Testing Procedure

ROTUNDA EXHAUST BACK PRESSURE TESTER 105-00020

Exhaust backpressure is difficult to measure without the use of specialized equipment. The Rotunda Exhaust Backpressure Tester 105-00020 is now available. It is designed to convert PFE sensor (Pressure Feedback EGR sensor or its signal circuit) signals to a voltage signal, measurable on Rotunda Digital Volt-Ohmmeter 014-00407 or equivalent.





The Rotunda Exhaust Backpressure Tester 105-00020, Figure 1, consists of the following parts.

^ Exhaust Backpressure Test Control Block

^ PFE Sensor

^ Pressure Test Adapter Fitting

^ High Temperature Silicone Hose

ROTUNDA EXHAUST BACKPRESSURE TESTER 105-00020 - HOOKUP AND RECOMMENDED TESTING PROCEDURE

NOTE:
IF THE VEHICLE WILL NOT START, REFER TO THE POWERTRAIN CONTROL/EMISSIONS DIAGNOSIS MANUAL, SECTION 2A, FOR DIAGNOSTIC ROUTINES.

If the vehicle starts and runs, proceed as follows:

1. Visually inspect the exhaust system for obvious damage that may cause a restriction. Service as required.

WARNING:
BE CAREFUL WHEN WORKING WITH HOT EXHAUST SYSTEM PARTS. USE PROTECTIVE GLOVES AND CLOTHING. ALLOW AMPLE COOLING TIME IF NECESSARY.

2. Remove the H02S (HEGO) sensor from the catalyst inlet pipe and disconnect the electrical connector.

a. Insert the Rotunda threaded adaptor fitting (part of 105-00020) into the H02S (HEGO) sensor boss in the catalyst inlet pipe.

b. Connect the high temperature silicone hose (part of 105-00020) to the fitting and to the backpressure test control block (part of 105-00020).

CAUTION:
DISCONNECT THE H02S (HEGO) SENSOR ELECTRICAL CONNECTOR TO AVOID EXCESSIVELY RICH AIR/FUEL MIXTURE DURING TESTING.

3. Disconnect the vacuum line from the single Thermactor Air Control Valve for the 7.5L engine. Disconnect the vacuum line from all of the Thermactor Air Control valves for the 5.8L engine. Plug the vacuum hoses for all applications.

4. Connect the backpressure test control block (part of 105-00020).

a. Connect the EBT Control Block to the input jacks of the DVOM.

b. Be sure the Negative (common) jack of the EBT Control Block is plugged into the Negative (common) jack of the DVOM.

c. Set the DVOM on a fixed DC millivolt range and not in the autorange mode.

NOTE:
THE DVOM MUST BE CAPABLE OF READING ONE TENTH (0.1) OF A MILLIVOLT DC (FOR EXAMPLE 99.9 MILLIVOLTS).

d. Connect the EBT power lead to the vehicle cigar plug receptacle.

e. Adjust the EBT control knob to read zero on the DVOM.

5. With the transmission in NEUTRAL/PARK, start the engine and allow it to reach a stable engine speed. The engine should be at normal operating temperature.

6. Open the throttle to 3000 engine RPM and hold for at least 3 seconds.

7. Read the voltage on the DVOM.

NOTE:
"+" READINGS ARE PRESSURE, "-" READINGS ARE VACUUM.

If DVOM reading is negative, recheck DVOM calibration to EBT control block.





8. Observe the voltage readings.

^ If the voltage is higher than the values shown in the following Exhaust Backpressure Chart, then a restriction exists in the exhaust system. Proceed to Step 9.

^ If the voltage is 12.4 millivolts or lower for the 7.5L engine, or 5.9 millivolts for the 5.8L engine, the power complaint is due to a cause other than a restricted exhaust system. See TSB 92-22-6 for additional diagnostic routines under "Supplemental Diagnostic Instructions."

9. Disconnect the muffler and repeat Steps 6 and 7 to isolate it from the exhaust system.

a. It may be easier to unbolt the exhaust pipe hangers from the side rail.

b. On some vehicles, it may also be easier to remove the fuel tank heat shield to allow for additional clearance for the exhaust pipe to move laterally to rear, away from the catalyst.

10. If the voltage is above 11.5 millivolts, for the 7.5L engine, or above 4.0 millivolts for the 5.8L engine, the catalyst is restricted.

NOTE:
IF THE CATALYST IS FOUND TO BE RESTRICTED, REFER TO TSB 91-12-11 OR SECTION 15A OF THE POWERTRAIN CONTROL/EMISSIONS DIAGNOSIS SERVICE MANUAL (PC/ED) TO DETERMINE THE ROOT CAUSE OF THE CATALYST RESTRICTION.

If the voltage is below 11.5 millivolts for the 7.5L engine or below 4.0 millivolts for the 5.8L engine, replace the muffler and tailpipe assembly.

NOTE:
VISUALLY INSPECT THE INSIDE OF THE CATALYST FROM THE REAR OPENING. IF THE CATALYST IS EMPTY OR IF LOOSE INTERNAL PIECES ARE PRESENT, REPLACE THE CATALYST ALSO.

11. Repeat Steps 6 and 7 after all repairs are complete, to verify successful diagnosis of the restricted part.

a. If the voltage still reads 12.5 millivolts or higher (7.5L only) or 6.0 millivolts or higher (5.8L only), then a restriction still exists in the exhaust system.

b. Be certain that the voltmeter has been carefully calibrated to zero during tool setup and to the actual testing procedure.

c. Use the knob on the control block for zero calibration.

NOTE:
THE ABOVE PROCEDURE IS ONLY INTENDED FOR THE VEHICLES LISTED IF THEY ARE FITTED WITH THE 7.5L ENGINE. VOLTAGE SPECIFICATIONS STATED IN THIS ARTICLE ARE NOT ACCEPTABLE FOR OTHER ENGINE/VEHICLE APPLICATIONS.

NOTE:
IF THE PFE SENSOR SUPPLIED WITH THE TOOL KIT IS SUSPECTED OF BEING FAULTY, IT MAY BE PURCHASED THROUGH THE NORMAL PARTS CHANNEL. THE PART NUMBER IS LISTED IN THE PARTS BLOCK.