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Fixes for 21 Problem Vehicles

(See the discussion on how these vehicles were chosen as "problem vehicles.")

Ford Escort/EXP/Lynx, 1984-85, 1.6 liter, feedback, carbureted

This vehicle, although certified for high altitude use, typically shows symptoms of altitude enrichment, failing emissions at 40+ grams per mile (gpm) of carbon monoxide (CO).  Once preliminary checks and verifications have been performed, the vacuum-operated power valve may be modified by weakening or replacing its spring, thereby delaying power enrichment (under load).  Additionally, fuel metering jets, air corrector jets and emulsion tubes may be replaced to enlean the fuel mixture slightly.  Ignition timing may be advanced two to four degrees over specification for driveability.

General Motors Truck 1/2, 3/4, 1-ton 5.0 and 5.7 liter, 1982-86, carbureted

These full-size trucks with Rochester Quadra-jet carburetors typically need fuel metering jets two steps (sizes) lean from factory calibrations.  Power valve mounting and settings must be checked for proper operation.  As with any truck that may have been heavily loaded and/or run rich for extended periods of time, catalytic converter efficiency must be tested.  An oil change and functional check of the positive crankcase ventilation system is recommended.

General Motors Truck S-10 1/2 ton & Blazer, 1984-85, and
Jeep Cherokee, 1984-88, non feedback, carbureted, 2.8 Liter

Carburetor problems abound in this application, with main circuit fuel enrichment occurring too soon at this altitude. Use of a positive crankcase ventilation (PVC) valve for a 1985, 5.0 liter V8 is suggested for altitude compensation.  Fuel metering jets could be changed to one size smaller; however, jet availability is questionable.  Metering rod height may be adjusted slightly to enlean the mixture throughout the range.  Fuel float height (and buoyancy) should be checked and adjusted to factory specifications.  These carburetors are subject to throttle shaft wear, which induces a vacuum leak, often compensated for by mixture enrichment adjustments (treating the symptom).  Throttle shaft bushing and resetting of mixtures, minimum air flow, and timings are necessary in this case.

Ford Mid/full size Cars, 1984-87, 3.8 liter & 5.0 liter, central fuel injection (throttle body injections)

This early fuel injection system is comparatively complex and not well understood.  A Ford EEC-IV scanner or dual trace digital oscilloscope is needed to check:

1)  that the two oxygen sensors (3.8L only) are switching together and nearly equal in amplitude (the 5.0L has only one O2 sensor);
2)  fuel control at idle, by forcing the mixture rich, then lean;
3)  fuel control at light and medium loads.

The air injection system can switch upstream and downstream (of the oxygen sensor) for catalytic converter overheat protection. Air injection will switch to upstream at 30-45 seconds at idle, forcing the mixture rich.  Ensure that air switches downstream as the throttle is opened.  The air injection switching solenoids are located below the coolant overflow bottle and are subject to rust, dirt and corrosion.

This system is extremely sensitive to low manifold vacuum, a typical condition for an older vehicle at our altitude -- check timing chain, ignition timing, carbon buildup, exhaust back pressure and leaking EGR problems. Verify manifold absolute pressure frequency at or near 102 hz at idle.  Fuel pressure is another critical area.  It is not uncommon for these systems to be running at or near 45 psi.  Ford specification is 37-40 psi; check and adjust to the low end of this specification at the throttle body as necessary.  (Do not set below 37 psi!).

Ford Taurus, 1986-87, 3.0 liter, fuel-injected

The early Tauruses have had several recalls and service campaigns, most of them unrelated to emissions.  The recurring problems with emissions have to do with the following:

1)  Faulty ground circuits for the computer and its inputs/outputs.
2)  Oxygen sensors are unusually problematic, both when new and aged -- check switching, amplitude and bias.
3)  Power steering leaks -- the oxygen sensor is located below the high pressure power steering hose.  If the hose leaks, it will leak directly onto the oxygen sensor, fouling it and driving the mixture rich.
4)  Fuel pressure regulator failures resulting in high fuel system pressures and high CO.
5)  Manifold absolute pressure (MAP) sensor failures, sending a faulty load signal to the computer, resulting in over-enrichment and CO problems.

Ford Truck 300 CID 6-cylinder, 1984-1985, feedback, carbureted

This pickup/Bronco application typically suffers from abuse, neglected service and a poorly designed manifold and feedback system. The vehicle will drive tolerably in open loop all the time, suffering from poor driveability and fuel economy (typically in the 8-10 mpg range). Check the mixture control solenoid on the carburetor for a varying duty cycle.  A fixed 50 percent duty cycle is the open-loop default value.  Check the oxygen sensor for switching (frequency) and amplitude. Verify powers and grounds to the ECU (computer - located inside, above the accelerator pedal). The ECU and 02 sensor grounds are on the firewall in the engine compartment low on the driver's side. Other critical inputs to the computer are the coolant temperature sensor and manifold absolute pressure (MAP) sensor (frequency-based), which both ground ahead and above the battery on the right inner fender well.

Manifold leaks are very common (both intake and exhaust).  Check all manifold, carburetor mounting and manifold preheat bolts and gaskets for tightness and leaks (flowing propane around gasket areas makes this fairly simple).  The exhaust manifold is a relatively thin casting and can be badly warped, particularly if the vehicle has been heavily loaded, used for towing or driven through deep water.  A leaking exhaust manifold will dump raw oxygen on the O2 sensor and can make the feedback system attempt to go rich.

Carburetor throttle shaft wear is an area for particular attention. Some of the carburetor screws have a propensity for vibrating loose and having the carb literally disassemble itself in normal use.  When rebuilding the carburetor pay careful attention to the reassembly of the accelerator pump components -- the washer goes on top of the diaphragm, not below it. This problem has been seen on several remanufactured carburetors as supplied from the rebuilder. Rejetting should be considered a last resort.

Air injection switching (upstream, downstream), exhaust system integrity, and catalyst function are areas of common problems on these trucks as well.

Chrysler, 1982-85, 2.2 liter K-Cars, feedback, carbureted

With the popularity of these early K-Cars, a wide variety of problems can exist.  Before spending too much time diagnosing, a check of basic engine mechanical condition is in order (compression, leaks, oil pressure, etc.). Air injection system function/switching is a common problem area, as is a weak ignition system, a dead catalyst, and soaked fuel evaporated canister.

The proximity of the ECU to the battery causes corrosion, resulting in electrical connector and harness problems. The early versions (1982-83) of this application may be low-altitude certified only and need rejetting and/or air bleed changes.  Check for availability of a high altitude compensation package.

Honda Accord, 1984-85, feedback, carbureted

As any repair technician who has attempted to repair one will attest to, this application is one of the most difficult to diagnose of any vehicle.  Basic mechanical condition of the engine driveline should come first -- check for oil consumption/smoke and clutch/transmission slippage in particular.  Many of these vehicles are low-altitude certified and have not been retrofitted with the available high altitude kit. Speed sensor switch function is a critical input for proper emissions function, although the driveability may not be affected.

The vacuum/emissions control "black box" on the passenger side firewall has long been a mystery, even in Honda service literature.  Local Denver-area technicians and instructors have teamed up to explain its intricate and varied functions in a separate volume of the High Altitude Repair Validation Study.  This volume is available through the Air Pollution Control Division; summarizing it is beyond the scope of this report.

General Motors, 1982-84, 307 CID V8 (Y-VIN), feedback, carbureted (E4ME carb)

The so-called Y-VIN Oldsmobile (which also includes some Buick, Cadillac, Chevrolet and Pontiac) is an example of a frequently-failing vehicle without a singular common failing component.  The Y-VIN 307s usually need a full front-to-back diagnosis, but here are some common problem areas:

1)  Basic carburetor, mixture control, solenoid setup and adjustments to GM factory specifications.
2)  Vacuum hose condition and routing should be carefully checked.
3)  Air management and air switching function must be verified, to ensure upstream/downstream/dump functions.
4)  Fuel-laden evaporative canisters seem to be fairly common.  Also, verify canister purge valve function.
5)  Components frequently found to be faulty are the thermal vacuum valve, throttle position sensor, and manifold absolute pressure sensor.
6)  Check for fuel leaks around the carburetor well plug.

The E4ME carburetor in this application behaves very differently between idle and loaded operation.  When verifying repairs on a Y-VIN, it is very important to load the engine on a road test or on a dynamometer to verify the various fuel management strategies.

Chrysler Car & Truck 318 CID V8, 1982-87, feedback, carbureted

This first-generation feedback system suffers from questionable design/engineering and durability problems. The computer (ECM) location, (behind the kick panel, near the battery, or on the air cleaner housing) makes it subject to damage and/or loosening of connectors.  Electrical harness connectors, particularly in truck applications, are subject to high or open resistance - perform voltage drop tests for all powers and grounds.

Ensure that the oxygen sensor is switching properly (frequency and amplitude) off idle, as several of these applications have an idle switch on the throttle position sensor that intentionally puts the vehicle in open loop operation at idle.  Multiple temperature switches determine when the vehicle may go to closed loop operation -- carefully check these switches according the Chrysler specifications.  Power valve problems and low vacuum combine to make these vehicles run rich to very rich under load.

Ford Mid-size Car 200 CID, 1982-83, non feedback, carbureted

This last bastion of the non-feedback Ford cars suffers a variety of neglect and altitude-related problems including a power valve in the carburetor that opens too soon due to low vacuum (tall gearing was feature for fuel economy); Exhaust Gas Recirculation (EGR) leakage at idle due to carbon build-up; and malfunctioning air injection.

This is the old thermactor system that was designed to protect the catalytic converter by "dumping" air after 90 seconds at idle. Typical of inline 6-cylinder applications, manifold and carburetor base gaskets are prone to leakage.

Finally, the catalytic converter should be checked for function.

Toyota (& Chevy Nova), 1982-89, various 4-cylinder w/electronic bleed control valve

This application is somewhat unique in that in meters filtered ambient air into the intake manifold for air fuel ration control. The electronic and vacuum systems are subject to tampering and misconnection so the first check should be for correct vacuum routing and electrical connections.  There is a secondary air filter on the intake manifold vacuum port (the "gas filter," due to its appearance) which is an often-neglected maintenance item. Check and replace as necessary.  All vacuum lines should be thoroughly checked for obstructions (tampering with BB's or screws).

Some of these vehicles came equipped with a high-altitude compensation valve, which should be checked for function.  The throttle/idle switch should close at idle and open off idle. The air bleed control valve itself should be checked next, according to Toyota (GM) procedures. This valve can be partially disassembled and the valve disc, seat and solenoid can be replaced as subassemblies.

Lastly, the carburetor function should be checked for cleanliness, leaks, proper circuit function etc.

Nissan, 1982-87, feedback, carbureted

This Hitachi carbureted application suffers a litany of sensor problems, so ensure all powers and grounds are good as well as sensor signal ranges at the computer (ECM).  The carburetor's mixture control solenoid is prone to wear and loss of sealing.  The factory repair calls for replacing the entire carburetor top assembly at a cost of several hundred dollars for the part.  With some careful modification of the air horn, Nissan part number 16197-61A03 (solenoid) will fit. Be sure to grind away only enough of the air horn so that the solenoid fits flat.  Be sure to carefully clean all traces of ground metal from the carburetor before reassembly.  The availability of this part number from Nissan is questionable, but it is available from some aftermarket sources.

Finally, check the catalyst for efficiency, as they tend to go dead after years of running out of fuel control.

Jeep, 1982-1991, full size Wagoneer 360 CID, non feedback, carbureted

The last of the full size Wagoneers was never fitted with feedback fuel control.  This fact, combined with a typical lack of good maintenance, results in this application being one of the highest carbon monoxide (CO) emitters on Colorado's roads. There is no singular fault with this application, rather any one of a combination of the following possible faults has been seen:

1)  Weak power valve that opens too easily - poor vacuum to the valve itself
2)  Clogged or stuck exhaust gas recirculation valve
3)  Worn timing chain
4)  Low vacuum as a result of 2) & 3) above
5)  Accelerator pump stroke set too long
6)  Malfunctioning air injection system - burned out check valves etc.
7)  Clogged or burned catalytic converter

Carburetor main jets were usually #56 from the factory, which is too rich for our altitude. As a last resort, the carburetor could be rejetted to #54, #52, or #50 jets. This is a Motorcraft (Ford) carburetor and corresponding part numbers are C3UZ-9533A-54; C3UZ-9533B-52; and C3UZ9533C-50. Particularly when going as lean as 52 or 50, be sure to check for driveability and oxides of nitrogen, as NOx emissions will increase with the enleanment if the EGR system is not functioning.

Hyundai-Mitsubishi-Dodge Colt, 1984-89, various, feedback, carbureted

This Mikuni carbureted system was common to several imports (Asian and American badged) of this vintage.  These tended to be economy models and routine maintenance is often lacking.

Verify fuel control by monitoring O2 sensor/MC solenoid dwell.  If the engine is not in control but the O2 can switch, start checking the other sensor inputs. The Coolant Temperature Sensor (CTS) seems to be a recurring problem preventing closed loop operation. Check Throttle Position Sensor (TPS) for a clean sweep and good ground.  This TPS typically fails to a wide open throttle (high volts signal).

The main body ground in many of these applications is located near the hood latch (underhood). Other recurring problem areas confounding fuel control are:

1)  High altitude compensation valves (check recalls)
2)  EGR System Function/faults
3)  Plugged catalytic converter
4)  Cracked exhaust manifold

Once the ability to regain control has been established -- that is, all powers, grounds and systems have been verified according to the vehicle manufacturer's service information -- the carburetor needs to be checked for:

1)  Basic carburetor integrity -- this carb uses a phenolic plastic floatbowl assembly that is prone to cracking, leaks and stripped threads.
2)  Mixture Control Solenoid (MCSd) sealing 'O' rings can leak, springs weaken, or they're just dead electrically.  The OEMs generally don't have parts available; however, they can be found aftermarket.
3)  Next check carb calibration-jetting, air bleeds and secondary air valve setups.

When the vehicle finally achieves fuel control, the catalytic converter should be checked, as it tends to die of neglect in this application.

Various - Audi, Mercedes Benz, BMW, Volvo - Mid 1980's Bosch CIS-E systems

This feedback controlled continuous injection system has a "black box" mixture control unit mounted directly on top of the fuel distributor.

NOTE:  Do not puncture wires on these applications!  Back probe connectors so as not to damage shielded wires -- puncturing the O 2 sensor wire on these cars will ground the sensor signal. The computer signal to the mixture control unit is a varying current (milliamps) -- not voltage.  High current is a lean command and low current is a rich command. Throttle switches are unique in this application as well -- a three way switch -- idle/off/WOT (wide open throttle).

Fuel system pressures, filters and contamination should be checked according to the manufacturer's specs.  (Replacing a fuel distributor is expensive.)  Some applications have a fuel pre-pump located in-tank.  If the customer complains of poor hot driveability, suspect a pre-pump problem.

Vacuum leaks are not uncommon in this application, with injector (O.D.) seals a prime area. These European import systems do not fail often, but when they do, some of their peculiarities make them difficult for the uninitiated.

Jeep/Ford various trucks, 1982-88, with Carter YF non feedback carburetor

Several of theses 4- and 6-cylinder truck applications suffer from wear and tear, with warped/cracked/loose manifolds, carburetors falling apart, etc.  A preliminary check of the vehicle's condition/value and owner advisement of potential repair costs may be in order.

Remember the basics -- check for vacuum leaks/loose manifold and carb bolts, check and adjust ignition timing, reset idle. Two problems are:

1)  Air injection system disabled or inoperable;
2)  Catalytic converter condition (is it hollow?).

Moving on to the carburetor itself, a quick check is made for leaks, loose or missing parts and overall condition. Then check for:

1)  Altitude certification and hardware;
2)  Check and adjust float level to manufacturer specs;
3)  Verify jet sizes to manufacturer specs ("strip kit" jet sets for Carter AFB carbs will fit);
4)  Jet needle height can be adjusted, if necessary.

Toyota 1981 20R engine - Celica, Corona, Truck, feedback, controlled air injection

This early import feedback system actually ran a non-feedback engine's exhaust through a feedback controlled catalytic converter system. The computer controls air injection management feeding the catalyst or dumping air to shut it down, based on the catalyst overheat switch, O 2 sensor and other inputs.  Verify sensor inputs, then check for ability to switch air.  Verify vacuum hose routing.

A fairly common failure in this application is the diaphragm in the auxiliary accelerator pump in the carburetor.  The AAP allows a longer accelerator pump stroke when cold.  A failed diaphragm will result in very high CO due to sucking gasoline straight through the accelerator pump circuit. 

If this vehicle is failing for hydrocarbons (HC), check the manifold and gasket at #4 runner (where the EGR valve is ) for leaks.

Mazda RX-7 Rotary, mid-late 1980's, non feedback, carbureted

Although the rotary engine is very different internally, control strategies and external hardware are comparable to its reciprocating engine cousin. Check for the following:

1)  Air injection system function-switching;
2)  Thermal reactor/catalytic converter burnout.

The carburetor on this application is relatively dependable. Rejetting is not recommended. If a rotary fails for hydrocarbons (HC), with low CO and a good ignition system, suspect worn apex seals on the engine rotors -- this situation is analogous to low compression in conventional engines.

Ford Tempo/Topaz, 1984 only, 2.3 liter, feedback, carbureted

This application, while normal in most respects, has some unique fuel metering features not found before or since. The carburetor main metering circuit is jetted lean, with an auxiliary fuel valve adding fuel based on the computer's signal.  The computer's electronic signal is converted to a modulated vacuum signal through a vacuum Feedback Control Solenoid (FCS) external to the carburetor.  The modulated vacuum acts upon the auxiliary fuel valve in the carburetor body. With zero vacuum, the auxiliary fuel valve piston is down, enrichening the mixture.  With 5 inches vacuum (the minimum delivered by the feedback solenoid) the piston moves up, closing the auxiliary fuel valve and making the engine run off of its basic (lean) jetting. The skewing of the calibration of the auxiliary piston can be a problem, as can misrouted vacuum hoses, and grounds/signals to the feedback control solenoid.