CURING THE FUEL INJECTOR BLUES In 1992, General Motors introduced a very unique fuel injection system on its 4.3-liter Vortec V6 truck engine. Dubbed Central Port Injection, or CPI, the system looked and functioned much differently than previous designs. The CPI system utilizes one main injector that pressurizes six plastic tubes housed in the intake manifold. Each tube terminates in a spring-loaded poppet BY JEFF MASTERMAN Armed with a pressure gauge, a good scope and a working knowledge of its trouble spots, you can purge the demons that plague GM s CPI system. 20 July 2000
GM s 4.3-Liter Vortec V6 July 2000 21
CURING THE FUEL INJECTOR BLUES The manifold tuning valve (MTV) can be removed from the top of the intake to allow visual inspection of the pressure regulator area without removing the plenum. If there s fuel laying in the intake or if the area around the regulator is very clean, the regulator diaphragm is probably leaking. Photos: Jeff Masterman being dumped directly into the crankcase or passenger-side cylinders. Fortunately, the manifold tuning valve (MTV) can be removed to allow easy inspection of the regulator if leakage is suspected. Fuel pressure leak-down with no external signs of leakage is another common problem on the CPI system. Like most injection systems, leak-down can result in long crank times or a stumble after restart. A quick way to isolate the cause of a leak-down problem on this system is to cycle the key and allow the fuel pump to run for a couple of seconds. Once the pump stops, simply pinch off the supply line. If the pressure holds, the cause is in the tank. If it doesn t, the intake will have to come off for a closer look at the internals. Inadequate fuel pressure or volume can also cause long crank times or a no-start condition. GM lists the pressure range for the valve that supplies fuel to its corresponding cylinder. The main injector is pulsed twice per crankshaft revolution, with pressurized fuel opening each poppet as system pressure approaches approximately 48 psi. The main injector and fuel pressure regulator on the CPI system are housed in a molded plastic piece with the six plastic tubes attached. GM dealers service this assembly as a single part number, and it s quite expensive to replace. The good news is that replacement pressure regulators, as well as fully reconditioned assemblies, are available via the aftermarket, which can save your customers substantial dollars on their repair bills. Common Problems Leaking pressure regulators are fairly common on the CPI system. Since the regulator is housed in the intake, a ruptured diaphragm often results in fuel Notice in this photo how the passenger side of the intake is much cleaner than the driver s side area. This is partially due to PCV flow, but can also be an indication of a leaking fuel pressure regulator (arrow). 22 July 2000
The EGR valve on the Vortec V6 sticks open frequently as a result of carbon buildup. While the intake is off, it s always a good idea to remove the valve and clean off as much carbon as possible to prevent rough idling. CPI system at 54 to 64. However, many techs have encountered hard-start or no-start complaints with pressure readings as high as 56 to 57 psi on a cold engine. This problem is amplified by the fact that many fuel pressure gauges have seen a rough life and can be off by as much as 2 to 5 psi. So always check the accuracy of your gauge before working on a CPI system. Finally, remember that the fuel pressure shouldn t drop during heavy acceleration. If it does, the volume of fuel being delivered needs to be evaluated. You ll find that many customers don t replace their fuel filter as a maintenance item, and it s not unusual to see a filter that s racked up 100,000 miles. Running for long periods against a restricted filter can kill even the best of fuel pumps. It can also cause big-time problems for the CPI system. Testing has shown that even slight changes in operating pressures can result in significant differences in the volume of fuel flowing through the poppet valves. EGR Woes EGR valves have been known to stick open on CPI engines due to carbon buildup. The result will be a rough idle, often accompanied by a failed emissions test due to high HC levels. These engines use an electronically controlled EGR valve with a built-in position sensor. Scan data can provide a quick check for this type of valve. With the key on and the engine off, just compare the EGR Desired percentage to EGR Actual. If the Actual percentage is higher than the Desired, the valve needs to be cleaned or replaced. The poppet valves on the CPI system have a tendency to stick open. If this occurs, a large amount of fuel will enter the combustion chamber. Fouled spark plugs can be a tip-off to this problem. If you look at the scan data, The intricate design of the CPI plenum can make it difficult to use propane during a cylinder balance test to determine if a misfire is due to a lean cylinder. July 2000 23
CURING THE FUEL INJECTOR BLUES you ll find that the Long Term Fuel Trim number will be negative, as well. An improper spray pattern can cause one cylinder to run leaner than the rest, which could result in a rough idle and high HC during an idle emissions test. Performing a cylinder balance test can help you isolate the problem cylinder. Once the cylinder is identified, add propane at the throttle body until the rpm peaks. Then repeat the test. If the weak cylinder now contributes, the poppet valve for that cylinder should be suspect. One caveat: The intake design of the 4.3-liter Vortec V6 can prevent the propane from being evenly distributed among the cylinders. This can result in misleading test This ASNU machine used by Linder Technical Services can be used to both test and recondition CPI injector assemblies. It has an adjustable pressure function that s great for demonstrating the effect of fuel pressure changes on the volume of fuel delivered to the engine during cranking. Photo: Jim Linder Left: Here, the pressure of the ASNU machine is adjusted to around the middle of the specified range of the CPI system. The tester pulses the main injector a number of times using a programmed frequency. The result is 54 milliliters (ml) of fuel delivered at each poppet valve. Right: Now the pressure is reduced to 54 psi still within spec, but at the bottom of the range. Notice how the volume of fuel delivered drops to 42 ml. That s a 23% reduction in volume due to just a 6-psi drop in fuel pressure. This can create a no-start or extended cranking times on a cold engine. Photos: Jim Linder 24 July 2000
CURING THE FUEL INJECTOR BLUES Once the intake plenum is removed from the top of the engine, the spray pattern of a suspect poppet valve can be observed. Use caution during this type of test; excessive cranking can cause engine damage in a hurry. One look at this waveform tells you that the main injector on the CPI system is driven by the PCM using the peak-and-hold method. The pattern was captured by the scope connected to an external connector located behind the alternator. results. When in doubt, remove the intake plenum. An off-the-vehicle injector tester is by far the best choice for testing fuel flow on a CPI engine. If one is not available, there s an alternative test that can be performed after the plenum is removed. Begin the test by removing the coil wire from the distributor and securely ground it to prevent the engine from starting. Leave the ignition primary connected so the main injector will pulse. With the intake plenum off, the weak cylinder s plastic line can now be pulled out of the intake and the poppet valve placed in a container. Observe the spray pattern while an assistant cranks the engine briefly. Since the main injector is concealed by the intake, a noid light cannot be used for no-start diagnosis. However, there is a connector behind the alternator that can be used to check the power feed and the PCM-controlled ground side of the injector with your scope. While the scope is not connected at the injector, grabbing a waveform at this location tests the whole circuit. Note that GM uses a peak-and-hold driver arrangement on the CPI system, similar to its TBI setups. If the waveform indicates a problem, the intake will have to be removed to check the rest of the wiring. With a working knowledge of the system and an accurate fuel pressure gauge, you can win the diagnostic battle with GM CPI. Just remember what makes this system so different from the more common ones you see every day and you ll be on your way to having found a cure for the fuel injector blues. For a free copy of this article, write to: Fulfillment Dept., MOTOR Magazine, 5600 Crooks Rd., Troy, MI 48098. Additional copies are $2 each. Send check or money order. 26 July 2000