Question about 1998 GMC Yukon

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1998 Yukon high idle sound with bad gas mileage.

1998 GMC Yukon, 5.7 liter, 135k miles.

Transmission seems to shift fine in all gears, but when driving down the road it seems to drop in power and it sounds like it is kicking up the rpm's but the tach shows no change at all. It will go in and out like that for the time I am driving it. Fuel consumption increases dramatically, used 1/4 tank to go 40 miles. Does not drop out of gear at all and there is no check engine light coming on but I know there is a problem. Crank position sensor maybe, transmission, vacuum leak, plugs and wires, fuel filter? But again no light on. I am stumped...

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1 Answer

What you have is the torque converter clutch,cycling on and off.
When it is off.your rpm will go up and your milage will go down
First-check to see if your brake switch, on the back of the brake peddle, is loose or misadjusted. When you apply the brakes,the first thing to happen, is that you unlock the converter,so your trans is not connected directly to the motor,or you would stall. Then below 42 MPH,your trans is working off the converter, as a pressure multiplier ,based on rpm
Second- switch okey. You remove trans pan and change TCC Solenoid.

Let me know if if you need more guidance

Posted on May 08, 2010

Testimonial: "Thank you for the quick response. Figured it to be something with the trans. Gives me a good place to start and a good reason to clean out the other stall in my garage to work on it. Thanks again!!"

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