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Computerized Scan Tools Are a Tech's Best FriendsPosted 7/10/2002By Jeff Bach
Technology is now the technician's best tool. With OBD II vehicles making up a large portion of the jobs entering most independent repair shops, it's time to decide which side of the fence to be on. The easiest way to get rid of those time-burning, troublesome diagnostic driveability jobs for some shops has been to just send “those cars” to the dealer. This frees up the shop to do the more profitable cut-and-dry work, not to mention the cost savings from not having to buy all those expensive brand-specific scan tool subscriptions. I see more cars than ever now that are here for one reason: the owner can no longer fix it himself. This makes me the bad guy, but it seems the lesser of two evils. A few customers come to me with the upfront statement: “All I want you to do is diagnose the problem; I'll hang the parts.” This is not because they are tightwads; they just love to work on their own cars. It's that sense of pride that comes with doing it yourself. Some of these guys have built their cars from “totals” or maybe even overhauled their own engines, and have done a nice job. When it comes to diagnosing OBD II vehicles, the guessing group is finding out the part that's named in the trouble code isn't always the part with the problem. And trying another PCM often just causes the water to get deeper. Sometimes the reluctance to pay a shop to diagnose one's car accompanies a certain attitude that can cause anxious tongue biting, either from resisting the temptation to spit technical terms and three-letter acronyms out about the vehicle's problem to re-ensuring the skeptical customer of your diagnostic prowess. Or, the temptation to send him down the road with the knowledge that the lack of his contribution to your overall profit margin will not be sorely missed. Temperance, I believe, is the operative word here. A recent job I had, which fits well into this subject, was a '96 Corvette that came to the shop following a call from a customer who was price shopping with the Yellow Pages, calling only the shops that list "computer diagnosis" in their ad. Right off the bat, I know this guy is sharp. First, he wanted to know if we had “the computer tool” that works on his car. Apparently some of the shops he had talked with have a “computerized scan tool” but it may not be able to work on his specific model and they wouldn't know for sure until he came in and they tried to hook it up. Then he wanted to know how much we would charge him to replace his O2 sensor and clear his computer. He was going to supply the part. My first instinct was to reply with something cliché like, “I don't know, we don't usually work that way ... Tell you what, why don't you call Perkins and see how much they want to make your breakfast. Tell them you have your own eggs and pancakes, you just want them to cook 'em like you like 'em. Let me know what they tell you; that way I'll have something to compare against.” I bit my tongue, however, and thought for a moment about what that says for our industry. Then I just said, “pre-cat or post-cat, Bank 1 or Bank 2 and would you like us to record the freeze frame failure records prior to clearing the codes so you'll have something to go by on the outside chance that an O2 sensor is not the problem?” “Let me call you back” he said. “Good answer,” I thought. Later that afternoon I got the call, and this time I got a lot more info and a little less skepticism. Turns out he had a diagnostic sheet from a dealer with two trouble codes, a PO171 fuel trim system lean Bank 1 and a PO174 fuel trim system lean Bank 2. The customer had done some Internet research and concluded he probably had a bad O2 sensor because most of the people who had either of these codes needed O2 sensors. After an initial consultation, the customer left the car for me to diagnose. The freeze frame failure records for both codes indicated the car was at cruise speed under light load and both banks were reading + 25 percent LTFT.
With the help of my test equipment and the customer's knowledge, it didn't take him long to convince me he should try replacing the fuel pump and filter to see if it would help, even though the pump was still within specs. The new pump showed obviously significant improvements in the numbers (Figure 2).
Three weeks later I get another call from this guy. The MIL light came back on, but now only while on a long downhill grade. The guy was more understanding than I had expected and didn't even mention the fuel pump he probably could have gone with a while longer without replacing it. He left the car and trusted that I would find the “new problem.” I was a little surprised and quite a bit dissatisfied with myself when I discovered it had the same two codes stored. Somehow I was missing an obvious problem. I checked everything I could think of that could affect both banks equally. I tested the injectors with my current probe, scoped the ignition, did numerous power balance tests, sprayed around the intake and injectors, and even did a little Internet sleuthing myself in the iATN fix database before posting the problem to the Vette experts. I got a few helpful suggestions, but nothing I hadn't tried. I was getting pretty frustrated and found myself once again left with nothing but a prayer. “Lord, got time to look at this Vette with me?” I still had my needle-nose pliers in my hand when it hit me that I hadn't pinched off the booster line. I had no sooner squeezed the vacuum hose than the LTFT started counting back ... all the way to 0 percent. The vacuum port is mounted in the middle of the intake, so it affected both banks. I have to admit it was a little more difficult to get the customer to believe that his booster was turning on his MIL. (Keep in mind that this car never really had a driveability concern, and no sign of a braking problem, just a MIL on). He was sure the O2 sensors were the problem. I couldn't find anyone local that had a booster in stock, which told me it's not a common item to go bad on these cars. I ended up getting an overhaul kit for it.
After repairing the booster, I looked at the numbers again and noted changes in the calculated load, MAF, injector pulse and IAC, but most were fairly small changes. The LTFT was significant, however, and the best indicator of the lean condition. The customer was all smiles again as I showed him the data and then he admitted that the booster wasn't even on the parts list he had in mind for replacing to get the light out. "Guess you can't do nothing on these cars without one of them high-dollar computerized scan tools," was his final comment. "Nope," I said.
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I was suspecting an overall lean condition since the O2 activity looked good for all the sensors. With no vacuum leaks around the intake, injectors and MAF detected, I went on to a fuel pump test. I got 41 PSI at key on and 38 at idle. This was on the low end of the spec and the current waveforms showed that the pump was on the weak side (Figure 1).
A test drive showed the long-term fuel trim was now at 11 percent for Bank 1 and 13 percent for Bank 2 under the same driving conditions indicated by the failure record. I didn't like it, but he was happy.
When I got the booster off I could see without disassembling it what the problem was (Figure 3). There was a hole in the plastic housing apparently caused by physical contact with an adjacent structure on the cowl support. Did I mention this was one of those cars with the seam in the steering wheel cover glued together and a suspicious looking sag in the dash on the passenger side where the SIR is supposed to be?
