Diagnostic Dilemmas: Servicing Jeep and Ford Feedback Carburetors
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Diagnostic Dilemmas: Servicing Jeep and Ford Feedback Carburetors

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Lately, it seems, more people are “going retro” by rehabilitating older cars and trucks. Perhaps it’s the longing for the simplicity of a bygone era or perhaps it’s the sentimental attachment to a particular vehicle that’s driving the retro movement. Although in the Rustbelt East, the survival rate of these vehicles is probably very low, their survival rate is very high in the arid West.

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In many areas of the country, vehicles rust very little and the only limitation on vehicle life is parts availability and the technical know-how needed to maintain and repair these vehicles. Their survival rate is high enough to encourage several aftermarket companies to offer surprising complete lines of parts for older vehicles such as Jeeps and domestic pickup trucks. Other, smaller suppliers have bought up many obsolete parts inventories and are supplying them to the retro market. It takes some searching on the Internet, but it’s surprising how many parts sources exist for early pickup trucks and Jeeps.

Early OBD I
This month’s story is the last of a series on tuning and maintaining carbureted engines. During this series, I’ve touched on a few electronic or feedback carburetor issues. At the risk of repetition, I’ll summarize the operations and diagnostics of OBD I computer control systems.

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To begin with, the feedback carburetor was a stop-gap measure employed by auto manufacturers to comply with EPA emissions standards until manufacturing capacity could get up to speed producing the then revolutionary fuel injection systems. In brief, the most popular systems were the General Motors’ C-3 systems that employed two- and four-barrel Rochester carburetors using pulse-modulated metering rods to control the low-speed air/fuel (A/F) ratios. Unless some were manufactured for use in California, it was rare to find a closed-loop carburetor on any GM truck lines in the early ’80s.

Although Chrysler was one of the first domestic manufacturers to convert exclusively to electronic fuel injection, some Chrysler Corporation engines used a feedback version of the Holley/Weber staged two-barrel carburetor on some of its 4-cylinder engines. Ford followed up with the variable venturi feedback carbs used on some luxury lines, pickup trucks and police vehicles. Ford also used a two-barrel Motorcraft carburetor on its Ranger and Bronco II series that employed a stepper motor to control A/F ratio by metering air around the closed throttle plates. Jeep used a similar arrangement to control A/F ratio by grafting a stepper motor arrangement to the familiar Carter two-barrel BBD series carburetors.

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Time has taken its toll on the Rochester C3 and Ford variable venturi carburetors. These carburetors were not designed for amateur hands. Eventually, most feedback Rochester carbs had the fine-thread metering rod height adjustment screw threads stripped and the variable venturi carbs had the flow bench-adjusted main metering jets irrevocably twisted out of calibration by amateur mechanics. Most of these surviving vehicles have been converted to conventional carburetors and distributors. Unless the vehicle has collector value, the cost of repairing either of these systems usually exceeds the value of the vehicle itself. If you do choose to repair feedback carbs, you’ll need the special tools required to make the main metering adjustments.

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In all of the above systems, a professional-grade DVOM and a professional two-channel lab scope are the diagnostic tech’s best friends. Last, but not least, severe oxygen sensor contamination is quite common on these old systems due to normal engine oil consumption and fuel contamination. If you’re diagnosing a fuel trim issue and you’re in doubt about the oxygen sensor, it will save a lot of time if you replace the relatively inexpensive single-wire sensor as a first step. When testing, remember that these oxygen sensors have no heaters and the system will go into open-loop operation when oxygen sensor temperatures drop below 600° F.

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OBD I Diagnostics
General Motors, Ford and Chrysler popularly introduced their on-board diagnostic systems around 1981. The General Motors system has always been the most user-friendly and comprehensive system to work with. Chrysler’s on-board systems were among the first to include bi-directional controls to help diagnose system failures. Whereas the GM system was oriented toward emissions diagnostics, the Chrysler systems were oriented toward cranking, no-start diagnostics.

The computing capacity of these early systems was low and their reaction speed was relatively slow. Although either system could recognize an open, short or short-to-ground condition, neither had enough computing capacity to recognize more than just a few sensor calibration failures. The “live” data streams provided by both systems were, however, a revolutionary diagnostic development at the time.

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Ford took another path with its EEC-IV system in the early ’80s. It doesn’t take long to discover that the EEC-IV system generally doesn’t do a good job of storing diagnostic trouble codes. An open-circuit oxygen sensor, for example, might run for days before the ECM will recognize the failure and store a trouble code. Although the EEC-IV system could recognize catastrophic sensor failures, the EEC-IV system relies heavily on a 60-pin breakout box that provides “live” data by being connected between the ECM and the ECM wiring harness. Originally, the sensor voltage values were interpreted by using a DVOM to measure the voltage, resistance and frequency produced by a sensor. Today, a lab scope will provide a much better picture of circuit activity.

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Ford also uses a key-on, engine-off (KOEO) scan tool diagnostic mode to automatically test circuit integrity of its electronics. A key-on, engine-running mode (KOER) is used to dynamically test sensors such as the oxygen, MAP and throttle sensors. The KOER mode provides the most accurate sensor data because it requires dynamic testing modes, such as the “code 10” snap throttle test.

Servicing Jeeps
The on-board diagnostic system built into the Jeep feedback carb system will not store trouble codes and does little more than provide data indicating various switch positions and basic sensor values. Notable failure points on the old Ford MCU-based system used in these Jeeps were corrosion at the ECM connector, which is located behind the glove box on some models, and corrosion on the ignition coil terminals.

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A quick way to evaluate fuel control on the Jeep feedback carb is to look down the auxiliary air inlet with the engine hot and running to observe the position of the stepper motor pintle valve. If the idle A/F ratio is adjusted correctly, the pintle valve should be about midway in its travel. Assuming the engine has engaged closed-loop operation, the oxygen sensor voltage should also be about 0.5 to 0.7 volts. A scope or DVOM can be tapped directly into the oxygen sensor lead to perform this test.

The distributor used with the Jeep feedback carburetors is a Motorcraft unit that incorporates the standard mechanical and vacuum advance mechanisms. In some cases, the mechanical advance on some Jeeps failed to operate because the secondary spring in the advance mechanism has too much tension. A spring with slightly less tension can be installed to allow full mechanical advance at about 3,500-4,000 rpm. In most cases, Jeeps of this era use the “blue grommet” Motorcraft ignition module to operate the Motorcraft ignition coil. The ignition module is generally located in a highly vulnerable position under the washer reservoir mounted on the inner driver’s side fender panel. Keep in mind that the blue-grommet ignition module is a high failure rate part. If the module is receiving key-on power and the distributor pickup meets resistance specifications, test the B- terminal of the ignition coil for dwell angle or duty cycle. If the module isn’t producing dwell or if the engine is stalling due to intermittent ignition failure, replacing the relatively inexpensive module is a good first step.

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EEC-IV Diagnostic Checks
I’m going to cover the carbureted EEC-IV system using the Motorcraft 2150-A carb because it has technical similarities to the feedback variable venturi carburetor and the Jeep feedback Carter carburetor. Although I recommend checking for trouble codes first, it’s also important to verify correct MAP frequency, which is 159 Hz at sea level. Data usually includes altitude corrections for these MAPs. MAP failures are infrequent, but can cause no-code, intermittent rich condition operating failures.

Next, because these TP sensors are often directly exposed to raw gasoline, inspect the condition of the throttle sensor mounted on the end of the carburetor throttle shaft. If the sensor looks contaminated, it should be replaced.

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Always inspect the condition of the ECM ground wire, which is attached directly to the negative battery terminal. The voltage drop from the wire to the negative battery terminal should be less than 100 millivolts (mV). When you’re testing voltage drops in EEC-IV system electronics, keep in mind that voltage drops of 80-100 mV are relatively normal for these systems.

Last, check base timing by disconnecting the single-wire spark-out (spout) connector located near the distributor. The base timing on most of these engines is 10° BTDC. The computed timing will generally range 20° or more at idle. If you’re using a breakout box, flipping the “computed timing” switch to “off” will drop the engine to base timing.

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Feedback carb engines generally must exceed 160° F coolant temperature and the oxygen sensor temperature must exceed 600° F before it activates to put the system into closed-loop operation. While saying this, keep in mind that some Asian and a few domestic vehicles might use an operating strategy that places their systems in closed-loop operation at idle speeds. On EEC-IV systems, once the ECM begins controlling A/F ratio via the carburetor stepper motor, the system is operating in closed-loop operation.

Richening the fuel mixture with some aerosol carb cleaner sprayed into the intake should produce an O2 voltage reading of 0.9 volts. Disconnecting a large vacuum hose, such as the brake booster hose, to lean the fuel mixture should drive the oxygen sensor close to zero volts. If the O2 sensor can’t be driven rich or lean, the sensor has either a rich or lean “bias,” which means the sensor should be replaced before any further carb adjustments are made. If the vehicle is being road-tested, the carburetor should average about 0.5 oxygen sensor volts during steady-throttle, cruise conditions.

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Idle Speed Authority
Feedback carburetors use either an electrical or vacuum-actuated external idle speed control (ISC) that acts on the throttle lever to control idle speed. The ISC generally had to be adjusted to maximum authority, which is the highest permissible idle speed. Many of these ISC systems also act as a dashpot, which means that, as the vehicle decelerates and the clutch is disengaged, the ISC will maintain perhaps 1,200-1,500 rpm for a few seconds to reduce emissions and stalling caused by an over-rich fuel mixture occurring as the throttle snaps closed. Each system requires a different min/max adjusting procedure, so have a manual close at hand.

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The maximum idle authority on the ’85 2.8L is adjusted by turning on the key and allowing the ISC to fully extend. Next, disconnect the ISC, start the engine and adjust the ISC to produce 2,000 rpm. Minimum authority is achieved by fast-idling the engine more than 1,000 rpm, disconnecting the ISC after it retracts and adjusting the base idle speed to 750 rpm. When the ISC is reconnected, the engine speed should be 800-900 rpm for manual transmission and 850 rpm for automatics in the drive position.

TFI Systems
Very briefly, the early thick-film ignition (TFI) modules had nearly a 100% failure rate. In some cases, the module would cause an intermittent stall. In most cases, the failure pattern of the TFI module is a cranking, no-start situation. In addition, the plastic material used in the distributor’s magnetic pickup assembly rapidly deteriorates due to age. Replacing the module and the pickup as an assembly is always recommended when servicing TFI systems.

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