Fuel Supply Debugging

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Fuel Supply Debugging

Postby MostMint » Tue Sep 01, 2020 5:18 pm

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. While there are certainly variations vehicle to vehicle, this should work for others as well.

C4 Fuel supply debugging:

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
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 has 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.
[quote="Basement Paul"]Is that a mint rocketship on the hood?? :shock:

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