Overheating - Code 51 - Map Sensor Calibration Test (CHR0901OS)

OVERHEATING - CODE 51 - MAP SENSOR CALIBRATION TEST

TECHNICAL INFORMATION TIP

OVERHEATED ENGINE

YEAR(S): 1987.5-89

MAKE: Dodge

MODEL: Caravan

ENGINE: 3.0L

TRANSMISSION: Automatic or Manual

COMPUTER SYSTEM: SBEC

FUEL SYSTEM: Port Fuel Injection

IGNITION SYSTEM: Hall Effect

DRIVEABILITY CATEGORY: Engine Performance

SYMPTOM/SYMPTOM: Overheating, Lean misfire, lean codes, runs rough

FAULT TYPE: Symptom and trouble code

TROUBLE CODE: Code 51

COMPLAINT/CONDITION

The vehicle had overheated and the engine had been damaged. The new engine was installed by a shop specializing in engine rebuilding. The vehicle continued to overheat with the new engine and had a lean engine misfire. Two radiators were installed, the overheating persisted. The vehicle was setting a lean code 51 and the engine repair facility referred the customer to a driveability repair shop. HC was high, CO was low, O2 was 3% to 5% (looks like a lean condition). Customer said the vehicle did not run lean until the new engine was installed.

The vehicle would overheat if driven for long periods of time. The vehicle would set code 51. The oxygen sensor was below .45 volts (verifying the lean condition). The MAP sensor showed 13" to 14" of vacuum. Applying 19" of vacuum to the sensor resulted in a reading of 19" on the scan tool.

TROUBLESHOOTING TECHNIQUE

The technician began to investigate valve train problems which could cause overheating. He checked valve timing and all engine mechanical specifications. All checked out. He checked all grounds and feeds at the computer. (Poor grounds at the computer could result in low readings on sensors.) Adding propane to the air induction system would result in vacuum increasing. The technician noticed that the Idle Speed control motor was open and would shut down with the propane flowing through the air induction. All indications pointed to a vacuum leak, but none could be found.

FIX

Following the manufacturer's computer diagnostic recommendations allowed all sensors to be checked out at the computer. The diagnostic code showed the problem to be a lean condition, not computer related. The causes of a lean condition could not be found in the engine mechanical system checks. There were no external vacuum leaks. The only problem of low vacuum seemed to come from the idle speed control motor opening fully to keep the engine at 800 RPM's. Any problem with a computer output device requires checking all voltages at the computer. The Chrysler MAP sensor has a particular procedure which involves a Map sensor calibration check. The technician recorded the voltage on the MAP with 20" of vacuum and 5" of vacuum. He subtracted the lower reading from the higher reading. The voltage difference was .9 volts. The specifications call for the voltage difference being between 2.33 and 2.85 volts. The MAP was replaced.

The lean condition was gone, the engine did not overheat, and the code did not reset. The technician concluded that the engine overheating which damaged the engine was probably related to this problem. The earlier conclusion from the customer and previous repair shop that the problem only started after the new engine had been installed was in error and had served to throw the technician off initially. However, following the proper procedures showed that the problem could be resolved.

The technician recorded the voltage on the MAP with 20" of vacuum and 5" of vacuum. He subtracted the lower reading from the higher reading. The voltage difference was .9 volts. The specifications call for the voltage difference being between 2.33 and 2.85 volts. The MAP was replaced.

Non-turbo: 2.33 to 2.85 volts

Turbo: 1.13 to 1.39 volts

If the voltage does not fall within the acceptable range, replace the MAP sensor.

Courtesy of Service Council of California

REFERENCE NUMBER

CHR0901OS