactor system.

I then ran a KOER test with the TAD vacum line disconected. This would default the thermactor air downstream to the catylitic converter during the entire KOER test. this resulted in a code 44 and a code 94, as expeirienced by everyone that runs aftermarket heads not drilled for smog. As i said earlier, these codes are set by the oxygen sensor not seeing the air injected through the heads into the exhaust.
Disconecting the TAB vacume line has the same effect, no thermactor air into the heads becase the TAB is defalting to venting the smog pump into the engine bay.
Therfore only a blockage in the air passage could produce a code 44 without a code 94

I got further comfirmation when I ran the KOER with vacum applied (With my trusty mightyvac) to the TAD valve through the entire test. this time I got a code 91 (HEGO indicates system lean (Left HO2S)) I understand this to be happening because the thermactor air is being routed through the head and into the exhaust system through the entire test and the o2 system is reading a lean condition the entire test from the extra thermactor air being present. I assume on a car with a properly functioning Thermactor system(without a blockage in the head somewhere like mine) a code 41(HEGO indicates system lean(right HO2S)) would accompany the code 91 under identical circumstances. Again, the only way a code 91 could occure without a code 41 is if there is a blockage.

Does this theory hold water or is there some other explanation for the results I have gotten?
 
sln583 said:
Probst lists code 44 as right side specific and code 94 as left side specific. I realize other code manuals do not, but I think his are accurate.
Examine the diagram for the Thermactor Air System I posted, or crawl under you car and see for yourself. There is no plumbing in the system that splits the smog pump output between sides. It is either into both heads or into the catalytic converters. Therefore codes 44 & 94 are not side specific. Probst, as good as he is, missed that one.
The O2 sensor codes are side specific since there is an O2 sensor in each side of the H pipe.

sln583 said:
I experimented with my vacum guage and found this not to be true. I dont think the solenoids for the TAB and TAD are responsible for setting the code 44 and 94.
A missing or open circuited TAB or TAD solenoid will set codes 81 & 82 respectively. However, if the TAB or TAD solenoid sticks and does not switch vacuum on and off, you can get the 44/94 code. That's why I say to remove the output hose from the TAB & TAD valves to check and see if the airflow changes directions when vacuum is applied to each control valve. Also check to see that the TAD & TAB output changes when the solenoid's computer control side wire is grounded.

To resolve any questions about blockages in the air tube that runs between the heads, remove the tube and run a plumbing snake or coat hanger through the tube. You should be able to make it exit both ports. Check the heads to make sure someone didn't leave a plug in one of the air tube ports. Both heads are the same, the air tube ports are on the front and rear surfaces of the heads. The front side gets a plug and the back side is left open for the air tube. If someone wasn't careful, they could easily put the head on with the air tube port plugged and facing the firewall.
 
Examine the diagram for the Thermactor Air System I posted, or crawl under you car and see for yourself. There is no plumbing in the system that splits the smog pump output between sides. It is either into both heads or into the catalytic converters. Therefore codes 44 & 94 are not side specific. Probst, as good as he is, missed that one.
I disagree, Maybe Im not doing a good job explaining this. The air tube that goes into the back of the heads is where the air is being split between sides. it goes into the right head on the right side and out into the right side exhaust manifold and then past the right side o2 sensor. It also it goes into the back of the left head and into the left side exhaust manifold and then past the left side oxegen sensor. Why would there be 2 codes for one problem. What sensor is the computer using to detect the code 44 and 94 if not the o2 sensors. I dont think Probst missed it, he just didnt explain it very well or pay much attention to Thermactor in his book because he thought of it as outdated technology I guess.

The O2 sensor codes are side specific since there is an O2 sensor in each side of the H pipe.
Exactly, and thats the same reason the trouble code for the thermactor air entering the heads, or lack therofe, is side specific. It enters the exhaust upstream from the O2 sensors and the 02 sensors, one on either side, detect its prescence or its absense.

A missing or open circuited TAB or TAD solenoid will set codes 81 & 82 respectively. However, if the TAB or TAD solenoid sticks and does not switch vacuum on and off, you can get the 44/94 code.
What do you think is telling the PCM that the TAB and the TAD arent switching, and causing the code 44/94 to be thrown?


That's why I say to remove the output hose from the TAB & TAD valves to check and see if the airflow changes directions when vacuum is applied to each control valve
.
I did that, and they are both switching, applying vacum to the TAB causes it to vent the air into the engine compartment. Applying Vacum to the TAD causes the air to flow into the back of the heads.
Also check to see that the TAD & TAB output changes when the solenoid's computer control side wire is grounded.
I verified that they are both activating.
To resolve any questions about blockages in the air tube that runs between the heads, remove the tube and run a plumbing snake or coat hanger through the tube. You should be able to make it exit both ports
.
This I did also, no blockages where present in the tube. I went so far as blowing it out with compressed air.
Check the heads to make sure someone didn't leave a plug in one of the air tube ports. Both heads are the same, the air tube ports are on the front and rear surfaces of the heads. The front side gets a plug and the back side is left open for the air tube. If someone wasn't careful, they could easily put the head on with the air tube port plugged and facing the firewall
That I have not considered, I will remove the pipe again and make sure there isnt a plug in the hole.
Where does the air come out of the head and into the exhaust manifold? through one exhaust port or all 4. Is it possible its clogged on that end?

I am going to convince you I am right about this.
Forget what you think you know about upstream Thermactor and a code 44/94 and look at it from a fresh perspective.
Again, thank you for your help.
 
I think the theory is still sound. My right side harness was melted and a couple of wires had been spliced in. I think the Right side o2 wasnt able to switch enough for the computer to detect the lean condition caused by the thermactor air, only on the right side. Im not sure exactly why I wasnt also getting a code 41, it seems the O2 sensor was working just enough not to throw that code. Or maybe the code 41 requires the O2 to do something, and it wasnt.
Maybe I can try blocking off the air passage on the back of just one of my heads and see if the code 44 trigers on its own.
 
On a side note, The only TAB code I know of is code 82. But there is nothing that detects the air being pushed into the atmosphere. So you only get a Circuit failure code. So the 02's have nothing to do with the TAB.

The TAD on the other hand, if it goes bad it will throw code 81 in a KOEO, but will throw code 94 and 44 in the KOER since both banks are affected by the one solenoid. So 94 and 44 are related to the TAD air flow, which is monitored by the 02s on either bank, and not the TAB solenoid and where it pushes the air.

This make the 94 and 44 codes side specific.
 
On a side note, The only TAB code I know of is code 82. But there is nothing that detects the air being pushed into the atmosphere. So you only get a Circuit failure code. So the 02's have nothing to do with the TAB.

The TAD on the other hand, if it goes bad it will throw code 81 in a KOEO, but will throw code 94 and 44 in the KOER since both banks are affected by the one solenoid. So 94 and 44 are related to the TAD air flow, which is monitored by the 02s on either bank, and not the TAB solenoid and where it pushes the air.

This make the 94 and 44 codes side specific.

No argument here.I will attempt to confirm this hypothesis.
 
I had to know, so I went out to the garage to remove all doubt abouy my theory. I warmed the car up, and then ran a KOEO and KOER. All tests passed with code 11. I then loosened the drivers side(I decided to use the drivers side (Code 94) as aposed to the passenger side (code 44) for 2 reasons 1. I have never gotten a code 94 before. and 2. the drivers side was easier to get at.) end of the thermactor crossover pipe from the back of the head and slid an index card in between to block the air coming out of the pipe and into the cylinder head, then I tightened down the bolt.
I then ran the KOER again, this time I got a code 94, Left side thermactor inoperative.
I removed the index card and ran the KOER and got a code 11 again.

There is no doubt left in my mind that the Code 44 and code 94 are side specific.
 
I learn something new every day. I will definitely take some time to study the "why it works that way" conclusions that have been arrived at here. Thanks for the input.

Joe R.