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A 'Walking' Skip That Stumped the Hotline

When I described what I was seeing on the Power Balance screen, no one I spoke with at Ford had the foggiest idea what the problem could be.
Monday, October 1, 2001 - 00:00

When I described what I was seeing on the Power Balance screen, no one I spoke with at Ford had the foggiest idea what the problem could be.

When our dispatcher went shopping for the car of his dreams, he bought his wife a gorgeous 1998 Continental at the Lincoln-Mercury dealer. It was in perfect condition, the test-drive went well, and he plunked down the cash.

However, after a week or two, he discovered the reason why the vehicle was traded in: It had a rough idle and a peculiar skip or surge unlike anything he had experienced. His wife noticed it first when she was driving around town, but he was initially unable to duplicate her complaint. A few days later, however, he was at the wheel and the problem surfaced again. He drove the car to work, had a work order written up and assigned the ticket to me.

Even a surge that is easily duplicated can be one of the most stubborn driveability concerns to repair. Some late '90s Tauruses had an innate EGR surge that Ford's service engineers dealt with by writing a new program. After flashing the new program into the Powertrain Control Module (PCM), it wouldn't lock up the torque converter until 50 mph. The Continental had no such PCM reflash available at the time, and there were no related TSBs. I was, shall we say, "measurably impressed" that the dispatcher had been gracious enough to give his Continental problem to me.

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. He knows them well enough to have a general idea where a problem's source lies, even if it's an intermittent one.

Unable to duplicate

To illustrate the point, I once received an e-mail message from a woman in northern Alabama asking for advice concerning a '96 Ford Explorer that was sometimes hard to start. It would occasionally run at a very low idle or stall. She had taken the car to a couple of shops. None were able to duplicate or repair her concern, and each had charged her a handsome diagnostic fee.

I had seen this problem before; it is common on many Explorers of this vintage. My answer was that she probably had an intermittently sticking Idle Air Control valve. I gave her the basic part number (9F715), and told her to purchase one at the Ford dealer. Her reply came a couple of weeks later: The Explorer had never run better.

My point is that I knew where to find the weak link in the chain, and I was able to point her in the direction she needed to go.

The Continental, however, was a different story. I had never worked on a Continental of this particular generation, and a lot had changed since the last time I had opened the hood on one of these babies.

The car idled predictably and smoothly. From a rolling start to highway speed, it was as cloudlike as they come - a truly beautiful machine. 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 that I didn't need to invoke the 'loose nut behind the wheel' label.

Test drive:

Rooting around under the hood revealed nothing out of the ordinary. The connectors were all seated, and no wires were chafing the vacuum lines. The wires were all routed properly, and unless there was some nasty little hidden gremlin under there that I missed, the engine compartment passed my visual exam.

Visual inspection:

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 Ford's money in good conscience. Another test-drive went pretty much like the first.

Code and data scans:

The dispatcher understood my dilemma and promised he'd bring it back as soon as the problem reoccurred. A week later, the Continental was back. It had a random idle miss, similar to a high-output Mustang 5.0L.

"Next time it happens??????"

Ford Mustangs, pickups, SUVs 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 engine 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 into 'Drive' usually smooths the idle on an engine with a mildly aggressive cam grind, and that's one way I verify that a random idle miss is caused by camshaft overlap. But during this test-drive on the Continental, I felt an irregular surge at road speed.

Back at the shop, I reconnected the WDS to the data link, but there still were no codes. The fuel trim readings didn't look much different either, but because the problem was present this time, I punched my way through the menu to the Power Balance screen. That's 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 a time, and it was walking its way through the firing order again and again. I blinked, looked at the screen and wondered if the WDS had a virus. This was like a 'Candid Camera' prank.

Back online with the Continental

I disconnected the WDS and plugged the Continental into the old SBDS dinosaur. A few minutes later, I was looking at the same weird pattern on the SBDS Power Balance screen. Some technicians will start throwing parts at a problem like this, but I decided it was time to place a phone call.

When I described my problem, the Ford Hotline representative was as puzzled as I was. He mentioned a possible problem with the cam sensor; because the PCM uses that input to determine which injector to fire during the sequential injection process, it made some sense to take a look at it. According to the hotline database, motor oil was found in some cam sensor connectors. I was to check for oil in the connector, and if any was present, I was to clean it out with electrical contact cleaner and replace the sensor.

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. 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 unfulfilled, but the guy was definitely intrigued by what I described.

Two shots across the bow

The Continental's fuel system has gone the way of more and more late-model vehicles. In the past, the fuel was piped from the high-pressure pump in the tank through the fuel rail. It then moved through the fuel pressure regulator on the rail and back to the fuel tank. Eventually, engineers realized 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. Now there are mechanical and electronic returnless fuel systems.

Fuel supply system overview

The mechanical system usually has the fuel pressure regulator built into the pump, which is actually a pretty good idea. Chrysler has been doing this for a few years; Ford Rangers got mechanical returnless systems in the late '90s.

As for this particular Continental, it 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 was 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, next to the wheel well, and the trunk liner has to be pulled back in order to access it. The Ford 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. This allows the Fuel Rail Pressure (FRP) reading to be monitored by your scan tool along with all the other data.

The WDS has a Pressure/Vacuum (PV) transducer, which can be connected to real-world pressures. It 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 PV transducer to graph the fuel pressure while monitoring the electronic FRP PID. What I saw there got my attention.

Pressure points to the problem

The PV transducer graph showed a peculiar pattern of downward spikes on what would otherwise have been a flat line at or around 60 psi. Shop manual specs on this car call for 55 to 85 psi. Don't be fooled when checking fuel pressure on late-model Fords; the pressures were once fairly universal at 35 to 45 psi, but now they're all over the place. See the '98 Pressure Chart.

When the graphs on the screen were stacked, and I compared the FRP PID graph to the PV transducer graph, the pressure readings were fairly close to each other. However, the electronic PID showed none of the downward pressure spikes I saw with the PV transducer. It dawned on 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, and the idle smoothed. When I returned to the Power Balance screen, the walking skip was gone.

Because the fuel system was returnless, any air making its way to the fuel rail is trapped 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 saw on the graph, and it manifested as a skip that mysteriously followed the firing order.

Why a "walking skip"?

I once worked on a Dodge van with a returnless fuel system and an unusual skip. Its problem was similar, but the skip stayed with cylinder number one. Because the fuel rail is tilted slightly on that engine, 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 air bubbles were able to move around.

I couldn't find any external leaks, and because the whole fuel system is pressurized from the pump to the rail, the air had to be originating in the pump. I didn'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 the last time I spoke with him, there were no further problems.

Cavitation at the pump

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