Tech to Tech
Tackling Rich O2 Sensor Readings
To my knowledge there is only one reason for an oxygen (O2) sensor to give a rich indication, and that is that the engine is running rich. You may recall that last month causes for lean O2 sensor readings were discussed.
As has been discussed in past columns, there is a close relationship between exhaust gases, the O2 sensor and the injector pulse width. Keeping this in mind, let’s consider the possibilities for rich O2 indications. Remember that when the O2 is in command, then the injector pulse should give the opposite response (O2 lean = pulse wide, or O2 rich = pulse narrow).
Rich O2, Narrow Pulse And Rich Exhaust Gases
When an O2 sensor reads rich, the injector or mixture control (MC) solenoid (on
feedback carburetors) has a narrow pulse and the exhaust is rich, what are the possible
causes? Probably the most common problem here is oil contaminated with fuel. This is
usually the result of an underlying problem such as a misfire, a defective positive
crankcase ventilation (PCV) system, a dead O2 sensor, a bad coolant sensor, or any
condition that causes the engine to run rich. It drives me crazy to see how many techs
overlook this obvious problem.
First, fuel contamination should have been identified during the initial inspection by sniffing the oil. Second, this is easily tested for with an infrared (gas analyzer). Pulling the PCV when running should not drop the CO by more than 1 percent on a properly functioning system. Third, and probably most important, any engine that has failed emissions probably needs the oil changed! If an engine has been running improperly, fuel will make its way into the crankcase. Do yourself and your customer a favor and change the oil. Sure, you may want to let the PCV system pull the fuel out now that the system is fixed. Just remember that the learned mixture values in the computer will have to constantly change to update, and the car may not run well until these values stabilize. And while rare, a crankcase explosion is possible.
PCV systems also can create a rich condition. Remember that the PCV system is a controlled vacuum leak. The PCV system has to carry oil, water and combustion gases out of the crankcase into the intake manifold for reburning. The components that the PCV is expected to evacuate are also the building blocks for sludge. The PCV system is very prone to plugging from this sludge. Make it a habit to service this system on every car in for a tuneup or emissions, or a driveability complaint. You can run a quick check of the PCV system by installing a vacuum gauge on the dipstick tube. How hard is that?
Fuel evaporative emission systems are another simple reason many engines run rich. This is another one that drives me nuts. Don’t make your job any harder than it already is! Eliminate the easy stuff first. Many of these systems suffered from high failure rates at higher mileage. Diagnosis is simple, just disconnect the evaporative hoses and see if the rich condition improves. How many techs are checking the air filter on the bottom of GM canisters? How many techs are checking the older canister vacuum diaphragms? Some canisters are full of fuel from customers over fueling the tank or parking facing down on a steep slope. Be sure to check for this and educate customers to prevent future problems.
High fuel pressure and vacuum-controlled fuel pressure regulators with leaking diaphragms can also cause a rich condition. The leaking diaphragm should have been found in the early inspection phase of diagnosis. When to test fuel pressure is more of a judgment call. This is largely dependent on how hard it is to test. A system with a schrader valve on the rail should be tested right away, but with a TBI unit that takes a half an hour to test, you may want to eliminate some easier possibilities first. The tip off to high fuel pressure and leaking diaphragms is usually an incredibly short injector on-time.
Leaking injectors also have a similar effect, except they usually will not affect all cylinders. A cylinder balance test, along with recording the changes for all four exhaust gases, will usually pinpoint leaking injectors. Also running the cylinder balance test at idle and 2500 RPM will help pinpoint this type of problem. The injectors will usually clear up at 2500 RPM because the amount of fuel needed exceeds the leakage. This test is also helpful in diagnosing vacuum leaks as they become less influential at higher air flow. Leaking injectors can be tested with pressure drop testing after confirming that the system drops are not the result of a leaking fuel pressure regulator or the often overlooked check valve in the fuel pump on the delivery side. If rubber lines are available, they can be clamped off to eliminate these problems. At my shop, we had hoped to test for leaking injectors by shutting off the engine and trying to start with the throttle floored (clear flood mode). The clear flood mode is entered when the computer sees wide open throttle (WOT) and less than 400 RPM. We have often been told no fuel is injected in clear flood mode. This is not always true, many manufacturers go to a 20:1 fuel ratio and the car may start. Also, residual fuel in the intake or crankcase may cause the car to start. We have not found this test to be a reliable indicator of injector leakage.
Carburetors that are stuck rich can be caused by many factors. First and most common are needle-valve/seat and float problems. A worn needle and seat needs replacement. A heavy float from fuel absorption also needs replacement. Flooding after a tuneup can be a common problem. This is usually caused by fibers from the new fuel filter or particles dislodged during filter replacement getting jammed in the needle and seat. To help prevent this problem, put some fuel in a new filter prior to installation to wet the element. Careful cleaning of the filter area, especially when Teflon tape has been used, should also help prevent contamination.
When the needle and seat are stuck from foreign material after a filter change, disassembly can usually be avoided using the following technique. First, clamp off the fuel line and run the engine until the carburetor is almost out of gas. This opens the needle and seat wide open. Quickly open the clamp on the fuel line. The rush of gas through the needle and seat will usually flush the offending particle loose. Most of these particles are small fibers from the filter and won’t affect carburetor operation. With tougher cases, sometimes after running the carburetor out of gas we have to remove the fuel line and use spray carb cleaner and air under pressure to flush loose the jam. Keep in mind that Teflon tape is used in these applications because Teflon reacts with practically nothing. Thus, a particle of Teflon stuck in a carb passage will never dissolve. This can ruin a carburetor! Clean Teflon particles very carefully from fuel line connections!
Carburetors can also run rich from stuck or malfunctioning MC solenoids, leaking power valve diaphragms, stuck power valves and internal leaks. It is important to understand the functioning of the type of carburetor you are working on. Some carbs control fuel for mixture adjustment and some control vacuum leaks. The CCC Quadrajet carb controls fuel off idle and controls a vacuum leak to set idle mixture.
After running rich for an extended period of time, an engine, converter and O2 sensor may be carbon laden and not functioning properly. Many times a good run at freeway speeds will clean things up and improve operation without replacement.
Wide Pulse Readings
When the O2 reads rich, the injector pulse or MC solenoid read wide and the tailpipe
gases are rich, what are likely causes?
The key here is that since the O2 and pulse width are not opposite, this shows the O2 is not in command. As discussed last month, what has greater authority than the O2? The MAP sensor, the TPS sensor, the coolant sensor and the air charge temperature sensor. Look for a defective input when the O2 is no longer in command. Poor grounds can also be the primary cause if the sensor voltages are wrong.
Just remember to form a game plan that has two lists, the most probable causes and the easiest to test. In the process of elimination, sometimes it makes sense to knock off five easy items that take only five minutes to test rather than going after the most probable cause that may take 30 minutes to test.
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AutoInc. Magazine ®, Vol. XLIV No. 7, July 1996