Motor Age Garage: Troubleshooting Traps

Jan. 1, 2020
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 tester came off the MAC tool truck for a cash exchange of about $1,700.

Troubleshooting techniques, political candidates and news stories can cause more problems and more work if they aren't vetted.

Motor Age Garage 2001 Monte Carlo troubleshooting problems transmission problems transmission won't shift automotive aftermarket vehicle repair repair shop repair shops

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 tester came off the MAC tool truck for a cash exchange of about $1,700.

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 1980s. This machine cost $2,000 and was outfitted with 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 I 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, a battery and a quick turn-through of the motor? You immediately can condemn a bad motor that way. If the light ever goes out during that test, the motor is junk. That test can be done easily 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 two amps with a humming fuel pump tells you the tank is empty. A normal pump that is actually moving fuel will show about eight 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.

Two More Bad Performers

In addition to the Monte Carlo we'll discuss here, we encountered a 2004 Dodge Stratus with just less than 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 $3,000 ignition system adapter) to paint the pattern. With a ragged No. 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.

The 148,000 mile Monte Carlo came to me via a phone call from a colleague, who 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 Harvesting

With the hood up on the Monte Carlo, we checked the engine vacuum – 18 inches idling with a steady eight inches at about 2,500 rpm 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 cars with a 1 in the 8th VIN digit – specs are higher if the 8th digit is K).

Our DTC scan with the Genisys gave a bevy of codes:

P0108 MAP Circuit high voltage

P0128 ECT below thermostat regulating temp

P0404 EGR Open Position Performance

P0463 Fuel Level Sensor Circuit High Voltage (the ever popular fuel tank sending unit problem so common to GM cars of this vintage)

P1404 EGR Closed Position Performance

Scoping the ignition system, we saw a flat-lined No. 4, leading us to a spark plug inspection. The air filter looked good, but the fuel filter probably had been in that same spot since leaving 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, which led us to check the ignition system. We put the exhaust back on.

Checking the plugs was tough. Those smooth and shiny Delco ceramics can vulcanize to wire boots in a hot engine compartment like there is no tomorrow. 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 temperature 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 4,000 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. It had to be backpressure.

The Monte Carlo got a universal cat that was actually a pretty good fit ($167) and 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

But the Stratus wasn't finished. I drove it to lunch and found that the cold misfire had returned, because No. 1 had scorched the inside of a 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 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 its troubleshooting, so I do it on everything, especially when I'm chasing an odd concern. The Stratus got new wires and a 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.