Discerning Veracity When our VAT40 died at the Ford/Jeep dealer, the shop foreman and service manager opted for what appeared to be a suitable succedaneum. The new ARBST came off the MAC tool truck for a cash exchange of about $1700. The VAT40’s impeccable ability to condemn an alternator stator had spoiled us dreadfully and I got burned a time or two with the new machine, which would tell us the stator was bad when it was actually good. I found myself doing an o-scope test on every alternator to back up the diagnosis when I got the red ‘stator’ light on the machine. Another non-credible test was the injector flow meter procedure on the Rotunda injector cleaner/tester supplied to Ford dealers in the late 80s. This machine cost 2000 and was outfitted a pump, a regulator, and a small fuel tank with flow meters with color bands for the various injectors – it was designed to flow fuel through the injector at a fixed pulse width, and if the floating ball in the glass tube indicated fuel flow that was in the colored band matching the injector, that nozzle passed the test. In theory it was great, but in the shop it was a liar. I repeatedly saw injectors pass the flow test before and after cleaning, but the vehicle would run better anyway, so that flow test was a waste of my time and dumped it from my routine. Then there are tried and true methods of troubleshooting that don’t cost much. Remember my method of wiring a test light in series with a cooling fan motor and a battery and a quick turn through of the motor? You can immediately condemn a bad motor that way. If the light ever goes out during that test, the motor is junk. That test can easily be done at the fan relay socket without removing the fan if you’re a sharp cookie. Using the in series amp measurement function of a cheap DVOM (set the meter up for 10 amps and use it like a jumper wire to bypass the fuel pump relay), a reading less than 2 amps with a humming fuel pump tells you the tank is empty. A normal pump that is actually moving fuel will show about 8 amps depending on the vehicle. And low-cost tests like this aren’t limited to electrical stuff. Bump the A/C compressor clutch that fails to engage hot, and if it clicks in, the air gap needs to be set. Fouled Up Five Hundred At the risk of belaboring the unreliable test equipment point, let’s take a peek at a 2005 Ford with 68k on the odo, a vehicle that kept throwing the wrench light and “Fail Safe Engine” message and a nasty failure to accelerate. This unit has electronic throttle control with three potentiometers in the pedal, a motor and a sensor on the throttle body, and a PCM strategy to limit throttle response if the pedal pots don’t agree with each other. About once a day this Five Hundred would drop into Fail Safe Engine mode without warning but a key cycle would temporarily return things to normal. With the Genisys software freshly updated (3.0, 2007)and the DTCs retrieved (P2106 Throttle Control Actuator Forced Limited Power and a P2135 Throttle Pedal Position switch A/B Voltage Correlation), we dove into the datastream and pulled up the three pots. The graph pattern on Sensors 2 and 3 should track together (although with slightly different voltages) and the trace on sensor 3 should be an inverted plot of the other two. While moving the throttle and watching the three pedal pot traces on the Genisys screen, I noticed that the three graphs were producing the right pattern sometimes and a dreadfully wrong pattern some other times. The pedal assembly is about sixty bucks, and so I ordered one. Nothing changed. Found TSB 6-21-1 that listed a gaggle of codes including the ones we got. It said to replace the throttle body and reflash if the unit was built before 3/9/2005. Well, this one was built a lot earlier than 3/9/05, so we put the updated throttle body on it and I had a graduate of mine re-flash it at the Ford store. The Five Hundred’s driver still had the same concern after a couple of days. This was getting interesting. Connecting my $2000 four trace Interro PDA scope to the three pot wires, I wanted to see if I got a screwball set of traces like the Genisys was showing me. The traces were beautiful on screen, but with the scope connected the PCM wouldn’t allow the throttle body respond to the pedal movement. Disconnecting the scope, we got throttle actuation. Apparently the Interro had shorted the pot signal enough that the PCM didn’t trust what it was receiving. It was disgusting to me that I couldn’t depend on the $2500 Genisys OR the $2000 Interro PDA scope to help me with this one. Back to the Ford store after a bevy of diagnostics and a call to the hotline the decision was made that PCM (neatly under the 80k emissions warranty) needed replacing. I suspected as much, but my high dollar tools disappointed me. Two More Bad Performers In addition to the Monte Carlo, we encountered a 2004 Dodge Stratus with just under 70,000 miles and a cold misfire. The owner was out of town, so I’d have it for a few days. We used the Interro (with a $3000 ignition system adapter) to paint the pattern. With a ragged #1 spark line and high snap patterns on the other three, the Stratus 2.4L got a fresh set of Champion Platinums and a verifying test drive. Everything seemed normal on the first couple of test drives, but we weren’t done with the Stratus: We just thought we were… More on that later. The 148,000 mile Monte Carlo came to me via a phone call from one of our nursing instructors – she told me the transmission on her Chevy was shifting late and hard. A short test drive was all it took to determine that the 2001 Monte Carlo was seriously underpowered – it took some pretty deep throttle to reach 30 mph and with that perceived load and speed, the transmission shifts were definitely late and hard, but the problem was an engine performance concern. A quick check of the transmission fluid showed it to be clean and full. Data Mining With the hood up on the Monte Carlo, we checked the engine vacuum – 18 inches idling with a steady 8 inches at about 2500 and a heavy and irregular induction backfire. Fuel Pressure was healthy all the way through the problem range, hanging in at about 50 psi (normal for 8th VIN 1 – specs are higher if the 8th digit is K). Our DTC scan with the Genisys gave a bevy of DTCs – P0108B - MAP Circuit high voltage P0128B ECT below thermostat regulating temp P0404B EGR Open Position Performance P0463C Fuel Level Sensor Circuit High Voltage (the ever popular fuel tank sending unit problem so common GM cars of this vintage) P1404A EGR Closed Position Performance Scoping the ignition system we saw a flat lining #4 leading to a spark plug inspection. The air filter looked good but the fuel filter had probably been in that same spot since the assembly plant and human lungs couldn’t shove any air through it. We yanked the exhaust and it didn’t rev any better that way – that led us to check the ignition system. We put the exhaust back on. Checking the plugs was tough. Those smooth and shiny Delco ceramics (why the heck to they do that?) can vulcanize to wire boots in a hot engine compartment like there are no tomorrows – I generally have to destroy the wires and put a new set on many of the 3800s I’ve done, and my students didn’t do much better on this one. Plugs and wires got rid of the induction backfire; the car ran smooth, but still wouldn’t pull your hat off. Troubleshooting Trap Textbooks teach a troubleshooting procedure that focuses on the normality of exhaust temperature at various places. The idea is that the exhaust is supposed to be hotter coming out of the catalyst than it is going in, and just about everybody who has done any troubleshooting knows about that method. So you get an infrared temperature gun, start the car, raise it on a lift, and take readings. We did. The exhaust going into the cat was 320 degrees, which is a little hot, but behind the catalyst it was reading 430 degrees. So what does that mean? Is the cat bad or good? Well, it runs like a bad cat, but what do we do with those temp readings? We learned from them – that test isn’t always right, so beware!! With the header pipe and converter completely removed from the manifold, the engine still wanted to cut out on the lift, but that was the 4000 rpm rev limiter GM programmed in. The problem was that it sounded almost exactly like the engine had sounded the first time – had we been confused by a rev limiter and an induction backfire? A test drive with open exhaust went beautifully. Monte got a universal cat that was actually a pretty good fit ($167) and the Chevy was back on the road, albeit still needing a thermostat and a fuel level sensor (see DTCs), but the customer didn’t want those concerns fixed on this trip. Finishing The Stratus The Stratus wasn’t finished – I drove it to lunch and found that the cold misfire had returned because #1 had a scorched the inside of spark plug wire boot, making a nice carbon track that tended to give the spark an easy path to ground that wasn’t through the center electrode. Oddly enough, the new Champion plug we put in #1 had a center electrode resistance of 57,000 ohms. The others read 17,000, 18,000, and 26,000. The old plugs read just over 5,000. I know Champion doesn’t like it when I mention measuring the center electrode, but Ford calls for it in their trouble shooting, so I do it on everything, especially when I’m chasing an odd concern. The Stratus got new wires and fresh set of Champion platinums for good measure. Concluding Thoughts Troubleshooting these days is almost impossible without sophisticated diagnostic equipment to gather data, but a real technician will learn to depend as much as possible on common sense whenever the high dollar boxes fall on their flat little faces and questionable troubleshooting procedures fail. Yanking the cat and driving the car turned out to be the test that made the grade on the Chevy, and a $10 ohmmeter smoked out a faulty new spark plug on the Stratus. A good head, good hands and steel wrenches are still the best tools out there. R.W.M.
Posted on: Fri, 10 Oct 2014 09:56:36 +0000
Trending Topics
Recently Viewed Topics
© 2015