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Harold's Fuel Pump StoryPosted 7/5/1998By Jeff Bach
"Hey boss, you got time to put on a set of plug wires?" comes a familiar voice from behind the car on which I was working (Harold calls everyone "boss"). I was bent over the rocker panel backward with my head driven as far up under the dash as I could get it. I had a flashlight in my mouth and was fumbling with the air conditioning (a/c) panel, counting over to the third button (the auto button) and the fourth button (the defrost button), cycling between the two, trying to see if the up/down mode door actuator was working. You would be surprised how many times you can count from the wrong end from that position. It's kind of like writing left-handed in the mirror while looking at the image instead of the paper. (I practice this for 45 minutes every night to maintain my edge.) "I think that's what is wrong with it," Harold says. "I already put a set of plugs, a distributor cap and rotor on it. That made it a little better, but it still misses something awful. I thought maybe you could hook it up to your machine and tell me what was wrong with it." Is it 6 p.m. already? (I think to myself). I talked to Harold on the phone earlier and listened to him explain the problem his daughter was having with her '88 Mustang convertible with the HO 5.0 eng. I told Harold at the time that I couldn't look at it until 6 p.m. because I had to get this a/c problem fixed for a dealer. "I got off work a little early so you would have plenty of time to fix it," Harold says. "Awlbub bwebb awt iaun auw cupubl ub maunuts," I mutter. "What's that ya say boss? I can't hear ya." Now I know Harold does not understand flashlight talk, so it looks as if I will be finishing this one up after I look at Harold's car. "I said pull it in the middle stall and we'll put it on the scope." He pulls it in and revs the engine. "Whapoomp. Pow! Pop. Pop. Pop. Purrrr," says the engine as Harold snaps the throttle. I'm thinking it sounds like a lean miss, but I know Harold is convinced of the wires. I hook the ignition scope up to it and watch the spark lines as he revs the engine. Spark lines confirm my suspicion. "You're right," I say. "It could use a set of wires, but that won't cure the problem." "What's wrong with it then, timing belt?" Harold asks. "I don't think so," I say. "Oh no, please don't tell me it has a burnt valve; it's only got 60,000 miles on it." I can see Harold should have been a mechanic instead of a roofer. He has the logic down pat: It goes, it's doing this, it has to be this, or this, or this, or that - arranged in order by cost starting with the least expensive then working its way up the list. I grab my lab scope with the current probe still hooked to it and roll under the rear of the car on a creeper. I clamp the current probe around the fuel pump feed wire, set the scope, and get the pattern displayed in Figure 1. This one is a no-brainer. At a pump speed this slow, it's no wonder the thing ran bad. "It's definitely a bad fuel pump," I say. "No way," Harold says. "Can't be the pump. That fuel pump was replaced less than a year ago." "Well I don't know how old it is, but it's definitely shot," I say. "How do you know it's the fuel pump when you haven't checked the fuel pressure?" "I used a more reliable test," I explain to him. I tested the fuel pump current waveform signature with my FW Bell CG100-A milliamp current probe and my Fluke 97 digital storage lab-type oscilloscope, and from that test I can tell the pump is running at 3,225 rpm, and is drawing less than 2 amps of current on average; the low peaks are way too low and the high peaks are too high. I can tell it also has a bad comutator. I download the scope to my computer, print the waveform and show him where the problem is. I also show him what a good waveform looks like. "Maybe we should put a gage on it to be sure," Harold says. "If I had a gage on it and was questioning the pump's integrity, I'd put my probe on it to be sure," I tell him. "You're the boss," he says. I can tell he remains skeptical. I order a new pump and pull the tank. The old pump looks like an aftermarket brand that I have had trouble with before. When the new pump arrives, I install it and scope the pump again and get the waveform in Figure 2. The new pump is spinning at 6,600 rpm and the car runs perfectly. The look on Harold's face is worth the effort to get the waveforms. I tell Harold to drive the car around the block to be sure it was not going to start missing again. He does and, when he returns, he says the new pump runs quieter than the other one ever had. "Man, you might be onto something there with that probe thing," Harold says, as he tries not to smile and give away that "I-thought-you-might-be-out-in-left-field" look. He says, "I sure never would have guessed it was a fuel pump." He then asks the inevitable question: "Can I get a copy of that picture of my fuel pump?" "No problem," I say, as I pull his bad and new waveforms together and print out the picture in Figure 3. I figure once the word about the current probe gets out, it will get to be like the old Sun Scopes when they were new. Everyone will want to start getting their fuel pumps looked at with "that new probe thing."
I stuff myself back up under the dash of the Buick and try to remember where I was. I think to myself, "That might make a good article. Someday I'll write the 'Harold's fuel pump story' and give him a copy of that too, if it ever gets published."
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