1993 Buick Lesabre Erratic Idle

Engine performance isn't really my specialty area, but before you get "wrapped around the axle" and too involved. Unplug the small connector on the side / rear of the generator, then see how the engine reacts. If the performance problems clear up, I'll be back tomorrow night to tell you how to proceed.

These have a lot of problems with the coil pack and ignition module witch is under the coil pack but if it runs worse after the tune up I would make sure you don't have a couple of plug wires on wrong or maybe check the spark plug gaps if you didn't before you installed them.

Yup, not many people catch that, but I'll share some trivia. In an effort to always use correct terminology in the classroom, we used to have "DC generators" in cars through the 1950s. Chrysler developed the first "AC generator" and used it on 1960 models. They called it an "alternator" and copyrighted the term. You will rarely see the term "alternator" used in any other manufacturer's service manual. While the industry-standard term today is "AC generator", everyone will know what you mean when you say "alternator".

To address what you were told about the battery running dead, that is correct too. The generator keeps the battery charged and it does that while the engine is running. When the generator stops working, on today's cars with way too much electronics on them, a good, fully-charged battery can keep the engine running for perhaps as much as an hour; much less if the head lights, heater fan, wipers, and radio are on. The test I described earlier by unplugging the connector is only done long enough to determine if the running problem clears up. That can take as little as a few seconds, or as long as a five or ten-minute test drive. The battery will last that long. Once you know if that solves the problem, you reconnect the plug, then have the charging system repaired.

All AC generators develop three-phase output current. Similar to what is used in most factories. Because of its very high efficiency. The problem is that can not be stored in any way. To store it in a battery, it has to be "rectified" by six diodes, which are just one-way valves for electrical current flow. While the older DC generators developed a real smooth and steady direct current that could go right into the battery, rectified AC current rises and falls. The three phases overlap each other resulting in a much smoother current, but still not nearly as smooth as with the older generators. That pulsing voltage can cause problems with electronics, and GM has a real big problem that is not seen in any other car brands. Due to the current design they started using in '87 models, their generators develop a lot of harmful voltage spikes that the battery is supposed to filter out. As the battery ages and the lead flakes off the plates, they lose their ability to do that. The resulting voltage spikes can damage the generator's internal diodes and voltage regulator, and interfere with computer sensor signals. This is where disabling the generator by unplugging it will stop the generation of those voltage spikes, and hopefully stop the engine running problem. You only have to run the engine for up to a few minutes to know if the problem clears up. That is just the first diagnostic step and is done first because it takes so little time. You moved on to a later test that involves spending money on parts. If that takes care of the running problem, that will be fine, but if the problem persists, do my quick test and see what happens.

AGGGGHHHH! STOP! STOP! STOP before you destroy the car! You are getting some miscommunication from people who understand selling parts. You must never ever disconnect a`battery cable with the engine running, and this is especially true with '87 and newer GM vehicles. Also, they apparently led you to believe you're supposed to unbolt the output wire on the back of the generator. That will stop it from charging, but it's real easy to let the socket or wrench touch that nut and anything metal on the car at the same time. That will give new meaning to fireworks. Go back and read my first reply. The plug is a small one on the side of the generator near the back. There are usually four places in the plug but there may be as little as one brown wire in that plug. Some cars have two wires, but it's very uncommon to find three or four wires. All you have to do is unplug that plug long enough to see if the running problem clears up. If it clears up at idle while you're under the hood, that test will take five to ten seconds and you're done. Sometimes the running problem is only noticed at highway speed. If so, you have to drive at highway speed with the plug disconnected. GM's generators develop huge voltage spikes that can destroy the internal diodes and voltage regulator, and interfere with computer sensor signals. Unplugging the small connector stops it from charging and stops it from developing those voltage spikes. That can potentially stop those spikes from interfering with sensor signals that cause the Engine Computer to do weird things.

You were also misled by the person who scared you that this would allow the battery to run dead. With the generator not working, it is true the engine will be running on just the battery, but you'll get at least a good half hour of driving time, and more if you don't turn on the heater fan, radio, lights, and anything else that uses electricity. If your battery is more than a few years old, you might only get 15 to 20 minutes, but that is still plenty of time to see if the running problem clears up. Once you know that answer, you can pull over on the side of the road, leave the engine running while you reconnect the plug, and the generator will instantly start charging the battery again. At that point you can drive like normal. If you have to drive five miles through the city to get to where the running problem shows up on a highway, just don't disconnect the plug until you get to where the problem typically occurs. Hop out, unplug the connector, drive the car a mile or two, hop out and reconnect the plug. All you need to do is see if the problem acts differently when the generator isn't working. If it does, the generator is the main suspect.

For a more technical answer, here is a copy of a reply I've posted a number of times:

DO NOT DISCONNECT ANY CABLE WITH THE ENGINE RUNNING!

Every year I did a demonstration on the generator test bench for my students to show what can happen when you do that. It was real easy for the voltage to reach over 35 volts. That WILL destroy any computer on the vehicle, the generator's internal diodes and built-in voltage regulator, and any light bulbs that are turned on.

The thinking is that if you disconnect either cable and the engine stays running, the generator must be working but a lot of them will stop working due to the voltage regulator responding to the dips in the "ripple" voltage being produced. That will make a perfectly good generator appear to be bad so that test is not valid.

If a mechanic is caught pulling this stunt he will typically get one verbal warning. For the second offense he will be fired. It's that big a deal.

Some generators respond to the high points in the ripple. That momentary higher voltage goes right back to the field winding and creates a stronger magnetic field. That stronger electromagnet creates a higher output voltage which again creates a stronger electromagnet. It's a vicious circle and voltage can keep on rising until something gives out. The main thing that smoothes out that ripple so it doesn't affect the voltage regulator or the generator is the battery.

Three things are needed to generate the output current. They are a magnet, (electromagnet, in this case), a coil of wire, and most importantly, movement between them. That's why the belt needs to make it spin. One thing that can save you from doing damage by removing a battery cable is not raising engine speed. Generators are relatively inefficient at low engine speeds and their output voltage is less likely to rise to dangerous levels, ... As long as you don't raise engine speed.

One other thing to keep in mind is batteries give off explosive hydrogen gas. Regardless if your generator is working or not there is going to be a big spark when you remove a battery cable with the engine running. Either the generator's current will be recharging the battery, and that can be up to 20 amps, or the battery is going to be supplying the car's electrical systems, and that can easily be over 30 amps. That kind of current is going to create a big spark when a connection is broken or reconnected. Small arc welders run as low as 40 - 60 amps and look at the sparks they create. The reason we don't hear about more battery explosions is because people are careful to not disconnect the cables when there is current flowing through them. It's also why there are huge warning labels on all battery chargers to be sure they are turned off before connecting or disconnecting them from the battery.

Another common generator problem is one defective diode out of the six. You will lose exactly two thirds of the generator's capacity but system voltage will remain normal or it could even be just a little high from the voltage regulator responding to the greatly increased dips in the ripple voltage.

It's always a good idea to wear safety glasses when working around car batteries, but if you still insist on removing a cable while the engine is running, a face shield makes more sense, and have plenty of water on hand to wash any acid off the vehicle's paint.

Ford used to have a really nice generator design that allowed testing right on the back of the unit. Only Chrysler alternators are easier to diagnose. Unfortunately the engineers don't really care about ease of service on GMs and many other brands.

The way you tell if the charging system is working is to measure the battery voltage while the engine is running. It must be between 13.75 and 14.75 volts. There still could be a bad diode though. You need a professional load tester to test for that. Ripple will be very high and the most output current you will get will be one third of the generator's design value. That is not enough to meet the demands of the electrical system under all conditions so the battery will have to make up the difference, until it runs down. You still could have a generator problem. Either have it load-tested or use a home battery charger to fully charge your battery at a slow rate for an hour, then see if it is dead again the next day.

Nope. That was just a quick test to see if the generator was causing the problem. There is still the possibility for some computers to not operate properly due to low system voltage, but that is less likely when the battery is still charged enough to start the engine.

The next tests are going to require a scanner to view live sensor data. The first thing to look at is where the Engine Computer is placing the idle speed motor. If it is increasing throttle opening when the idle speed goes down, it is trying to correct that but without success. If it is increasing throttle opening when idle speed goes up, it is doing so in response to something it perceives as a need to do that. Then you have to look at the other sensor readings, temperature sensors in particular.

You can also look at the oxygen sensors as they switch between rich and lean. They normally do that about two times per second. Those sensors are just monitoring what has already burned in the engine but it can be useful in finding a mixture that stays rich or lean too long. The most common is too lean. That's almost always due to a vacuum leak. A smoke machine works best for finding that. You inject a white, non-toxic smoke at 2 psi, then watch for where it sneaks out. A mixture that stays too rich is not real common. A defective fuel pressure regulator is the main suspect, and the vacuum hose going to it. If that vacuum hose has a leak, the regulator will "see" low vacuum which equates to acceleration. Low vacuum is pulling the fuel out of the injectors with less force, so the regulator bumps up fuel pressure to offset that. That way the total of vacuum pulling and fuel pressure pushing on the fuel remains constant.

Spark-related misfires can be confusing too. When that occurs, unburned fuel and air go into the exhaust. The oxygen sensor will detect the oxygen and report that as a lean condition, but you'll smell the unburned fuel at the tail pipe and assume the mixture is too rich.

I almost forgot to mention to look at the fresh air tube between the mass air flow sensor and the throttle body. Chrysler is the only manufacturer that has never used that sensor. For all other brands, a leak in that hose will cause stumbling, running too lean, and other symptoms. Any air that sneaks into the engine that doesn't go through that sensor will not be measured and won't be included in the fuel metering calculations. Be sure that tube doesn't have any cracks and the clamps are tight.

I'm sensing you're describing a misfire as an erratic idle. Don't overlook worn spark plugs and wires.

The vacuum gauge and vacuum readings have nothing to do with a vacuum leak. Neither one means you have or don't have the other. I should have picked up on that comment earlier. A vacuum leak can occur anywhere air can sneak into the engine without going through the mass air flow sensor. That will upset the fuel / air ratio but will have no noticeable effect on the reading on the vacuum gauge. Only Chrysler engines will still run okay with small vacuum leaks because their main sensor for fuel metering calculations is the MAP sensor, which measures intake manifold vacuum. On almost all other cars the mass air flow sensor is the main fuel metering sensor. That's why those engines can't tolerate a vacuum leak.

Start by inspecting the rubber and hard plastic vacuum hoses. You can also pinch off various hoses while the engine is running to see if one changes the idle speed and quality. If one does, work your way along its branches to isolate the leak.