To make it even more of a mystery, the pony had some quirks. It never misbehaved when it was cold -- you had to drive it for a few minutes before it would buck and act up. Also, if you held your foot on the gas pedal, it would buck and surge until it would die. It sometimes would happen when turning and other times would stop happening when turning. At times it idled fine and other times it would die. But, there was one thing that you could do that would always make it straighten itself out -- "Just stomp on it." Once your right foot was firmly planted on the floor with the gas pedal hammered down, the engine would run as if nothing was wrong. That was fine, as long as there was no one in front of you and you didn't break the speed limit!
When Tim first started on the job, the computer was full of codes. Tim figured the most important ones were the MAP and Throttle Position Sensors. Replacing these two sensors seemed to lighten things up a bit, and it ran better. The rest of the codes disappeared, but the bucking continued. Tim thought it was acting as if the fuel pressure was off and the fuel pump was at fault. His gauge said the pressure was only 24 psi, when it was supposed to be at least 35. The pump also flunked the fuel volume test. It turned out there were two pumps, one in and one outside the gas tank. The one in the tank is for transferring fuel out of the tank and the other is for pressurizing the fuel.
Tim replaced the gas filter. No difference. After replacing the outside pump, the fuel pressure came up to 30 psi. He squeezed off the return fuel hose and the fuel pressure instantly shot up even higher. He installed a new fuel pressure regulator, but the problem remained.
"Maybe my gauge is off?" Tim wondered. It was at this point he asked Mark, the shop foreman, to enter into the fray. Mark hooked up his own gauge and found the pressure to still be just a hair over 30 psi. "Let's try running it with the pressure regulator vacuum hose disconnected, which will make the pressure stay at 40 psi.” No difference. The bucking and dying was still there.
"Time to connect BOB, the break-out-box," said Mark. While Tim drove, Mark probed the pins in the BOB.
"Man, this thing is really running lean. The oxygen sensor even goes into negative volts! But, when you give it gas, it straightens itself right out. Why? What does going full throttle have to do with this puzzle?" "Geez, Louise...why me? Why do I always get the winners?!" lamented Tim. "I still think it has something to do with the fuel pressure being low?' Mark replied, "No way, no how. We ran it with 40 pounds of pressure and it still acted the same!"
"Maybe it's a bad computer. Maybe the owner fried it when he was installing that clutch. Maybe he shorted the battery or something and cooked the computer?' said Tim."Yeah. It could be that the computer is turning on the EGR valve and making it run too lean?' said Mark. "Stop the truck and let me disconnect the EGR valve. Here, give me a pen to plug up the vacuum hose?" No, don't use a ball-point pen?' said Tlm. "Why not?"
"Because the engine will suck the ink right out of the pen. Ever see what that ink does to the inside of an engine?"
"No..," “Well, ifs not a very pretty sight. Here, use this pencil instead.”
Stabbing the pencil into the vacuum hose effectively disabled the EGR, but made absolutely no difference in the way the truck ran. "Maybe its the timing?' said Mark. "Let’s try putting the engine into base timing mode. Here, let me switch to distributor timing while it is acting up and see if there is any difference. This will tell us if there is a problem with the distributor or the knock sensor."
No such luck.
"Maybe the computer can't hold the injectors open long enough or something like that. Let’s check out the injectors. Let’s look at their pulse width. The computer fires them in groups: one-two-four and three-five-six. Maybe one group is weaker than the other?' Mark measured the pulse width of the injectors using pins #58 and #59 of the BOB.
"Wow! I didn't expect to see this. Every time the truck starts to buck, the pulse width goes full rich. I'm reading numbers as high as 20 milliseconds. But, every time you put the pedal to the metal, it goes down to around 10 milliseconds. I don't get it?!" said Mark.
"Maybe the computer is trying to hold the injectors open and can't. Maybe one of them is shorted and is making the others fail to open. That would cause a rich mixture?' said Tim. “Yeah, you’re right. It follows then, that if I short out pin #58 or #59, it would hold the injectors open and make the mixture richer-artificially. Let’s try it again and see if this trick tames this pony.” As the truck ran along and seemed to run out of gas, Mark tried shorting out the injectors. For a very short period of time, he grounded the set of injectors, then the other set. No difference. “I gotta get away from this monster and think.” said Mark. “Park it. I’m going to run a few errands and think this over.”
As Mark drove along, he was thinking, “Why would the computer make it go lean? Why does the oxygen sensor read normally and then go totally lean? Why does the computer make the injectors go rich but the mixture stay lean? Then, what does flooring the gas pedal have do with it? What does the injector pulse have to do with it? Why does it run so badly at 17-20 milliseconds pulse width, yet straighten itself out when it runs at 10 milliseconds pulse width? Could the computer be failing to open the injectors, even though I am reading a large pulse width? Why?”
Suddenly it hit him right between the eyes. The oxygen sensor was lying to them. The 02 sensor was telling the computer the mixture was lean, when it fact it wasn’t. The computer was actually making the mixture so rich that it was flooding the engine. Somehow theo oxygen sensor was intermittently losing its voltage signal. “Maybe the oxygen sensor lead got caught or pinched when that guy replaced the clutch-and ti is intermittently grounding out the signal. That would do it!” Mark thought. “That’s it! Then, when you floor it, the computer goes into the fixed operating mode and ignores the oxygen sensor. The pulse width narrows and it runs fine.”
Mark couldn’t wait to get back to the shop and stopped at a payphone to call Tim. “Hey, check the O2 sensor lead. I bet it is grounding somewhere around the engine.” By the time Mark got back to the shop, Tim had the Bronco up on a lift and was all smiles. “You’re right on the money. Take a look at how that lead melted against the exhaust manifold. The wonder didn’t put it back into its clip holder and it is grounding out.” said Tim. And all it took to tame the Bucking Bronco was a little electrical tape.
