Engine Misfire Or TCC Chuggle?

A customer complains of an intermittent misfire, surge or chuggle that occurs at highway cruising speeds on a late model front wheel drive vehicle. The vehicle is equipped with a V6 engine and an electronic four speed transmission. The technician checks the vehicle for trouble codes, but there are no codes stored. As the condition is intermittent, the technician is having difficulty duplicating it–what could be the cause? Is it a lean engine misfire or a transmission issue?

If this condition is encountered, begin your diagnosis with a good visual inspection of the engine wiring harness. Make sure the harness has not rubbed through on any of the engine or transmission brackets, brake or fuel lines, then road test the vehicle and command the Torque Converter Clutch (TCC) ON. This should eliminate the chuggle.

Continue your diagnosis by inspecting the TCC brake switch and verify the switch is not cycling ON and OFF while driving. If the TCC slip speed is less than 30 RPM, the cause will be an engine performance issue and the lean misfire diagnosis should be followed. If commanding the TCC ON did not eliminate the chuggle the cause will most likely be internal transmission leaks

If the visual inspection did not reveal any wiring issues, the TCC brake switch tested good and commanding the TCC ON while driving does not eliminate the condition, use the scan tool to take a snapshot of the event. Use the scan tool or scope to graph the TCC duty cycle and look for a saw toothed pattern that ranges from 40 to 80 per cent. If this style of pattern is displayed on the graph then the pressure control solenoid is out of range and is causing the chuggle/misfire. If a scope or scan tool is not available, a transmission pressure gauge can be substituted–the gauge needle will oscillate when the condition is evident.

To correct the condition the pressure control solenoid will have to be replaced. Once the pressure solenoid has been removed it would be a good idea to inspect the main line pressure regulator valve which is located behind the pressure solenoid. The regulator valve should be inspected for sticking and binding in the valve body bore. If the valve is binding in the valve body, the valve body will have to be removed for cleaning. With the valve body on the work bench carefully push the valve against the return spring and verify that the valve will return to its original rest position. If the valve does not snap back to the original position, it will have to be removed for further inspection.

Remove the valve and inspect it for scratches and scoring, use your fingernail to determine if any scratches are too deep. Move your fingernail over the valve surface and if the nail catches any scratches, the entire valve body will require replacement. If the scratches do not catch on the fingernail, clean the valve and bore with brake cleaner and dry with shop air. When cleaning the valve and bore make sure the machined surface of the valve body is facing down so any debris will fall out of the bore. Use clean transmission fluid to reassemble the valve body and verify that the regulator valve will move freely within its bore. Re-install the valve body.

After replacing the pressure control solenoid or overhauling the valve body, the transmission shift adapts will have to be cleared. To clear the transmission shift adapts disconnect both battery cables and leave them disconnected overnight. Road test the vehicle to verify that the condition has been corrected. If the chuggle/misfire is still evident after this procedure, the vehicle electronic control or transmission control module will have to be replaced. Once the repair has been confirmed the technician can confidentially return the vehicle to the customer.

For more information on automotive technology visit CARS OnDemand training at: www.cars-council.ca