In today's modern, high-tech world, it's easy for a well-trained technician to develop an attitude of narrow-mindedness when diagnosing a problem.
Technicians, with experience, become diagnosticians. That is, they learn to listen to the complaint, test various systems on the vehicle and most often, trace the problem to a single cause. Because we get used to doing this a certain way, it's always possible to fall into a 'pit' of narrow-mindedness when confronted with an exceptional problem. What if a vehicle, for example, comes in with a single symptom that is being caused by numerous faults?
Case in pointMy experience involved a clean, low-mileage '94 Camaro Z28, with the beloved LT1 V8 power plant under the hood. The customer's only complaint on the repair order was "check engine skipping." On the initial test-drive, it appeared that this was nothing more than a basic secondary misfire.
The 350-cubic-inch LT1 has proven to be a fairly durable V8 engine, producing both adequate horsepower and torque to propel the Camaro down the road or on the strip, if you happen to be a fan of drag racing. It is a standard single-cam, push rod motor with multiport fuel injection and a roller cam. The designers opted for the use of GM's Opti-spark ignition system, which incorporates a very slim distributor; it's mounted on the front of the engine, behind the water pump, and it is driven off the end of the camshaft. The system offers adequate spark across a wide range of rpms, but as many of us whose experience in diagnosing this system have found, there are a couple of weak points - not the least of which is moisture intrusion.
The initial diagnosticsAfter an initial test-drive to verify the customers complaint, it seemed apparent that this car was suffering from nothing more than a secondary ignition miss. The vehicle had a slightly rough, unstable idle and a minor miss under normal acceleration, but would tend to break down violently under a hard acceleration or heavy load. A quick check was made to ensure that the battery was adequately charged and the alternator was operating correctly. This is something that is often overlooked in preliminary diagnostics. However, it should be checked first because it can have a profound effect in contributing to the fault. I also checked that there were no aftermarket accessories installed, which could be contributing to the problem. All other systems appeared to be functioning normally.
Most technicians who are experienced with this engine would see these symptoms and might begin to think, "internal distributor problems," or something similar. I was thinking along those same lines, but little did I know that I had fallen into that narrow-minded trap.
The ignition systemLet's take a look at what I thought was the root of the problem: the Opti-spark ignition system. On the 5.7L LT1 engine, the Opti-spark consists of a distributor assembly, control circuits for ignition timing in the Powertrain Control Module (PCM), a separate ignition coil and control module assembly, primary and secondary wiring, and the spark plugs. All ignition timing is controlled by the PCM, and there is no ignition bypass mode built into the ignition control module as in other ignition systems. The distributor itself is fairly durable, but it does tend to have a problem with moisture intrusion and carbon tracking inside the cap and housing. Another problem tends to stem from the location and routing of the spark plug wires: These wires are very close to the accessory drive belt and pulleys, and if one of the wire retainers pops loose, well, there goes a wire or two.
I decided to connect a secondary ignition scope and see what was going on. The scope displayed a good, normal secondary ignition pattern. The only discrepancy I could find was an occasional drop in the secondary firing line on cylinder number two. When loaded, the engine would begin to miss, and the firing line would drop off more often.
Upon closer inspection, I found that one of the wire retainers had popped loose. The number two plug wire was situated against the air conditioning compressor pulley, and it was shorting to ground. I promptly installed a new plug wire on the vehicle and placed it in the retainers as it should be. Problem solved, right? Wrong.
While the new wire did take care of that miss under normal acceleration and clear up that pattern on the scope, it did nothing for the rough idle or the breaking down under hard acceleration. Another look on the scope showed a very erratic pattern at higher rpm. I decided my next step should be to look inside the distributor. The water pump and crankshaft front pulley must be removed, so it's not a quick job like removing a basic distributor cap. Upon removal and disassembly of the distributor, it was apparent that this unit was suffering from a serious case of moisture intrusion and carbon tracking, which can cause some pretty bad secondary problems. Another trip to the parts department and a short time later, we were back in business ... or so I thought. My high rpm problems were now gone, but the vehicle still had that rough, unstable idle. In addition, it now had developed a light surge. Here we go again, I thought.
Connections and groundsAfter another very short test-drive, I was ready to start thinking of a different career choice. Not really, though. We've all run into this sort of thing: No matter how simple the problem seems, you just can't get rid of the vehicle. It continues to haunt you, and you think of the next job you could be working on ... the one that could make you some real money. How- ever, this little irritant had to be dealt with first.
I now took a little closer look at some of the PCM data on my hand-held scan tool. Contributing factors, such as vibration and corrosion, can cause some interesting problems such as faulty, loose or corroded electrical grounds and terminal connections. These can reek havoc on computer and sensor data.
While having a co-worker drive the vehicle, I noticed some peculiar things happening with some of my sensor data. The Throttle Position Sensor (TPS) readings, which are normally around 0.50V at closed throttle, were slowly drifting above and below that point at idle while my partner's foot was off the gas. I also noted the same type of drift while holding the throttle steady at any speed or throttle angle. The manifold absolute pressure (MAP) sensor reading, normally around 1.50V at idle, also had some peculiar characteristics: It was fluctuating wildly.
Many other pieces of sensor data showed these same awkward attributes. I concluded that I either had a PCM malfunction or a faulty ground or connection. I started with the PCM itself, which is mounted under hood at the passenger side cowl area on this vehicle. A prime location, if you like your computers moist and tender from runoff water in the cowl area. The connectors appeared clean and tight, and I could find no wiring problems in the harness leaving the PCM. I checked the connections and wiring at all affected sensors with the same results.
Power and ground connections were next on my list. Most all checked out, that is until I hit the major electrical grounds in the engine compartment. Several of the grounds affect the integrity of PCM and sensor data including G100, G102, G103, G105, G106 and G109. The most significant of these would have to be G102, which is at the left-hand front of the engine block, just below the head. It contains four ring terminals, three other ground wires and a braided ground strap. This is one of the primary points for the PCM to ground, as well as the ignition coil module and various sensors.
Here I discovered yet another problem. Upon visual inspection, it appeared to be a good stable ground, but once the nut was removed, corrosion began to flake off the terminals and the stud. A good thorough cleaning and a couple of new star washers brought the integrity of the ground back to within normal parameters. Abracadabra! Magically my PCM and sensor data returned to their normal readings. That is, except for that pesky MAP sensor voltage.
How about that idleEverything was looking pretty good now, apart from the fact that I still seemed to have a somewhat rough, unstable idle. Remember that awkward MAP sensor reading that I spoke of earlier? Well, I still had it. That, coupled with the idle problem, led me to investigate the possibility of a vacuum leak or a faulty MAP sensor. The MAP sensor checked out ok, and just to be on the safe side, I tried a known good sensor in its place. Nothing beats having a known good part lying around to try sometimes - it can be a real time-saver. The idle was still the same. I searched over the entire engine for a possible vacuum leak, but to no avail. I finally came to the exhaust gas recirculation (EGR) valve, and what do you think I found? The valve pintle was completely covered with carbon; it would hardly open and would not close at all. Because of the age of the valve and the excessive carbon buildup, I opted to replace, rather than clean it. After replacing the valve, the idle was now "as smooth as glass." A final test-drive confirmed that everything was back to normal.
While this repair turned out to be a little more costly than the customer or I had anticipated, it was an experience that I will not forget. We all run into a vehicle like this one from time to time, and it tends to be our own short-sightedness that allows us to suffer the consequences. In this age of modern diagnostic procedures, we tend to lose track of the big picture and get ahead of ourselves. The lesson here is to always try to think 'outside of the box.'