1995 GMC C2500 Black soot and smells like gas

Tiny
NSENYK117
  • MEMBER
  • 1995 GMC C2500
  • V8
  • 4WD
  • AUTOMATIC
  • 215,000 MILES
I have soot in my exhaust pipe and if I hold my hand in front of it while someone revs it up I will have it all over my hands with the gas. It does NOT smoke however. I know its running rich but its not smoking. New plugs, wires, cap, rotor, ignition coil. It does effect performance a little. The engine idles a little rough but does not stall. I have maybe 2 plugs that look like they could be fouling but the others seem ok. I am thinking fuel pressure regulator. How can I test it to be sure before I replace it?
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Sunday, September 13th, 2009 AT 6:05 PM

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Tiny
DR LOOT
  • EXPERT
The way to check the fuel pressure regulator is to move the vacuum line and it will have full press, if nothing changes it is not the fuel pressure regulator. You failed to mention that you change the fuel filter, or check the fuel pressure? All symptoms are fuel related.
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Sunday, September 20th, 2009 AT 4:19 PM
Tiny
NSENYK117
  • MEMBER
Yes the fuel filter is new. It is a 350 TBI so I havent checked the fuel pressure. The only vacum line coming out of the TBI goes to the MAP sensor. No SES light or codes. It acts like its missing, rough idle, shudder. Its not major and its not stalling or smoking. I have repaired all the vacum leaks I could find. Engine temp sensor is good. At this point I was going to start checking sensors. Im starting to wonder if its burning a small amount of oil to effect performance but not make a cloud of smoke?
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Sunday, September 20th, 2009 AT 7:05 PM
Tiny
DR LOOT
  • EXPERT
Checking the fuel pressure is critical, if there is no engine light on, I don't believe it is a sender. It is fuel related, you failed to mention your engine size so I will assume it is a 5.7. This is how to check the fuel pressure regulator:
FUEL DELIVERY NOTE:For fuel system pressure testing, see BASIC TESTING - 5.7L article. Fuel Pressure Regulator (Port Fuel Injection)Fuel pressure regulator is a vacuum-controlled diaphragm type, which uses manifold vacuum to modify fuel pressure to compensate for engine load fuel requirements. Connect fuel pressure gauge to fuel pressure service port. Start engine and note fuel pressure. Disconnect vacuum hose from fuel pressure regulator. Fuel pressure should increase 4-10 psi (.28-.70 kg/cm2 ). If pressure does not increase 4-10 psi (.28-.70 kg/cm2 ), check for presence of manifold vacuum at signal line. If vacuum is not present, check for kinked, cut or split vacuum hose or plugged throttle body vacuum port. If vacuum is present and no pressure change occurs, replace fuel pressure regulator.

You mentioned there was no vacuum line, so I will give you the instructions to check the mechanical regulator as follows:

Fuel Pressure Regulator (Central & Throttle Body Injection)Fuel pressure regulator is mechanically controlled by internal spring pressure. Regulator is adjusted at factory and is not serviceable. If fuel pressure is too low, check for restricted delivery line. Also, check fuel pump pressure and volume. If fuel pressure is too high, check for restricted fuel tank return line or fuel filter. If no faults are found and pressure is too high or too low, replace fuel pressure regulator.

When you are done performing the above diagnosis, you can perform the fuel system pressure test.

FUEL SYSTEM PRESSURE TEST WARNING: Begin fuel system trouble shooting and diagnosis with fuel system pressure test. Relieve fuel system pressure before disconnecting any components or installing fuel pressure gauge. TBI Turn ignition off for 10 seconds. Turn ignition on, and listen at fuel tank for fuel pump operation. Pump should run 2 seconds (20 seconds on models with fuel module). If fuel pump runs, go to next step. If fuel pump does not run, go to step 7). If fuel pump runs, turn ignition off. Verify fuel tank has fuel. Relieve fuel pressure. See FUEL SYSTEM PRESSURE RELIEF. Remove air cleaner, and plug air cleaner vacuum ports (if equipped). Disconnect fuel line between throttle body and fuel filter. Install Fuel Pressure Gauge (J-29658-D) and Adapter (J-2968A-85) between steel fuel line and flexible hose, on outlet side of fuel filter. Turn ignition on, and note reading on pressure gauge. If fuel pressure is 9-13 psi (.63-.91 kg/cm2 ), no problem exists. If pressure is less than either of the low readings, go to step 5). If pressure is greater than either of the higher readings, turn ignition off, and bleed fuel pressure. Disconnect fuel return line downstream of pressure gauge. Insert return line into a gasoline container. Turn ignition on. If pressure is now as specified, correct restriction in fuel return line between disconnected point and fuel tank. If fuel pressure is greater than either of the higher readings, check for restricted return line (including fuel filter) downstream of pressure gauge. If no restrictions are present, replace fuel pressure regulator assembly. Check for restricted fuel line between in-tank fuel pump and pressure regulator. If fuel line is okay, disconnect injector connector. Turn ignition on. Gradually pinch fuel pressure gauge outlet hose. Note pressure.
CAUTION:DO NOT pinch off fuel return line completely. DO NOT allow fuel pressure build-up to exceed specification, as damage to fuel pressure regulator may occur. If pressure is greater than 13 psi (.91 kg/cm2 ), replace fuel pressure regulator. If pressure is less than 9 psi (.63 kg/cm2 ), check for faulty fuel pump or incorrect part. Check fuel pump coupling hose and pump inlet filter in fuel tank. On models with dual fuel tanks, check for faulty fuel tank selector valve and meter switch. Apply 12 volts to fuel pump test connector. For fuel pump test connector location, see under hood engine component views under COMPONENT LOCATIONS in SYSTEM/COMPONENT TESTS - 5.7L article. If fuel pump now runs, repair open in fuel pump relay drive or power circuit or repair faulty fuel pump relay. To test relay, see SYSTEM/COMPONENT TESTS - 5.7L article. If fuel pump does not run with 12 volts applied to fuel pump test connector, repair open in fuel pump power or ground circuit, or repair faulty fuel pump.
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Sunday, September 20th, 2009 AT 10:17 PM

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