1991 Chevy Corvette - Fuel Pressure Problems

Technically Speaking

James Berry Nov 6, 2013 0 Comment(s)
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Q:

I own a 1991 Corvette that’s developed an unusual problem: It will not start when cold unless I squirt ether starting fluid into the intake manifold, at which point it starts immediately. Once started, everything works perfectly—until I shut it off.

I would like to repair the car myself to save some money. I have a good knowledge of auto repair and the normal do-it-yourselfer’s set of tools. Is it possible that I just need a good tune-up?

A:

It sounds like your Corvette is an ether junkie, but I think with a little rehab we can get it back on the right track. As we have discussed in earlier articles, it takes three things to make an engine run: compression, a fuel/air charge, and spark. You’ve already eliminated two of those elements, so we can focus our diagnostics on what appears to be a fuel problem.

Common fuel-system problems on C4 Corvettes include the following:

  • Faulty fuel pump
  • Restricted fuel filter
  • Faulty fuel-pressure regulator
  • Leaking fuel injectors
  • Faulty oil-pressure sending unit
  • Faulty pickup coil (located inside the distributor)
  • Throttle-position-sensor (TPS) voltage too high
  • Faulty mass-airflow sensor (MAF)
  • Faulty coolant-temperature Sensor
  • Faulty engine controller

Let’s start by installing a fuel-pressure gauge on the Schrader valve on the passenger-side fuel rail, near the rear of the engine (Image A). When the key is turned to the Run position, the fuel pump will activate for two seconds, pressurizing the fuel rails and giving a reading of between 40 and 42 psi of pressure. The reading should jump to 38-40 psi once the engine is started.

Fuel Pressure 2/8

A

Fuel-Pressure Diagnostics

1. If no fuel pressure is present, use a test light to verify that all of the fuses are working.

2. Assuming the fuses check out, you’ll need to gain access to the fuel pump (Image B). Fortunately this is fairly easy on C4 Corvettes. Just remove the cover under the gas-door lid to reveal the pump.

Fuel 3/8

B

  • Next, have someone turn the key to the Run position and see if the pump activates. If not, tap on the pump while someone cranks the engine. If the vehicle starts, the pump has an internal failure and will need to be replaced.
  • If the vehicle does not start, check for battery voltage and ground at the fuel pump using a Digital Volt Ohm Meter (DVOM) while someone cranks the vehicle. If voltage and ground are present, the fuel pump has an internal failure and will need to be replaced.
  • If voltage and ground are not present while the vehicle is being cranked, check to see if the fuel-pump relay is being energized. A great way to test a circuit is to remove the top of a relay and simply look inside. This will allow you to see if the relay’s coil energizes and shuts the contact when that particular circuit is energized.
  • You can also manually close the contact on the test relay by pressing the points closed with your finger. If the fuel pump comes on and functions properly, the fault is neither the pump nor the wiring from the relay to the pump.
  • At this stage, possible faults include the battery voltage being supplied to the coil of the relay (or any circuit controlling this voltage), the ground being supplied to the coil of the relay (from the engine controller), and the relay itself.

3. For the fuel-pump relay to become energized, oil pressure must reach 4 psi. At that point the switch will close, allowing the fuel pump to run. An abnormally long crank time or a no-start could indicate a faulty oil-pressure switch (Image C).

Note: GM had a run of bad oil-pressure switches controlling the fuel pump. These defective switches can cause a long crank time. There are two oil-pressure switches on C4 Corvettes; one controls the gauge and normally has one tan wire, while the other switch is longer and has two wires.

Oil Pressure 4/8

C

4. If the pump runs but there’s no or low fuel pressure at the fuel-pressure gauge, you’ll need to check for a restricted fuel filter. With today’s fuels containing alcohol (which is a cleaner), sediment will tend to wash out of the fuel tank and become trapped in the filter. As a result, changing fuel filters regularly is more important than ever. Remember, the fuel pump is cooled by fuel flowing through it. If the fuel filter becomes restricted, there will be less volume passing through the pump, which could cause it to overheat and fail.

5. Next, check for a leaking fuel-pressure regulator. A leaking regulator (usually caused by a ruptured internal diaphragm) can allow raw fuel to be drawn through the vacuum line and into the intake. This often results in a rich fuel mixture or even flooding.

6. The next step is to check for leaking fuel injectors. Install the fuel-pressure gauge, pressurize the system, turn the key off, and note the time it takes for a pressure drop of 20 psi to occur. If the time exceeds 20 minutes, you likely you have a faulty fuel-pump check valve or pressure regulator, or a leaking injector.

To locate which part has failed, turn the key on for a few seconds to pressurize the fuel system and clamp the rubber portion of the supply line closed using a pair of clamping pliers. Turn the key off and again note the time it takes for a reduction of 20 psi in fuel pressure to occur. If it takes longer than your previous measurement, the fuel-pump check valve has failed, and the pump will need to be replaced.

If the times are similar, pressurize the fuel system again, but this time clamp the return line instead of the supply line. Once again note the time it takes for 20 psi of fuel pressure to drop. If the time is considerably longer than before, a faulty pressure regulator is the likely culprit. If the time is less than 20 minutes, suspect leaking injectors.

To confirm this, raise the fuel rail slightly out of the inlet manifold without disconnecting any fuel lines, and place a piece of paper under each injector. Pressurize the system by cycling the key on for a few seconds, then turn the key off and wait roughly 10 minutes. If you have a leaking injector, one or more of the pieces of paper will have fuel stains.

7. Finally, you’ll need to check to see if the injector is being energized. This occurs when the engine controller sees oil pressure of greater than 4 psi as well as reference pulses from the distributor. It will then energize the injector drivers, which will pulse the injectors on for 4-millisecond periods. To test for reference pulses, install a noid light (Image D) on the injector plug and check for pulses while cranking the engine. If no pulses are present, the most likely fault is either a defective pickup plate in the distributor or a failed engine controller.

Noid 5/8

D

Scan-Tool Diagnostics

If fuel pressure seems to be good, it’s time to connect a scan tool and check for fault codes. If a fault that could affect the fuel system is present, you’ll need to address it at this time. If not, with the key in the On position and the engine not running, select the “View Data” section to determine the sensor values and voltages for items related to the fuel system.

One of the first items to look at is the voltage for the throttle-position sensor, or TPS (Image E). This reading should be approximately 0.50 to 0.54 volts. If the engine controller sees more than approximately 0.60 volts, it may assume that the engine is flooded and the driver has pressed the accelerator to clear the flooded condition. In this scenario, it will restrict fuel flow to help resolve the perceived problem. This is known as the “clear flood mode.”

Tps 6/8

E

It’s possible to adjust some TPSs by loosening the torx screws attaching the sensor to the throttle body and rotating the sensor until proper voltage is achieved. Some sensors are not adjustable and may need to be replaced if the closed-throttle voltage exceeds 0.60 volts.

Next, with the key still in the On position and the engine not running, rotate the throttle smoothly to wide open and monitor TPS voltage. It should progressively climb without dropping and be between 4.5 and 5.0 volts at WOT. If the voltage drops during this test, suspect a faulty TPS or wiring.

Your next step is to monitor the mass-airflow sensor, or MAF. Most C4 Corvettes use a MAF to determine how much air is being pulled into the intake manifold. The exceptions are the ’84 and ’90-’93 models, which use a speed density system known as Manifold Absolute Pressure (MAP). In 1994 the Corvette went back to the MAF-based system but used the speed density approach as a backup. Before we go on, let’s take a look at how these systems operate.

Manifold Absolute Pressure Sensor (MAP) Operation

Map Atmospheric 7/8

F

This system uses a sensor called a MAP, which measures the manifold absolute pressure and compares it with the atmospheric pressure outside the intake manifold (Image F). This information, along with the manifold air temperature (MAT), coolant temperature (CT), and engine rpm, is used by the ECM to determine the amount of air entering the cylinders. A MAP sensor has a vacuum line that runs from the sensor to the intake manifold. This line should be inspected for cracks, which could lead to incorrect sensor readings.

The TPS and the MAP are both three-wire sensors that share some of the same qualities:

One wire sends a constant 5-volt reference voltage from the engine controller to these sensors when the key is in the Run position.

One wire returns the voltage from the sensor to the engine controller.

One wire is ground.

Mass Airflow Sensor (MAF) Operation

This system uses a sensor called a MAF, which contains an extremely fine wire inside its housing. The ’85-’89 C4 engines use a Bosch sensor that heats the wire to 100 degrees (C). The ’94-and-later C4 models use an AC/Delco sensor that heats the wire to 200 degrees (C). The amount of current required to reach the desired temperature is monitored by the engine controller. More airflow will cool the wire, requiring more current to heat it. By constantly reviewing these parameters, the engine controller can precisely calculate how much air is passing through the intake.

A simple test for either of these sensors is to use a scan tool to monitor voltage with the engine running. Note the voltage at idle, then snap the throttle and look for a voltage change. If a change occurs, the sensor is most likely functioning properly. Most good scan tools will have a specification section listing the range in which each sensor should perform.

Coolant Temperature

Coolant 8/8

G

On a cold engine, use your scanner to verify that the coolant-temperature reading at the engine controller is approximately the same as ambient (Image G). If not, suspect a faulty coolant-temperature sensor or wiring, or the engine controller itself. Note that while these are not common failure areas, they should nevertheless be checked.

The foregoing might sound like a lot of work, but if you take your time, use common sense, and work step by step, you should be able to find your problem and help get your Corvette off the ether.


Questions?

Got a question for our Tech Corner expert? Just jot it down on a paper towel or a lightly soiled shop rag and send it to us at VETTE Magazine, Attn: Technically Speaking, 9036 Brittany Way, Tampa, FL 33619. Alternatively, you can submit your question via the Web, by emailing it to us at vette@sorc.com. Be sure to put “Technically Speaking” in the subject line.


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