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Fuelish ThoughtsPosted 2/12/2000By Michele Pogue
My duties as an ASE certified technician for Linder Technical Services are much more involved than just "fixing broke cars." Two days every week, I work with the "injector wizard" in our fuel injector reconditioning room. When injector cores enter the reconditioning room, they are considered "bad" until we prove them to be good. This involves many steps, including an ohm test, current draw test, leak test and spray pattern test. The injectors are then cleaned and back flushed, and then flowed and matched into balanced sets. When I'm not working in the fuel injection room, I am diagnosing cars with driveability problems or answering technical questions on our hotline. Although the GM Vortec central port injection (CPI) unit has been around now for several years, I still get calls on the hot line weekly from technicians needing help diagnosing these systems. I would also say that on the average, we ship two to three reconditioned CPI units every week. This tells me that these systems are still a problem and that technicians are still having trouble with diagnosing them properly. This article may offer some helpful hints. First, let me start with a brief description. The CPI unit is sometimes referred to as a "pod" or "spider." This unit consists of a main body that houses a fuel injector and a fuel pressure regulator, and six small tubes that deliver fuel to each cylinder.
Fuel Pressure Problems
We say the system running pressure should be slightly lower than KOEO (normally between 59 to 61 psi), but never go below 57 psi. There is a 30 percent fuel delivery loss at 56 psi, which can cause hard starting and driveability problems.
Leaks Case in point, a used car lot brought me a 1995 Chevy Blazer with about 80,000 miles. The truck obviously sits a lot and when customers want to test drive the vehicle, sometimes it runs really poorly and pours black smoke when they first start it. Other times it runs fine. I hooked up the fuel pressure gage and turned the key on- 59 psi. I turned the key back off and expected to see the pressure bleed down, but even after several minutes, it held right at 59 psi. Knowing all the problems with these CPI units, I assumed even before I had pulled the truck in that the pod would be the problem. So, now I stood scratching my head.
To my surprise, I saw a leaking regulator. No, it wasn't leaking right at that moment, but one side of the regulator was very clean, while the other was dirty and oily, indicating that regulator had leaked! I grabbed a reconditioned unit from the "injector wizard," installed it and the problem was solved! I was glad I took the extra few minutes to double check!
IMTV The IMTV is also a very handy tool for diagnosing problems with CPI units. The valve sits on top of the intake manifold in the center and is held in place with two torx screws. If you suspect a problem with a CPI unit, before you remove the manifold, take just a few seconds and remove the IMTV. With the valve out, use a flashlight to see down into the intake. Look closely for clean areas that would indicate fuel had been leaking and washing the area clean. This can save you a lot of time if you are not completely sure if the CPI unit is a problem. Many of you have become very familiar with the CPI unit that I just discussed. These units have been updated for 1996 and are slightly different. The new units are called central sequential fuel injection (CSFI). These new units still resemble the old CPI units, but their operation is much different. You may see these in 6- or 8-cylinder varieties. These "pods" contain not one, but six (or eight) injectors and a fuel pressure regulator, all of which can be replaced separately without replacing the entire assembly.
The fuel injectors fire sequentially, which makes proper installation of the tubes critical. I had a call on the hot line a few weeks ago from a technician that had installed one of these CSFI units in a truck and wanted to know if the tubes had to go back in the same spot. The answer is a definite yes! On the old CPI unit, it wouldn't have made any difference because all of the cylinders were getting fuel at the same time, but on a sequential system, the tubes must be in the proper order. For those of you wondering, on the side of the "pod" each one is labeled. Fuel pressure on these new systems is slightly higher, 60 to 66 psi with pump running and engine off. Sticking poppets also seem to be a problem with these systems. GM has an on-the-car cleaner designed to free sticking poppets. I recently heard from a technician at a GM dealer that he had to run the system up to 80 psi before he had any luck with the cleaner. We have just recently started seeing these systems in our fuel injection room. As more and more go through the system, we will have more detailed information on testing and repairing these CSFI systems.
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Copyright (c) 1996-2008. Automotive Service Association. All rights reserved.
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At the end of each tube is a poppet nozzle that contains a check valve and spring to regulate fuel flow. Fuel will flow from these nozzles when pressure exceeds 37 to 43 psi. The fuel injector has six small holes in the bottom that supplies fuel to all cylinders at the same time. This system is not sequential. If there is a problem with either the injector or the fuel pressure regulator, the entire unit must be replaced. These components cannot be replaced individually.
KO/EO should be 54 to 64 psi and should not bleed down. (In a minute, I'll address what to do if the pressure bleeds down.) According to OEM specifications, system running pressure should drop 5 to 10 psi at idle, but the actual test information we have compiled by flowing and reconditioning hundreds of these units differs slightly.
I was just about to turn my attention in a different direction when I decided I would remove the IMTV and take a look down inside just to be sure. I grabbed up a flashlight, climbed under the hood (if you were 5 feet tall, you would climb inside, too!) and looked down inside the plenum.
