No-Code DiagnosisA Step-by-Step Guide to Success

Jim Halderman

Topics to Be Discussed

• Where to start diagnosis?

• Why following a procedure rather than just performing tests works best

• How to look at scan tool data (PIDs) in a sequence order to reduce diagnostic time.

• How to use fuel trim to diagnosis fuel delivery problems.

• Many case studies as examples

Where do you start?

• Verify-If you can not verify the customer concern, you can not verify the repair

• This is hard for some customers to understand. “ Don’t you believe me?”

• Have the owner drive is instead of the technician

• Is the troubleshooting procedure explained to the customer?

Step #2

• Visual inspection

• An older technician once told me that “the vehicle will tell you want is needed”

• True?

Turbo Ford Case Study(ran rough after cylinder head replacement)

Checked Air Vane Sensor

Verified the Cam Timing

Verified Ignition Timing

Checked the Valve Clearance

Checked Injectors

Tachometer fluctuated when running

• What can cause that to happen?• Bad coil?• Poor connections on coil?• Bad ignition control module?• Bad tachometer?• Poor ground?

Grounded Cylinder Head

Ran OK with the Jumpers

Turbo Ford Story Conclusion

• Cylinder head was not properly grounded to the block

• Sealant was used on the head bolt threads• The ground from the battery was connected to the

block; not the head• Spark plugs need to be properly grounded• Poor ground caused feedback to the tachometer

Step #3

• Check for diagnostic Trouble Codes (DTCs)

• Could be performed before step #2

• Check for pending codes too

• Check that all monitors have run

• Could there be a driveability problem without a DTC? yes

Monitors

Continuous: Misfire, Fuel System, and ComprehensiveNon-Continuous: Evap., EGR, O2 Sensors, O2 Sensor Heaters, Catalyst, HeatedCatalyst, A/C System, Secondary Air, and Warm-upsNOTE:In emission areas, a specific number of monitors need to be “complete” or “ready” in order to perform an emissions test.

Monitors vs. DTCs

• If a monitor cannot run, then a DTC cannot be set

• Always check to see if the all of the monitors have run and passed

• Some require certain temperatures

Step #4

• Check for any technical service bulletins (TSBs)

• Why not use the resources of many before you?

• I would also suggest using www.iatn.net

• Identifix (free to NATEF certified programs)

Step #5

• Check scan tool data• Look at the “high

authority” sensor information

Step #6

• Narrow the problem to a cylinder or system• The systems could be the fuel, ignition or

emission control system• The cylinder could be just one cylinder or a

bank of cylinders

Step #7

• Find the root cause• The root cause may not be obvious but has to

be found and repaired to prevent a comeback

Step #8

• Verify the repair • Use the same conditions used to verify the

problem to verify the repair• Clear DTCs (not if going to an emission

test????)• Write the story on the work order• The three Cs (Complaint, Cause and

Correction)

Dash Warning Light On?

• Check Engine• Check Engine Soon• Maintenance Required• Service Vehicle Soon• Air Bag• Side Air Bag• Trac

P0304 Example

No-Code Diagnosis

• Many times are tough to locate

• Keep the basics in mind

• The primary purpose of OBDII is emissions-not driveability!

2004 Prius Case Study

• Poor fuel economy (25 mpg instead of normal 40+)

• No codes• Scan data (PIDs) looked normal • Found right front disc brake caliper stuck.• No drop in performance noticed by the driver

Rough Idle; Surge; No Codes

7 5 6 8

Base line for Sensor Values(Except as mentioned)

• Normal operating temperature (cooling fans cycled twice)

• Idle (closed throttle)

• All accessories off

• In Park or Neutral

• Closed loop

Skewed Sensors

• A skewed sensor gives variable readings that appears to be accurate

• However the sensor may be contaminated or dirty and sending incorrect information to the PCM

• Does the PCM know the sensor is skewed?

Data Stream Step #1• Before starting the

engine, connect the scan tool.

• This step is very important, especially if the driveability concern is hard starting or cold driveability.

Data Stream Step #1 (continued)

• Key on/Engine off (KOEO) and look at the values for ECT (engine coolant temperature) and IAT (intake air temperature).

• Basically, the same sensor and the two temperatures should agree.

ECT = IAT• The two temperatures

should be the same (within 5 degrees).

• Both should measure the ambient air temperature.

• If the two indicate different temperatures, the one closer to the ambient air temperature is the one most likely to be correct.

ECT = IAT (continued)• The ECT sensor has a

higher authority than the IAT and is therefore more likely to be the cause of a starting or cold running problem.

• The ECT is the only sensor used by the PCM when the ignition key is first turned from on to start.

Quick and Easy Metric Temperature Conversion

• Double the Celsius degree number• Add 25• Should be close to the Fahrenheit

temperature• Example: 50 degrees X 2= 100+25=125• Actual= 122

Data Stream Step #2MAP= BARO

• Another sensor to check is the MAP sensor because it is a high-authority sensor, especially on speed density controlled engines.

• The MAP reading at KOEO should be atmospheric pressure (about 29.50 in. Hg.), depending on altitude and weather conditions.

• An easier value to remember is that it should be about

4.6-4.8 volts

MAP Sensor Authority

• The MAP sensor is a high-authority sensor on an engine that uses the Speed-Density method of fuel control.

• If the exhaust is rich, try disconnecting the MAP sensor.

• If the engine now runs OK, then the MAP sensor is skewed or giving the PCM wrong information.

MAP Too High or Too Low

• The sensor could be skewed.

• Check the power and ground of the sensor.

• If 5-volt reference (Vref) is low, check other sensors that also use the reference voltage.

Data Stream Step #3IAC Counts

• After the engine starts, observe the IAC counts or percentage.

• The IAC is used to control idle speed by changing the amount of air bypassing the throttle plate (just like depressing or releasing the throttle pedal).

IAC (continued)

• On a warm engine (cooling fans cycled twice), the IAC counts should be 15-25 counts or percentage.

• If the IAC commanded position is low, a vacuum leak (speed density engines mostly) could be indicated.

• The extra air decreases the vacuum and the MAP sensor reads this drop as an increase in load. The PCM adds fuel, increasing the engine speed.

IAC Too High

• If the IAC position is higher than normal. This could indicate a dirty throttle plate(s) or a vacuum leak on a MAF engine.

Higher IAC • A vacuum leak on a MAF

engine is actually false air not measured by the MAF sensor. This can cause the engine speed to decrease due to the leaner-than-normal air-fuel mixture. The mixture causes the PCM to increase engine speed and commands a higher IAC position.

• Note: Some minor vacuum leaks can cause the IAC to drop just like on a speed density engine.

Data Stream Step #4MAF Sensor

• Look at the amount of air entering the engine.

• There should be:– 3 to 7 grams per second (g/s)– About 1 volt (analog MAF sensor)– About 0.5 lb. per hour– About 13-19 kilograms per hour (kg./hr)

Normal MAF Readings

• Use a scan tool to look at the grams per second.

• Warm the engine at idle speed with all accessories off. Should read 3 to 7 grams per second.

• GM 3800 V-6 should read 2.37 to 2.52 KHz.

• If not within this range, check for false air or contamination of the sensor wire.

MAF Sensor Diagnosis

• If the MAF sensor wire were to become coated, it cannot measure all of the incoming air.

• A normal warm engine at idle should be 3 to 7 grams per second.

• Rapidly depress the accelerator pedal to WOT. It should read over: – 100 grams per second (scan tool) or– higher than 7 kHz (digital MAF sensor)– 4 volts (analog MAF sensor)

Visual Inspection

• Look for a very dirty filter.

• Look for a K & N filter that has been over-oiled.

• Look for fuzz on the sensing wire from fibers coming off of the filter paper.

Fuzzy MAF

Mass Air Flow (MAF)-False Air

Usually affects operation in drive; may run OK if driving in reverse.

MAF Sensor Authority

• High-authority sensor

• If in doubt - Take it Out!

• If the MAF sensor is disconnected, the PCM substitutes a backup value.

• If the engine runs OK with the MAF disconnected, then the MAF has been supplying incorrect information.

Data Stream Step #5Injector Pulse-Width

• On a warm engine, the injector pulse-width should be

1.5 to 3.5 milliseconds.

Injector Pulse-Width Too Long• If the PW is higher than

normal (higher than 3.5 ms):– Extra load on the

engine, such as the AC is on or other accessory.

– Engine has a vacuum leak or some other fault, causing a leaner-than-normal air-fuel mixture.

Injector Pulse-Width Too Short• If the injector pulse width is

shorter than 1.5 ms, the

engine could be getting fuel from some other source such as:– from the EVAP system

(purge valve stuck open)– from the fuel in the

crankcase being drawn in through the PCV system

– from a bad fuel pressure regulator

Data Stream Step #6 Oxygen Sensor

• Look at the oxygen sensors for proper operation.

• Upstream sensors should fluctuate higher than 800 mv and lower than 200 mv.

Oxygen Sensor Authority

• The O2S is a high-authority sensor when the engine is operating in closed loop.

• The sensor can add or subtract up to 25% from the base pulse width (some vehicles even more).

• If the sensor is skewed, it can create a driveability problem.

• If in doubt, take it out.• If the sensor voltage is not connected, the

PCM will go into open loop.

Downstream Oxygen Sensor

• The downstream oxygen sensors should be relatively stable and not show too much change in the voltage.

Data Stream Step #7Fuel Trim

• Short-term fuel trim (STFT) is used by the PCM to correct for exhaust readings that are slightly rich or slightly lean.

• STFT can add or subtract fuel quickly.

• STFT can add or subtract only a limited amount of fuel.

Long Term Fuel Trim

• Long-term fuel trim (LTFT) can add or subtract more fuel than STFT.

• LTFT is slower than STFT.

• The purpose of LTFT is to keep STFT within plus or minus 10%.

LTFT and STFT Diagnosis

• I suggest that you ignore STFT.

• STFT is what is happening this instant.

• LTFT gives a history of the exhaust mixture and is more helpful when it comes to a diagnosis.

• Add the two together to get total amount of added or subtracted fuel.

• LTFT = +8%, STFT = -3%, total = +5%.

LTFT Numbers • Ignore any fuel trim numbers less than

10%.

+LTFT Too High

• Look for a vacuum leak.

• Look for a cracked exhaust manifold.

-LTFT Too High

• Check for too high fuel pressure.

• Check for a leaking fuel pressure regulator.

Fuel Trim Example

• LTFT = +12%

• STFT= +2%

• How is the engine running now?

Fuel Trim Example

• Answer:– The engine is operating OK now because the

PCM has compensated for a slightly lean air-fuel mixture by increasing the injector pulse-width by about 14%.

– Look for a vacuum leak or low fuel pressure.

Fuel Trim Cells

• Computers use 16 or more cells for fuel trim corrections.

• Look at the fuel trim numbers in the cell where the problem is occurring.

• For example, looking at the fuel trim numbers at idle, will not show what is happening under a load at highway speed.

Plotting Snap Shot Data

• The trick is to look at the range of sensor values along the side.

• The range represents the high and the low values recorded for that sensor.

Pin Down to a System or Cylinder

• Compression Test• Cylinder Leakage Test• Running (Dynamic) or

Relative Compression Test

Pontiac Grand Prix Case Study (1997 3100 V-6)

• Misfiring at idle on cylinders one, three, and five only

• No DTCs

• When first started, I noticed slow, jerky cranking even with the ignition disconnected (this was not a customer concern).

• What would you do first?

Visual inspection discovered a corroded cable

Injector inspection showed that the wiring for the front and rear banks were switched.

ATF was used through the intake to check to see if the valves were sticking.

Spark testers were installed to check the ignition system for proper operation

Based on information from the archives of www. iatn.net, the intake gasket area was

checked for possible leaks

Spraying around the injector O-rings caused a misfire

Checking the misfire counters confirmed that we had discovered the root cause

Final Result

• Replacing the injector O-rings solved the misfire on cylinders 1, 3, and 5.

• Switching the injector harness back to the correct bank of the injectors solved the slow, jerky cranking

What I Learned

• I learned to use the misfire counter whenever working on a possible engine misfire

• The misfire counters picked up misfires that could not be detected otherwise.

Fuel Delivery System CheckTest Drive Analysis

• Low power complaint– Drive will consist of WOT.

– Please check the oil level first!

– Watch MAP and O2 sensor reaction.

Must maintain at least 850mV

Test Drive Analysis

• If MAP does not go high (4.6-4.8 volts); there is an intake restriction.

• Look for clogged air passages.

• If O2S voltage goes low; the fuel system is likely going lean.

• Look for a weak pump or a clogged fuel filter.

Fuel Pump Pressure Test• All fuel pumps should produce a maximum

pressure (deadhead pressure) about two times the operating pressure.

• Normal operating pressure is 35 psi to 45 psi.(returnless systems are higher)

• Maximum pump pressure should then be between 70 psi and 90 psi.

Quick and Easy Test

• Remove fuel pump relay and install a fused jumper lead between terminals #30 and #87.

• Use a stethoscope and listen for fuel returning to the fuel tank.

• If fuel is not heard, then either the pump is weak or the regulator is defective.

Checking Fuel Pressure

• Connect the gauge to the Schrader valve.

• Record the pressure with engine at idle speed.

Checking Fuel Pressure (continued)

• Rapidly accelerate the engine and watch the pressure gauge.

• If the pressure drops more than 2 PSI, the fuel filter is partially clogged or the pump is weak.

Case StudyChrysler mini van no-start

• Customer stated that it would not start unless he pounded on the dash. Finally, the vehicle wouldn’t start at all.

Step #1-VerifyEngine did not startNo sparkNo Squirt

Step#2- Visual Inspection

• Yep the customer pounded on the dash

• So hard that the needles fell off

Step #3 Check DTCs

Step #4

• Checked for TSBs-nothing that was related

• Checked archives of www.iatn.net

• Bingo- found several fixes for wiring being burned near the CPK sensor

Step #5 Scan Data

• No engine RPM while cranking

Step #6- Narrow Down

Step # 6-Continued

Step #7 Root Cause

Step #8-Verify Repair

Summary• Check monitor status and DTCs early in the

diagnostic process• Always follow the same routine while

diagnosing problems• Use all resources possible including:

– iatn.net– Identifix (free to NATEF certified programs)– Factory and/or aftermarket service information

Contact Information

• jim@jameshalderman.com