CMFI fuel injection question.

My '93 Safari with a 4.3L W code(CMFI) engine will not start. It has compression and spark but no fuel.

I checked the sensors with a voltmeter and chilton manual and they all seem to be good, but the fuel injectors refuse to fire.

I was wondering if someone out there had any suggestions on how to troubleshoot this type of fuel injection system or could point me in the right direction.

Reply to
BartB
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Do you have fuel pressure?

Reply to
Jim

Here are a few things you should know about this system:

1)Low fuel pressure (by even a few pounds )can cause a hard start/no start, especially cold. PSI is critical, should be around 58-62 for proper starting.

2)The fuel pressure regulator is prone to leakage (inside manifold,on injector assembly). Remove access cover on top of manifold to inspect. Signs of leakage should be obvious. Replacement regulators can be found at

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for a decent price.

3) The actual injectors are "poppet" nozzles that open when pressurized by the electronic injector at the top of the 'spider" They are prone to sticking. If stuck, you will probably need a professional injection cleaning done. Techron by chevron is good to prevent this problem.

So, check your fuel pressure for proper psi and to see if it holds. That should help your diagnosis. Good luck

Reply to
CURLY

Yes, I checked the pressure and it is about 55 PSI at the schrader valve on the back of the manifold.

Reply to
BartB

Vortec fuel pressure specs are;

54 psi engine running (vacuum applied to pressure regulator) 64 psi engine not running (no vacuum applied to pressure regulator)

Looks like you don't have enough fuel pressure. Vortec poppet nozzles will not open without either sufficient fuel pressure at atmospheric or sufficient pressure differential across the poppet with the engine running.

Try a squirt of primer fuel into the throttle body, often times this will get a vortec running when the pump is getting weak or if the pulsator has a small leak. Observe usual safety precautions when handling liquid gasoline.

Also, make certain that you have strong spark put at the end of the plug wires, the coil wire on vortecs likes to short out against the intake manifold.

Reply to
Neil Nelson

Thanks for the suggestions.

I tried squirting starting fluid in the engine and it ran for a couple of seconds then died. The fuel pressure went up for a second to above

60 then back down after it died.

The fuel pressure holds and I didnt see any signs of a leaking regulator.

Is the central unit just a solenoid or is it some kind of armature assembly? I dont think all the poppets could be bad. What type of voltage signal goes to the unit? With key on engine off it reads 11-12 volts. There is resistance across the two connections on the unit of about 1.5 ohms.

I am totally stumped by this problem.

Reply to
BartB

professional

Central Multiport Fuel Injection (CMFI) which is one injector with six tubes running from it, and is serviced as one expensive unit. CMFI have poppet nozzles located just above the intake valves in the intake manifold.

If you put in a test light you may be able to see electrical actions. Then again, if the pressure at the poppet nozzles is reduce due restrictions at the top in the injectors, you have no choice but to increase pressure to 64.

Here's more bad advice: If you manually supply more current to the injectors just enough, you may get it open long enough to build up pressure and open the poppet nozzles, thus, avoiding the starting fluid.

Take my advice with a grain of salt. And somebody better correct me...

Reply to
Tibur Waltson

You've now determined that the engine is not getting sufficient fuel.

Fuel pressure should go down once the engine is running because manifold vacuum is being appliied to the vacuum side of the fuel pressure regulator.

Did you actually remove the tuning valve from the top of the plenum and look inside? Did you see any areas that are washed clean? Not that I think at this point that it is a fuel pressure regulator leaking, but it's good to be thorough...

The central unit is one single solenoid.

It's always possible, but you measured the fuel pressure and it is too low, too low of fuel pressure will keep all of the poppets from opening.

Key on (ignition) voltage is fed to one side of the CPI unit on the pink/black wire, the ECM pulls the circuit to ground on the dark blue wire, completeing the circuit causing the injector to pulse. The specification for injector resistance is 1.3 ohms, so yours looks to be in spec there.

This system has to have 58-60 psi fuel pressure in order to run, that means with your accurate fuel pressure gauge connected to the rail, key on or fuel pump diagnosic lead jumpered, fuel pump cycles/running and primes the rail, you -have- to have at least

58 psi showing on the gauge. Common and expected failures include;
  • Weak fuel pump, can;t deliver necessary presure or volume.
  • Torn or ruptured pulsator, the pulsator is the piece that connects the fuel pump outlet to the steel tubing that exits the fuel sending unit on top of the tank, the diaphram in the pulsator tears leaking fuel back into the tank, insufficient pressure and quantity to feed injector assembly. The pulsator is best eliminated with a length of proper high pressure flexible fuel hose and clamps. It's merely there to dampen noise.
  • Plugged injector poppets. All you can do is replace them with new or remanufactured CPI unit.
  • Leaking fuel pressure regulator, evidenced by observing a washed clean area inside the intake plenum as viewed by removing the vortec tuning valve.
  • Cracked or broken fuel nuts. The fuel nuts are the semi-rigid plastic tubes that connect the frame mounted fuel lines to the CPI unit inside the intake manifold, the fuel pressure test port is part of one of the fuel nuts. When they crack and leak, raw fuel leaks into the intake manifold to the point where it will puddle a few inches deep in the lowest points of the manifold. This extra fuel does nothing to assist the engine in running but may has been evidenced at an earlier time by indications of rich running conditions such as extremely poor fuel economy or a check engine light on with a rich mixture code (45) stored -or- by monitoring the O2 sensor and having observed high O2 sensor voltage. (above paragraph applies to ruptured fuel pressure regulator also.)

The GM CPI system is neither durable nor is it reliable, it has a long history for frequent expensive repairs.

Reply to
Neil Nelson

Thanks for the info guys.

Just one more question, if I deadhead the pump by pinching off the return line should the pressure go higher and how high will it go? If it does go higher that eliminates the pump as the problem and means it is most likely the regulator or poppets that are bad and the whole unit should be replaced($350 ouch).

I have the top half of the manifold off now and there is no sign of leakage at the fuel nuts, regulator or tubing.

Reply to
BartB

Yes.

Dunno, I'd expect it to go to 90-100 psi. There really isn't a specification on this...

Sounds like you've been shopping for a CPI.

As long as you're that far, you can pull the CPI out and give each poppet a cleaning with some aerosol carb cleaner.

I'm betting that the problem is the fuel pump.

Reply to
Neil Nelson

Since Neil's not here yet, I'll jump in.:-)

The dark blue wire takes the ground path through a transistor. However, the ECM must be "on" and running to tell the transistor to complete the circuit path to ground.

Just as I suspected.

The voltage take it's path through the solenoid and shows up on the dark blue wire.

You need a test light to observe flashes on the harness wires when cranking.

Reply to
Tibur Waltson

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