Rick's Free Auto Repair Advice

Transmission slips – GMC Safari

Transmission slips – GMC Safari

Rough engine and slipping transmission on 2004 GMC Safari van with the 4.3L V-6 engine.

Before you even think of adding anything to your engine or transmission, clean your mass airflow sensor (MAF) sensor. It’s one of the known causes of engine and transmission issues.
The MAF sensor is located in the air duct between the air filter box and the throttle body. The sensor contains heated wire elements that are cooled by the inrush of fresh air. The sensor measures how much power is required to keep the wires at a set temperature. The MAF tells the PCM not only how much air is passing by the MAF, but also the air’s density (mass). Once the PCM knows mass, it can calculate the correct amount of fuel.
Engineers know that fine particles can pass through an air filter. So MAF sensors are designed to conduct a self-cleaning procedure at regular intervals. It’s similar to a self-cleaning oven, where the wire elements heat up and literally burn off any contaminants. However, in some cases, instead of burning off the contaminants, the self-cleaning procedure just bakes it. So the wire elements wind up with a “jacket” of insulating crud. The insulation tricks the MAF into reporting less airflow than is actually moving across the elements. So the PCM calculates an air/fuel mixture that’s too lean. And that can cause a rough engine.
Here’s where the story takes a twist. In addition to using MAF data to calculate fuel mixtures, GM also uses MAF information to command transmission line pressure changes. For example, if the sensor is clean when you stomp on the accelerator, the PCM sees a huge inrush of air, figures that you’re going for pedal-to-the-metal acceleration, and boosts transmission line pressure to lock the transmission clutch packs and prevent slipping. But if the MAF is contaminated and the PCM thinks there’s less air than is actually moving into the engine, it reduces transmission line pressure and the clutch packs slip. That’s why I recommend cleaning the MAF as your first step.
Here’s how to clean a MAF sensor. Purchase a can of aerosol MAF Sensor Cleaner at any auto parts store. Disconnect the electrical connector to the MAF (engine off) and loosen the clamps holding the sensor in place. Remove the MAF from the air duct. Spray the cleaner directly at the wire elements, soaking them completely. Let the wires soak for the recommended time and repeat the procedure. Do NOT attempt to clean the wires with a toothbrush or cotton swab. They are delicate and break easily. Let the unit air dry and then re-install it. If the cleaning helps, but doesn’t eliminate the problem, replace the sensor. If the transmission still slips and the engine doesn’t run smoother, the MAF probably isn’t the cause. Take it to a shop and have them diagnose the problem. Continuing to drive a vehicle with a slipping transmission can cause extensive damage.

© 2012 Rick Muscoplat

Posted on by Rick Muscoplat

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