Hey all. Not sure where to put this so here it will be. I just spent several days tracking down a ridiculous fuel electrical grounding problem on my 89 C4. Pump worked sometimes and sometimes not more than 20 PSI. Bottom line is I had the ground wire on the B pillar disconnected for a roll cage project. Somehow a weak ground existed through the chassis or other wires - which really threw me for a loop! As I went through this odyssey of finding that I got real familiar with the fuel supply system. Putting this here hoping I can save someone time down the road - helping justify how many hours I spent on this problem.


If you are past the obvious and fighting an obscure problem:

1. Basic testing setup
   1. Put a pressure gauge on the fuel injector rail. Good pressure should be close to 45 PSI.
      
   2. Set up a switched wire power source to connect to G slot on ALDL port. Key not needed for this to work
      
   3. Use these to check progress as you change things and want to check. Will save a lot of unnecessary engine cranking, not to mention hot exhaust that all this gas you are draining out can find.
      
2. Electrical problems
   1. Fuse – labeled “FP”. Bad or corroded
      
   2. Relay – under hood on engine side of brake booster. Failed, connections corroded
      
   3. Ground – connected to passenger side B pillar interior side, corroded, loose or disconnected. Pump can still work but cannot build pressure
      
   4. Connection between wiring harness and sending unit
      
   5. Connection on sending unit – three wire harness connects to male plug to get into gas tank. These can be corroded, particularly on a car that sat with low/no fuel for a long time
      
3. Flow problems (factory manual says flow rate should be 1/2 pint in 15 seconds)
   1. Fuel filter - clogged
      
   2. Fuel pressure regulator – stuck open or shut, also possibly vacuum activation not working properly
      
   3. Fuel sock – dirty or pinched, tank is dirty
      
   4. Fuel pump – not operational
      
   5. Injectors blocked/ not activating
      
   6. Fuel lines blocked/pinched – can resolve by blowing compressed air through the lines. Just figure out a way to catch the gas coming out at the other end.
      
   7. Fuel pump to sending unit connection leaking. With low gas in the tank you can see the movement of fuel around it. If the pulsator is still in there it could have a bad seal, be rusted out. I understand the newer pumps do not need this and you can use fuel hose and clamps
      
4. Can eliminate all electrical items as follows
   1. Obtain a pigtail to go directly to pump, wire it directly to the battery (use a fuse and a switch). Set up your wiring so it will not spark inside the tank!
      1. Bench test the pump it should cycle (don't run it dry more than a couple seconds)
         
5. Odd mechanical flow considerations
   1. Has any of the original line been replaced? Look for hose patches, or replaced sections. This could result in pinching, or pieces of hose obstructing on the inside
      
   2. My C4 came with water in the fuel lines which created rust inside the lines. This had a propensity to block up the filter and the screen in the tank. It was so bad I actually ran water in the lines and kept blowing it out with air until the water came out with no rust bits.
      

 

My car sat for a few months and when I tried to start it it would only crank. I measured the battery voltage and it was 12.6 at the battery and inside the Corvette I measured 10.5 volts at the fuse panel hot side. If you have a situation like this the only repair is to disconnect the battery from the Corvette and then clean each terminal and the post where they are attached. After cleaning mine the fuse panel shows the same voltage as the battery is showing. It is amazing how much things work when they are getting the proper voltage! The little Post is where all the fusible links get their UN-fused battery power. This connection is shown in the FSM and is easy to find. Follow a red wire from the battery positive connection it will lead to the post. On my 1988 C4 (Coupe) it is mounted on a frame rail behind and under the battery. You will need to remove the small section of body cover behind the battery to see it more easily. This point is where 7 individual fusible links are powered by the battery by a direct cable to the battery positive terminal.

Later on I had a Fuel Pressure Supply line gradually plug up internally and it drove me nuts. I had the pressure but not the Volume of gasoline the Engine requires. I had to compare the flow rate with a rubber line using a gallon jug marked off. I simply detached the fuel line at the right front wheel well and connected a hose that went to the bottle where I was able to see my problem in easier terms. The fuel pump pressure was 43 psi but the flow was less than a quart of fuel expelled over a period of time. I then used a hose the same size as the brake line and bypassed the fuel supply line by attaching it to the Fuel Pump Output hose barb where the fuel comes out of the fuel tank heading forward. The difference in volume was huge, it took only a few seconds of unrestricted fuel to see the problem. Now when the car is going to sit I put the anti-ethanol additive to keep the fuel fresh for the next time. I also learned the hard way to flush brake lines more often. I bought a tool that makes it a little less like "work" and almost fun so I do it more often.

Classic Tube in Upstate NY would be happy to make you a nice new set in steel or S.S.. Funny thing is I had a brake line fail right at the entrance point of the ABS Control system behind the driver on my 1988 C4. I ended up getting a discount as I ordered a new set of fuel lines and a complete 1988 Brake Line kit.

Fuel Pump relays like to go bad so I keep a spare in my glove box. This same relay is used in a lot of places and they are cheap. One tool that I particularly love when diagnosing an electrical issue is called a "Power Probe". It is one tool that can replace many. I applied full battery voltage at my fuel pump to see if anything different happened.

While you are at it be sure that your EGR valve is still functioning. I started to replace my EGR and ended up replacing 8 injectors, EGR Vacuum solenoid, Fuel pressure regulator diaphragm and all the spark plugs and wires among part of my shopping list.

Clean all battery connections, it is critical to your Corvette to have clean battery connections.