Auto Service World
Feature   March 1, 2004   by Rick Cogbill a.k.a. Slim Shambles

Lots of Jet-ta Black Smoke

"Sorry I'm late," I puffed as I burst through the front door. "Did I miss the meeting?"...

“Sorry I’m late,” I puffed as I burst through the front door. “Did I miss the meeting?”

Quigley, our service writer didn’t even glance up from his computer where he was going over the weekly work schedule. “How could the meeting be over; you’re the one who called it.”

He had me there. I looked into the coffee room, but it was empty. “Where is everybody? I can’t have a shop meeting without staff!” Quigley jerked his thumb over his shoulder. “In the shop. Beanie’s bought another car.”

I found the crew standing around the open hood of a 1992 VW Jetta. The 1.8 litre gas engine was running rough, and black smoke filled the air. “Whew!” I plugged my nose. “Running a little rich, aren’t we?”

Beanie nodded glumly. “Sure is. I bought this from a guy at trade school who needed some cash. I thought I’d fix the rich mixture problem and resell it, but we’re stumped!” My two other technicians, Tooner and Basil, stood there with their arms crossed, puzzled looks on their faces. An inline fuel pressure gauge was installed in the fuel line, so the thought of excessive fuel pressure had obviously crossed their minds.

“No luck?” Basil shook his head. “It’s a bit of an odd one. It runs fine until it goes into closed loop. Once it warms up.” — he spread his hands — “this is what we get.”

Tooner grunted his agreement and went to find his hand held meter. “Can’t get no data stream,” he said over his shoulder. “It wasn’t available until 1993. I’ll have to start testing things manually.”

I understood his reluctance, as we didn’t do a lot of imports in our shop; we usually left that to Dieter at The Import Shop. We knew a little about the Volkswagen fuel injection systems, but it was limited knowledge at best. Tooner started with the O2 sensor wire, thinking the sensor might be stuck low, causing a rich condition. Instead, he found the opposite. “Hey, there’s 1.5 volts on this line! I thought O2 sensors could only produce up to .9 of a volt, even when fully rich.”

Beanie scratched his head. “Faulty sensor? Maybe we should call up Dieter for a new one.”

Basil disagreed. “I don’t think so, Beanie. Even if the sensor could produce that much voltage, it would be driving the computer lean.” He pointed to the black sooty tailpipe. “And that is obviously not the case.” Tooner caught on to what Basil was getting at, and unplugging the oxygen sensor, tested the wire again. “Hey, even with it unplugged, I’m getting 1.5 volts on the computer side. I think we’ve got a faulty computer here.”

I held up my hands. “Don’t be so hasty. Let’s go back and check the basics — sometimes stray voltages are caused by faulty grounds.”

So the hunt began. Tooner crawled under the dash to check out the fuse panel, known for their tendency to cause problems, while Beanie checked the connections at the computer. Since the shop meeting was on hold, Basil and I grabbed a cup of Quigley’s coffee before it got too stale.

“Y’know, Basil,” I said as we stood in front of the Jetta, “I would think that a good place to find a broken or loose ground wire would be someplace where there’s vibration.”

Basil nodded slowly and took a sip. “Yes, you have a point.”

“So, it’s probably not going to be where the wiring is tight and secure, like in the harness.”

He shook his head. “No, not usually.”

“And,” I continued, “corrosion would probably be a factor.”

“Hmm, could be,” he agreed, rubbing his chin as we stared at the car. I drained my coffee. “In fact, I would say that since it’s related to engine performance, it only makes sense that the ground wire for that circuit would be attached to the engine.”

Basil shot me a look. “Is this an observation based on an intimate knowledge of Volkswagen wiring circuits?” I grinned. “Nope. It’s based on this loose wire I noticed near the cylinder head.”

Basil took a closer look. A ground wire attached to the heater hose outlet on the cylinder head had become corroded, and had broken loose. After repairing the connection, we tested the circuit again. This time we found only .5 volts coming from the computer, which we later learned was normal for this car.

“Wow!” exclaimed Beanie. “She purrs like a kitten now. How can I ever thank you guys?”

“Sounds like it’s time for a test drive,” said Basil. “I’ll go get Quigley.”

Beanie looked confused. “Test drive? Where are we going?” “Frankie’s Donuts,” I said. “I’ve still got a shop meeting to conduct, and you get to supply the refreshments.”

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