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Discussion Starter #1
Hello everyone.... New guy here with a few question please. Just picked up a high mileage (175k) 08 Crew Z71. Need to work out all the used vehicle bugs. The main issue I am having is at 45-50 mph under light throttle the trans goes in to top gear and the rpms are really low about 1500. For a brief period there will be a metal rattle sound or maybe grind heard and felt mildly from the floorboard. It will go away if throttle is pushed and the rpm's go up or if ground speed is raised to about 55 mph. Really don't feel like a loose component, feels drive-train related. Maybe, this is a common occurrence with these vehicles? Hopefully, it is not a torque converter or transmission related. Other than that it shifts quick and is very smooth.
Second question, not as important. The coolant is yellow. Should I flush it out and replace with dex-cool or just leave it alone?
Thank you all for the help.
 

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When you say "top gear" I think you might be feeling the "torque converter lock-up" which our trucks have. If you turn off the radio and pay attention when you're driving your truck, you will feel 1-2, 2-3, 3-4 shifts and then you will also feel what is like another gear shift. It's the torque converter "lock-up" which is designed to remove any slippage in the torque converter. It's designed to lockup at about 50 MPH an disengage at about 45 MPH. It also engages and disengages when under load or accelerating. A good scan tool that provides live data and provides transmission codes, will be able to let you know if there's a problem. It works off fluid pressure and I think is the earliest indication of low fluid level. Since you just bought the truck and probably have no idea what the past service records are, I'd change the transmission fluid and see if that helps at all.
As far as the coolant goes, the previous owner may have replaced a hose, water pump or thermostat and just grabbed whatever coolant he had and put in there. The only problem I see with using either of these coolants is that they don't "play well" together. If you have any idea that you might have the two mixed together, I'd drain, flush and refill with whichever you like the most.
When I purchased my truck at about 180,000 miles, I assumed that no real service had ever taken place an I changed the oil, transmission fluid, rear differential, front differential an transfer case fluid, replaced fuel filter, air filter and spark plugs. That was a pretty expensive trip to the parts store that day, but I wanted to make sure I was giving this use truck the best chance I could to be dependable and trouble-free. It also gave me a great opportunity to become familiar with the truck and where everything was and how it was all put together.
Change that transmission fluid and see how that works. In the "how-to" section here you'll find tons of information on how to do all the things I've suggested if you're not mechanically inclined. Also, there's very little that you'll experience with your truck that we have not already "been there" and found out how to fix it, so let us help when you need it. You've found a very valuable "support team" here an we like the help!
Welcome!!
 

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Discussion Starter #3
Thank you OldGuy for your support. You were correct, it happens when the converter locks and the rpm's are about 1500 at about 45-50 mph and a demand to accelerate. If the rig is going faster say 55mph it will not happen. Hooked up a scanner and there are no codes. Checked the fluid level and it is full. While the scanner was hooked up I did notice the trans fluid temp stays at 32 degrees. Also, I never get an egr command. The egr stays at 0 duty cycle and never opens. Not sure if there is a trans temp sensor or an egr and my scanner just has it on the screen as a default? Don't want to change the fluid if I have to replace the converter. Thank you again for the help.
 

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Discussion Starter #5
Thank you for you support. I believe my problem has been found.

 
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