TS-03, Troubleshooting - Fuel System

Introduction

This troubleshooting guide is intented to help identify and repair Fuel System problems. The following is a list of problem symptoms discussed in this guide:

Excessive Fuel Consumption

Possible Causes
Checks
Procedure
Dirty Air Filter Check / Change Air Filter N/A
Bad O2 Sensor (if equipped). Check sensor and replace if necessary. FUEL-18
Poor Ignition System Function Check the condition of the distributor cap and rotor.
Check the condition of the spark plugs and spark plug wires.
IGN-04

Strong Fuel Smell In Passenger's Compartment

Possible Causes
Checks
Procedure
Fuel Level Sender Gasket Using ELECT-13, locate the fuel level sender and check the condition of the gasket around the sender cap (1985.5 and newer). If it appears hard/brittle, remove the sender and replace the gasket. The part number for the gasket is 944-201-327-00.

Early 944s aren't as susceptable to this problem as they use an o-ring seal between the sender and the top of the gas tank. The sender is held in place by 5 cheesehead screws each with it's own seal ring. You can get fuel smell from the early cars as well if any of these seals go bad. The part number for the sender o-ring is 477-919-133. The seal rings for the cheesehead screws are part number M 013 802 1.

ELECT-13
Fuel Tank Ventilation System The fuel tank has a ventilation system which is designed to prevent pressure buildup in the tank by removing fuel vapor. The system has hoses that run from the tank along the bottom of the vehicle on the right hand side, into the engine compartment, along the back of the firewall, to a charcoal filter canister in the left fender well (must remove the interior fender well cover to locate canister).

If any of those fuel lines is loose, cut, or leaking, you will get a strong fuel smell inside the passenger's compartment. Also, if the charcoal cannister itself is split or otherwise leaking you'll get a raw fuel smell in the passenger's compartment.

N/A
Fuel Filler or Fuel Reservoir System Leak There are a number of rubber hoses and rubber sleeves associated with the fuel filler and fuel reservoir system which can cause a fuel smell in the passenger's compartment if they develop holes or if their hose clamps come loose. The filler system and reservoir components are located in the rear hatch area behind the carpet on the fuel filler side of the car and behind the paneling on the inside of the roof pillar on that same side. See the diagrams below for exploded views of the system.

Early Fuel Filler and Reservoir (Pre-1985.5)

Late Fuel Filler and Reservoir

One specific problem related to the fuel filler/reservoir system is the fuel system ventilation valves. When these valves stick, it can result in fuel vapors leaking into the passenger's compartment. If you get a noticeable "whooshing" sound (sorry for the use of the non-technical teminology) when you remove the fuel filler cap, there's a good chance that the ventilation valve is stuck and is causing your fuel smell.

For the early fuel system, there are two valves. There is a shut off valve (#19 in the Early Fuel Diagram) and a ventilation valve (#20). The part number for the shut off valve is 928 201 083 02. The part number for the ventilation valve is 810 201 186 A.

The late fuel system only uses one valve (#17 in the Late Fuel Diagram) which is the same shut off valve used on the early cars (928 201 083 02).

Another specific problem is a leaking fuel cap gasket. For the early cars the gasket (#30 in the Early Fuel Diagram) is part number 477 201 553. For the late cars, the gasket (#5 in the Late Fuel Diagram) is part number 944 201 553 00. I suspect it's the same part for both and Porsche just changed to a Porsche specific part number later on.

Apparently another problem which can result in the fuel cap not sealing properly is elongation of the fuel filler neck (#12 in Early Fuel Digram, #7 Late). Over time, as the fuel system is repeatedly filled, the weight of the fuel nozzle hanging in the fuel filler neck can cause the neck to distort in shape. When this happens the fuel cap will not properly seal and you'll get fumes into the passenger's compartment. The fuel filler neck is a sleeve which is replaceable. The part number for early cars with chassis code up to 94EN4 00060 is 931 201 281 01. For all other 944s is 931 201 281 02.

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Clark's Garage 1998