Question about 1982 Porsche 911

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Fuel Mixture I have just failed emissions inspection -- NOx too high and HC very low. No CO. Believe I am running too lean. How do I adjust the fuel mixture?

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I'll do some research on this one and get back to you. I'm not really familiar with the Porsche carbs.

Posted on Sep 27, 2008

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How fix that problem


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Nitrous Oxide NO is created when an engine's combustion chamber temperature reaches over 2500F. 1. Lean Fuel Mixture - Lean fuel mixtures cause high NOx. A lean fuel mixture exists when less fuel then required is delivered to the combustion chambers or when more air then necessary is added to the fuel. In either case the lack of gasoline needed to cool the combustion chambers down is not present. Combustion temperatures increase causing high nitrous oxide emissions. A lean fuel condition may be due to a vacuum leak/s and/or defective fuel control components, such as the Air Flow Meter, Engine Coolant Temperature Sensor, and O2 sensors.
2. Defective EGR System - The Exhaust Gas Recirculation system is designed to reduce NO. The EGR system consists of an EGR valve, EGR pressure sensor, vacuum hoses, and one or more vacuum switching valves or solenoids. newer vehicles may use an electronically controlled EGR valves, which do not require vacuum lines or switching solenoids.
The EGR system's job is to re-route a small amount of exhaust gas back into the intake manifold to help reduce combustion chamber temperatures. As mentioned above NOx is created when combustion chamber temperatures reach above 2500F.
By recirculating exhaust gas back into the intake, a small amount of the air/fuel mixture is replaced with inert gas, reducing combustion temperatures.
3. Defective Catalytic Converter Some vehicles operate without EGR valves. Non-EGR equipped vehicles rely heavily on the Catalytic Converter to assist in the reduction of NO. These vehicles have tendencies to develop CAT problems sooner then those which are equipped. If you own a non-EGR equipped vehicle, and have failed the emissions test for high NOx, pay close attention to the Catalytic Converter.
4. High Engine Mileage - Over an engine's lifetime, carbon build-up develops in the engine's combustion chambers. The more miles on your engine, the more carbon build-up on the pistons, cylinder heads and valves. Carbon build-up decreases the available space for the air/fuel mixture to combust, and causes higher cylinder compression. High compression results in high temperatures and high NOx. Keep in mind this problem is usually seen in vehicles with over 150,000 miles which have been poorly maintained. The solution to this problem is called De-Carbonizing. It will remove a good amount of carbon out of an engine. This will increase combustion space, lower compression and lower NOx.
5. Engine Overheating - Inadequate engine cooling can will high NOx. If your vehicle's cooling system is not working efficiently, high NOx will be created. Remember high NOx nitrous oxide is created when an engine's combustion chamber temperatures reach over 2500F. You will want to make sure your vehicle's cooling system is working properly, and your vehicle's temperature gauge is always indicating normal.

Carbon Monoxide is a by-product of incomplete combustion. Carbon Monoxide exceeding maximum limits, can be due to a number of emission failures ranging from inadequate air intake to defective engine computer sensors. This condition is referred to as a "Rich Fuel Conditon".
1. Dirty Air Filter - The number one overlooked emissions component, yes, "emissions" component is the engine air filter. A dirty air filter will absolutely restrict air flow, thus disturbing the proper 14.7:1 air/fuel ratio required for optimum fuel combustion.
2. Faulty Oxygen Sensor The Oxygen Sensor is responsibly for delivering information to the ECU or ECM relating to the oxygen content in the exhaust stream after it has left the combustion chambers.
The engine control computer will determine how much fuel to inject into the combustion chambers based on this data. The more oxygen in the stream, the more fuel the computer will deliver, and visa-versa. A defective O2 sensor will cause increased carbon monoxide emissions.
3. Defective Manifold Absolute Pressure - The MAP sensor determines the level of vacuum created during an engine's intake stroke, and sends this information to the ECU. During low vacuum the MAP sensor assumes the engine's throttle is in some degree open, meaning you've stepped on the pedal. It relays this information to the ECU. The ECU, in turn, sends commands to the fuel injectors, or carburetor, to increase fuel delivery.
A defective MAP sensor will not report the correct information to the ECU, thus disturbing air/fuel ratio. Usually when the ECU senses a defective MAP sensor it will learn to ignore its data, and rely on preset values, and other sensors such as the Throttle Position Sensor, and Engine Coolant Temperature Sensor; Fuel delivery will not be as accurate and high CO may result.
4.Defective Throttle Position Sensor - Obviously a very important emissions sensor; the TPS relays information regarding the position of the air intake system's throttle plate. The throttle plate, located after the engine air filter and before the intake manifold controls the amount of air entering the combustion chambers. It is usually manipulated by the gas pedal via a cable. On late model vehicles the throttle plate may be controlled electronically. A defective throttle position sensor will confuse the ECU into thinking the vehicle's operator is demanding more or less fuel, when neither is really neccessary. Most often a faulty TPS will cause high CO, as an engine's ECU always prefers to send more fuel rather then less, in an effort to avoid a lean fuel mixture and subsequently higher engine temperatures.5. Defective Engine Coolant Temperature Sensor - Low engine temperature requires more fuel. When the ECU is unable to determine what the engine's accurate temperature is, it will not adjust fuel delivery properly; resulting in high CO. As explained above, the Engine Control Computer prefers to send more fuel rather then less to avoid a lean fuel mixture.

Hydrocarbon HC. Hydrocarbons are basically raw fuel, otherwise known as Gasoline. High Hydrocarbon (HC) emissions are almost always a sign of poor fuel ignition. However, it's not always that the engine's ignition system is responsible for high Hydrocarbon emissions.1. Improper Ignition Timing - Engine ignition timing is measured in degrees before or after Top Dead Center. Example of an ignition timing failure would be in the case where an engine's ignition timing is required to be set at 10 degrees Before Top Dead Center and instead is set to 15 degrees BTDC. This fault will not only cause a smog check "functional failure", but will increase Hyrdocarbon (HC) emissions as well. California allows 3 degrees +/- off of the manufacturer's required setting. Newer vehicle's may not have a distributor, and and no timing adjustment will be needed. On these engines timing is electronically controlled by the ECU.
2. Defective Ignition Components Your vehicle's ignition system consists of the ignition coil/s, distributor, distributor cap, distributor rotor, ignition wires, and spark plugs. If any of these components are defective the engine will produce high hydrocarbons. A common reason ignition components perform poorly is due to carbon build-up. High ignition voltage traveling through the air pockets within these components form carbon. Carbon acts as an insulator between paths of electricity, decreasing the energy required at the spark plug to ignite the air/fuel in the combustion chambers properly.
3. Lean Fuel Mixture - Any condition which will cause unmetered air to enter the intake manifold, and ultimately the combustion chambers, will cause high hydrocarbons (HC). This condition is called a lean miss-fire. Such faults as vacuum leaks and gasket leaks will cause lean fuel/air mixtures. Broken, disconnected or misrouted vacuum hoses will do the same. It is also important to note that many engine components rely on engine vacuum for proper operation. If any of these components are defective, externally or internally, they may cause large vacuum leaks as well.
4. Defective Catalytic Converter - A defective catalytic converter may be responsible for high HC, CO, and NOx emissions. The Catalytic Converter, commonly referred to as the CAT is a component designed to continue the combustion process within itself and emit a more thoroughly burned and less harmful emissions containing exhaust. The most accurate way to find out if your vehicle's CAT is working efficiently is by using an exhaust gas analyzer. Unfortunately this tool is fairly expensive.
Some obvious symptoms of a bad CAT could be any of the following:
a. Major loss of power over 15-25 mph. This may be an indication that the catalytic converter is plugged up and restricting exhaust flow.
b. Strong sulfer or rotten egg smell emitting from the exhaust on an otherwise good running vehicle. This may be an indication that the Catalytic Converter isn't burning fuel completely, instead storing it, then releasing it as hydrogen sulfide.
c. Loud rattle being heard from inside the CAT. This may indicate a broken Catalytic Converter substrate. You may want to insure this sound is not due to loose exhaust components.
5. Defective Air Injection Components - Faulty smog pump and related emissions system components will cause high HC. The air injection system is designed to introduce additional oxygen, after the metering system, to the engine exhaust as it exits the exhaust manifold, or directly before it enters the Catalytic Converter; thus burning whatever remaining fuel (HC) in the exhaust completely.
6. Low Cylinder Compression - This fault is one of the less common high HC causing problems. Reasons an engine may have low or no compression in one or more of its cylinders may include things such as burned intake or exhaust valve/s, defective valve guides and/or seals, defective piston rings, and burned head gasket/s. A wet/dry cylinder compression test will diagnose this fault. More then often if such a problem exists it will be very apparent. You should notice rough idle.

Feb 19, 2015 | Cars & Trucks

1 Answer

I failed emissions test. The HC and CO both failed. I was told by a family member that it sounds like the catalytic converter and I wanted to know how much that would cost or estimated cost to replace


Rich fuel mixture is leaking fuel injectors,failed oxygen sensors

Need clean air filter,new spark plugs

An Exhaust Converter is only good for 12 to 14 years max then
it has to go

This is a social web site we use as you do so prices are are over the internet less labor of coarse

Mar 02, 2013 | 1997 Mercury Cougar XR7

1 Answer

1994 cadillac sedan 4.9 engine will not pass smog. Noxious too high.i replaced egr, erg solenoid,changed oil, still nox is high.hc and co pass


The HC is high because the CO is high, CO is caused by to much fuel being delivered. You will want to check the Oxygen sensor and make sure it is functioning properly. You will want to check the fuel pressures, including rest pressure. We have seen leaking injectors cause this problem. You will want to make sure the thermostat is working correctly also.

First thing to do is check over the sensors that control the fuel mixture as it sounds like you're runnign too rich. This could be due to a faulty water temp sensor. If this is the problem then the ECU (Engine Control Unit / aka computer) will be leaving the car constantly 'on choke' and not reducing the fuelling as the engine warms - so unplug the sensor and clean the contacts on both sides with some switch / electrical cleaner (not WD40) and inspect the wiring for any signs of damage to the insulation especially where it runs over other components and through the bulkhead

If the engine is running well and the state of tune and ignition timing is good (15 degrees BTDC) then most likely the converter itself is bad. Usually NOx readings will go down as HC and CO readings go up, and as a result it is extremely rare for a car to fail an emissions test with all three things if the Catalytic Converter is working properly.

A bad oxygen sensor can also cause a failure similar to this, but will usually cause HC/CO readings to be high (but NOx readings would be low), or the opposite - high NOx but low HC/CO readings. The O2 sensor is located in the exhaust manifold just before the bulge of the catylst and at the point where the 4 pipes join together.

Unfortunately there is no good way to test the Cat. Highly specialized equipment is necessary to do this, however a temperature test might clue you in to one that is bad.

After driving the vehicle for several miles, immediately check the temperature of the converter at the very front and the very rear of the unit itself. The rear should be at least 100 degrees (F) hotter than the front. If the two are close to the same temperature then it is proof the converter isn't working very well.

I have to note though that temperature testing is far from conclusive. I've seen converters that pass test with flying colors but still ended up needing to be replaced.

Aftermarket converters tend to be less effective than factory converters because they are not built specifically for your vehicle. Instead, a universal unit is welded into some exhaust pipes so that it will fit. Generally this isn't an issue, but if you live in a 'green' state such as California where emissions standards are very strict, it could pose a problem.

Keep us updated.

Aug 02, 2011 | 1994 Cadillac DeVille

1 Answer

I have 1993 pontiac bonnevile, it will not pass emissions the nox levels are to high this is the fourth try. what can i do. also my check engine light comes on as well.


high NOX indicates your engine is running high combustion temps. Two of the major reasons for lean mixtures are intake leaks and clogged fuel injectors. The reason the engine computer cycles between rich/lean when running is because the cat needs lean mixtures to deal with HC and CO and a rich mixture to deal with NOX. If you are running lean the cat can't lower your NOX. I would first get your fuel injectors flushed and de-carbonize your intake/combustion chambers.

May 31, 2011 | 1997 Pontiac Bonneville

1 Answer

1989 Toyota 22re fails California Emissions Test. Rebuilt the engine 20K or so miles ago. Has passed most tests with just a little bit of timing adjustments. At 15 MPH, (1670 RPM) HC measures 153...


HC is hydrocarbons or unburnt fuel.(raw fuel ).NO no such gas measurement.did you mean CO this is carbon monoxide or incomplete combustion.(all the fuel is not being burnt).if you mean NOX this is nitrates of oxide and is related to a heat problem.(motor running hot,bad catalytic converter).its probably getting too much fuel and not burning it all,possibly tune-up problem,carburetor(if it has one),bad fuel injection system(dirty injectors),computer(if it has one)may be getting a wrong signal.these are in reference of high HC and CO.for high NOX cooling system not keeping motor cool enough,cooling fan problem(weather clutch fan or electrically controlled cooling fan),poor cooling system circulation,bad catalytic converter.not a running lean problem as the other gases indicate rich problem.the converter may not be able to convert the emission gases to H2O and CO2 out the tailpipe.

Apr 24, 2011 | 1989 Toyota Pickup SR5

1 Answer

What are the causes for NXo reading


A high NOx reading can be caused by any conditions that make the engine run lean. Vacuum leaks will dilute the fuel/air mixture with excess air leading to lean running. Lean mixtures lead to high combustion temperatures which produce NOx.

A malfunctioning EGR valve will not recirculate enough exhaust gas to keep combustion temperatures in check. The primary purpose of this valve is to lower NOx emissions are high RPM. It does this by delivering some low oxygen content gas back into the intake stream to dilute the incoming mixture. By decreasing the oxygen, combustion temps are lowered.

By the same logic, a misfire can cause high NOx. If the fuel/air mixture is not burned, excess oxygen in the exhaust (catalytic feed gas) will cause a dramatic decrease in catalytic converter efficiency in reducing NOx

Jan 24, 2011 | 1988 Toyota MR2

2 Answers

1996 jimmy failed an emission test


Did you change the air filter. The fuel filter will have no impact on emmision's but the air filter is a very common problem.If you usually drive it at low rev.'s then before you send it for emmisions test you will need to put it in a low gear and floor it for about a mile the reason is when it is being tested the engine is reved up to around 3500 rpm and any build up of unburned fuel or oil in the exhaust system will put up the emmisions reading.

Aug 19, 2009 | 1996 GMC Jimmy

2 Answers

Emissions failing with high NOx


Change the lambda/oxygen sensor in the exhaust.

May 21, 2009 | 1994 Mitsubishi Galant

2 Answers

Emissions failing due to high nox


NOx emissions are the result of high combustion chamber temps pure and simple. NOx readings go up whenever there are hot spots that exceed about 2400 degrees. Things like lean mixtures contribute to high chamber temps, as does over-advanced ignition timing.Lean mixtures are usually caused by some failed components or by air (vacuum) leaks in the intake.
Poor fuel quality contributes to detonation, another cause of high chamber temps.
  1. Check to make sure the vacuum system is tight.
  2. Try a full tank of high octane fuel(run at least half a tank through before testing)
  3. Have the tester do the rolling tests with the trans locked in first gear.
  4. Make sure the oxygen sensors is working.
If you have done all of this and still fail, You will need a new catalyst

May 20, 2009 | 2000 Mitsubishi Galant

1 Answer

1994 Mazda Protege 1.8 SOHC failed emissions HELP!


have your EGR system checked . sounds to me like its not working or clogged

May 03, 2009 | 1994 Mazda Protege

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