A-8 Engine Performance / L1 Advanced Level ASE Test Preparation   

 

Composite Vehicle
is equipped with either a MAP sensor or a MAF sensor.

RPM: is a magnetic pickup but the scan tool just shows the RPM

LOAD: the MAP sensor puts out an analog voltage. Idle at 18"Hg will give us 1.0 v, and wide open throttle with 0"Hg will put out 4.5 v. The MAF sensor option is also an analog voltage. Idle will be about 0.5 v and wide open throttle will be about 4.5 v

TPS: is an analog potentiometer that gives us a variable voltage, with 1.0 v at closed throttle idle, and 4.5 v at wide open throttle.

ECT: Engine coolant temperature sensor that is a negative coefficient thermister which gives 3.5 v at 32 degrees F and 0.5 v at 195 degrees F.

O2: Normal zirconium oxygen sensor where values below 0.45 v are leaner than 14.7:1 and values higher than 0.45 v are air-fuel ratios richer than 14.7:1.

LFTRIM: Given in %, where +% is adding time to the injector pulse width to richen the mixture, and -% is subtracting time from the injector pulse width. Normal values can be between + 10% and - 10%.  Maximum range is -25% to +25%.

 


Red Kap Automotive Mechanics Clothing

Scan Tool Diagnosis #2

The questions below have be reprinted with permission from Professor Steve McAfee, www.Smogsite.com

Composite Vehicle Specifications
are located on left side, under menu

1. An engine lacks power. It starts fine and idles fine, but when driven down the road the engine lacks power and the car won't go fast. The scan tool data was taken while trying to cruise down a city street. What's wrong?

Frame

-8

-7

-6

-5

-4

-3

-2

-1

0

TPS: (v)

3.2

3.2

3.2

3.2

3.2

3.2

3.2

3.2

3.2

MAP: (v)

4.4

4.4

4.4

4.4

4.5

4.5

4.5

4.5

4.5

ECT: (v)

0.5

0.5

0.5

0.5

0.5

0.5

0.5

0.5

0.5

O2: (v)

0.9

0.9

0.9

0.9

0.9

0.9

0.9

0.9

0.9

RPM:

1700

1700

1700

1700

1800

1800

1800

1900

1900

INJ: (ms)

6.4

6.4

6.4

6.4

6.5

6.5

6.5

6.5

6.5

LFTRIM: (%)

-6

-6

-6

-6

-6

-6

-6

-6

-6

A spark plug is fouled.
The air filter is plugged.
The catalytic converter is plugged.
The TPS has an intermittent glitch.

 

2. An engine surges intermittently when driven at cruise speeds. All on it's own it seems to go a little faster, then a little slower, then a little faster again. And the customer says his fuel economy isn't as good as it used to be. The scan tool data was taken at cruising speed as it surged. What is wrong?

Frame

-8
-7
-6
-5
-4
-3
-2
-1
0

TPS: (v)

2.4
2.4
2.4
2.4
2.4
2.4
2.4
2.4
2.4

MAP: (v)

2.6
2.6
2.6
2.6
2.6
2.6
2.6
2.6
2.6

ECT: (v)

0.5
0.5
3.2
3.4
0.5
0.5
0.5
0.5
0.5

O2: (v)

0.6
0.2
0.9
1.0
0.7
0.3
0.7
0.1
0.8

RPM:

2300
2300
2400
2450
2350
2300
2300
2300
2300

INJ: (ms)

5.1
5.1
5.8
5.9
5.1
5.1
5.1
5.1
5.1

LFTRIM: (%)

+3
+3
+3
+3
+3
+3
+3
+3
+3

Intermittent injector.
TPS intermittent.
Timing belt jumped a tooth.
Corroded connection at ECT.

 

3. An engine failed a smog test. The customer says, "It just doesn't seem to run very well." Take a look at the following scan tool data and tell us what you think. (PS: the data was collected while the technician was adding propane to the air intake.):

Frame

-8
-7
-6
-5
-4
-3
-2
-1
0

TPS: (v)

1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0

MAP: (v)

1.1
1.1
1.1
1.1
1.1
1.1
1.1
1.1
1.1

ECT: (v)

0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5

O2: (v)

0.0
0.0
0.0
0.1
0.1
0.1
0.2
0.2
0.2

RPM:

800
800
810
800
810
800
810
800
800

INJ: (ms)

1.9
1.9
1.9
1.9
1.9
1.9
1.9
1.9
1.9

LFTRIM: (%)

+6
+6
+6
+6
+6
+6
+6
+6
+6

Bad and sluggish O2 sensor.
High fuel pressure.
MAP sensor out of range.
ECT sensor out of range.

 

4. An engine is running so rich it has black smoke coming out the tailpipe. It almost doesn't run at idle. Needless to say, it failed a smog test at idle. And the customer said he was just working on it to try and make it run better. In this question, we only have a snapshot of one frame that shows what is going on. The problem is not intermittent, so it should be enough information for you to get an idea. What do you think is wrong?

RPM:

2560

MAP:

3.6 v

TPS:

1.8 v

ECT:

0.6 v

IAT:

2.1 v

O2:

0.9 v

LFTRIM:

-23 %

INJ:

4.3 ms

MAP sensor hooked up to ported vacuum signal.
Thermostat keeps engine too cold.
O2 sensor is dead.
Fuel pressure way too high.

 

5. A car failed a smog test at idle, but passed the test at cruise RPM. The 4-gas emissions readings at idle were:

HC: 136 ppm
CO: 0.6%
CO2: 14.1 %
O2: 0.6%

Use the Scan tool data captured at the right and the readings above to decide what do you think is wrong.

RPM:

900

MAP:

1.1 v

TPS:

1.0 v

ECT:

1.0 v

IAT:

2.0 v

O2:

1.2-1.8 v

LFTRIM:

-3 %

INJ:

2.1 ms

EGR valve stuck partly open.
MAP sensor out of range.
#2 spark plug wire fell off.
Thermostat keeps engine too cold.

 

6. A car failed a smog test at idle, and the cruise RPM results weren't very good either. At idle it was too rich, as the emission readings below indicate:

HC: 124
CO: 1.4%
CO2: 13.9%
O2: 0.1%

As usual, make your best choice from the four choices:

RPM:

900

MAP:

1.1 v

TPS:

1.0 v

ECT:

0.5 v

IAT:

2.0 v

O2:

0.9 v

LFTRIM:

-14 %

INJ:

1.9 ms

Intake Air Temp sensor has open ground circuit.
Fuel pump pressure below specs.
Vacuum line fell of fuel pressure regulator.
PCV valve opening flows too much air.

 

7. A car failed a smog test at idle. Strange, it has almost the same values as another question, but for a different reason. Look at the emission readings below and the scan tool data to the right and take your best guess. Oh yes, the customer mentioned just as he was leaving that the yellow light was on in the dash now, and it hadn't been in the past. (MIL, check engine light)

HC: 144
CO: 1.6%
CO2: 13.9%
O2: 0.1%

RPM:

900

MAP:

0 v

TPS:

1.0 v

ECT:

0.5 v

IAT:

2.0 v

O2:

0.9 v

LFTRIM:

-0 %

INJ:

2.3 ms

Fuel pressure is high.
Bad catalytic converter.
Plugged exhaust system.
Computer is in limp mode.

 

8. A car failed a smog test. (We seem to be having a lot of failed smog checks in our shop lately.) It passed at cruise RPM, but failed at idle. So we looked at the scan tool data and displayed it for you on the right. Pretend you are a highly paid consultant that goes around diagnosing engines that shops can't fix and this is your next job. The idle emission readings are below. What's wrong with this car?

HC: 465
CO: 1.35%
CO2: 12.9%
O2: 0.1%

RPM:

900

MAP:

1.8 v

TPS:

1.0 v

ECT:

0.5 v

IAT:

1.3 v

O2:

0.2-0.8 v

LFTRIM:

-5 %

INJ:

2.3 ms

Gap on #2 spark plug is way too small.
MAP sensor vacuum line has a cut.
EGR valve is stuck partly open at idle.
Both B and C.

 

 

 
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