A Beautiful Car When our dispatcher went shopping for the car of his dreams, he bought his wife a gorgeous ’98 Continental at the Lincoln-Mercury dealer. It was in perfect condition, the test drive went well, and he plunked down the cash. After a week or two, he discovered the apparent reason the previous owner had traded it in. His wife noticed it first when she was driving around town, but when he tried to verify her complaint, he was initially unable to duplicate what she had experienced. A few days later, however, he was at the wheel and was surprised to note a rough idle and a peculiar skip or surge unlike anything he had experienced. He brought the car to work with him, had it written up, and passed the ticket along to me. Even a surge that can be easily duplicated can be one of the most stubborn driveability concerns to repair. Some late ‘90’s Tauruses had an innate EGR surge that Ford’ service engineers dealt with by writing a new program. After we flashed the new program into the Powertrain Control Module (PCM) it wouldn’t lock the Torque Converter up until 50 mph. The Continental had no such PCM reflash available at the time, and there were no related TSB’s. I was, shall we say, “measurably impressed” that the dispatcher had been gracious enough to give his Continental problem to me. Unable to Duplicate When a competent driveability guy draws a job on a vehicle he’s familiar with, he’s been around the engine block a few times and is likely to know the weak links in the chain well enough to have a general idea what to do about a problem, usually even if it’s intermittent. For instance, my service manager handed me an e-mail message from a lady from north Alabama a few months back asking for advice concerning a ‘96 Explorer that was sometimes hard to start and would occasionally run at a very low idle or stall. She had taken the car to a couple of shops, neither of which had been able to duplicate or repair her concern, and each of them had charged her a handsome diagnostic fee. It was her daughter’s vehicle, and she was concerned that it might leave her daughter stranded somewhere. I had seen this sort of thing before, and found myself wondering who hadn’t seen it, especially if they had worked on very many Explorers of this vintage. My return e-mail basically outlined the idea that she probably had an intermittently sticking Idle Air Control valve, and I gave her the basic part number (9F715) and told her to send her son to the Ford place to buy one. Her return e-mail came back a couple of weeks later. The Explorer had never run better. The point is that I knew where the weak link in the chain was that could cause her problem, and I could give her some idea of which direction she needed to go. The Continental was a different story. I had never worked on a Continental of this particular generation and vintage, and a lot had changed since the last time I had opened the hood on one of these babies. Test Drive: Check! The car idled predictably smooth and from a rolling start to highway speed it was as cloudlike as they come. A truly beautiful machine, this car would put a guy like me to sleep at the wheel on a long drive. Still, I knew there was a problem lurking somewhere or the dispatcher wouldn’t have brought it in. He’s a levelheaded guy and I knew him well enough to know I didn’t need to invoke the “loose nut behind the wheel” idea that we’re so prone to hang on drivers that complain about problems we can’t duplicate. Visual Inspection: Check! Rooting around under the hood revealed nothing out of the ordinary. The connectors were all seated and no wires were chafing the vacuum lines were all seated and routed properly, and unless there was some nasty little hidden gremlin under there that I missed, the engine compartment had passed my visual exam. Code & Data Scans: Check! I plugged the WDS machine into the DLC, went through the preliminaries, and brought the car’s computer online. As is usually the case on a surge, a scan of the OBDII system revealed nary a code. The datastream looked good too, with fuel trims hovering near zero on the line. With the problem as intermittent as it was, I wasn’t in the least surprised. I saw nothing on which I could spend Henry Ford’s money in good conscience. Another test drive went pretty much like the first. “Next Time It Happens…” The dispatcher had been around for awhile, and he totally understood my dilemma. He’d bring it back as soon as the problem re-occurred. A week later, he had it back, and it had a random idle miss, about the way a high output Mustang 5.0L does. Mustangs, Ford pickups (including SUV’s) and vans with aggressive camshaft overlap will idle a bit rough in neutral, and some customers complain about it. But a regular passenger car engine is cammed differently and should idle smoothly. A 3.0L in a Ranger or an Aerostar generally doesn’t idle as smoothly in neutral as a 3.0L in a Taurus for the same reason. Dropping the transmission in drive usually smooths the idle on an engine with a mildly aggressive cam grind, and that’s one way I would verify that a random idle miss was being caused by camshaft overlap. Jeep 4.0L engines from ’91 up through about ’96 have a similar feel. As for my test drive on the Continental, this time I felt a nice irregular surge at road speed. Back Online With the Continental Back at the shop, I reconnected WDS to the data link, and there were still no codes. The fuel trim readings didn’t look a lot different either, but since the problem was present this time, I punched my way through the menu to the Power Balance screen, where I hit pay dirt. What I saw there was the strangest thing I’ve seen in years. A ghostly misfire was making its way across the screen one cylinder at the time, and it was walking its way through the firing order again and again. I blinked, looked at the screen, and figured maybe the WDS had a virus or something. This was like a candid camera prank. I disconnected the WDS and plugged the old SBDS dinosaur into the Continental. A few minutes later, I was looking at the same weird pattern on the SBDS Power Balance screen. Some guys will start throwing parts at a problem like this, especially when it’s under warranty, but I tend to be a bit more surgical than that. I figured it was time to place a phone call or two. When I described my problem, the Ford Hotline guy I spoke with was as puzzled as I was. He mentioned a possible problem with the Cam Sensor, and since the PCM uses that input to determine which injector to fire during the sequential injection process, it made a little sense to have a look at it. According to the Hotline database, some Cam Sensor connectors had been found to have motor oil in them for some reason or another. I was to check for oil in the connector, and if I found any, I was to clean it out with electrical contact cleaner and replace the sensor. Two Shots Across the Bow The Cam Sensor was clean, but I replaced it and the Crank Sensor as well. There was no change in the way the car idled. Now I had two empty shell casings but no kill, and I was still mystified by the walking skip, which hadn’t changed. A quick call to the driveability guy at the local Lincoln Mercury dealer left my quest for information on this weird phenomenon unfulfilled, but the guy was definitely intrigued by what I described. Fuel Supply System Overview The Continental fuel system has gone the way of more and more late model vehicles. In the “olden days,” the fuel was piped from the high pressure pump in the tank through the fuel rail, through the fuel pressure regulator on the rail, and back to the fuel tank, but engineers everywhere began to agree that it was a bad idea to put fuel through the combined stress of heat and pressure in the engine compartment and then pipe some of it back to the fuel supply. Nowadays there are mechanical returnless fuel systems and electronic ones. The mechanical system generally has the fuel pressure regulator built into the pump, which is a pretty good idea, actually. Chrysler has been doing that for a few years, Ford Rangers got mechanical returnless systems in the late ‘90’s. As for this particular Continental, was blessed with an electronic returnless system. The fuel pump is operated by the PCM through a relay and the inertia switch, but that’s where the similarity with the old system ends. A special module has been added, which controls the current flow to the fuel pump based on what the PCM sees at the electronic sensor mounted on the fuel rail. Fuel rail temperature is monitored as well. The Fuel Pump Module is a small electronic box with six wires connected to it. It is mounted in the right front corner of the trunk, right next to the wheel well, and the trunk liner has to be pulled back in order to access it. Taurus received this same setup in the 2000 model year. One interesting tidbit is that the Taurus fuel system went electronic in 2000 and then reverted back to a mechanical returnless system in 2001, possibly because of troubles with the Fuel Rail Sensors on 2000 models. We still have a Shrader valve on the rail to check live pressure, but a scan tool PID has been added through the PCM’s datastream so the Fuel Rail Pressure (FRP) reading can be monitored with your scan tool alongside everything else. Pressure Points to the Problem The WDS has a Pressure/Vacuum transducer, which can be connected to real-world pressures and will produce a graph on the datastream screen when the transducer is selected as an input. I decided to follow Ford’s shop manual procedure and use the Pressure/Vacuum transducer to graph the fuel pressure while monitoring the electronic Fuel Rail Pressure PID, and what I saw there got my attention. The Pressure/Vacuum transducer graph showed a peculiar pattern of downward spikes on what would otherwise have been a flatline at or around 60 psi (shop manual specs on this car call for 55-85 psi. be careful not to get fooled when checking fuel pressure on late model Fords. The pressures were once fairly universal at 35-45, but now they’re all over the place (see 98 pressure chart). When stacked the graphs on the screen and compared to the Fuel Rail Pressure PID graph to the PV transducer graph, the pressure readings were fairly close to each other, but the electronic PID showed none of the downward pressure spikes I was seeing with the PV transducer. It became apparent to me that I was seeing the equivalent of a pressure gauge needle bounce on the PV transducer graph. There was air trapped in the fuel rail. Briefly opening the valve on the PV transducer, I saw fuel laced with air bubbles race through the clear hose leading to the drain pan. The transducer spikes went away, the idle smoothed, and when I returned to the Power Balance screen, the walking skip was gone. Why a “Walking Skip”? Since the fuel system was returnless, any air making its way to the fuel rail has to stay there until it eventually spits its way through an injector. This air bubble was apparently expanding and contracting as the injectors operated, causing the low pressure spikes I was seeing on the graph to be manifest as a skip that mysteriously followed the firing order. I worked on a Dodge van with an unusual skip and a returnless fuel system a few years back, and it had a similar problem, but the skip stayed with cylinder number one, because the fuel rail is tilted slightly on that engine and the air bubble had stayed right at the mouth of the number one injector. The Continental’s fuel rail was mounted perfectly level and the bubble or bubbles were able to move around. Cavitation at the Pump There were no external leaks that I could find, and since the whole fuel system is pressurized from the pump to the rail anyway, the air had to be originating in the pump. I couldn’t believe the dispatcher or his wife would let the Continental get low enough on fuel to draw in air. It was a simple matter to replace the fuel pump, and there were no further problems..
Posted on: Tue, 23 Jul 2013 08:26:33 +0000
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