Check sticky poppet nozzle to fix truck’s idle

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This article was published 14/07/2006 (7069 days ago), so information in it may no longer be current.

Q: I have a 1996 GMC K1500 with a 5.7-litre Vortec gasoline engine. It has a solid miss at an idle, but pulls well on all eight cylinders when the accelerator is pressed. The mechanic who looked at it can’t figure out why, other than to suggest that the ECM is the problem. I have tried fuel treatments, including the GM version, but with no change. I have installed new plugs, wires, rotor, and cap with also no change. The truck does 19 miles per gallon on a combination of highway and urban driving. Your thoughts please.

A. GM used a unique gasoline fuel-injection system on their V6 and V8 truck and van engines from 1996 till about 1999. They called it CSFI or Central Sequential Fuel Injection. CSFI has small individual fuel injectors mounted in a group in the intake manifold instead of in the engine’s intake ports. These injectors are the same in operation as other electric injectors used in other engines and are controlled by the computer in the same manner. What makes CSFI different is in how the fuel is delivered to the intake ports.

A small plastic tube is moulded to the end of each electric injector.

This tube carried the fuel to the individual ports. At the end of each tube, a spring-loaded injector/spray nozzle called a poppet nozzle directs the fuel into the port. This poppet nozzle only opens and sprays fuel when fuel pressure inside the plastic tube is greater than the spring pressure of the poppet nozzle, about 42 psi or higher.

Because your truck pulls well during acceleration, It would appear that all the injectors and poppet nozzles are delivering fuel. The miss at idle, when only small volume of fuel is injected, is likely caused by one of the poppet nozzles sticking so that fuel is not sprayed out in a fine mist. GM has had some problems with these poppet nozzles sticking and even concentrated injector cleaner may not clean them. A special adapter that forces higher pressure nitrogen gas through the poppet nozzles is used by GM repair shops to help clean difficult injectors/poppet nozzles, and that is what I would recommend you have done.

If cleaning the system with nitrogen doesn’t work, individual injector/poppet nozzle assemblies can be replaced, or you can replace the complete injector/poppet nozzle package with an new fully electric injection assembly. This assembly uses new style injectors that are small enough to fit into the position of the original poppet nozzles.

Q. I have a 1999 Astro van with a 4.3-litre engine in it. It has 230,000 kilometres on it. Last winter, it got so that you could not start it. It would crank and crank until the battery went dead. Then you would boost it and get it going. When it did start it would seem to be flooded and sometimes would put fuel in the pan. The engine light will come on sometimes. Now it seems not to have the power on the road. I had it to the shop and this is what they put on it and nothing helped it: fuel pump, coil and crank sensor. Now they say it may need a map sensor. Do you have anything that might help me?

A. This engine has the same CSFI fuel injection system as used by the previous GM truck (above) but the problem is much different. The first thing I would check is fuel pressure. Correct minimum fuel pressure is critical on these systems. Even one or two psi below minimum specs (60 to 66 psi with fuel pump running, engine stopped) is enough to cause all the symptoms you describe.

There are several things that could cause low fuel pressure. You have already replaced the fuel pump, but it could still be faulty. A fuel pressure check is essential. If pressure is low, check for voltage at the fuel pump itself. A faulty fuel pump relay or corroded electrical connection may be dropping the voltage to the pump so it is not operating correctly. Boosting the vehicle may help, because you are supplying additional current to the vehicle during cranking that can help overcome electrical resistance in the fuel pump circuit.

Finally, if fuel pressure is good, then clean and tighten all ground connections between the battery, body and engine. A poor ground connection can also cause the problems your van is experiencing.

Jim Kerr is an experienced mechanic, instructor of automotive technology and freelance journalist. You can e-mail questions to Jim at the address below.

kerr.jim@sasktel.net