Emissions Problem - Very High Hc

Dart68 said:

Update -

The car finally has the correct CATs with a functioning secondary air injection system going to them. New O2 sensors, spark plugs, cap and rotor, oil and filter. There are no identifiable vacuum leaks and it passes all the tests I have thrown at it with the exception of the 44 and 94 codes. I got a tip from another forum to check for clogged air passages in the head to loosen the crossover pipe (which I removed and cleared previously) and start the engine and listen to see if exhaust gasses were coming from the loosened area of the crossover pipe. I got nothing so my conclusion is that the heads are blocked and that is what is setting the codes. It always passes the cylinder balance tests and no other codes have been set.

I did make sure that the air pump and valves are working correctly. If I may make a suggestion for anyone have problems in this area is to simply test the functionality of the air injection system. Just disconnect the hoses leading to the crossover and the CATs at the pipes. Start the car cold and check to see if air is coming from the hose that was connected to the crossover pipe. If yes, it's working cold. Now let the engine fully warm up and checking the hose going to the CATs, bring the RPM up and check to see if air is coming from that hose and none coming from the other hose. If it passes this simple test then you have effectively checked the entire air injection system from the air pump to the diverter valves including the two solenoids. If it doesn't pass, then it's time to check individual components. Please correct me if my conclusion in wrong.

I took the Mustang to be emissioned after all that and it failed again for the same, high HC. However, the reading was much lower, 3.74 ppm vs. 9.47 ppm (previously). The limit is 2.50 ppm. The spikes in the HC occur at around 20-25 mph then settle lower but not low enough for the average reading. Interestingly, the CO spikes a bit at the same time but drops quickly. This is a dynamometer test and its run twice with the same result both times. The HC stays highest as the car is slowly decelerating down to idle and doesn't return to near 0 until the car has been idling for a few seconds.

At this point I don't know what else to do. I am suspicious of the ECM in that it might be causing the high emissions some how. Sometimes the car will be idling normally around 850 and will suddenly idle up to 1500. The only way I can bring it back down is to shut if off for a minute.

Anyone got any TNT? HELP!

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That's a sign of a TPS failure either due to a bad TPS sensor or loss of the TPS signal ground. Dump the codes and see it you get a TPS code 53 or 23

High HC – Lean misfire, vacuum leak, common misfire due to worn or weak ignition system components. On rare occasions, an overly rich mixture may be the cause. Do the ethanol/E10 fill up as suggested.


Check the O2 sensor voltages at the computer. Your O2 sensors may be marginal, but not bad enough to se the code 41/91
Code 41 or 91. Or 43 Three digit code 172 or 176 - O2 sensor indicates system lean. Look for a vacuum leak or failing O2 sensor.

Revised 11-Jan-2015 to add check for fuel pressure out of range

Code 41 is the passenger side sensor, as viewed from the driver's seat.
Code 91 is the driver side sensor, as viewed from the driver's seat.

Code 172 is the passenger side sensor as viewed from the driver's seat.
Code 176 is the driver side sensor, as viewed from the driver's seat.

Code 43 is not side specific according to the Probst Ford Fuel injection book.

The computer sees a lean mixture signal coming from the O2 sensors and tries to compensate by adding more fuel. Many times the end result is an engine that runs pig rich and stinks of unburned fuel.

The following is a Quote from Charles O. Probst, Ford fuel Injection & Electronic Engine control:
"When the mixture is lean, the exhaust gas has oxygen, about the same amount as the ambient air. So the sensor will generate less than 400 Millivolts. Remember lean = less voltage.

When the mixture is rich, there's less oxygen in the exhaust than in the ambient air , so voltage is generated between the two sides of the tip. The voltage is greater than 600 millivolts. Remember rich = more voltage.

Here's a tip: the newer the sensor, the more the voltage changes, swinging from as low as 0.1 volt to as much as 0.9 volt. As an oxygen sensor ages, the voltage changes get smaller and slower - the voltage change lags behind the change in exhaust gas oxygen.

Because the oxygen sensor generates its own voltage, never apply voltage and never measure resistance of the sensor circuit. To measure voltage signals, use an analog voltmeter with a high input impedance, at least 10 megohms. Remember, a digital voltmeter will average a changing voltage." End Quote

Testing the O2 sensors 87-93 5.0 Mustangs
Measuring the O2 sensor voltage at the computer will give you a good idea of how well they are working. You'll have to pull the passenger side kick panel off to gain access to the computer connector. Remove the plastic wiring cover to get to the back side of the wiring. Use a safety pin or paper clip to probe the connections from the rear.

Disconnect the O2 sensor from the harness and use the body side O2 sensor harness as the starting point for testing. Do not measure the resistance of the O2 sensor , you may damage it. Resistance measurements for the O2 sensor harness are made with one meter lead on the O2 sensor harness and the other meter lead on the computer wire or pin for the O2 sensor.

Backside view of the computer wiring connector:


87-90 5.0 Mustangs:
Computer pin 43 Dark blue/Lt green – LH O2 sensor
Computer pin 29 Dark Green/Pink – RH O2 sensor
The computer pins are 29 (L\RH O2 with a dark green/pink wire) and 43 (LH O2 with a dark blue/pink wire). Use the ground next to the computer to ground the voltmeter. The O2 sensor voltage should switch between .2-.9 volt at idle.

91-93 5.0 Mustangs:
Computer pin 43 Red/Black – LH O2 sensor
Computer pin 29 Gray/Lt blue – RH O2 sensor
The computer pins are 29 (LH O2 with a Gray/Lt blue wire) and 43 (RH O2 with a Red/Black wire). Use the ground next to the computer to ground the voltmeter. The O2 sensor voltage should switch between .2-.9 volt at idle.


Testing the O2 sensors 94-95 5.0 Mustangs
Measuring the O2 sensor voltage at the computer will give you a good idea of how well they are working. You'll have to pull the passenger side kick panel off to gain access to the computer connector. Remove the plastic wiring cover to get to the back side of the wiring. Use a safety pin or paper clip to probe the connections from the rear. The computer pins are 29 (LH O2 with a red/black wire) and 27 (RH O2 with a gray/lt blue wire). Use pin 32 (gray/red wire) to ground the voltmeter. The O2 sensor voltage should switch between .2-.9 volt at idle.


Note that all resistance tests must be done with power off. Measuring resistance with a circuit powered on will give false readings and possibly damage the meter. Do not attempt to measure the resistance of the O2 sensors, it may damage them.

Testing the O2 sensor wiring harness
Most of the common multimeters have a resistance scale. Be sure the O2 sensors are disconnected and measure the resistance from the O2 sensor body harness to the pins on the computer. Using the Low Ohms range (usually 200 Ohms) you should see less than 1.5 Ohms.

87-90 5.0 Mustangs:
Computer pin 43 Dark blue/Lt green – LH O2 sensor
Computer pin 29 Dark Green/Pink – RH O2 sensor
Disconnect the connector from the O2 sensor and measure the resistance:
From the Dark blue/Lt green wire in the LH O2 sensor harness and the Dark blue/Lt green wire on the computer pin 43
From the Dark Green/Pink wire on the RH Os sensor harness and the Dark Green/Pink wire on the computer pin 29

91-93 5.0 Mustangs:
Computer pin 43 Red/Black – LH O2 sensor
Computer pin 29 Gray/Lt blue – RH O2 sensor
Disconnect the connector from the O2 sensor and measure the resistance:
From the Red/Black wire in the LH O2 sensor harness and the Red/Black wire on the computer pin 43
From the Dark Green/Pink Gray/Lt blue wire on the RH Os sensor harness and the Gray/Lt blue wire on the computer pin 29

94-95 5.0 Mustangs:
Computer pin 29 Red/Black – LH O2 sensor
Computer pin 27 Gray/Lt blue – RH O2 sensor
From the Red/Black wire in the LH O2 sensor harness and the Red/Black wire on the computer pin 29
From the Dark Green/Pink Gray/Lt blue wire on the RH Os sensor harness and the Gray/Lt blue wire on the computer pin 27

There is a connector between the body harness and the O2 sensor harness. Make sure the connectors are mated together, the contacts and wiring are not damaged and the contacts are clean and not coated with oil.

The O2 sensor ground (orange wire with a ring terminal on it) is in the wiring harness for the fuel injection wiring. I grounded mine to one of the intake manifold bolts

Check the fuel pressure – the fuel pressure is 37-41 PSI with the vacuum disconnected and the engine idling. Fuel pressure out of range can cause the 41 & 91 codes together. It will not cause a single code, only both codes together.

Make sure you have the proper 3 wire O2 sensors. Only the 4 cylinder cars used a 4 wire sensor, which is not compatible with the V8 wiring harness.

Replace the O2 sensors in pairs if replacement is indicated. If one is weak or bad, the other one probably isn't far behind.

Code 41 can also be due to carbon plugging the driver’s side Thermactor air crossover tube on the back of the engine. The tube fills up with carbon and does not pass air to the driver’s side head ports. This puts an excess amount of air in the passenger side exhaust and can set the code 41. Remove the tube and clean it out so that both sides get good airflow: this may be more difficult than it sounds. You need something like a mini rotor-rooter to do the job because of the curves in the tube. Something like the outer spiral jacket of a flexible push-pull cable may be the thing that does the trick.

If you get only code 41 and have changed the sensor, look for vacuum leaks. This is especially true if you are having idle problems. The small plastic tubing is very brittle after many years of the heating it receives. Replace the tubing and check the PVC and the hoses connected to it.

You said that it passed the cylinder balance test, so that pretty much eliminates the ignition as being a possible source.

If the purge canister valve is stuck open or someone removed it and hooked the tubing back together, it would cause high HC,. The engine would be constantly sucking gas fumes from the tank.

High HC can also be caused by a lean misfire. That can be caused by vacuum leaks from a multitude of places or the failure to cap the odd vacuum lines where someone decided to disable the emissions equipment. That includes the evaporative vent system plumbing described below; a break in the line or loose or missing connection will be a vacuum leak.

See the info below for more...



Code 85 CANP solenoid - The Carbon Canister solenoid is inoperative or missing.

Revised 11 –Jan_2015 to add warning about vacuum leaks due to deteriorated hose or missing caps on vacuum lines when the solenoid is removed.

Check vacuum lines for leaks and cracks. Check electrical wiring for loose connections, damaged wiring and insulation. Check solenoid valve operation by grounding the gray/yellow wire to the solenoid and blowing through it.
The computer provides the ground for the solenoid. The red wire to the solenoid is always energized any time the ignition switch is in the run position.

If you disconnected the carbon canister and failed to properly cap the vacuum line coming from under the upper intake manifold, you will have problems. You will also have problems if the remaining hose coming from under the upper intake manifold or caps for the vacuum line are sucking air.

Charcoal canister plumbing - one 3/8" tube from the bottom of the upper manifold to the rubber hose. Rubber hose connects to one side of the canister solenoid valve. Other side of the solenoid valve connects to one side of the canister. The other side of the canister connects to a rubber hose that connects to a line that goes all the way back to the gas tank. There is an electrical connector coming from the passenger side injector harness near #1 injector that plugs into the canister solenoid valve. It's purpose is to vent the gas tank. The solenoid valve opens at cruse to provide some extra fuel. The canister is normally mounted on the passenger side frame rail near the smog pump pulley.



It does not weigh but a pound or so and helps richen up the cruse mixture. It draws no HP & keeps the car from smelling like gasoline in a closed garage. So with all these good things and no bad ones, why not hook it up & use it?


The purge valve solenoid connector is a dangling wire that is near the ECT sensor and oil filler on the passenger side rocker cover. The actual solenoid valve is down next to the carbon canister. There is about 12"-16" of wire that runs parallel to the canister vent hose that comes off the bottom side of the upper intake manifold. That hose connects one port of the solenoid valve; the other port connects to the carbon canister.

The purge valve solenoid should be available at your local auto parts store.

Purge valve solenoid:



The carbon canister is normally mounted on the passenger side frame rail near the smog pump pulley.
Carbon Canister:

If timing is bouncing around, I may be wrong but sounds like distributor related. The shaft has no play? The spark advance is built into the thick film module, right? Possible advance problem in their. You can pull it, oreillys has a tester. If ok put heat sink grease reinstall. And 3 rd you can test pick up coil in distributor. Sorry I just have I phone can't send test procedure. One of the other guys probably have specs. If you have a factory cd/ manual. I think 800 ohms is for a good one, but don't quote me. The last question is your using light with spout out? I'm sure you are. Let us know what you find. Moose!


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Oh yeah, the other thing you might want to check is a worn timing chain. Pull distributor cap, put a socket and ratchet on lower pulley. See how long it takes for rotor to turn. A lot of play in chain can cause problems. It should turn right away.


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Moosee1955 said:

If timing is bouncing around, I may be wrong but sounds like distributor related. The shaft has no play? The spark advance is built into the thick film module, right? Possible advance problem in their. You can pull it, oreillys has a tester. If ok put heat sink grease reinstall. And 3 rd you can test pick up coil in distributor. Sorry I just have I phone can't send test procedure. One of the other guys probably have specs. If you have a factory cd/ manual. I think 800 ohms is for a good one, but don't quote me. The last question is your using light with spout out? I'm sure you are. Let us know what you find. Moose!


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Wrong answers again.

The TFI module testers at the auto parts stores are next to useless. Ask @DuderMcMerican about his misadventures with TFI modules and the auto parts stores people that tested them. It's a sad soap opera; most of the employees haven't got a clue what a TFI module is or how to properly use the tester and how to tell when the tester isn't working.

The computer controls the spark advance. That's why the SPOUT is present; remove the SPOUT and you get no spark advance signal from the computer. With no SPOUT, the timing is locked at whatever you set it when the distributor was installed and you tightened down the bolt that holds it in place.
With the SPOUT in place, the timing will bounce around several degrees when the engine is at idle. I have personally watched it bounce around on my on 5.0 Mustang when I stuck the SPOUT back in to confirm that the spark advance was working.

Well everything I was taught years ago about electronic ignition and tfi modules was wrong. And this guy was no gomer. I like your explanation, makes sense. So tell me Professor, what besides giving spark, does the thick film module actually do on Ford vehicles. I'm all ears!


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Moosee1955 said:

Well everything I was taught years ago about electronic ignition and tfi modules was wrong. And this guy was no gomer. I like your explanation, makes sense. So tell me Professor, what besides giving spark, does the thick film module actually do on Ford vehicles. I'm all ears!


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You asked for it.
Tmoss's explanation is from and Electrical Engineer's standpoint and gives some excellent insight as to how it works.
Tmoss’s explanation of how TFI modules work
OK guys here is the straight skinny........If uyour TFI is gray - buy a gray one..........NOTHIN TO DO WITH AUTO OR MANUAL........

Most technicians who deal with Ford drivability and no-start problems have become very familiar with the Thick Film Ignition (TFI) system. Ford started using the six-pin TFI module with the EEC-IV computer system in 1983, and for years it remained basically unchanged.

The early TFI system, which Ford calls the "Push Start" TFI system, uses a gray TFI module. Originally, the module was mounted on the distributor. In the late '80s, Ford began to relocate it away from the distributor on some vehicles to provide better protection from the effects of engine heat, but system operation remained the same. It uses a Hall effect pickup (stator) in the distributor, which generates a battery voltage, 50% duty cycle square wave, called the PIP signal, to the EEC-IV PCM and the TFI module. The PCM processes this signal and sends out another battery voltage, 50% duty cycle square wave, called the SPOUT signal, to the TFI module. As long as the TFI module is receiving a SPOUT signal, it will fire the coil at the rising edge of that signal (except during engine cranking, when SPOUT is ignored) and the vehicle will run with the amount of timing advance commanded by the computer. If the TFI module does not receive the SPOUT signal, it will fire the coil at the rising edge of the PIP signal, and the vehicle will run at base timing. This is true on all TFI systems.

Ignition dwell with the Push Start (gray module) system is controlled by the TFI module alone, and increases with engine rpm. The Ignition Diagnostic Monitor (IDM) signal on a Push Start TFI system comes from the coil negative circuit and is filtered through a 22k ohm resistor to pin #4 on the EEC-IV computer. The computer monitors this circuit to verify a coil firing for each PIP signal, and sets codes if it sees missing or erratic signals. Another feature that is unique to the Push Start TFI system is the start input on pin #4 of the module connector. This is wired into the starter relay trigger circuit, and signals the TFI module that the engine is cranking. When the module sees battery voltage on this circuit, the SPOUT signal is ignored.

In the early '90s, Ford began using a different TFI system on certain vehicles -- the Computer Controlled Dwell (CCD) TFI system. The TFI module on CCD TFI is always black in color. There are a few major differences between the two systems. As the name implies, with the CCD system, the computer controls primary dwell. The CCD TFI module still ungrounds (fires) the coil at the rising edge of the SPOUT signal, but now the falling edge of the SPOUT signal (which had no meaning to the Push Start TFI module) is used by the CCD TFI module to ground the coil. The PIP signal remains the same 50% duty cycle square wave, but SPOUT signal duty cycle varies according to how much dwell is desired by the computer.

Another major difference between the two systems is the IDM circuit. Pin #4 on the CCD TFI module, which was the start circuit input on the Push Start TFI module, is now the IDM signal, sent directly from the TFI module to pin #4 on the EEC-IV computer. This signal is still a filtered (low voltage) version of the ignition primary waveform, but is filtered internally in the TFI module rather than through an external resistor. There isn't any start circuit input to the CCD TFI module; the module infers engine cranking from a low rpm input from the PIP signal.
Since these two TFI systems are so significantly different, yet so similar in appearance, parts application problems will inevitably occur. A gray Push Start TFI module will plug right into a CCD system, and vice versa. To make matters worse, parts books are often incorrect on TFI module applications! With the incorrect TFI module installed, the vehicle will run, but drivability and MIL (malfunction indicator lamp) problems will result. For instance, if a gray Push Start TFI module is installed in a CCD system, the computer will not be able to control ignition dwell, and the MIL will illuminate with memory codes for the IDM circuit set, as the gray TFI module is incapable of generating an IDM signal to the computer. If a black CCD TFI module is installed in a Push Start system, dwell will remain fixed, since the SPOUT signal duty cycle never changes. If in doubt about which TFI module belongs on a particular vehicle, consult the ignition system-wiring diagram for the vehicle. If the wire going to pin #4 on the EEC-IV computer comes directly from pin #4 of the TFI module, it is a CCD system. If not, it is a Push Start system.