For an automotive technician, catching an intermittent driveability problem in the act is kind of like the feeling a trapper has after catching a wise old red fox. You must have a good knowledge of your prey's habits and think like it does.

I remember my brother Greg coming home from running his traps many times discouraged that he had been outsmarted by one of these elusive critters, sometimes even feeling he had been laughed at by the animal. On one particular occasion, I remember him having a set that had been defecated on and then buried by the fox (as evidenced by the tracks and droppings).

Each time he was bested, I remember him telling his account of what went wrong or how next time he would be better prepared.

Then came the day when his diligence had paid off. He had dug a whole at an angle and buried some dead fish in the bottom of it. Then, a few inches from the surface, he set a trap and covered it loosely with the fresh dirt. He had been ever careful not to leave any human scent in the area, wearing gloves and hip waders that he kept on a line outside in the breeze.

He would also use some of the foulest smelling lure imaginable. Greg buried a fish and sprinkled some fox urine around the freshly dug dirt. He surmised that the smell of fish (one of the fox's favorites) would bring a fox to the area; the animal would think another fox had buried its dinner and was planning on returning later. Of course, only the biggest, "baddest" fox around would dare steel another fox's meal without first sizing up the competition.

He must have been right because, when he came by the next morning after checking his traps, he had a big red by both front legs. I never saw a brighter beam on Greg's face than the day he came through the front door holding that big red fox - the prettiest you ever saw.

After that, full of the confidence from his experiences, Greg caught foxes with increasing regularity and became somewhat of an expert on trapping them. (Once you nail the tough one, the rest are gravy on the table.)

This month's driveability problem is one of those tough ones. It came from a friend of mine that has his own shop. He was the first guy I worked for as a mechanic right out of vocational school. The car was a 1988 Buick with a 2.8 engine. When warm, the car would occasionally stall at stops. It also would idle too high for long periods of time and then set a code 35. He had tried a lot of parts after cleaning the throttle body and setting the TPS. They included an MAF (mass air flow) sensor, an ECM (electronic control module), an IAC (idle air controller) motor, a new TPS (throttle position sensor) and a PROM (programmable read only memory).

I could get it to act up with some regularity after letting it run for about a half hour. The scanner would show zero IAC counts while the idle was high. I figured this was going to turn out to be a wiring problem - maybe a pin loose at the ECM or IAC plug. I had been looking for a chance to play with my new CRT Current Probe's low scale. IAC current was pretty low. I clamped the probe around one of the IAC wires and adjusted my scope to show a good picture.

I saw this pattern and knew I had a bad ECM.

I knew it because I was used to seeing IAC voltage signals in the past and have always seen the voltage reverse polarity at equal times. The one in Figure 1 (figures will be displayed in a new browser window) was not at all what I expected to see. The blue arrow to the left of the grid is the zero current reference so everything above that is positive current; everything below is reverse current. The other strange thing was the weird dips and humps.

I was fairly confident about ordering a new ECM, but I was unsure about the condition of the IAC motor.

After plugging in the new ECM, I got the pattern in Figure 2, which looked much better. I didn't know for sure what I was looking at, but I knew it looked uniform. I was happy with my new toy. I had already printed the waveforms and was thinking about how neat it was to look at the IAC current. I know how the doctors who got to play with the first sonogram machine must have felt.

The problem was that after it warmed up, the car started idling too high at stops again. It even set the code 35 again with the new ECM. I looked to see that the signal hadn't changed back to what it was, which it hadn't. Now I started to get that feeling. (You know, the one in the pit of your stomach.) I started thinking that there was still something funny about the signal. I was getting waves on the bottom and humps on the top that didn't mirror each other.

I grabbed an IAC motor off another car and plugged it in to see if it looked the same.

This was probably not the smartest thing I have ever done since this car had a stalling complaint also; but it turned out to give me one of the best lessons I have had all year, because when I plugged this one in and got the pattern in Figure 3 (keep in mind that to get these things to pulse, I am grounding the diagnostic connector), I noticed that the pintle was not moving in and out. I have done this before and found sticking IAC motors, but never while seeing the current wave form.

Even though this one was not moving, at least the bottom pattern mirrored the top one.

I took it over to the solvent tank and gave it a good cleaning. I plugged it back in and took another reading.

Now I have my fox!

The dip in the charging curve of the IAC coil, shown in Figure 4, makes more sense than anything I had seen yet on this car. It is what I call "The Gull Effect." I first noticed it on an old '82 Corvette MC solenoid. I noticed that when I held down the solenoid while watching the current wave form, the dip went away. I had seen this on PFI injectors also, but not as exaggerated. (Figure 5)

I see this now on any solenoid or relay circuit that has a coil with a moving core. "The Gull Effect" takes place when the solenoid plunger, injector pintle or relay core moves in the windings. The movement causes a fluctuation in the magnetic field as it is building, usually about two-thirds of the way up the inductive curve. (I'm sure this is simple academics to an engineer, but hey, I'm just a guy with a scope and a probe. Heck, I feel like Davy Crockett.)

Even the slight movement in a TCC solenoid causes "The Gull Effect" to take place (see Figure 6).

Refer to Figure 7 to see what a sticking PFI injector looks like. v Next, take a look at Figure 8. This was the shot I got after cleaning the stick- ing injector.

Now, take a look at Figure 9. I have copied several of the segments of the waveforms that I have studied and cut away the rest of the pictures to illustrate why I call this "The Gull Effect."

What do you think?

Jeff Bach is the owner of CRT Auto Electronics, an ASA-member shop in Batavia, Ohio. For more information on this topic, contact Bach at (515) 732-3965. His e-mail address is northstarguy@zoomtown.com

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