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Gas Mass Flow Sensor Error
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Overview
| CODE | REASON | EFFECT |
| Fault Code: 751 PID: P183 SPN: 183 FMI: 2 LAMP: Amber SRT: |
Gas Mass Flow Sensor Error. The sensed gas flow does not match the desired gas flow. |
Possible reduced performance. |
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Gas Mass Flow Sensor Circuit |
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Circuit Description
The electronic control module (ECM) uses the gas mass flow sensor to measure fuel into the engine and to adjust the fuel control valve based on this measurement. The gas mass flow sensor is powered by battery voltage.
Component Location
The gas mass flow sensor is located on the air/fuel mixer housing on L Gas Plus engines.
Shop Talk
The L series uses a supply voltage converter in series with the voltage supply to the gas mass flow sensor to provide the needed 15.5 VDC to operate correctly. The possible fail modes are the following: Improper fuel pressure or out of fuel, a stuck or sticking fuel control valve not correcting gas flow according to gas mass flow sensor, restriction due to deposits in the laminar flow screen and oil contamination in the mixer housing and gas flow sensor.
Troubleshooting Steps
| STEPS | SPECIFICATIONS | |
|---|---|---|
| STEP 1. | Check the fault codes. | |
| STEP 1A. Check for active Fault Codes 455 and 511. | Fault Codes 455 and 511 active? | |
| STEP 1B. Check for an inactive fault code. | Fault Code 751 inactive? | |
| STEP 2. | Check the fuel system. | |
| STEP 2A. Check the fuel supply | Fuel supply sufficient? | |
| STEP 2B. Check the primary fuel inlet pressure. | Fuel pressure between 517 to 1138 kPa [75 to 165 psia]? | |
| STEP 2C. Check the secondary fuel inlet pressure. | Is pressure above 75 psia? | |
| STEP 2D. Check for a stuck open or sticking fuel control valve. | Secondary fuel pressure drop within 50 seconds of key on? | |
| STEP 2E. Check the laminar flow screen. | Deposits or evidence of oil? | |
| STEP 2F. Check for oil in the air fuel mixer housing. | Evidence of oil? | |
| STEP 3. | Clear the fault codes. | |
| STEP 3A. Disable the fault code. | Fault Code 751 inactive? | |
| STEP 3B. Clear the inactive fault codes. | All fault codes cleared? | |
Guided Step 1 – Check the fault codes.
| Guided Step 1A – Check for Fault Codes 455 and 511 for being active. | |
|---|---|
Conditions
Action
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Fault Code 455 or 511 active? |
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| YES | NO |
| No Repair | No Repair |
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Fault Codes 455 or 511
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| Guided Step 1B – Check for an inactive fault code. | |
|---|---|
Conditions
Action
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Fault Code 751 inactive? |
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| YES | NO |
| No Repair | No Repair |
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Procedure 019-362
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Guided Step 2 – Check the fuel system.
| Guided Step 2A – Check the fuel supply. | |
|---|---|
Conditions
Action
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Fuel supply sufficient? |
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| YES | NO |
| No Repair |
Refuel the vehicle. |
| Guided Step 2B – Check the primary fuel inlet pressure. | |
|---|---|
Conditions
Action
Note: All pressure sensors on Gas Plus engine are absolute pressure sensors. |
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Fuel pressure between 517 to 1138 kPa [75 to 165 psia]? |
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| YES | NO |
| No Repair |
Replace the primary regulator. Refer to the OEM troubleshooting and repair manual. |
| Guided Step 2C – Check the secondary fuel inlet pressure. | |
|---|---|
Conditions
Action
Note: All pressure sensors on Gas Plus engine are absolute pressure sensors. |
|
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Is pressure above 517 kPa [75 psia]? |
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| YES | NO |
| No Repair |
Replace the secondary regulator. Refer to Procedure 005-047 in the C8.3g, C Gas Plus and L Gas Plus Troubleshooting and Repair Manual, Bulletin 3666206. |
| Guided Step 2D – Check for a stuck or sticking fuel control valve. | |
|---|---|
Conditions
Action
Note: If the fuel control valve is stuck open, raw fuel will be dumped into the engine and out the exhaust. This will cause the gas mas flow sensor to read more fuel than is commanded by the ECM. This will allow Fault Code 751 to occur, but the fuel control valve is actually the root cause. |
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Secondary fuel pressure drop within 50 seconds of keyswitch on? |
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| YES | NO |
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A defective fuel control valve has been detected. Replace the fuel control valve. Refer to Procedure 019-102. Use INSITE™ electronic service tool to reset the fuel tables. |
No Repair |
| Guided Step 2E – Check the laminar flow screen. | |
|---|---|
Conditions
Action
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Deposits or evidence of oil? |
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| YES | NO |
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Replace the fuel shutoff valve. Refer to Procedure 019-050. |
No Repair |
| Guided Step 2F – Check for oil in the air fuel mixer housing. | |
|---|---|
Conditions
Action
Note: Check the fuel filter for excess oil due to improper maintenance. If there is more than one ounce of oil in the fuel filter than there is a possibility that oil has contaminated the fuel system. The customer will need to check their gas compression station for oil carry over. |
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Evidence of oil? |
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| YES | NO |
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Replace the gas mass flow sensor. Refer to Procedure 019-101. Clean the fuel system Refer to Procedures 005-009 and 005-047 in the C8.3g, C Gas Plus and L Gas Plus Troubleshooting and Repair Manual, Bulletin 3666206. Note: Other objects in the fuel mixer housing and fuel system will possibly need to be changed if there is significant evidence of oil contamination. Use INSITE™ electronic service tool to reset the fuel tables. |
No Repair |
Guided Step 3 – Clear the fault codes.
| Guided Step 2A – Disable the fault code. | |
|---|---|
Conditions
Action
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|
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Fault Code 751 inactive? |
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| YES | NO |
| No Repair | No Repair |
| Guided Step 3B – Clear the inactive fault codes. | |
|---|---|
Conditions
Action
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All fault codes cleared? |
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| YES | NO |
| No Repair | No Repair |
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Repair complete
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Appropriate troubleshooting charts
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