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4.2L Lurching at Crawl Speeds with a Mopar MPI/EFI Conversion

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I have a problem with accelerator lurching at crawling speeds since I installed my MPI/EFI conversion kit to my 4.2 six.  I thought I recalled seeing a remedy for this in my Ludel's CJ rebuilders manual but for the life of me I can't find it.  Any suggestions?

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  • Moses Ludel changed the title to Lurching at Crawl Speeds with my MPI/EFI Conversion
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Hi, ArkansasCrawler...Separating issues, it's worth mentioning that the EFI kit throttle linkage can be "jumpy" and hypersensitive.  When Bill Burke and I taught Tread Lightly 4WD Clinics with off-pavement, low range driving, we would emphasize resting the right side of your accelerator foot against the transmission tunnel and rolling the throttle open by leaning your foot into the pedal.  I bring this up to help determine whether the lurch at crawl speeds is EFI/MPI related or a hypersensitive throttle in bumpy crawl conditions.  You indicate that the system has lurched at crawl speeds since the kit was installed.

If the problem is not the pedal sensitivity, EFI system lurching is usually either a vacuum, Idle Air Control (IAC) or MAP issue.  MAP issues can be vacuum related or loose electrical connections.  Vacuum leaks should be chased down first, including the vacuum in the EVAP system and the check valve on the brake booster.  If you have the earlier conversion system with the two-rail EFI and a fuel return to the fuel tank, the fuel return must be unrestricted and unaffected by fuel sloshing in the tank.  The rollover check valves at the tank must be working properly to prevent fuel sloshing into the fuel filler system or affecting EVAP functions.  A loose or poorly sealing fuel cap can create havoc, especially when jostling around off-pavement with fuel pressure changes and fuel sloshing around in the tank.  

Idle Air Control and MAP are possibilities.  MAP is a perishable item over time.  Before condemning the MAP sensor, I would check wiring, connectors and grounds, especially at the EFI splices into your chassis electrical.  Grounds are critical...The MAP sensor is easy to replace if defective, but I'm not a fan of arbitrarily changing parts.  The Idle Air Control Valve could be involved, though they are seldom defective. 

Check for vacuum leaks first, grounds and connections.  The throttle position sensor might be involved here, a defective or out of voltage range TPS will create an unstable idle and tip-in condition, though usually at all times, not just when crawling.  It's more likely that the Idle Air Control (IAC) would cause a surge condition. 

The TPS affects the rpm stability at idle, while the IAC tries to stabilize the idle speed to the factory/PCM rpm preset.  This can sometimes create a drivability issue.  Consider this:  The IAC wants to stabilize the idle while you're braking against the engine in low range.  If the idle lurching is an effort to stabilize the idle speed, some of this is unavoidable.  The rpm might surge while you try to drop the engine speed below the factory preset for idle.  Axle gearing and load involve other sensors that create an IAC response.  You're off the throttle, but the IAC is on it!

Some issues are inherent drivability quirks of EFI and the IAC's effort to hold rpm at a steady, often faster than desired speed.  I recall testing the EFI/MPI Jeep 4x4s (1991-up Mopar systems) when new and discovering that the downhill behavior ("deceleration") in low range crawls was unsettling.  The engines would struggle to hold a higher rpm while the vehicle was under braking in gear.  ABS automatic brake pumping on loose down slopes was a whole other story.

Here is how the Idle Air Control system works.  See whether this is what your driving issue could be:

"IDLE AIR CONTROL MOTOR DESCRIPTION...The IAC stepper motor is mounted to the throttle body, and regulates the amount of air bypassing the control of the throttle plate. As engine loads and ambient temperatures change, engine rpm changes. A pintle on the IAC stepper motor protrudes into a passage in the throttle body, controlling air flow through the passage. The IAC is controlled by the Powertrain Control Module (PCM) to maintain the target engine idle speed.

OPERATION...At idle, engine speed can be increased by retracting the IAC motor pintle and allowing more air to pass through the port, or it can be decreased by restricting the passage with the pintle and diminishing the amount of air bypassing the throttle plate. The IAC is called a stepper motor because it is moved (rotated) in steps, or increments. Opening the IAC opens an air passage around the throttle blade which increases RPM.

"The PCM uses the IAC motor to control idle speed (along with timing) and to reach a desired MAP during decel (keep engine from stalling). The IAC motor has 4 wires with 4 circuits. Two of the wires are for 12 volts and ground to supply electrical  current to the motor windings to operate the stepper motor in one direction. The other 2 wires are also for 12 volts and ground to supply electrical current to operate the stepper motor in the opposite direction. To make the IAC go in the opposite direction, the
PCM just reverses polarity on both windings. If only 1 wire is open, the IAC can only be moved 1 step increment) in either direction. To keep the IAC motor in position when no movement is needed, the PCM will energize both windings at the same time. This locks the IAC motor in place. In the IAC motor system, the PCM will count every step that the motor is moved. This allows the PCM to determine the motor pintle position. If the memory is cleared, the PCM no longer knows the position of the pintle. So at the first key ON, the PCM drives the IAC motor closed, regardless of where it was before. This zeros the counter. From this point the PCM will back out the IAC motor and
keep track of its position again. When engine rpm is above idle speed, the IAC is used for the following:

1) Off-idle dashpot (throttle blade will close quickly but idle speed will not stop quickly)
2) Deceleration air flow control
3) A/C compressor load control (also opens the passage slightly before the compressor is engaged so that the engine rpm does not dip down when the compressor engages)
4) Power steering load control

The PCM can control polarity of the circuit to control direction of the stepper motor. IAC Stepper Motor Program: The PCM is also equipped with a memory program that records the number of steps the IAC stepper motor most recently advanced to during a certain set of parameters. For example: The PCM was attempting to maintain a
1000 rpm target during a cold start-up cycle. The last recorded number of steps for that may have been 125. That value would be recorded in the memory cell so that the next time the PCM recognizes the identical conditions, the PCM recalls that 125 steps were required to maintain the target. This program allows for greater customer satisfaction due to greater control of engine idle.

Another function of the memory program, which occurs when the power steering switch (if equipped), or the A/C request circuit, requires that the IAC stepper motor control engine rpm, is the recording of the last targeted steps into the memory cell. The PCM can anticipate A/C compressor loads. This is accomplished by delaying compressor operation for approximately 0.5 seconds until the PCM moves the IAC stepper motor to the recorded steps that were loaded into the memory cell. Using this program helps eliminate idle-quality changes as loads change. Finally, the PCM incorporates a 'No-Load' engine speed limiter of approximately 1800 - 2000 rpm, when it recognizes that the TPS is indicating an idle signal and IAC motor cannot maintain engine idle.

A (factory adjusted) set screw is used to mechanically limit the position of the throttle body throttle plate. Never attempt to adjust the engine idle speed using this screw. All idle speed functions are controlled by the IAC motor through the PCM."

There is a wire hook-up for checking stored PCM engine trouble codes with your EFI Conversion.  On the earlier Mopar EFI Conversion system, I always hooked these wires to a dash LED lamp.  There is a key-cycle check for codes.  The key-cycling method and an LED lamp will work in the field.  There is a scan tool/diagnostics connector built into the later Mopar EFI Conversion wiring harnesses.  If your system has wiring with a diagnostics plug connector, it will likely take a Chrysler type scan tool adapter.

This should provide some ideas.  Separate the issues:  driving challenges like the throttle jumpiness versus a mechanical issue.  Take into account what the system is designed to do, including the IAC's functions and overrides when you're trying to maintain a slow idle

If this prompts questions, let me know...



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  • Moses Ludel changed the title to 4.2L Lurching at Crawl Speeds with a Mopar MPI/EFI Conversion

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