Diagnostics
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1. Define the problem in objective terms, e.g. The CO level is too high (5%).
2. Determine what has possibly changed or happened prior to the problem, e.g.
Both the fuel pressure regulator & O2 sensor were replaced at the recent 60K service.
3. Review similar problems which have occurred in the past, e.g.
A bad vacuum line to the fuel pressure regulator caused a 4% CO problem.
4. Evaluate all the possible causes, e.g. vacuum line, fuel pressure regulator, O2 sensor,
temperature sensor, throttle position sensor, fuel injection unit, or mass air flow sensor.
5. Localize the problem, e.g. Test the O2 sensor voltage & the temperature sensor resistance
for the proper values and check the connectors & wiring for them.
6. Substitute a known good unit for the potentially bad unit, e.g. Replace the O2 sensor.
7. Test for the problem, i.e. Check the CO level.
8. Repeat the process at step 4, if the problem still exists.
Typical Sensor Values
Temperature Sensor - Hot ~ 300-400 ohms, Cold ~ 2,000-3,000 ohms
Throttle Position Sensor - 0-5 volts
Air Flow Meter - 0-5/12 volts
Mass Air Flow Sensor - 0-5 volts
O2 Sensor - .10-.80 volts
Throttle Body Switch - Closed ~ 0 ohms, Open > 100K ohms
Altitude Sensor Switch - Closed ~ 0 ohms, Open > 100K ohms
Pressure Sensor - 0-5 volts
Magnetic/Reluctance Pickup (speed/reference) Sensor - 600-1,000 ohms, .10-1.0 volts A.C.
Knock Sensor - > 100K ohms, ~ 100mv A.C.
Hall-Effect Sensor - Switch = Closed ~ 100 ohms, Open > 10K ohms - Module = 0/5 volts
Typical Ignition Coil Values
Bosch TCI - R(primary) = .50 ohms, R(secondary) = 6200 ohms, N = 90,
L(primary) = 3.6 mH, L(secondary) = 30 H
Bosch CDI - R(primary) = .30 ohms, R(secondary) = 1400 ohms, N = 70,
L(primary) = .20 mH, L(secondary) = 1.0 H
Electronic Diagnostic Tool Use
Powers/Grounds => incandescent/LED test light, multimeter set to D.C. volts
Pickup/Ref/Speed/Knock sensors => multimeter set for A.C. volts, oscilloscope
Hall sensors/switches => LED test light, multimeter set to D.C. volts
CHT/ECT sensors, contact switches (TS) => multimeter set to ohms
AFM/MAF/MAP/TPS sensors => multimeter set to D.C. volts, OBDI/II scanner
O2 sensors => multimeter set to D.C. volts, oscilloscope, ODBI/II scanner
TCI ignition signal => incandescent test light, oscilloscope
CDI ignition signal => oscilloscope
Fuel injector signal => incandescent bulb, LED test light, OBDI/II scanner
Diagnostic Fault Codes (DTC) => ODBI/II scanner, LED code flasher
CAN data => ODBII scanner, data bus analyzer
Initial Checks
First, check all the battery connections & engine grounds and also the smaller power leads & grounds.
Then check all fuses in all fuse boxes. Solder all other connections that may have been just crimped.
Disconnect & clean all connectors, i.e. check for corrosion/oxidation. Next, check the battery voltage
and make sure that the battery provides an adequate cranking speed for the engine to start. Once the
engine starts, again check the battery voltage, i.e. the battery voltage should increase once the engine
starts if the alternator is O.K. Also, make sure that the alternator isn't overcharging.
No-Start
Always begin with a simple check for spark (15-25mm) at the main coil or at a cylinder coil.
Next, check for an injection signal with a small injector light. If both of these are present,
then check for the proper fuel pressure.
If any of the above tests fail, then check that both the ignition and the fuel injection control units
(or the integrated unit) have proper +12 and ground inputs. Vehicles with a check engine light
generally have an additional continuous +12 input. Main fuel injection relays and fuel pump relays
can be a problem. The timing signals, engine speed and reference, should be checked for ohms
and A.C.volts (inductive pickups) or a switching signal if a Hall pickup (a semiconductor diode
or transistor) is used. The speed and reference sensors sometimes have been reversed in error.
Some vehicles may have a third signal, e.g. a distributor or cam sensor. These usually must be
synchronized with the reference sensor.
Newer (post 1990) vehicles may have only one signal which provides both speed and reference,
and a distributor or cam sensor which is generally used for sequential fuel injection and/or direct
ignition systems (DIS). If either the spark or the injector signal is present, then the timing signals
are most likely present since they are common to both the fuel injection and the ignition systems.
Then in most cases the other system or the integrated fuel & ignition system has failed, if one
signal is present but not the other.
The injectors must have a +12 at one side, as the control unit provides a pulsating ground to the
other side for operation. Inductive discharge ignition systems (early points ignition and late model
vehicles) have +12 at one side of the ignition coil. The ignition signal is usually a pulsating ground.
Capacitive discharge (CD) types of ignitions (CDI) never have a +12 coil input, only a ground and
a pulse input. The Bosch CDI unit produces a buzzing sound from the DC to DC converter when
operating, indicating that its power source is present.
A standard test light can be used when checking for a signal on an inductive discharge system,
but not for a CD ignition. A scope must be used for this test to prevent CD damage. Use an LED
type of tester when checking for injection pulses. If trigger points are used as an input for an ignition
unit or a fuel injection control unit, use a standard test light connected to +12 to check for a pulsing
ground signal from the trigger points. A weak spark (<15mm) usually results from a bad ignition coil.
If the above tests are OK, then the problem is probably not electrical. Other inputs such as engine
temperature, air flow, O2, TBS, i.e. throttle body switch - idle and wide open throttle (WOT)
switches, or the TPS (throttle position sensor), will generally not prevent an engine from starting or
attempting to start. If the engine has been altered mechanically, e.g. during a parts removal and
replacement, the timing from the flywheel, damper pulley, distributor/spark plug wires or other
mechanical sources may have changed affecting the overall engine timing and thus starting.
The typical EFI fuel pressure is approximately 2-3 bars. The typical CIS system fuel pressure is
approximately 5-6 bars. On CIS fuel injection systems, the control pressure set by the warm-up
regulator (WUR) can significantly affect starting. The CIS-K system with the WUR has a control
pressure of approximately 1.5 bars cold and reaches about 3.5 bars when hot. These values vary
based on the engine, e.g. turbo or non-turbo. The CIS-KE system has a differential pressure of
approximately 1.5 bars cold to .5 bars hot set by the electric hydraulic actuator (EHA) unit.
The control pressure is checked at the base of the fuel distributor near the pressure regulator
source line. The system pressure is checked at the top of the fuel distributor near the cold start
fuel line. Hookup plugs are provided for connecting both pressure test lines.
In addition to the WUR, the CIS-K unit provides additional fuel enrichment for cold running based
the oil temperature switch. The fuel accumulator dampens/buffers large fuel pressure demands,
e.g. during heavy acceleration, and thereby helps to maintain a constant system pressure. A faulty
accumulator can cause the fuel pressure to reach a very high value under de-acceleration stalling
an engine.
Intermittent Running
A number of intermittent problems may arise from; bad coils, distributor caps with radio inference
covers (black plastic shields), fuel pressure regulators or accumulators, distributor rotors which
breakdown to the metal shaft, or timing sensors. Additional problems may result from poor electrical
connections, e.g. +12 & grounds, or bad relays, e.g. main fuel injection relays (small cube or DME).
Some aftermarket capacitive discharge (CD) ignition units have become intermittent with varying
engine temperatures. This may result from units that are filled with a potting compound that can
cause thermal expansion problems for electronic components and circuit boards. This problem has
occurred in other types of electronic systems where a potting compound was used.
As with any engine related problem, always try to isolate the problem to either fuel or spark. Also
check the potentially problematic part, e.g. fuel pressure regulator, when the intermittent problem
occurs and not during normal running times. Try and determine how different units and sensors affect
the engine when it's running good, e.g. disconnect the CIS K/KE Lambda unit, or an O2/temperature
sensor and monitor the starting/running. Also, measure voltages and resistances of possible intermittent
control units or sensors when the engine is running properly to establish a base value to check against
when the engine runs poorly.
Later model vehicles with check engine lights will usually store codes relating to a sensor problem
or help in diagnosing an intermittent problem. Alarms, either OEM or non-factory units can cause
problems. Also, an intermittent overcharging alternator may be a problem. Many older vehicles may
have problems from wiring connections which were not originally soldered but only crimped together.
These connections must be cleaned and soldered for reliability.
Generally, most sensors do not become intermittent with the exception of air mass meters or TBS.
When very hot, ignition coils or sensors (e.g. temperature, Hall/magnetic pickup) may be intermittent.
Occasionally, the difficult intermittent running problem results from bad fuel injection or ignition
units. A light taping on the bad unit may help determine if it's really bad, in some cases because
of an intermittent connection internally. Most situations will require a temporary replacement of the
unit to determine the problem, which is the ultimate test and generally the only test. The key point
about an intermittent problem is that tests are only valid when then intermittent problem occurs.
Poor Running
A poor running situation is generally less difficult to analyze since testing can be done continuously
with the engine running versus trying to start an engine or waiting for an intermittent problem to
occur. Before electrical testing is begun, it is assumed that all pressures and air leaks have been
checked. Scanners (BMW MODIC, Porsche Hammer/ST2, Snap-On MT2500, & Ottoscan),
multimeters, or signal scopes can be very valuable in analyzing a poor running condition. Also, LED
test lights which provide a green LED (ground level) and a red LED (+12V level) can be very
helpful for checking levels and changing signals in quick troubleshooting. Additionally, a three gas
analyzer, i.e. CO, HC, & O2, is generally a necessity for a comprehensive diagnosis.
Specifically, bad spark plug wires or their routing near sensors, spark plug connectors not properly
pushed on spark plugs, or overcharging alternators can cause strange running problems. O2 sensor
wires which short together or to ground can cause major problems. Also, high impedance sensor
inputs such as the O2 sensor input, are easily affected by ignition spark radiation. As in the no-start
case, improperly timed/aligned flywheels, damper pulleys (e.g. with rubber couplings), or timing
sensors can definitely affect a running condition. Marginal timing sensors, or their alignment, can
result in hard starting problems or intermittent poor cold running, inadequate performance, or high
RPM misfiring, e.g. worn distributor shafts.
The proper fuel pressure value is very critical for an ideal running condition. Improperly adjusted
throttle sensors can significantly affect a running condition, e.g. an idle switch not closing. Variable
camshafts not properly setup can cause a severe idling problem. On CIS fuel injection systems,
the auxiliary air regulator (AAR) and the WUR can significantly affect the cold idle running. Also,
check for air intake leaks or leaky vacuum lines on fuel pressure regulators, which may cause a
poor idle. EGR valves which are open at the wrong engine conditions, e.g. during cold running,
can cause problems.
A weak spark may cause problems under load conditions. A strong spark can usually jump about
15 to 25mm from the coil wire to ground. Also, a weak injector signal the result of a bad +12 at the
injector or a poor injector pulse can cause a loss of power as could a loss of or drop in fuel pressure.
Another common running problem results from water and/or corrosion in electrical connectors. These
conditions can affect the values which input sensors (temperature or TS) provide the fuel injection
or the ignition unit. As always, bad +12 and ground connections can be major sources of problems.
A poor running engine may contribute to an emissions test failure. A high CO (carbon monoxide)
level may result from a number areas, e.g. a bad O2 sensor, an incorrect fuel pressure, a bad
temperature sensor, or a faulty air flow/mass meter. High HCs (hydrocarbons) may be caused by
some mechanical problems, e.g. leaky injectors, fouled spark plugs/weak spark, intake air leaks,
weak cylinders, or an incorrect timing. The HCs will reach a minimum value at the optimum timing.
High NOX (nitrous oxides) levels usually result from an advanced ignition timing, a very lean mixture,
or a bad catalytic converter. The O2 sensor has the most effect on the CO value.
If the CO needs to be adjusted, the internal wheel of an AFM (air flow meter) or the adjusting screw
of a MAP (manifold air pressure) needs to be adjusted for a CO change over the full range of RPMs.
The CO adjusting screw/knob only adjusts the CO at idle. Most AMSs (air mass sensors) used on
late model vehicles can't be adjusted for idle or the full range without major changes to the unit.
If the CO is less than .5 to 1.0% before the catalytic converter without the O2 sensor and the other
inputs, i.e. the fuel pressure, the temperature sensor, and the TPS, are correct, then the AFM or the
AMS is potentially bad. The off-idle CO should be checked at 2000 to 2500 RPMs.
On the CIS system, the mixture adjustment affects the full range and must be adjusted after proper
WUR setup and after the control pressure reaches the value set by the normal operating temperature.
The CO is typically set to 1.0% - 1.5% at 2000 RPMs before the catalytic converter without the O2
sensor by means of a gas analyzer. The CO level usually falls to less than .20% with the O2 sensor.
Vehicles with OBDII ('96 and later) have extensive fuel injection diagnostics and monitoring systems
to determine the effectiveness of the emissions controls. These include; the functioning of the catalytic
converter, the O2 sensor, the adaptation of the fuel injection unit to parameter changes, e.g. air leaks
or fuel pressure, and the functioning of the secondary air injection system. The OBDII standards
require that the fuel injection system go thru a self-test at engine startup and cycle thru six readiness
modes based on certain driving conditions over various time periods. Some late model vehicles,
e.g. 2003, may have 15 or more readiness modes.
Once this cycle is completed, readiness "flags" are set indicating that the vehicle is in an acceptable
state for an emissions test. If the fuel injection system is reset because of an existing check engine light
or because of an impending fault code, the readiness process must be re-initiated requiring two driving
cycles, "trips", about 20 minutes each. No indication of the readiness status is provided to the driver.
A bad catalytic converter can cause many problems especially with late model vehicles that have two
O2 sensors. Early vehicles that have a bad catalytic converter will lack performance or run/rev poorly
above idle. Vehicles with two or more O2 sensors can have additional problems such as intermittent
fault codes indicating cylinder misfires on a bank of cylinders. This can occur during hard acceleration
versus idle to light acceleration conditions. A problematic catalytic converter may also be indicated by
an OBDII readiness code not being complete after the proper driving interval.
Lastly, it's always possible that the fuel injection unit or the ignition control unit may be the problem.
Again, a light taping on each unit may help determine if either is the source of the poor running
condition, i.e. because of possible intermittent connections internally. Given the complexity of most
control units, determining a poor running problem the result of a control unit, even by use of a scanner,
is difficult. Usually, the control unit must be replaced temporarily with another unit.
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