When the shop manual comes up short, the only trouble tree matrix available may be the one you build in your head.
It can be quite easy to get lost in a shop manual's troubleshooting matrix to the point that you don't even know why the next step is necessary or why you are performing it. In too many cases, we come to the end of the matrix only to discover that we haven't fixed anything.
The point is that you can't switch off your powers of logic when you start following the steps in a trouble tree. The instructions at the beginning of a matrix may be where the answer lies: check fuel quality, etc. I worked on a brand-new 1989 Taurus rental car for a few days before discovering that someone had pumped some diesel into the fuel tank before returning the vehicle.
One strong point to be made here is that engineers don't always write good shop manuals because they are so familiar with their products, they think everybody knows them as well as they do. If that weren't enough, problems can develop in the field that they never encounter or even consider. That's what technical service bulletins (TSBs) and special service messages are all about.
Hot potato
This month's story is about a 1992 Lexus SC300. According to the owner, it had been to half-dozen or more shops – none of which had been able to do anything except replace parts and make suggestions. While the car did run, it didn't run well, and you could have thrown a match at the gasoline mist that was coming out of the tailpipe to make a nice pyrotechnic display.
This young fellow had purchased a throttle body for his Lexus from a salvage yard and installed it himself. Although the part cost him more than $500, gasoline mist was still blowing out the tailpipes in a poisonous cloud. In true Lexus form, this throttle body has two throttle plates stacked in it with a throttle plate position sensor for each plate, an actuator for the Traction Control throttle plate and a stepper motor-style Idle Air Control.
When the book doesn't cover it
VEHICLE: 1992 Lexus SC300
As a result of having experienced one disappointment after another, he had a jaded view of virtually every shop in the area – even those he hadn't visited. This view was reinforced because every shop that had worked on the car charged him for repairs that weren't repairs at all. This wasn't an intermittent concern, either. It still boggles my mind to see shops taking in jobs and then turning out cars that run just as badly or worse and charging the customer as if they had fixed something.
At the college where I teach, jobs like this are a powerful challenge for burgeoning students. I pledged to this customer that we would try to find the problem without spending any more of his money than needed. It might cost him something to have it fixed. But my students don't get paid labor, and it wouldn't cost him anything to have us check it.
Digging in
This little machine is equipped with a 3.0L I-6 similar to the powerplant Toyota stuffed under the hood of the Cressida. It's a peppy little mill when it's running right. This one did run – but not well. Initially it seemed that whatever was causing the problem was affecting all the cylinders equally, because it didn't appear that any one cylinder was misfiring.
"When a fuel-injected engine is getting too much fuel," I explained to the students, "we have to determine whether the Powertrain Control Module [PCM] is making the decision to put too much in there or whether the fuel is coming from an unauthorized source over which the PCM has no control."
For an easy first check, I pulled the fuel pressure regulator vacuum line and looked for fuel there. This condition would indicate that the regulator diaphragm had been breached and fuel was passing through that vacuum hose into the intake. The line was dry.
A peek into the 1992 Lexus symptom chart was rather vague. I had been to an identical chart a few months ago on a 1995 LS400 no-start we tangled with and found it to be no help at all. I started building a trouble tree matrix in my own mind. Those of us who are diagnosticians at heart have all done that without labeling our thought process as such, and in many if not most cases, it's the best way – if not the only way – to find the answer to a problem like this one. A plan of attack that doesn't involve expensive parts swapping is always wise.
In so doing, we work a little harder to check the stuff that makes sense and find out what's not wrong so we can home in on what is wrong like missiles on a target. In some cases, a known-good part is the only choice. I hate those situations, especially when the known-good part is costly and/or isn't on the shelf.
Question 1: Should we pull the flash-out codes? Maybe, but this fuel dump wasn't black smoke, it was a white mist of raw gas. It made more sense to check the fuel pressure. It was a foregone conclusion that there would be oxygen sensor/fuel mixture issues.
Question 2: Should we check the fuel pressure? Good choice under the circumstances. It didn't seem likely that it would be an ignition concern.
Connecting the fuel pressure gauge to this Lexus was an adventure. Toyota and Lexus engineers, as well as other Asian automakers, have trouble understanding the need for a fuel pressure test port. Even some of the domestic platforms have been moving away from that tech-friendly feature for some reason. On the mid-'90s Lexus LS400 V8, you simply watch a small screw on the pulsation damper to see if it moves when the key is switched on. If it does and the stuff in the rail is gasoline and not water or diesel, you have enough pressure to get the car started if all else is OK.
Not so with our SC300. The fuel supply on this car runs through the filter underneath the driver's side rear with simple inverted flare nut fittings (one possibility for a pressure test point). It then travels to the fuel rail through a line that connects at the engine with a banjo bolt. In our fuel pressure test kit, I found a fitting that would replace the banjo bolt and provide a connection point for the fuel gauge quick coupler. It was fairly simple to make this connection on a lift in our shop, but it would be a drag on a rainy highway at night somewhere in the Utah wilderness.
If the pressure was really high, the fuel pressure regulator or return line would need further investigation.
With the gauge installed and the key switched on, the pump ran for two seconds. The fuel pressure bounced up to a healthy 40 psi, but it didn't hold pressure after the fuel pump's two-second run. It should. Even with no fuel leakage into the engine, a loss of residual fuel pressure causes hard starting with a hot engine.
Closing in
Now that we had identified the fuel pressure loss as a concern, we needed to find out where the fuel was going. There were no external leaks, so the fuel had to be going one of three places, and the data already gathered pointed toward one or more injectors. On our SC300, I wanted to do a systematic diagnosis before arbitrarily diving into a nuts-and-bolts injector inspection.
A. Block the return line, and check for fuel pressure leak-down. We pinched the return line near the fuel rail – it was a plain old rubber hose – with pliers to see if the regulator was dumping the fuel back through the return.
Result: The fuel pressure still didn't hold at pump shutdown with the return line pinched off.
B. Block the pressure line to see if fuel is bleeding back through the fuel pump check valve. I already suspected a mechanically leaking injector, but for instructional purposes we took measures to pressure-up the fuel system and block the pressure line at the filter to see if we still had pressure loss. If there was no pressure loss here, then the pump was the source of the leak-down, and the plot would thicken.
Result: The fuel system still wouldn't hold pressure. The problem had to be a leaking injector.
C. Remove the fuel injectors and mechanically inspect each one for leakage. Removing the injectors on this 3.0L is no small feat. The upper intake has to be removed, and some of the fasteners aren't easy to access. Furthermore, there are more than just a few hoses and connectors. Finally, we peeled our way down to the injectors, removed the fuel rail and the injectors, and checked each one by attempting to push compressed air through it with a rubber-tipped blower. Energizing each injector a few times to check for intermittent sticking made no difference.
Result: No mechanically leaking injectors.
D. Reinstall the injectors, and retest the system for leakage. This was where I moved away from conventional test procedures. The injectors were reinstalled but electrically disconnected; the fuel filter was removed; and the fuel pressure gauge was still connected at the banjo bolt point.
I attached a 3/8-inch fuel hose to a cylinder leakage tester, raised the lift and fed 40 psi of regulated shop air through the now-empty fuel line and fuel rail just forward of the fuel filter. This would give me an audible leak point that would be easier to pinpoint.
The air pressure showed up as 40 psi on the fuel gauge. I pinched my supply hose to trap the air, and it didn't leak down. This was getting even more interesting.
I left the air pressure on the system, lowered the vehicle and electrically connected the injectors. With a drain pan in place to catch the fuel from the pump, I switched the key on and saw the gauge's reading drop to zero. I had just dumped my trapped air pressure, and I could clearly hear it hissing through an injector into the intake. Disconnecting the injector connectors one at a time, I found that #4 was the open nozzle. It could be a shorted injector control wire. But disconnecting the PCM eliminated the concern, and I knew then that I had a PCM with a shorted driver.
Closing out the mental matrix
The list price on a new PCM is sizable. Checking with a networked salvage yard revealed less than 10 used Lexus SC300 PCMs nationwide. We opted to send this one off for repairs (there were no cores available), and in 10 days it came back repaired. The bill was much less than a new module.
With all that fuel dumping, an oil change was in order, but we did that while the PCM was away. I started the Lexus and ran it for a while to clear the puddled gasoline from the exhaust system. When the learning process was done, it ran like a top.
While "the book" mentions possible problems with the injector circuit in its troubleshooting table, our set of homemade tests brought real-world success to a problem that had stumped more than a few other techs. But let's be honest: If the problem had been an intermittent one, it would have been a whole lot harder to find.
RICHARD MCCUISTIAN is an ASE-certified Master Auto Technician and was a professional mechanic for more than 25 years. Richard is now an auto mechanics instructor at LBW Community College/MacArthur Campus in Opp, AL. E-mail Richard at [email protected].
