It seems lately I'm starting every article with "Last month, I ..." and ending them with "Next month ..." It's hardly professional writing, but trying to communicate complex diagnostic routines in small kernels isn't easy. This strategy issue keeps expanding the more I think about it; so just bear with me and we'll get to the end eventually. Look out, here it comes in the next line!

Last month, I gave the formulas for calculating fuel delivery as: Speed Density: Base + CTS + [(MAP + RPM) x VE] + TPS + ACT - EGR + -O2 + wildcards. Well, there are a few corrections I would like to make. First, CTS (coolant temperature sensor) should now be called by its new OBD II acronym, ECT (engine coolant temperature). Throttle position sensor (TPS) is now TP, or throttle position; I'll have a hard time changing to this one. ACT (air charge temperature) is now IAT (intake air temperature). The rest remain the same. We all need to get used to the new nomenclature so we can all speak the same language. Down comes the Tower of Babble!

Second correction: BARO! Speed density systems use the MAP sensor to take a barometer reading (atmospheric pressure) when the key is turned on. Barometric pressure is the weight of the air above you. If you are at sea level, the barometric pressure is 14.7 psi or 101.3 kPa or 29.92 in. Hg (inches of mercury), which is the most commonly used scale.

As you move up in altitude, the air becomes thinner, requiring less fuel to be injected to achieve the correct air/fuel ratio. This is why old carbureted cars working in high altitudes (over 5,000 feet) needed to have special calibrations. The general rule is to subtract 0.9 inches of mercury for each 1,000 feet of rise in elevation. Tucson, Ariz., is at 2,500 feet above sea level, so our barometric pressure should be 27.75 in. Hg. Checking our weather records shows the average in Tucson to be 29.8 in. Hg. I suppose living in a hot climate gives us higher atmospheric pressure. I have never fully understood why our pressure is higher than the formula predicts.

Checking my files for recorded MAP voltages for GM vehicles with the key-on/engine-off (KOEO) showed an average voltage of 4.4 volts. Using a GM MAP voltage to pressure conversion chart shows this to be 28.0 in. Hg, very close to the predicted value. This is the best way to diagnose a MAP sensor suffering from the most common mode of failure, "sensor drift." One that has drifted slightly from calibration can have serious effects on system performance.

The MAP sensor consists of a tiny diaphragm tied to stress-sensing resistors and sealing a small chamber with an absolute vacuum in it. The higher the pressure on the hose side of the diaphragm, the greater the stress on the diaphragm, causing a change in resistance. MAP is an acronym for manifold absolute pressure, and measures pressure relative to true zero, an absolute vacuum. If your compression gauge in the shop reads against true zero, then it would always read 14.7 psi to start. You may notice that many gauges have "psig" printed on the face; this is a pounds per square inch gauge, showing that the 14.7 psi of atmospheric pressure has been subtracted.

So, why does the MAP sensor drift and where does it drift to? It drifts because the diaphragm relaxes with time. This results in a lower voltage reading, which translates into a lower BARO reading, which translates into less fuel delivered, causing a chronic lean condition if the fuel delivery drops out of the O2 sensor's command window. It is also possible for the sensor to drift up or down if the circuit has more resistance in the ground or power and signal lines.

Summary:

• Higher voltage = add fuel = less vacuum = more diaphragm stress.
• Lower voltage = subtract fuel = more vacuum = less diaphragm stress.
• While they are becoming rare, watch out for GM vacuum sensors. This sensor does not use a vacuum in the reference chamber. Instead, it uses sea level atmospheric pressure.
• Using atmospheric pressure as the reference causes the voltage on this sensor to be the opposite of a MAP sensor.
• High voltage = subtract fuel = more vacuum = more diaphragm stress.
• Low voltage = add fuel = less vacuum = less diaphragm stress.
• GM is the only company I know of that used this "backward" reading sensor. These were generally only used in the early years, but be careful. Also note that some manufacturers, like Toyota, call their MAP sensor a "vacuum sensor."

Ford MAP Sensor Drift
The Ford MAP sensor outputs a frequency relative to the manifold pressure and, while it uses a slightly different method (variable capacitance) to measure the manifold air pressure, it still uses a reference vacuum. At sea level, key-on/ engine-off, this sensor will read 159Hz. Subtract about 3Hz for every 1,000 feet increase in altitude. That means Ford MAP sensors read about 153Hz in Tucson, KOEO. We have replaced many Ford MAP sensors that had shifted 10Hz to 143Hz. This causes a lean condition that the O2 sensor can't compensate for. According to the excellent TechTip published in AutoInc., April 1995, a shift of 2Hz can cause problems. For a copy of this tip, call AutoLine Telediagnosis at (800) 288-6210 and ask for customer service. The tip was written by Mitch Belew; I'm sure they will fax it to you for a reasonable fee. There are a zillion Ford's out there with this problem.

GM And Lack Of Information
In the early '80s, I had an elderly couple drop into the shop and request that I go for a test drive "right now," because an intermittent problem was occurring. I jumped in, started the car and off we went. No problem ... it ran great. When we returned, they described the problem. They had driven up Mount Lemmon (9,450 feet) from Tucson (2,500 feet), and near the top at idle, the engine stalled. On restart it ran fine, but concerned about getting stranded, they immediately returned. When they got to Tucson, the car ran rough and almost stalled. They drove straight to the shop, turned off the car and spoke to me.

One week later, they dropped the car off for a complete diagnostic. They were going to California and could not chance a breakdown. We ran every test in the book and took a long test drive. No problem found. They gladly paid the bill, but they were very concerned about the car. Three weeks later they came in for an oil change on their brand new car. They had traded in the year-old car because they could no longer trust its reliability and, if I couldn't fix it, then they didn't want it!

I learned later about the engine checking BARO when the key was on/engine off. Lack of information cost my customer the confidence in their car, and a big chunk of change. If I had known the cause, I could have put their mind at ease, and perhaps more importantly, I could have said in all honesty "they all do that!"

It seems the engineers that programmed the strategies into the computer never imagined that the barometric pressure could change so dramatically on a single trip. They did realize this later, and most cars since about 1988 will reset BARO on wide open throttle. The presumption is that by climbing a mountain, you will have to go to wide open throttle (manifold at atmospheric pressure or zero vacuum) at some time during the climb. Once again, whipped by some internal computer strategy! Sometimes I feel like I've been down to the whipping post!

Next month, more MAP punishment brought to you by GM, Chrysler and Toyota!

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