Torque converter likely cause of acceleration shudder

By: Jim Kerr Posted: 2:04 AM CST Thursday, Dec. 31, 2015

Question: I have a 1993 Intrepid ES with a 3.3L V-6 and automatic transmission. It is in good shape, but it sometimes shudders when moving at about 60 km/h in fourth gear (drive). It never shudders when accelerating or decelerating or in third gear. The gears shift very smoothly. It has been very recently serviced with new oil and filter which did not fix the problem. Computer analysis said it is OK to drive. How can I fix the shudder?

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Hey there, time traveller!
This article was published 31/12/2015 (3917 days ago), so information in it may no longer be current.

Answer: This problem sounds like it is occurring when the torque converter in the transmission is locking up. The torque converter locks up to eliminate any slip. This gives better fuel economy and reduces the heat build up in the transmission.

When the converter locks, an extra load is placed on the engine. This is not noticeable at higher engine speeds, but at low speeds it can cause a shudder. Sometimes manufacturers will modify the engine computer with a special program to delay converter lockup until a higher speed. I couldn’t find reference to one, but the transmission specialist at the Chrysler dealership may be able to access one through their technical assistance.

Another cause of the shudder can be bad spark plugs, plug wires or poor fuel-injector spray patterns. The extra engine load that occurs when the converter locks up places a heavy load on the ignition system. A misfire occurs and we feel it as a shudder. Bad injectors or a small throttle opening leans the fuel mixture and misfires occur more often.

Changing spark plugs and testing the plug wires for high resistance is a start. If new plugs don’t fix the problem, then the fuel mixture should be monitored with a diagnostic computer scan tool while the vehicle is being driven. A lean mixture at the shudder point would indicate a fuel-delivery problem.

Question: I own a 2003 Toyota Echo. When putting gas in it, the gas nozzle kicks off after only a small quantity of gas has gone in and keeps doing it until the tank is filled — which takes quite awhile. When the gas cap was first removed to gas up, air sucked into the tank. I pushed a wire down the gas-fill pipe and it seemed to go all the way to the tank. Then a service technician looked at it, blew compressed air down the gas-fill pipe and the problem stopped. However, after about a month the problem is back again. What could cause this, or is it a case of sabotage (somebody putting something into the fill pipe)?

Answer: Nobody is tampering with your car. The fuel tank vent is not working properly and when you fill the fuel tank there is no place for the air to escape the tank. The filler pipe is designed to block air from leaving the tank through the filler pipe when fuel is going through it. The air inside the tank must then go out the vent hose, through a two-way check valve and into the charcoal canister where the fumes are stored until the engine is started.

The charcoal canister also lets air back into the tank as fuel is being taken out by the fuel pump. A small rush of air going into the tank when you remove the fill cap is normal because the check valve in the vent pipe does cause a slight vacuum in the tank to reduce fuel vapour release to the atmosphere. Fix the vent and you should be filling up quickly again.

james.kerr@sasltel.net