Another "pulled these codes" post...a little help???


New member
Nov 30, 2009
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Laredo TX
Hello all.'s the story.  My kiddo got the Bronco for his graduation and he is experiencing some strange problems.  We rebuilt this 5.0 engine to stock specs about 2 years ago and over the summer it has been experiencing some issues that i think he might have induced.  He goes to school in another city and the 4x4 club buddies told him "put this on, put that on, take that emissions crap off for more hp" etc etc.  Sounds good and I'm actually glad these young people are getting in there and doing things mechanically...keeps the craft going if you guys no what i mean.  Unfortunately, most of them can't afford to undo their messups when they are done and need dear old dad to get them out of it.  So, here we are.  No CEL but a rough idle with significant vibration.  I'm thinking vacuum leak because I recognize a missing engine due to a vacuum leak when i see one.  WRONG...probably.  Pulled codes even though no CEL and got the following KOER:


He took off last night back to school and took the KOEO codes that i wrote down but I remember 33.  So still suspecting vacuum leak, i smoked the engine through the brake booster line and see the smoke coming out of the egr valve. This made some sense to me because if it is sucking air through the diaphragm that would explain the 41 engine lean code.  Replaced it and egr position sensor...settled it down some but still not back to normal.  I'm convinced that the egr issue is fixed but i didn't repull the codes to check.  I know the EGR vac solenoid is left but some carb cleaner to it did not find anything.  I may replace a 26 year old component anyway to make EGR circuit new.

The 81 code is my second concern...he took off the stock walker exhaust and cat and put on some magnaflow universal cat that DOES NOT have the connection for the secondary air diverter valve to the cats.  He is a welding student and i told him to weld a ****** on the exhaust pipe and reconnect the hose.  from my understanding this valve will be commanded to open when the TAD solenoid energizes and sends vacuum to the valve and opens to send the smog pump air to the cats instead of to the heads. This will prevent overheating in the exhaust system.   Is that right?  If so, any ideas on this code?  Thank you all for reading this long post...We repaired so many things this weekend and its good to see my son wrenching on this instead of staring at a tv, phone, or gaming system.  In advance, thanks for any assistance provided.



96 Bronco 5.0
Staff member
Oct 18, 2005
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Floating in the Pacific
Yo Edmund,

For DTC 81, see my reply @

Thermactor System (Smog/ Air Pump) Operational Description & pic; "...This pumps fresh air into the exhaust system, to burn left over hydrocarbons, lowering emissions. The computer uses 2 air valves (TAB & TAD) to control where the air flows depending on engine operation. .



Thermactor Air Bypass Valve (TAB) aka Air Control Valve; "...based on vacuum input from the Thermactor Air Bypass Solenoid, and the Thermactor Air Diverter Solenoid, this valve controls the flow of air from the air pump to the exhaust manifold and the catalyst..."


Thermactor Air Diverter Solenoid (TAD, AIRD, AM2); " controlled by the EEC-IV computer and provides vacuum to the Air Bypass, aka Air Control Valve. With vacuum present, air flows to the exhaust manifold. With no vacuum, air flows from the air pump to the catalyst..."

Thermactor Air Bypass Solenoid (TAB, AIRB, AM1); " controlled by the EEC-IV computer and provides vacuum to the Air Bypass Valve. With vacuum present, air flows through the secondary air injection system. With no vacuum, the air from the air pump is dumped to the atmosphere. Essentially, this turns the secondary air injection system on or off. The secondary air injection system adds air to the catalytic converter to improve the burning of NOx gases. This reduces emissions..."

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.." MIESK5 NOTE; If Engine Coolant Temperature Sensor (ECT) is Below 50 degrees F, then TAB is grounded & sends air to atmosphere; If Between 50 and 190 Degrees F, Bypass Valve sends air to Diverter and to Manifold; If Over 190 Degrees F, it is in closed loop & Air goes to Catalytic Converter; Bypass when at idle/Wide Open Throttle (WOT), and with failing Oxygen Sensor. The fastest way to see if vehicle is in open loop is to see where the air is going; to Catalytic Converter, it is in Closed Loop; to Atmosphere or Manifold it is in Closed Loop, provided the thermactor system is working; miesk5 Note, Air for the Thermactor system is cleaned by means of a centrifugal filter fan mounted on the air pump driveshaft. The air filter does not require any type of replaceable element. To prevent excessive pressure, the air pump is equipped with a pressure relief valve which uses a replaceable plastic plug to control the pressure setting. The Thermactor air pump has sealed bearings which are lubricated for the life of the unit and preset rotor vane and bearing clearances, which do not require any periodic adjustments. The air supply from the pump is controlled by the air by-pass valve, sometimes called a dump valve. During engine deceleration, the air by-pass valve opens, momentarily diverting the air supply through a silencer and into the atmosphere, thus preventing backfires within the exhaust system when deceleration supplies larger-than-normal amounts of unburned fuel to the exhaust ports. Check(s) valve is incorporated in the air inlet side of the air manifolds. Its purpose is to prevent exhaust gases from backing up into the Thermactor system. This valve is especially important in the event of drive belt failure and also during deceleration, when the air by-pass valve is dumping the air supply. The air manifolds and air supply tubes channel the air from the Thermactor air pump into the exhaust ports of each cylinder, thus completing the cycle of the Thermactor system. Beginning in 1968, a conspicuous decal listing all tune-up specifications which contribute to the effectiveness of the emission control equipment is located in the engine compartment..." &

"...And for the fuel that wasn't burned in the combustion chamber, we need extra airflow pumped into the exhaust system. This air with the heat of the exhaust creates further break down of HC, CO, and Nox into CO2, H2O, and N2. The catalytic converter can accept all of the airflow without fear of over heating during cruise. We need hot exhaust gasses to help complete combustion and converter operation. Air is pumped into the converter, but will dump to the atmosphere after several minutes to prevent overheating the converter. The converter is cooled by air passing under the vehicle..." by Ryan M.


33 EGR valve opening not detected. DTC 33 is triggered when the EVP sensor is not closing;

Testing & Operation; "...The EGR Valve Position (EVP) sensor monitors the position of the EGR valve pintle. The EVP sensor converts the mechanical movement of the pintle into an electrical voltage signal which is relayed to the PCM. The EVP sensor is a linear potentiometer in which resistance varies with the EGR valve pintle movement.

Voltage is fed to the EVP by the signal return circuit. As the EGR is opened the EVP directs more voltage to the EEC and less down the voltage reference circuit. The EVP sensor provides the PCM with information on EGR flow and EGR system failures. The EVP should read between 0.24 and 0.67 volts at idle with a closed EGR valve..." read more

Source: by Ryan M (Fireguy50) at excerpts;

Damaged EVP sensor

•Corroded or dirty connector

•Damaged EGR valve

•Faulty Vacuum system

•Broken wire in harness

•Grounded harness

•Damaged Computer

I know you did the vac leak test, but pull vac hose off @ EVP - I pull em off and use the straw sucking test; one finger over one end; and... ya get the idea?

I will reply to the other codes later if you need the info, let me know.

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