Fuel System Diagnosis. Cadillac Concours 1996

For 1990-2009 cars

## Fuel System Diagnosis Fuel Quality

### Fuel

Certain driveability problems can be related to fuel quality. The Fuel Related Driveability Problems table lists some of those conditions and which fuel characteristic may be the cause. However, fuel testing is limited. Therefore, all other system checks must be performed (e.g. Powertrain System, Fuel System, Ignition System, Emission System) before fuel quality is diagnosed. If these checks fail to solve the problem, gain information from the customer about when the condition occurs, when and where the fuel was purchased, and which octane level was used. Ask the customer to change fuel brands. If the symptom persists, perform the Fuel System Diagnosis (Alcohol in Fuel) test. If this is not conclusive, drain the tank and refill with a known good quality fuel.

## Fuel System Diagnosis Alcohol in Fuel

### Alcohol In Fuel

Certain driveability complaints such as hesitation, lack of power, stall, no start, etc. may be caused by an excessive amount of alcohol in fuel. These complaints may be due to fuel system corrosion and subsequent fuel filter plugging, deterioration of rubber components and/or air-fuel mixture leaning.

Various types and concentrations of alcohols are used in commercial fuel. Some alcohols are more detrimental to fuel system components than others. If an excessive amount of alcohol in the fuel is suspected as the cause of a driveability condition, the following procedure may be used to detect the presence of alcohol in the fuel. In this procedure, water is used to extract the alcohol from the fuel. However, the specific type of alcohol (methanol, ethanol or methyl tertiary-butyl ether - MTBE) will not be determined by this procedure.

### Testing Procedure

The fuel sample should be drawn from the bottom of the tank so that any water present in the tank will be detected. The sample should be bright and clear. If the sample appears cloudy, or contaminated with water, as indicated by a water layer at the bottom of the sample. This procedure should not be used, and the fuel system should be cleaned refer to Fuel System Cleaning .

1. Using a 100 ml cylinder with 1 ml graduation marks, fill with fuel to the 90 ml mark.
2. Add 10 ml of water to bring the total fluid volume to 100 ml and install a stopper.
3. Shake vigorously for 10-15 seconds.
4. Carefully loosen the stopper to release pressure.
5. Close the stopper and shake vigorously again for 10-15 seconds.
6. Close the stopper and shake vigorously again for 10-15 seconds.
7. Carefully loosen the stopper to release pressure.
8. Place the graduated cylinder on a level surface for approximately 5 minutes to allow adequate liquid separation.

If alcohol is present in the fuel, the volume of the lower layer (which would now contain both alcohol and water) will be greater than 10 ml. For example, if the volume of the lower layer is increased to 15 ml, it will indicate at least 5 percent alcohol in fuel. The actual amount of alcohol may be somewhat greater because this procedure does not extract all of the alcohol from the fuel.

## Fuel System Diagnosis Driveability Problems Table

#### Fuel Related Driveability Problems Table

Symptom | Action
--- | ---
Poor cold start | Low volatility Injector deposits Refer to Fuel System Diagnosis .
Poor warm up | Low Volatility
Poor cold weather performance | Low Volatility
Poor hot start | High Volatility
Poor hot weather performance | High Volatility
Vapor lock | High Volatility
Spark/detonation | Low octane

Poor fuel economy

High volatility

Injector deposits

Refer to Fuel System Diagnosis .

Intake valve deposits

Refer to Fuel System Diagnosis .

Low octane

Excessive exhaust emissions

Low volatility

Sulfur deposits

Injector deposits

Refer to Fuel System Diagnosis .

Intake valve deposits

Refer to Fuel System Diagnosis .

Reduced Power | Low octane Intake valve deposits Refer to Fuel System Diagnosis . Injector deposits Improper volitility
Rough/unstable idle | Injector deposits Refer to Fuel System Diagnosis .

Always begin the fuel and the air control system diagnosis with, Fuel System Check. This chart will identify other charts for specific checks, such as:

• | Fuel Pressure Check

• | Injector Flow/Voltage Drop Check

• | Power Balance Check

• | Idle Speed Control Check

## Fuel System Diagnosis Fuel System Check

### Circuit Description

Refer to
Engine Controls Components
Fuel Injectors
OBD II Symbol Description Notice
ESD Notice
and
Engine Controls Components
Fuel Injectors
OBD II Symbol Description Notice
ESD Notice
for an electrical diagram of this circuit.

The Fuel System Check provides a quick analysis of the fuel system. This chart then identifies more detailed diagnosis required based on the symptoms present.

The Fuel Leak/Odor Check provides a means of identifying a fuel system component that is causing a leak. This also provides information on how to diagnosis a fuel odor condition.

### Diagnostic Aids

• | Nylon fuel lines are not repairable and must be replaced.

• | Evaporative Emission Control (EVAP) system problems may cause a fuel odor condition.

• | Rich engine operation may also cause a fuel odor condition.

• | Check all component and circuit grounds.

### Test Description

Number(s) below refer to the step number(s) on the Diagnostic Table.

1. If the engine does not start or stay running and there is no fuel pressure check all components and circuit grounds.

2. The vehicle should be operated the same conditions as when the leak/odor occurs.

#### Fuel System Check

Step | Action | Value(s) | Yes | No
--- | --- | --- | --- | ---
1 | Was the Powertrain On-Board Diagnostic (OBD) System Check performed? | -- | Go to Step 2 | Go to A Powertrain On Board Diagnostic (OBD) System Check
2 | Were you sent here from a DTC table? | -- | Go to Step 4 | Go to Step 3
3 | Key On install the Scan Tool. Are any PCM DTC(s) set? | -- | Diagnose those DTC(s) first | Go to Step 4
4 | Crank the engine. Does the engine start and stay running? | -- | Go to Step 5 | Go to Fuel System Pressure Test
5 | Is there a fuel leak or odor complaint? | -- | Go to Step 10 | Go to Step 6
6 | Is there a fuel odor complaint? | -- | Go to Step 11 | Go to Step 7
7 | Is there a fuel gage complaint? | -- | Go to Electrical Diagnosis Instrument Panel Displays | Go to Step 8
8 | Is there a economy complaint? | -- | Go to Poor Fuel Economy | Go to Step 9
9 | Are there any other driveability symptoms? | -- | Go to Fuel System Pressure Test | Fuel System OK
10 | Try to duplicate customer complaint. Is there a fuel leak? | -- | Go Step 12 | Go to Excessive Exhaust Emissions or Odors
11 | Try to duplicate customer complaint. Is there a fuel odor? | -- | Go to Excessive Exhaust Emissions or Odors | Fuel system OK
12 | Repair fuel leak. Is the repair complete? | -- | Go to A Powertrain On Board Diagnostic (OBD) System Check | --

## Fuel System Diagnosis Idle Speed Control Check

### Circuit Description

Refer to
Engine Controls Components
IAC Valve and PSP/ Oil Level/ TR Switches
OBD II Symbol Description Notice
ESD Notice
for an electrical diagram of this circuit.

The Idle Speed Control Check verifies proper IAC valve operation.

### Test Description

Number(s) below refer to the step number(s) on the Diagnostic Table.

1. This check test the IAC valve to make sure that it is moving in and out without binding. Follow PCM DTC P1508 diagnosis to identify the cause of a faulty IAC valve.

2. This procedure checks for high minimum air idle which could cause a high idle at closed throttle. It is OK if the engine cannot stay running at minimum air idle.

3. This procedure checks for IAC ability to increase idle speed.

#### Idle Speed Control Check

Step | Action | Value(s) | Yes | No
--- | --- | --- | --- | ---
1 | Was the Powertrain On-Board Diagnostic (OBD) System Check performed? | -- | Go to Step 2 | Go to A Powertrain On Board Diagnostic (OBD) System Check
2 | Were you sent here from a DTC table? | -- | Go to Step 4 | Go to Step 3

3

1. Connect the scan tool.
2. Turn the Key to the On position.

Are any PCM DTC(s) set?

--

Diagnosis those DTC(s) first

Go to Step 4

4

1. Start and idle the engine.
2. Turn all of the accessories Off.
3. Connect the scan tool.
4. Select IAC System in Misc. Tests.
5. Command the engine speed up 1500 RPM, down to 500 RPM then back to 1500 RPM while monitoring the Engine Speed display.

Does the Engine Speed remain within the specified value of Desired Idle for each RPM command?

50 RPM

Go to Step 5

Go to DTC P1508 Idle Speed Low - Idle Air Control (IAC) System Not Responding

5 | Command IAC to minimum RPM (full closed position). Is idle above the value specified? | 500 RPM | Go to Step 6 | Go to Step 7

6

1. Inspect for vacuum leaks in the air intake system and vacuum system.
2. Repair vacuum leaks if any.

Was a leak found and repaired?

--

System OK

Go to Step 9

7 | Command IAC to maximum RPM (full open position). Is idle below the value specified? | 1500 RPM | Go to Step 8 | System OK
8 | Replace IAC valve. Refer to Idle Air Control Valve Replacement . Is repair complete? | -- | System OK | --
9 | Replace throttle body. Refer to Throttle Body Assembly Replacement . Is the repair complete? |  | System OK | Go to A Powertrain On Board Diagnostic (OBD) System Check

## Fuel System Diagnosis Power Balance Check

### Circuit Description

Refer to

#### Fuel Injectors

Engine Controls Components
IAT/ ECT/ TP/ MAP and BBV Sensors
Fuel Pump and Relay
OBD II Symbol Description Notice
ESD Notice
for an electrical diagram of this circuit.

The Fuel Injector Balance Test compares the performance of each of the cylinder by using the PCM to shut off fuel injection. This measures the performance of each cylinder by the amount of RPM drop caused by that cylinder not producing a power stroke. If proper fuel delivery has been determined, then a cylinder that does not show good performance (RPM drop) indicates either an ignition or mechanical problem.

### Test Description

Number(s) below refer to the step number(s) on the Diagnostic Table.

1. Idle quality should get worse when an injector is disabled. The engine should miss and the idle will become rough.

#### Power Balance Check

Step | Action | Value(s) | Yes | No
--- | --- | --- | --- | ---
1 | Was the Powertrain On-Board Diagnostic (OBD) System Check performed? | -- | Go to Step 2 | Go to A Powertrain On Board Diagnostic (OBD) System Check
2 | Has the Fuel System Check been performed? | -- | Go to Step 3 | Go to Fuel System Diagnosis (Fuel System Check)

3

1. Connect the scan tool and select Inj. Balance test.
2. Record change in engine speed and idle quality as each cylinder is disabled.

Were engine speed and idle quality changes for all cylinders equal?

--

Go to EVAP Control System Diagnosis

Go to Electronic Ignition (EI) System Diagnosis (Ignition System Check)

## Fuel System Diagnosis Injector Flow Drop Check

Refer to

#### Fuel Injectors

Engine Controls Components
IAT/ ECT/ TP/ MAP and BBV Sensors
Fuel Pump and Relay
OBD II Symbol Description Notice
ESD Notice
for an electrical diagram of this circuit.

The Injector Flow/Voltage Drop Check compares the fuel pressure drop when each injector is pulsed for a set amount of time. Since each injector is pulsed the same, the fuel pressure drop for each injector should be approximately the same. Suspect fuel injectors can be identified by either too high or too low fuel pressure drop. This check also determines if the voltage drop of the injector is within specification. Injector voltage drop out of specification can cause driveability conditions that are intermittent or that do not show up during an injector flow check.

#### Tools Required

J 34730-1A Fuel Pressure Gauge, J 39200 Voltmeter, J 39021 Fuel Injector Tester.

### Test Description

Number(s) below refer to the step number(s) on the Diagnostic Table.

1. Commanding the fuel pump on for 5 seconds will energize the fuel pump to build fuel pressure. Then selecting individual injectors will allow you to see the fuel pressure drop associated with each injector. If all eight injectors drop some pressure then the circuit is not open. Any injector that drops no pressure has an open in the circuit or a faulty injector.

2. Checking if a fuse was open causing either injectors 2, 3, 5, 8, or 1, 4, 6 and 7 not to operate.

3. Checking if CKT 739 or CKT 1239 is open from the fuse to C127.

Step | Action | Value(s) | Yes | No
--- | --- | --- | --- | ---
1 | Was the Powertrain On-Board Diagnostic (OBD) System Check performed? | -- | Go to Step 2 | Go to A Powertrain On Board Diagnostic (OBD) System Check
2 | Has the Fuel System Check been performed? | -- | Go to Step 3 | Go to Fuel System Diagnosis (Fuel System Check)

3

1. Place key in lock position.
2. Install fuel pressure gage, J 34730-1A, fuel injector tester, J 39021 and the Scan Tool.
3. Turn key On, select Misc Tests then select Output Tests and then FP Relay command the fuel pump On.
4. Monitor the fuel pressure, select Fuel Injector Flow Test on the fuel injector tester and test fuel injector #1.
5. Monitor the fuel pressure after injector test and subtract from the fuel pressure noted prior to the injector test.
6. Repeat steps 2 through 4 for injectors 2 through 8.

Does fuel pressure drop when each injector is tested?

--

Go to Step 7

Go to Step 4

4 | Do two or more fuel injectors have no pressure drop? | -- | Go to Step 5 | Go to Step 11

5

1. Place key in lock position.
2. Remove and inspect I/P fuse block injector 1 and injector 2 fuse.

Are both fuses OK?

--

Go to Step 6

Go to Step 14

6

1. Disconnect the fuel rail connector and key On.
2. Measure voltage to ground on pins N and P (engine harness side).

Do both pins measure greater than 10 volts?

--

Go to Step 15

Go to Step 17

7 | Select Coil Test on fuel injector tester and test all injectors. Did voltage reading for any injector exceed the specified value? | 9.5V | Go to Step 18 | Go to Step 8
8 | Inspect the highest reading and subtract the specified value. Was the voltage for any injector less than this value (more than .55 volts less than the highest)? | 0.55V | Go to Step 19 | Go to Step 9
9 | Have any injectors been replaced up to this point? | -- | Go to Step 20 | Go to Step 10
10 | Were the fuel pressure drops recorded in step 2 for all injectors within the specified value? | 2 PSI | Go to Fuel System Diagnosis | Go to Step 21
11 | Select Coil Test on the fuel injector tester and test injector which had no fuel pressure drop. Does coil test voltage measure between the specified value? | 5.74-7.0 V | Go to Step 12 | Go to Step 13

12

1. Reconnect the fuel rail connector, disconnect PCM connector C2, and Key On.
2. Measure voltage to ground on PCM pin for suspect injector.

Does voltage measure the same or greater than the specified value?

10V

Go to Step 22

Go to Step 23

13 | Check fuel rail harness for opens or shorts in suspect injector circuits. Open or short found? | -- | Go to Step 24 | Go to Step 25
14 | Check CKT(s) 739 and 1239 for shorts to ground. Was a short found? | -- | Go to Step 26 | Go to Step 27
15 | Measure resistance at the fuel rail connector (fuel rail side) across suspect injector circuits. Do any measure less than the first specified value or greater than the second specified value? | 8 to 25 ohms | Go to Step 16 | Go to Step 12
16 | Connect fuel injector tester to suspect injector(s) and select Coil Test. Does voltage reading from coil test measure between the specified values? | 5.74-7V | Go to Step 28 | Go to Step 29
17 | Repair open CKT 739 or CKT 1239 between the fuse and the fuel rail connector. Was a repair made? | -- | Go to A Powertrain On Board Diagnostic (OBD) System Check | --
18 | Replace injector(s) whose voltage reading exceeded the specified valve. Was a repair made? | -- | Go to A Powertrain On Board Diagnostic (OBD) System Check | --
19 | Replace injector(s) whose voltage reading was more than 0.55 volts less than the highest voltage reading Was a repair made? | -- | Go to A Powertrain On Board Diagnostic (OBD) System Check | --
20 | Duplicate conditions under which customer complaint occurred and verify proper operation. Is the system operating OK? | -- | System OK | --
21 | Replace injector(s) whose pressure drop was not within the specified value of the others. Was a repair made? | -- | Go to A Powertrain On Board Diagnostic (OBD) System Check | --
22 | Check PCM terminal contact and if OK, replace the PCM, refer to PCM Replacement/Programming . Was a repair made? | -- | Go to A Powertrain On Board Diagnostic (OBD) System Check | --
23 | Repair open between the fuel rail connector and the PCM in suspect circuit. Was a repair made? | -- | Go to A Powertrain On Board Diagnostic (OBD) System Check | --
24 | Repair circuit. Was a repair made? | -- | Go to A Powertrain On Board Diagnostic (OBD) System Check | --
25 | Replace injector. Was a repair made? | -- | Go to A Powertrain On Board Diagnostic (OBD) System Check | --
26 | Repair short and replace blown fuse. Was a repair made? | -- | Go to A Powertrain On Board Diagnostic (OBD) System Check | --
27 | Replace blown fuse. Was a repair made? | -- | Go to A Powertrain On Board Diagnostic (OBD) System Check | --
28 | Repair fuel rail harness for suspect injector. Was a repair made? | -- | Go to A Powertrain On Board Diagnostic (OBD) System Check | --
29 | Replace suspect injector. Was a repair made? | -- | Go to A Powertrain On Board Diagnostic (OBD) System Check | --

## Fuel System Diagnosis Fuel System Deposits

### Fuel System Deposits

Fuel system deposits can cause various driveability problems. Deposits usually occur during hot soaks after key Off. Poor fuel quality or driving patterns such as short trips followed by long cool down periods can cause injector deposits. This occurs when the fuel remaining in the injector tip evaporates and leaves deposits. Leaking injectors can increase injector deposits. Deposits on fuel injectors affect their spray pattern, which in turn could cause reduced power, unstable idle, hard starts and poor fuel economy.

Intake valve deposits can also be related to fuel quality. While most fuels contain deposit inhibitors, some do not and the effectiveness of deposit inhibitors varies by manufacturer. If intake valve deposits occur, fuel may be suspected. These deposits can cause symptoms such as excessive exhaust emissions, power loss and poor fuel economy.