Shuddering/Grinding Noise Only When Turning Off Engine and Throttle-Oil Pressure - SOLVED

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Billydunwoody

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Good Morning Ya'll,
08 Tahoe 4WD here with 99k miles and recently I have had two issues I wish anyone can help me out with.
1. Only when turning off the car, there is a shuddering noise. It is not present when starting or during driving, only when I turn the key off. This happens whether the a/c is on or off, it does not matter. This recently began happening.

2. Every now and then usually when idling at a stop light, the idle will drop a bit below 500, the oil pressure will drop a bit below 20 and the car will vibrate. Then the idle and oil pressure immediately comes back up and the vibration stops. There is no leaks, I have no added accessories and this also happens whether the a/c is on or off. Could this be a throttle body? I did clean the throttle body the other day after I felt this but it wasn't dirty at all and the truck has done it once since doing that.

Just as a history, in the last year I have changed with brand new oem parts: alternator, battery, both belts, brakes, control arms, water pump and all fluids. I take care of this car well so I want to see if someone can help me with these issues on my own before brining it to a mechanic.

Thanks!
 

B-train

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Point 1: Based on what it sounds like and comes from I would say it could be a heat shield, a transmission or motor mount going bad?

Point 2: there is another current thread on here about a 2007 Denali that doesn't restart and was acting similar to yours. He ended up changing the oil and running some Lucas stuff in it and it helped. Since yours is 15 years old,I'm assuming the oil pump o-ring is original and allows for air to get sucked into the pump causing cavitation. This will decrease oil pressure ans can mess with the VVT actuators.

I'd try a good oil change first off and see how it acts. Chime back in afterward and we can tell you ALL the stuff you should do based on what you observe. LOL
 

Fless

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#2 sounds like it might be an issue with the Idle Air Control (IAC) on the throttle body.
 
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Billydunwoody

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Point 1: Based on what it sounds like and comes from I would say it could be a heat shield, a transmission or motor mount going bad?

Point 2: there is another current thread on here about a 2007 Denali that doesn't restart and was acting similar to yours. He ended up changing the oil and running some Lucas stuff in it and it helped. Since yours is 15 years old,I'm assuming the oil pump o-ring is original and allows for air to get sucked into the pump causing cavitation. This will decrease oil pressure ans can mess with the VVT actuators.

I'd try a good oil change first off and see how it acts. Chime back in afterward and we can tell you ALL the stuff you should do based on what you observe. LOL
Thank you!
#1 How much labor is it to change bkth motor mounts?
#2 yes you are correct, I occassionally have the oil pressure drop and it’s proly the o-ring. I ran motor flush and changed the oil with good synthetic and its helped. Because its 4WD I know changing the oil pump and o-ring is very labor intensive as the diff is in the way. Also, the o-ring causing air to get in, can it cause catastrophic failures down the road or is it usually just the pressure dropping and coming back up on its own shortly after? I drive it 50mi daily and check the oil regularly with no leaks/loss.

I ordered a new GM throttle body so I will see on Saturday if thats the cause of the throttle issue. It could use it anyways, looked like when i was cleaning it the original one was still on the car and it looks pretty easy to replace.
 

B-train

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1. Motor mounts if you're mechanically inclined will take you a couple hours. There are posts on this site about tips and tricks. Cost of parts is probably around $150 or so. I think?

2. If you are experiencing that kind of oil pressure issue while driving and coming to stop lights, etc I would plan on doing the oil pump o-ring sooner than later. Starving the motor on a regular basis will cause premature wear, or catastrophic depending on the circumstances.

It's a crappy job on your back but can be done. Took me 5 hours in a shop with a concrete floor, some good jack stands, and and rolling floor jack to help with the diff. Much easier if you have access to a lift. Just make sure to buy GM OE parts or good aftermarket like Felpro.
 

Fless

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15 psi or so oil pressure at such a low idle isn't anything I'd worry about, at least not until the idle speed is fixed. I think the low idle is what causes the vibration; these engines don't like to idle that low.
 
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Billydunwoody

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Thank you both! Throttle body getting replaced Saturday and I will change the oil again too while I’m at it. Oil pressure every now and then goes to 15psi only when its really cold but it always stays above 25 when the car is warm and stays that way. Should I change anything else beaides the oil pump amd o-ring when it gets to that point?
 

B-train

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There's not much else under there besides the oil cooler lines, if you have them. You'll know if they are bad because they will be wet near the crimped connections
 
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Billydunwoody

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Wanted to update everyone.
The noise was getting bad so I took it to one of my friend’s friend who works on cars in his garage, he used to be a dealer tech. The following was done:
1. New throttle body
2. Cleaned the intake manifold, there was mud inside lol
3. Replaced oil pickup tube o-ring
4. Replaced oil pan gasket
5. Flushed the engine
Car runs smooth now. Idling at 25psi and goes up to 45/50 when driving. Hasn’t dropped at all even when it was 40 degrees out and it usually had issues during cold starts. He did say I have a small rear main seal leak but not to worry about that right now as it’s very small and that’s a tough job
 

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