Being a technician training and support center as well as an injection repair shop makes for some very interesting days at our Gasoline Alley location. First of all, we get to work with automotive service technicians from all over the country and we also get to service many different types of walk-in customers - from the occasional do-it-yourselfers to the troubled "I have been everywhere and just want it fixed" group.
It is not uncommon for the customer to come to our shop somewhat confused in regards to a particular service issue, and the technician or shop owner who created the confusion attending a training class the very same evening. When this situation comes up we address this as a group so that all the students may learn from the experience.
One such issue lately has been the correct service and repair of the GM light truck fuel injection systems used from 1992 up through the 2000 model year. For starters, this is a somewhat complex system that operates very different from any other unit on the market. It is also a "pattern failure" item with an above-average failure rate. Add a very high repair cost to that and we have a sure-fire winner.
1. Complex system difficult to service (requires training).
2. Higher-than-normal component failure rate.
3. Nothing you can do on the unit is low priced.
With this in mind, let’s start out with the 1991 CPI unit and discuss these systems upward through 1995 and throw in a case study analysis vehicle or two.
CPI Crab Unit
Central Port Injection (CPI) (see Figure 1) is offered on the V-6 L35 VIN W engine. This engine uses a swirl intake port for improved airflow and is called the Vortec. This engine is available on 1992 and up L, M, S and T model vehicles.
Increased fuel pressure is required to operate this unit, and the factory specification for this engine (CPI) applications listed at 54-64 psi.
Fuelish Tip #1
This specification to the untrained is very misleading. This unit needs the upper end of the fuel pressure specification (64 psi) to operate correctly and have 64 psi at the injector to correctly fire all six nozzles at 43 psi. Sixty to 61 psi is the minimum to run this engine correctly (key on engine off).
As shown in Figure 2, the fuel delivery changes quite a bit with just a slight change in pressure. In this flow test the unit flows 28% less volume at 55 psi compared to 60 psi. Many technicians have been misled by this. You must have 60 psi to start one of these units on a cold engine. Many times we hear that the engine will start and run if additional fuel of some type is sprayed into engine to start.
For example, one owner even stated that WD-40 was sprayed onto the headlamp to start engine. It took a few minutes to determine that the S-series Blazer was a Vortec CPI, and that spraying WD-40 on the headlamp would feed extra fuel (WD-40 in this case) to the air intake tube (mounted behind the headlamp area). How the owner of the Blazer figured this out, we don’t really wish to know, do we?
Correct fuel pump service on these vehicles requires a somewhat different approach with using:
1. Current Draw
Current draw may be viewed using the fuel pump test connector hooked with a jumper to battery positive and measured either using a multimeter or a current probe. We prefer to use the low current probe and view not only actual current draw but view pump waveform to confirm pump condition as well (see Figure 4).
2. Fuel Pump Pressure
Fuel pump pressure may be tested as usual (remember the tight specification shown above) and should be checked for pressure as well a leak down rate (see Figure 3).
3. Fuel Pump Volume
Fuel pump volume may also be tested using a conventional method of XX amount of flow in XX amount of seconds but we prefer to use the latest volume meter showing actual flow through a meter to determine the actual volume of the pump in gallons per hour - this unit must exceed 0.5 U.S. gallons per minute under all test situations.
This case study displayed good pressure and poor volume. Volume should be 0.5 gallons per minute for these vehicles.
4. Fuel Pump Current Waveform
Figure 4 shows a very good pump with a very smooth repetitive brush contact with motor commutator. Experience is required for this type of testing, but again, knowing what to look for is the key as well as viewing many pumps in your day-to-day service of vehicles.
Our experience has shown that this is the most correct method of analyzing the overall pump condition and determining, beyond a shadow of doubt, that the pump module does in fact have a problem and require replacement.
FUEL PUMP TESTING
Pressure (KOEO): 58-60 minimum
Pressure (KOER): 52-54
Pressure (Deadhead): Relief @72-100 psi
Volume: 31 gm per sec @51 psi (0.5 gallons per minute)
Amp Draw: 7 amps minimum
Current Waveform Rest Pressure: Smooth and repetitive, must hold pressure.
Fuelish Tip #2
Lab Scope Current Probe Utilization
The tool of choice for most technicians today is the digital storage oscilloscope (DSO). Almost every tool supplier has some type of a DSO offering. Some have proven better than others but in reality, any DSO is better than no DSO at all, and most are light years ahead of where we were just a few years ago.
The DSO gives the technician the ability to actually see an actuator or sensor in its operation under the actual working (dynamic) conditions. With this ability, there is a considerable amount of self training involved (as no provider has scratched the surface on training their product), and progress is being made daily on the utilization of the tool. As use of the DSO has increased, the working technicians have demanded better leads, batteries and external probes.
To current test a fuel pump requires an additional probe. This probe is a low current probe that can inductively clamp around a supply line and accurately measure a small current flow. By using this current probe and the DSO we can chart current flow and actions that previously couldn’t be seen. Today, lab scopes are like scan tools with many brands and many features. Discussing the good and bad points of different lab scopes has become a discussion that rivals the Ford versus Chevrolet debates and will go on forever. My opinion (called a Linderism in some parts) is that ANY lab scope is better than not having one at all. Lab scopes and Internet access are one in the same discussion. If you don’t have it, get it!
Probe Hookup
Most late model fuel injected vehicles have some sort of fuel pump control circuit that is fairly easy to access and test. The value of the inductive current probe is that by clamping around a single fuel pump feed wire and operating the pump we may determine the actual loaded "health" or condition of the pump under operating conditions.
Fuel pump amp draw, operating rpm, brush contact and even wear is easily determined.
Scope Set-Up
The majority of our fuel pump testing uses a voltage setting of 100mv per division and a time setting of 2ms. In some cases we expanded the time base to 5ms for a better look at the waveform produced by the pump.
CPI Service
Although the GM CPI unit is serviced only as a complete unit, we will discuss the basic operation of the components to aid the technician in understanding operational procedures of service.
The unit uses a single low impedance (1.5 ohms) "maxi injector" that distributes fuel simultaneously to six poppet nozzle assemblies. The injector controls fuel flow based on PCM command. A 20 micron final fuel filter basket filters the fuel entering the injector body.
After reconditioning a few hundred of these units we have found this injector to rarely fail, although a small quantity of units has shown a fuel leak out the connector. But, for the most part, this injector itself has been a very durable unit that seldom fails.
The fuel pressure regulator is mounted within the manifold and must respond to a biased manifold pressure against actual fuel pressure. Excess fuel is returned to the fuel tank through the fuel return line. The regulator itself has been a major "pattern failure" unit with units failing daily. These regulators will leak and cause fuel to blow out the regulator vent hole creating a very rich mixture on one bank (passenger side) of engine. It is not uncommon to have a regulator leak come and go with the unit leaking one minute and holding pressure the next.
Fuelish Tip #3
Regulator Leak Testing Vortec CPI Units
Fuel pressure testing as previously mentioned is a must with these units but regulator leakage may be somewhat intermittent. A tip is to check fuel pressure with the gauge while looking for a quick leakdown.
Another short cut is to remove the IMTV (Intake Manifold Tuning Valve) from intake manifold (shown in Figure 5). Once the IMTV unit is removed, visually inspect the inside of intake manifold. The manifold and injector unit should be very carboned up and dirty. Any clean areas indicate a fuel leak internal to the manifold plenum. Most times this is caused by the fuel pressure regulator leaking fuel from vent hole in the regulator body.
Other factors may be present (such as a cracked inlet fuel line) but usually the regulator unit has failed.
The V6 Vortec engine has also experienced somewhat of a problem with the EGR system. (See "In Search of Pattern Failures," Underhood Service, January 2000.) This engine seems to sometimes have an excessive build-up of carbon. This carbon often flakes off and sticks in the linear EGR valve causing the unit to stick open. Many times the fix for this is to replace the rather costly valve assembly. To prevent a comeback, the technician may wish to explore these services to keep the new unit carbon free:
• A service fix of a new PROM (or flash program on later vehicles) to exercise the opening periods at highways speeds to remove any future carbon chunks.
• An aftermarket screened gasket to prevent this condition from repeating (see Figure 6).
• A chemical cleaning kit, requiring chemical to remove and clean out the EGR passages.
Fuelish Tip #4
Code 32
A DTC 32 indicates a fault in the EGR system. It sets when vehicle speed is above 32 mph and engine parameters (i.e., INT, BLM, MAP, TPS) are at a specific value, then the ECM commands an off EGR signal for 2.5 seconds while observing the Intergator change. If INT doesn’t change by at least nine counts for the 2.5 seconds, a Code 32 sets. Also, anytime the actual EGR pintle position and desired EGR position is greater than 3% for two seconds, the code will set.
Central Sequential Fuel Injection (GM Trucks)
Central Sequential Fuel Injection (Central SFI) is used on 1996 and above light trucks to meet the increased emissions requirements. Central SFI is used on all 1996 truck engines except some carryover engines in G vans and P/G units over 8,500 pounds. This system is very similar to the previous CPI system used on the 4.3L V6 engine. The major difference is that in lieu of a single injector feeding all cylinders, the new design uses a single injector unit for each cylinder poppet nozzle. Each is then fired sequentially for precise fuel control.
Overall, system operation is very similar to the previous discussed CPI unit with fuel pressure being super critical. The fuel pressure specification is 60-66 psi KOEO. As mentioned, the fuel pressure is very critical for proper operation and may be somewhat misleading when a vehicle is close to specification. Close won’t work on these applications!
Fuel Flow Test at Different Fuel Pressures
Flow test is from left to right:
54 psi = 37 MILS
56 psi = 41 MILS
58 psi = 44 MILS
60 psi = 49 MILS
62 psi = 53 MILS
64 psi = 60 MILS
As shown above, for every second psi change in fuel pressure, the actual fuel delivered to that cylinder changes about 10%. As we have said in the past, fuel pressure is very critical. Our test unit (new OEM) is regulated to maintain 64 psi with some return beginning at 62 psi (KOEO). The poppet nozzle will begin to spray fuel at 48 psi (a poor stream at this pressure). Any pressure below 48-50 psi, this engine won’t even spray fuel at all.
Fuelish Tip #5
Injector balance on the 1996 engines requires a special harness adapter with the fuel injector test tool.
A fuel injector tester, switch box and harness adapters are required to perform injector balance on the 1996 engines as no scan tool supports cylinder balance on this model year. For 1997 and up vehicles, most scan tools support injector balance from the ALDL and this test may be performed without the adapters. Each injector is pulsed for a given period of time KOEO and must be within +/- 10 kpa (1.5 psi) of each other. Any injector failing this test is suspect of a problem.
Kent Moore developed their fuel displacement gauge just for this small (1.5 psi) pressure drop that utilizes a fuel chamber with a spring-loaded diaphragm. A dial indicator measures diaphragm movement when an injector is pulsed, which correlates with the actual amount of fuel delivered. This tester directly indicates fuel delivery volume for each injector, which is a far more accurate test.
Movement of the dial indicator is in the neighborhood of .120 to .150 and the technician is looking for a movement of no more than .026 plus (lean) or less than .026 (rich) difference between units!
Actual Case Study
1997 GMC Yukon (Figure 7)
Test procedure:
1. Connect the fuel displacement gauge to fuel rail, injector tester, switch box and harness adapter to vehicle.
2. Power up fuel pump using the Tech 2 Scan tool.
3. Bleed gauge to remove air into container.
4. Zero the dial indicator.
5. Select first injector to be tested.
6. Depress the "push to start test" button.
7. Record the dial indicator once the injector tester has completed its pulse cycle of the injector.
8. Repeat test on all injectors.
9. Calculate values and compare readings.
The highest and lowest readings could skew the average value of all readings. Therefore, exclude the high and low readings and calculate the average of the remaining. Subtract each individual reading (including the highest and lowest) from the average to identify rich or lean injectors. If the subtracted value for an injector exceeds 0.026", the injector is lean. If a subtracted value falls below -0.026", the injector is rich.
Once the service technician has determined that the fuel supply system is up to specification and a problem fuel injector has been found, the actual repair procedures may vary depending on where the vehicle is being repaired. Care must be taken during disassembly to prevent a comeback with either the same problem or a worse one!
If the injector balance has pointed out a defective or different flow rate injector, the vehicle should be checked for presence of a cylinder misfire code (to be discussed in a later article) and a history code of past injector problems (a good side of OBD II).
GM has issued a bulletin #87-65-07 addressing the "Rough Idle After Start When Vehicle Has Set Overnight/SCPI Poppets Sticking (Clean Fuel Injector Using New Procedure). This bulletin describes the service methods needed when these poppets stick closed using injector balance testing and replacing any and all failing injector units. Once units have been replaced it is suggested to add a 20-ounce bottle of port fuel injector detergent.
As a fuel injection shop, Linder Technical Services is currently doing testing using the available two-line fuel injection service units to fix these units on the vehicle by raising the pressure and opening stuck poppets, then applying a gasoline chemical solution to injector units. More research will be performed and on-car service may be the way for the independent repair shop with such equipment to handle this problem.
At this time we are servicing these units off the car on the flow bench and offer the service as a complete assembly only.
The problem of stuck poppets also seems to be a "neighborhood related" issue as different parts of the country have a larger amount of stuck/problem units than others. For example, the West Coast seems to have more of a problem than the Midwest.
If injector replacement is required, special care must be taken to remove and replace the injector unit. Step one in injector pod removal is to remove injector harness connector at injector pod. A thorough visual inspection must be made at this time to determine if fuel has leaked from the injector unit into the harness area.
Caution: These units have been known to leak from the injector upper area into the harness, up through the wiring and into the PCM/VCM unit. The customer complaint is usually the smell of gasoline from the engine compartment. Current GM warranty policy is three years/36 months on this problem with an eight-year/80,000 warranty only on the pcm.
We have serviced a few of these at our injector shop and leave the extent of the actual repair up to the customer (either a repair shop or walk-in customer). We feel as if the "correct" repair is to replace the injector that is leaking, the wiring harness and the PCM/VCM if leaking to this extreme.