‘Been there, done that’ -- Advice from experienced service technicians

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‘Been there, done that’ -- Advice from experienced service technicians


We had a 2004 Chevy Silverado 2500 in the shop, equipped with the 6.6L Duramax diesel engine. The MIL was on and the vehicle exhibited intermittent low power. Code P1093 was stored, which indicated low fuel rail pressure during power enrichment.

We first connected a vacuum gauge at the fuel service port. While cranking the engine, it should show three to four inches of vacuum. At wide open throttle (in park), you should see four to five inches. During a test drive under hard acceleration, you should see eight to 12 inches of vacuum. If vacuum is too low, you probably have air in the system. If vacuum is too high, you probably have a restriction, which could be a damaged/kinked fuel line or a clogged fuel filter.

If your vacuum checks meet specification, inspect the fuel return from the fuel pressure relief valve (located at the rear of the left side common rail), and check the fuel return rate of the injectors. In our case, we found high vacuum. Upon further inspection we found a mashed fuel line, the result of someone’s previous service. It looked like someone had used a pry bar that pushed against a section of the line.

Information courtesy of Teri McCoy, McCoy’s Tire and Auto Service.


One of the most frustrating things that can happen when a technician is trying to diagnose a concern is to find a lack of communication with the module or modules he is trying to diagnose. In most cases, there are some simple tests that can be done to quickly determine why a module is not communicating.

Let’s start with the most simple, yet very common issue: Check power to the data link connector. In many cases, Ford uses the same fuse for the data link connector and the cigarette lighter, so it is fairly common for that fuse to be blown. Power will always be on pin 16, the bottom, far right pin of the data link connector. Power loss will affect all of the modules, and if there is a lack of power to the data link connector, none of the modules will communicate.

If only one module has lost communication, test the communication circuits. There are many communication systems, but this tech tip will only cover the standard corporate protocol (SCP) and the controller area network (CAN). SCO is used in earlier-model Ford vehicles with OBD-II systems. CAN is used on most current model year Ford vehicles, and for this tech tip, we’ll only cover the high-speed CAN.


For a quick check of the SCP network, simply place your voltmeter across the SCP pins at the data link connector (top row second from left and bottom row second from left).

SCP voltage will usually range between 4.5 and 5.0 volts. If there is no voltage present at these pins when the key is on, unplug one module at a time until this voltage returns. In some cases, a shorted module will load down the network.

If the voltage is low no matter what module is unplugged, there is likely SCP wiring damage. There needs to be at least one module plugged in to get the voltage on the network.

When testing the CAN network, both voltage and resistance can be checked. Place a voltmeter or an ohmmeter across pins 6 and 14 to check the high speed CAN network.

When checking voltage, the CAN network will normally be around 2.5 volts. When checking resistance, ensure that the key is off. The resistance of the CAN network will be around 60 ohms. An explanation of this resistance can help diagnose a high speed CAN problem.

On a Ford vehicle, there are two termination resistors on the CAN network. These resistors are usually found in the PCM and the instrument cluster. Each resistor is 120 ohms.

When they are put together on the network, the network will show 60 ohms. With this in mind, if you check the resistance and it reads 120 ohms, you know that one of the modules is not on the network. Simply unplug the cluster or the PCM to see which one is on the network.

A low resistance value will usually indicate a shorted CAN circuit, or a shorted CAN module. Unplug all of the modules except the PCM and the instrument cluster and then see if, at any point, the resistance returns to 60 ohms. A high resistance will usually indicate a connector issue, corrosion or wiring issue.

Information courtesy of Identifix Inc. tech department. See


A customer driving a 2001 PT Cruiser came in complaining of a severe vibration that only occurred when vehicle speed reached about 40 mph, lasted up to around 55 mph, then went away, but he stated that this only happened when he began driving after the vehicle sat for a few hours or longer.

About five minutes after he arrived, I test drove the car and did not experience any vibration. Since we were very busy at the time, I asked him to wait until I had time to really delve into the vehicle. About an hour later I test drove the car again, and sure enough, it had a very severe vibration between around 40 mph up to around 60 mph, then it disappeared. Since it felt as though the vibration was coming from the front, with the vehicle on the lift, I performed a visual inspection of the front suspension and found nothing out of the ordinary. We removed both front wheels and ran them on our balancer, to find that the left front was severely out of balance, requiring about 100 ounces to correct. This was obviously a problem. I asked the customer how long this problem had been present, and was told that it only started about a week prior.

After checking for lateral and radial runout, I demounted the tire from the wheel, only to find a huge amount of tire “sealant” that had puddled inside the wheel and tire. Only then did the customer tell me that the tire had been losing pressure for a few weeks, and in an attempt to remedy the problem himself, he injected a massive amount of aftermarket sealant. The customer noted that he wanted to make sure that the leak was sealed, so instead of following the instructions on the sealant package (which specified one tube of sealant), he injected four tubes of the liquid sealant. Apparently, after sitting for an hour or more, the sealant “puddled” into a mass, creating a severe out-of-balance upon initial driving. After a few miles, the sealant apparently dispersed and “evened out” by centrifugal force.

We cleaned the tire and wheel, found a small nail puncture, repaired the puncture, installed a new valve and remounted and balanced the assembly, solving the problem. The moral: Never underestimate the problems that can be caused by the vehicle owner.

Information courtesy of Eric Bender, B&A Service Center.



A new customer had spent a few hundred dollars at another shop to address an idle surge and intermittent hard-crank on a 2005 Mercury Sable with a 3.0L DOHC engine. A number of components had been replaced apparently in a “shotgun” approach to attempt the fix, with no operational improvement. This included spark plugs, plug wires, coil, alternator, etc.

We suspected a sticking throttle plate and possibly a sticking IAC. We removed the IAC and cleaned it using throttle body cleaner, and disconnected the air intake ducting from the throttle body, and cleaned the throttle body plate with the same cleaner. The entire procedure took about 15 minutes. Idle returned to a normal, stable condition and the engine started immediately with no issues. Sometimes, the simplest fix is the answer.

Information courtesy of Mary Beth Allen, Thompson Car Care.


Whenever you’re dealing with a customer’s vehicle that has aftermarket aluminum wheels, pay attention when you remove the wheels. Some aftermarket wheels have larger hub holes than the vehicle’s hubs need, because the wheel maker may have made that wheel with a larger hub hole in order to fit a wide variety of vehicles.

In order to properly fit the vehicle hubcentrically (where the hub hole centers the wheel instead of relying on the wheel studs), if the wheel’s hub hole is too big for a snug fit to the hub, a plastic or aluminum hubcentric ring will be placed either on the hub or on the wheel. If a hubcentric adapter is in place, make sure it isn’t stuck to the hub. If you install an emergency spare or a different wheel, and the adapter stays in place on the hub, the replacement wheel (a wheel that has a hub hole made for that vehicle) will try to crush against the adapter, preventing the wheel from seating fully on the hub.

The result will be a stressed wheel and lateral runout. Just pay attention to see if a hub adapter is present. Sometimes, with age, rust or dirt, it’s hard to see right off the bat. If a vehicle does have hubcentric adapters, I like to remove them, clean the hubs and adapters, and apply a thin film of high temperature grease onto the adapters before reinstalling them. If you don’t look for these adapters and try to install other wheels that don’t need them, you’ll ruin the adapters and the wheels won’t mount flush to the hubs.

Information courtesy of Larry Ritz, Smitty’s Fleet & Auto.


Excess moisture contaminating diesel fuel isn’t uncommon. It’s very common for moisture to accumulate in underground tanks due to temperature changes, tank condensation, etc. On a Ford truck equipped with the great and very reliable Navistar 7.3L turbo engine, whenever a customer complains about intermittent surging, bucking or hesitation, the first item to suspect is the fuel filter.

With all of my customers who own the 7.3L Navistar engine, I suggest carrying at least one new spare fuel filter. Access is easy. After removing the cheap black plastic cover, open the fuel drain lever (located on the passenger side rear of the filter housing) and let the fuel drain out of the filter housing. This will prevent making a mess while removing and installing the filters. If the truck is parked on asphalt, be sure to place a drain pan under the engine first. The filters thread into the housing and feature two seals: an O-ring near the bottom of the filter and a square-cut O-ring at the top. Make sure that both of these O-rings are removed along with the filter. After removing the old filter, inspect the housing to make sure that neither O-ring was left inside the housing. Before installing the new filter, smear a bit of diesel fuel on each O-ring. Aftermarket filters, depending on the brand, will feature a raised male hex or a female hex hole. The really cheap filters will have a female hole that looks like a Torx. When you insert a 1/2-inch drive wrench or extension, this style likes to round-out. If you strip this hole out, you can use large channel-lock pliers to grab the raised ribs on the top of the filter. Stick with the premium brand filters for the best filtering and for the best removal/installation drive style.

Depending on the area of the country, the humidity conditions and fuel quality, expect to replace the fuel filter anywhere from every 500 miles to 1,000 miles. Remember: If the engine suddenly starts bucking or surging, change the fuel filter before spending time with a scanner. More often than not, the drivability issue will be caused by a water-laden fuel filter.   ●

Information courtesy of Brian Kneeley, Bobby V Diesel Fleet Service.

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