Lack of power. Wow that happened to me once...

How often does this happens? This particular fault is by far the most common in today's auto diagnostic world. Lack of power can come from many sources. Here we'll delve into some of the possible causes and how to address them. Enjoy...

 

HESITATION/LACK OF POWER DIAGNOSTICS STRATEGY

THEORY OF OPERATION

Diagnostic strategies are employed to speed up and accurately find specific electronic faults that affect all automotive systems today. The following strategy is meant to be used by any technician using a multi-channel oscilloscope/ diagnostics equipment. In today’s automotive diagnostics field, the proliferation of electronics has led to the need for using multi-channel scopes and test equipment in an increasing way. The use of such equipment speeds up the diagnostic process and allows sure diagnostics that will not result in the replacement of unnecessary parts, therefore, reducing repair costs. The following are strategies used to detect and confirm electronic faults in today’s vehicles. Provided that the engine has no mechanical problems and all cylinders are working without any misfires, a no-power or hesitation complaint will usually fall into 5 major categories of problem areas. These are the categories in order of importance.

• Fuel delivery. ( pressure and volume ) - Possible symptoms are lean/rich misfire or flooded engine.

• Spark reserve or enough magnetic buildup (saturation) in the coil - Possible symptoms are misfire, hesitation & lack of power.

• Exhaust backpressure.( clogged cat. ) - Possible symptoms are bogging down & lack of power.

• Ignition timing. - Possible symptoms are: Pinging and no power.

• Valve timing.( jumped timing belt ) - Possible symptoms are: Backfire. This systematic fault detection order will lead you in the right direction.

GUIDELINES FOR USING A MULTI-CHANNEL/DDC SCOPE and REPARING ENGINE PERFORMANCE FAULTS IN GENERAL

THE FOLLOWING ARE STEP-BY-STEP DIRECTIONS ON USING A MULTI-CHANNEL OSCILLOSCOPE TO PERFORM ELECTRONIC DIAGNOSTICS ON MODERN AUTOMOBILE SYSTEMS. THEY ARE MEANT TO BE FOLLOWED IN A SEQUENTIAL, STEP-BY-STEP MANNER. BEFORE PORFORMING ANY TESTS, A STARTER CURRENT COMPRESSION TEST SHOULD BE PERFORMED FIRST TO VERIFY THE MECHANICAL INTEGRITY OF THE ENGINE.

PRE-CONDITIONING STEPS and PRE-DIAGNOSING WITH THE SCANNER

First a determination must be made if the use of the Multi-channel/DDC scope or any electronic breakout-box system is called for. It may be possible to simply use regular diagnostic tools like a test light and DVOM to do the job in much less time. The use of oscilloscopes and more hard to connect equipment always adds hook-up time to the diagnostic routine. Using such equipment must out-weight the extra time drawback , as when dealing with circuit problems on complicated automotive system that can not be detected through simpler means. The following steps must be taken to determine if there is a need for the use of these tools.

1. A thorough visual inspection must be made to determine the condition of the following components: Battery posts, ignition parts, connectors, wiring harness, engine odors, ignition arching KV, ground strap between engine and block, chassis grounds, sensors and relays, oil & fuel leaks, etc.

2. Warm the engine to operating temperature and determine that there are no cooling or major mechanical problems.

3. Connect a scanner and decode the ECM. Make sure that all codes are recorded for future reference. See next Fig.

NOTE: The importance of a basic visual inspection can not be overstated. A large percentage of all electronic faults are resolved by simply doing a very thorough visual inspection. Remember the faster you find the problem the better off you will be. After assessing and analyzing the scanner codes and troubleshooting information, determine whether to continue with these steps or go to the Multi-channel scope/DDC hookup to continue the diagnostics. Scanner diagnostics using graphing software linked to a desktop computer can be very useful in diagnostics. It is important to correlate signals together so as to establish a relationship between signals.

NOTE: By choosing the RPM, MAF or MAP signal, LT fuel trims, and O2 sensor signals, it is possible to see the reaction of the O2 sensor while power breaking or preloading the vehicle. A momentary rich signal should be observed followed by a normal cycling O2 sensor signal, indicating that the ECM is in constant fuel control. The ECM should be in fuel control at all times except WOT and Deceleration. So long as the O2 sensor is cycling, the ECM is in control.

4. Perform a 4 or 5 gas analysis of the vehicle’s tail pipe emissions, so as to ascertain proper air/fuel ratio on combustion. A gas analysis can reveal the overall condition of all the different systems in the engine in a very fast and efficient manner.

High HC indicates degraded mechanical or ignition components.

High CO indicates a rich mixture, from either lack of air or excess fuel.

High O2 indicates a lean mixture, from either lack of fuel or excess air. High O2 can also be due to a continuously running secondary air injection system.

High NOx indicates EGR problems, high combustion chamber temperatures or an overheating engine.

High CO2 indicates good combustion. It is the only gas that should always be high (14 % or higher).

It is important to understand that as far as gases are concerned, CO is a determinant of excessive fuel and O2 is a determinant of excessive air in the mixture, CO2 is the result of any carbon based combustion and should always be high, NOx is the binding of nitrogen and oxygen molecules at high temperatures and is a measure of EGR or lean (hot mixture) operation, and HC is raw fuel exiting the exhaust (due to a misfire or any other mechanical fault).

Typical readings for all gases should be as follows. (Newer ULEV systems have much lower specifications). HC-under 100ppm CO-under 0.50% O2-under 0.5% CO2-over 14% Nox-usually under 850ppm.

NOTE: These numbers are to be used only as guidelines. LEV and ULEV vehicles have much lower readings.

5. Check fuel pressure and volume. Fuel pressure and volume should meet specifications and not be slightly over or under.

 

NOTE:  • When using a volumetric fuel pressure gauge, the following rule of thumb could be used. Generally, with engine running, SFI and PFI systems should have a 0.3 to 0.5 gal per minute of fuel flow, at a minimum, and TBI should have 0.4 to 0.7 gal per minutes of fuel flow. There are system variations and this is only meant to be used as general minimum rule.

• An oscilloscope should be used to check the integrity of the commutator windings inside the fuel pump motor, by using a low clamp-on amp probe to determine current load ( Amps ) and fuel pump RPM. A 3000 to 7000 RPM and a 4 to 6 Amps load is acceptable.

If the fuel system is found to be within normal parameters then again decide whether the use of the Multi-channel scope/DDC is called for. If fuel mixture problems are suspected, then there might be a sensor causing the problem. If not then go on to the next step.

6. Perform a full ignition check using an oscilloscope. Determine that there is a proper KV firing line, spark line KV and spark duration. ( normal coil saturation (dwell) should be at around 4 mS to 5 mS, with as much as 8 mS on some systems).

7. Perform a back pressure check using a back pressure gauge to determine if a clogged catalytic converter is the culprit.

8. Determine that ignition timing is up to specifications. Older engines may require a slight off specs adjustments to make up for slack in the timing chain or belt. Use your judgment.

9. Determine proper valve timing and/or timing-belt/chain operation.

DDC/MULTI-CHANNEL OSCILLOSCOPE OPERATION

The DDC is a Multi-Channel oscilloscope using software driven electronic break out box technology. The following steps must be performed when using the DDC or any other multi-channel scope/breakout box. However, this technique is applicable in using any multi-channel oscilloscope.

Perform a sweep test of the entire wiring harness. Start by checking all sensors and activate all solenoids and actuators to determine proper operation. ( The conditional tests on the DDC should not be performed under normal circumstances, since it would only reveal a problem in extreme cases, as in open or shorted grounds).

NOTEPay particular attention to the fact that sometimes unrelated components share the same ground or power feed circuit. A short in the A/C high pressure switch may cause a MAP sensor faulty code, due to the fact that they might share the same 5 volts reference circuit from the ECM. This is the case of the 95 Plymouth Voyager. You should never dismiss any out of specs sweep readings and assume nothing when in comes to diagnosing modern vehicle systems.

Perform the following signal waveform tests. Test all grounds while cranking the vehicle, and while running at idle and at 2500 RPM. The ground signal should not go above 300 mV in cranking mode and should not go above 100 mV in idling mode. Anything above those figures would constitute a ground signal problem. However, certain vehicles exhibit a high amount of interference on the ground lines. Such interference is normal, so long as the interference voltage waveform returns to 0.00 volts.

Power feed test. Test all 12 Volts ECM feed lines including the 5 Volts reference line to the sensors. Check for no sudden drop-outs of battery volts and correct voltage levels.

Dynamic signals. Start with the most important signals which appear at the beginning of a pin out list. A general rule should be distributor or ignition signals first ( pick up coil, crank, cam, ignition reference, ignition primary if needed, knock sensor to determine ignition retardation, and starter signal ), then fuel related signals ( injectors, MAF, TPS, MAP, IAT, O2 sensor ).

NOTE:  It is extremely important to arrive at a correlation between signals to establish a relationship. Notice the two examples bellow.

Example 1- If a crank signal scope trace has a spike that correlates with the ignition primary of cylinder # 1, then there is a relationship between the two signals and the ignition components are probably at fault. The use of PC-OVERLAY feature should be used to determine a possible correlation and therefore establish a relationship between two signals.

Example 2- If the vehicle is pre-loaded, the MAF signal should indicate excess air coming into the engine and the O2 sensor signal should show a momentary rich signal output and then start cycling again to show that the ECM is in control. Lack of fuel control by the ECM points to a fuel starvation problem or on the rich side an excess fuel problem. These causes should be further analyzed.

When checking the fuel related signals, the ECM should be in fuel control at all times with a few exceptions ( WOT & Deceleration ). See figure 1.

Sensor check . Manual multi-meter or Graphing multi-meter tests should be used, when necessary, to check sensors and generally slower moving signals like O2 sensor and MAF/MAP response.

Example 3- When checking MAF sensor response to a WOT actuation, the resulting meter or GMM waveform time-base should be around ½ a second or 500 mS. O2 sensor response should be in the order of once every second or 1 Hz. Also a TPS sweep should be performed, if needed. The manual multi-meter tests can also be used to see signal correlation and establish relationships between signals just as the scanner graphing or the oscilloscope function.

Proper diagnostics procedure is an integral part of good troubleshooting practice. By strictly adhering to these procedures, you can confidently arrive at the source of the problem at hand. It is important to always avoid the pattern failure approach to diagnosing problems. Over the years experience has shown the advantages and limitations of each and proper procedure clearly will always lead you in the right direction.

 

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Sensors                 O2-Sensor                  APP-Sensor                CAM-Sensor               CRK-Sensor               ECT-Sensor               IAT-Sensor                 Knock-Sensor              MAF-Sensor                MAP-Sensor                TPS-Sensor                 VSS-Sensor                FRP-Sensor                AFR-Sensor              

Actuators               Fuel-Injectors            Ignition-Coil           Leak-Detection Pump  GM-Series Alternator   Idle-Control Valve      EGR-System            EVAP-System      

Repair Strategies Current-Ramping      Lack of Power           Fuel-Flow Volume     Ignition Waveform       Ignition Testing          A/F Ratio Diagnosis    Minimum Air Rate      No Fuel-Pressure       No Injector Pulse      No-Start/No-Spark     General No-Start      

ECM/PCM Modes Cranking Enrichment   Warm-up Cycle         Open-Loop            Closed-Loop            Accel. Enrichment Deceleration Leaning   Idle Control             Low-Voltage Correction Clear Flood Mode     Selective Inj. Cut-Off   Limp-In Mode           Exhaust Variable Valve

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 Code Setting Criteria

Dodge CSC                 Ford CSC                    GM CSC                     Honda/Acura CSC        Hyundai CSC               Isuzu CSC                   Kia CSC                      Mazda CSC                 Mitsubishi CSC            Nissan/Infinity CSC      Subaru CSC                Toyota/Lexus CSC

SRS Airbag DTC

GM SRS Airbag Code   GM Truck SRS Code    Ford SRS Airbag Code  Ford Truck SRS Code   Dodge SRS Code        Dodge Truck SRS Code Acura/Honda SRS Code Isuzu SRS Codes         Mazda SRS Codes        Subaru SRS Codes Infinity/Nissan SRS       Kia SRS Codes     Hyundai SRS Codes     Mitsubishi SRS Codes Lexus/Toyota SRS

How to Get SRS Codes

Retrieving Dodge SRS   Retrieving Ford SRS     Retrieving GM SRS      Retrieving Honda SRS   Hyundai/Kia/Mitsu SRS Isuzu/Mazda/Subaru     Retrieving Toyota SRS  Nissan/Infinity SRS

SRS-Airbag Repair Guide

Deleting SRS Codes   Dodge SRS Location       Dodge SRS Operation     Ford SRS Location          Ford SRS Operation        GM SRS Location            GM SRS Operation           Honda SRS Location       Honda SRS Operation     Isuzu/Mazda/Suba Loc.   Isuzu/Mazda/Suba Ope Kia/Hyun/Mitsu Location Kia/Hyun/Mitsu Oper.       Nissan/Infinity Location   Nissan/Infinity Operation Toyota/Lexus Location    Toyota/Lexus Operation 

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