No matter how large a problem seems, it's usually the small stuff that plagues us.
Sometimes in life, people make things harder than they have to be. There's always someone who goes around making mountains out of molehills. Then again, there are those times when that molehill is a little hard to climb. More often than not, it seems to be the little things that give us the most problems. I was once given a book of inspiration called "Don't Sweat The Small Things." Well, sometimes you've got to sweat the small things and sweat them, you will.
I recently worked on a sleek 2000 Cadillac Eldorado that turned out to be quite a challenge. The beautiful black coupe had GM's 32-valve, dual overhead cam, V8 Northstar powertrain onboard. This engine, while at only 279 cubic inches, can produce more than 300 horsepower. When coupled with the electronically shifted 4T80-E automatic transmission, it really creates quite a package.
The drivability and electrical systems have proven to be quite durable on this Powertrain, and most of the time makes for pretty straightforward diagnostics if you are familiar with OBDII diagnostic systems. Even with the Eldo's elaborate on-board computer systems, an experienced technician can expect better than average diagnostic times in most cases.
However, there are those times when, despite your best efforts, things can get a little difficult. We have all had those repairs where the customer complaint cannot be verified. Even though we believe the customer has a legitimate complaint, the fault will not show itself once the vehicle is in the shop.
The Eldorado had been brought to our service department several times. Each time it was brought in, the problem could not be found, and the vehicle would be sent back to the customer. Like a boomerang, it would return in a couple of weeks with the same complaint. No matter how hard we searched, the problem the customer described could not be duplicated, and both the customer and I were becoming quite frustrated.
The returning menace
The repair order on the initial visit simply stated, "Check engine stalls." This statement was repeated on each consecutive repair order.
On the first visit, I reviewed the repair order, and then spoke with the service advisor to get a little better understanding of the customer's problem. The owner had simply stated that his wife was driving the car, and it had stalled several times. Most of the time, it would happen as she pulled up to a stop, such as a traffic light or stop sign. Once or twice it had stalled with her at highway speeds. Each time there had been no trouble in restarting the vehicle, and at this time the problem was little more than a nuisance. However, that feeling would soon change.
The first thing I did was to take the vehicle on an extensive test-drive with my diagnostic scan tool attached to capture any faults that might occur and to verify the customer complaint. After about 30 miles, I figured I was wasting both time and gas as the test-drive was going off without a hitch. I returned to the shop and began investigating any system that might be causing the stall condition. I searched for service bulletins or technical assistance cases that might relate to this type of problem.
I found numerous instances of erratic signals from the crankshaft position sensors that caused intermittent stall conditions on these vehicles. This seemed like a good place to start, but first I wanted to do some basic checks to verify that all other systems were operating correctly.
I checked for codes in the PCM with only code P1599 present, which is an engine stall or near stall detected. This code will set if the engine speed drops below 152 rpm after exceeding 400 rpm for the current ignition cycle and there are no reference pulses received from the ignition system for 0.75 seconds. A set of failure record data is stored in PCM memory to aid in diagnosis for this code, but in this instance it didn't tell me anything that I didn't already know: The engine had stalled. It was that simple.
I also checked for codes in the Vehicle Theft Deterrent (VTD) system to assure myself that there wasn't a security system issue causing the problem. No codes were present in the VTD system, and all theft system data appeared normal. Because there were no other codes present in any system, I moved on.
The fuel system was next on my list. The Northstar V8 uses multiport fuel injection with eight injectors mounted to a fuel rail on the intake plenum. These are 12-ohm injectors with a range of 11.4 to 12.6 ohms at coolant temperatures between 50ºF and 90ºF. Because they were easy to get to on this engine, I checked each injector, making sure it fell within specifications.
The fuel system
An in-tank fuel pump also is used in this application, and system operating pressure is specified as 48 to 55 psi. The fuel pressure would rise quickly when the key was switched on and would come to rest at an operating pressure of about 50 psi, which was good. As always with a problem like this, I took a fuel sample just to make sure that there was no water or contamination in the tank. The fuel looked good and was nice and clear.
Next I tested the electrical system. The battery and alternator both checked out fine. The battery would hold its own when put under a load test, and the alternator was doing all it could do, maxing out at about 14.7 volts and 140 amps when loaded. Cables and connections at battery, alternator, engine and chassis were all clean and tight.
Power and grounds
I moved on to the PCM and began checking each power and ground that it relied on for correct operation. The filtered battery voltage at each of the PCM battery and ignition feeds was around 12.5 volts. Voltage drops across the grounds for the PCM were negligible, running around 0.1 volts. I knew that I had good power and ground circuits to the PCM, so I inspected the connectors at the computer to verify that they were free of corrosion and had good tight connections.
The PCM is mounted in the breather box at the left front of the engine compartment, allowing for easy access to the computer and its connectors. The correct male terminal can be used to insert in the PCM harness connector and check the connector for a good tight fit, if you have one available. Never use anything but the correct terminal to probe the harnesses as damage to the terminals in the harness can occur. Everything was checking out here as well.
I now went to the ignition system. The vehicle uses an Ignition Control Module (ICM), which is mounted to the top of the rear camshaft cover. It monitors crankshaft and camshaft position sensors, creates 4X reference high and 24X crank signals sent to the PCM for ignition control, creates a camshaft reference signal sent to the PCM for fuel injection control, provides a ground reference to the PCM and provides a means for the PCM to control spark advance. Power and ground connections checked out here as well. All connections to the module, cam sensor and crank sensors were intact, clean and tight.
The ignition system
Nothing seemed to be causing this intermittent stall of mine, and I still hadn't been able to duplicate it myself. I finally decided that since no other resolution could be found, I would opt for the repeated occurrences of crankshaft position sensor failures that I had found in the technical service bulletin earlier. The Northstar uses two crank sensors for reliability, faster starting and as a built-in backup system in case one sensor should fail. Even though the sensors appeared to be working correctly at this time, I changed them both due to the intermittent nature of the problem.
After another good test-drive, we returned the vehicle back to the customer, reasonably sure that the problem had been fixed. We didn't hear a word from the customer, but then, like a boomerang, the vehicle came right back two weeks later.
This same scenario occurred twice more, each time the same in depth checks and the same results. The problem still could not be duplicated. By this time I had contacted a technical engineer for assistance, but was still unable to resolve the condition. The customer had even requested that we put extensive mileage on the vehicle to try to get the fault to occur. Still, no fault was detected. The customer reluctantly picked up his vehicle.
We finally had a breakthrough. The customer called and stated that the vehicle had stalled and would not restart. It was being brought in on the hook. I was never so happy to see a vehicle brought in on a tow truck.
Sure enough, the Eldo wouldn't start. I grabbed a scanner, fuel pressure gauge and a spark tester. With everything connected to the vehicle I hit the key. Guess what? The vehicle started right up! Just as I was about to throw everything down and go home, it stalled again and this time, wouldn't start.
The fuel pressure gauge rested close to zero. Oh, what a joyous site. The fuel pump fuse and relay were energized. I had power at both. I threw the car on a lift and checked the connector going to the in-tank fuel pump and found that it had power as well. As quickly as I could, I drained and dropped the tank to inspect and replace the fuel pump if needed. It all looked good in the tank and fuel sender assembly, but I installed a pump and sender assembly anyway. With it all back together, I hit the key. No start and still no fuel pressure. I couldn't believe it.
I again searched for the cause of the lack of fuel pressure. This time I grabbed a good quality voltmeter and began checking my voltages in the fuel pump circuit. Oh, I had voltage all right, but only about 6.5 volts. Not even enough to get the fuel pump squeaking. I traced the fuel pump feed circuit and ended up at the fuel pump fuse power feed in the back of the underhood fuse and relay center. The wire and terminal were extremely hot, as well as the fuse. When I finally got the fuse out, you could see where it had been arcing across the loose terminal connection and intermittently welding itself to the terminal. This, unfortunately, would occur when the car was in the shop and the circuit wouldn't break until the customer was driving it.
After a terminal and fuse replacement, we finally had a happy customer on our hands. With the use of a good quality voltmeter, I could have saved myself a little time, but it's hard to fix a problem that won't occur. Remember to always use the best tool for the job when you can. Sometimes, you just have to sweat the small things.