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 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.

  • 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.

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.