Tech to Tech
Using Four-gas Analysis
The purpose of the internal combustion engine is to convert fuel into energy as efficiently as possible. The
proof that this process happened correctly is found in the exhaust gases. Any disruption in the combustion
process upstream will be reflected in the exhaust gas.
The air injection system and catalytic converter work together to change these gases. I have seen a new Volkswagen Jetta with one spark plug wire off the plug and shorted that still passed emissions specs. Those of us that have worked on European cars have seen the exhaust sampling tube ahead of the catalytic converter. It is usually capped by a blue silicone rubber boot and is readily accessible under the hood. I wish all manufacturers had the wisdom to supply cars with this powerful diagnostic feature.
When this sampling tube is not available, some compensations can be made. First, the air injection must be disabled to get any semblance of a true reading! This could be a pulse air system that uses reed valves and the natural negative pressure pulses in the exhaust system or a pump-style system with air injected under pressure. Be sure to plug both the upstream air to the exhaust manifolds and the downstream air to the catalytic converter. When testing a warm engine, you might assume the air will be switched downstream. But I've seen many poorly seated diverter valves send small amounts of air upstream when they should be totally sealed. Be safe. Block all air sources using plugs in the hoses.
Blocking off the air to the converter will usually disable it enough to get better gas readings. Keep in mind that an ignition misfire will supply the converter with a perfect air/fuel mixture. In effect, the miss is supplying its own air. Also, many newer cars don't use an air injection! How do they get away with this? The oxygen (O2) sensor is allowed to swing way rich and way lean. This supplies enough carbon monoxide (CO) from the rich swing to give the converter what it needs.
The next solution is to let the converter cool down long enough to unlight. Then restart and take a reading as quickly as possible before the converter relights. This is fairly effective on a hard failure, but is not effective when watching for transient failures. Be careful using this technique -- some converters take 15 minutes to cool all the way to the core!
The final and surest technique is to take a sample ahead of the converter. This sample can't be taken from the O2 sensor hole, as this would disable the entire feedback system. Many times this sample can easily be taken from a disconnected manifold air injection line with the check valve removed. Keep in mind that if this method is used on a "Vee" engine, you may only be reading the gases from one side. The most time consuming and last resort is to drill or punch a hole in the exhaust pipe. There are many good tools sold for this job. I prefer the punch method because it leaves more metal for the sealing bolt to engage.
OK, so now we finally have a sample and we're ready to roll. Well sorry, but just one more warning! Remember that analyzers are designed to test samples at the flow rate the pump supplies. Do not allow your connection to pressurize your analyzer! This means you need to make a heat resistant tube that connects to the exhaust, but has a large enough inside diameter to accommodate your probe. Also, exhaust temperatures near the engine can be hundreds of degrees higher than your analyzer was designed for. The same warning holds true for extremely high tailpipe temps caused by misfire. Dynamometers cause high exhaust temps from the higher air flow and loading effect. Most "dyno" probes for gas analyzers have considerable heat transfer ability (cooling fins). Many an analyzer has been damaged by not heeding this warning.
Enough about connections. On to the gases! First and foremost, get a copy of a four-gas chart and get it laminated. Attach this copy to your analyzer. Most techs won't memorize this chart, but they will refer to it if it's connected to the machine.
CO is the first gas we look at. This tells us if the engine is running rich. Look at the chart in Figure 1, CO starts very high at 13.5 percent. It drops rapidly as we approach the perfect air/fuel ratio (AFR) of 14.7 to 1. At the optimum AFR, the CO flat lines as the mixture goes leaner. So CO shows how rich the engine is, but gives no indication of how lean an engine is past optimum AFR.
O2 is the next most important diagnostic gas. Note it is flat lined before 14.7 to 1. Then it starts to rise rapidly past this point giving a good indication of leanness. The O2 really blasts off when lean misfire is reached. Also, if the O2 is way too high and the other gases look fairly normal, look for exhaust leaks!
Carbon dioxide (CO2) is next on our list. This is an indication of combustion efficiency. As you can see in Figure 1, the CO2 is highest at the perfect AFR. Any deviation from this ideal will cause the CO2 to drop. The by-products of perfect combustion from a carbon-based fuel will be CO2 and water (H2O).
Hydrocarbons (HC) represent unburned fuel. This is a good indicator of misfire or incomplete combustion. HC will always exist in metal-based engines because fuel cannot be ignited next to a cool surface from temperature quenching.
Now that we've reviewed these basics, let's look at a common problem we see in the shop. Picture a vehicle with a CO at 3.0 percent (this is too high); O2 at 3 percent (also too high); with CO2 at 12.0 percent (this is too low); and HC at 300 parts per million (ppm), which is too high. These readings are taken ahead of the converter -- if they hadn't been, a healthy converter would have cleaned this up. These are idle readings. Now, if you studied the chart, you can see there is no point there that corresponds to these readings! No fair, you say? OK, just keep in mind that the chart in Figure 1 represents perfect combustion at the indicated AFR.
So what might the problem be? Well, how about a misfire on the O2 side of the engine? The extra O2 would drive the O2 sensor lean, causing the computer to respond with more fuel, accounting for the higher CO. This would also explain the lower CO2. But a solid misfire should cause higher HC -- in the order of 1,000 ppm to 2,000 ppm HC. Hmm ... could be an intermittent misfire.
Plugged injectors combined with intake valve carbon deposits cause a patchy air-fuel mixture in the combustion chamber. This is a mixture that is not homogenous (rich and lean patches in the combustion chamber). This leads to incomplete combustion. OK, the incomplete combustion causes O2 to be dumped into the exhaust, driving the CO higher, just as in the example above.
A leaking exhaust manifold gasket that allows air to be sucked in over the O2 sensor is also a commonly missed problem that can cause readings like the ones above.
Two final notes: first, the 14.7 to 1 AFR. This is by weight, not volume! One cubic foot of air weighs about 1.25 ounces. One cubic foot of fuel weighs about 50 pounds. This yields a weight ratio of about one to 700. Now multiply this weight ratio by the 14.7 of air to the one of fuel and you find the AFR by volume is about 10,000 to one. So it takes 10,000 gallons of air to burn one gallon of fuel.
Now let's talk about Lambda. Yes, it sounds like some weird German word, but once you understand it you should love it. Lambda is an equivalence ratio. This means it is a fraction, with your actual AFR over the ideal ratio of 14.7. So 12/1 over 14.7/1 is 12/14.7 = .82. The ideal ratio equals one and leaner ratios are greater than one. So what's so great about that? It gives you leanness or richness in a percent deviation from the norm. The 12 to 1 example is 18 percent richer than it should be.
The Lambda or AFR readout on your analyzer uses all gases in a complex formula to derive this value. Any false O2 (O2 not from combustion) will cause this reading to be way lean! Unless there is no upstream air, this reading is false, false, false!
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AutoInc. Magazine ®, Vol. XLIV No. 4, April 1996