Friday, June 3, 2011

2000 Buick Lesabre

This fine example of GM engineering has 137,050 miles on it. It has the 3800 K motor in it and the complaint is simple. When it gets hot it stalls out and doesn't want to restart too well. Let the car cool down and away she goes. My mind is already thinking crankshaft sensor, ignition module, bad ground for the fuel pump, ignition switch heating up and going open. These vehicles have always had issues with those parts. I start the car and the vehicle cranks well, fires right up and runs well. I have other vehicles to look at so I close the hood to build heat and let it run. I hook up my scanner to check codes and there are none. I am really not that suprised. I leave the scanner hooked up and go off to check the other problem vehicles. I come back to our Lesabre about 20 minutes later and it has stalled. I try to restart and it starts and immediately dies. I restart and try to feather the gas and it stays running barely. I get the distinct odor of a vehicle that is overfueling. I have been down this road before. I get out and open the hood. I disconnect the vacuum hose from the fuel pressure regulator fully expecting it to be wet with raw fuel. This is a very common intermittent issue on these cars. In fact, when I was on the techline I used to tell the techs over the phone my trick to add a piece of clear airline tubing from the aquarium store between the vehicles existing vacuum harness and the fuel pressure regulator and look for liquid fuel.
Dry as a bone as you can see! I hook my fuel pressure gauge just to see where that is.
Nothing wrong here. Well within specs. Now, lets look at some scan data. When you have a gross mismanagement of fuel you want to look at the big four. No, not Metallica, Megadeath, Anthrax, and Slayer( I have my tickets for Yankee Stadium) but RPM, Coolant Temp, Engine Load, and Throttle Position. So, that is what I do. Everything seems to be in order until I look at my MAF (Mass Air Flow) pid on my scanner while cranking. See below.

Yikes! That is correct 170 g/s (grams per second) during a crank event. Remember, our general rule g/s should basically be the same as you liter displacement at an idle and 40 times your liter displacement at wide open throttle. So we should see approximately 4.0-5.0 g/s at an idle and about 152 g/s at full throttle. we have 170 g/s just cranking! Do we have a bad MAF? It does seem lately that every other car I look at needs one. I am not convinced yet. I need to check the wiring.

Well the first thing I do is check for the proper open circuit voltage from the PCM (Powertrain Control Module) to the the MAF. the PCM sends out a voltage of 5 volts that the MAF will bring down to ground and release creating a square wave. The speed of this action is the frequency. The PCM uses this frequency to determine engine load. This input along with others helps the PCM to map fuel, ignition timing, egr, etc.
Nothing wrong here. I shake the harness and look at my min/max voltages. They don't waver. The remaining two wires at the MAF supply power and ground. Whenever possible I like to scope powers and grounds using the battery negative terminal as my ground. So I set my scope up to capture mode. I reconnect the MAF to load the circuit and I backprobe. I crank the engine and record.
Channel 1 is the power feed and channel 2 is the ground. Nothing wrong here. The reason I scope is to look for noise on either circuit. Something that could be missed with a multimeter. I reset my scope parameters and caught this on a start and stall.
Lots of ugliness. I was able to capture the MAF failing after it cooled down some. Check out the screen shot below.

This shot is at an idle. Notice we have great upstream O2 sensor activity, MAP kpa, and MAF value is at the expected 5.0 g/s. Then at about the 175 frame the MAF skyrockets and the MAP tanks, and O2 values peg. Let's get the cursor on that spike.

You are reading correct that is 307 g/s at an idle. To put it in perspective that would be a good value for a 7.7 liter motor at full throttle. The lesson here is never assume. Past experience and pattern failures are nice but you are better off looking at every vehicle as a brand new experience. Next.....