Thread: Engine "Knock"

Hello all...

I've recently purchased a Grand Prix GT (2003 w/87000 mi.).
Since I have changed the oil (Royal Purple Synthetic 5w-30), I have noticed a knock coming from what appears to be the botton end of the motor, hard to tell exactly. The knock is not rythmic (it comes and goes only at idle). If I raised the RPM's the knock goes away.
The oil looks good (clean), but I'm not sure on the oil pressure, I assume its ok...

I have read that there is some piston slap with these motors, but this sounds like lower end. I'm wondering if the previous owner put something in the their last oil change to hide this.

Any suggestions?


Thanks much in advance...

Sounds like a spun rod bearing.

Originally Posted by pbxcomm

I have noticed a knock coming from what appears to be the botton end of the motor, hard to tell exactly. The knock is not rythmic (it comes and goes only at idle). If I raised the RPM's the knock goes away.

Actually, this sounds exactly like a spun rod bearing (commonly referred to as a rod knock) in its beginning stages.

Take a video and post it.

Agreed. In my experience, a spun bearing knocking sound goes away once the RPMs are increased. Once the bearing wears more and more, the knock gets louder and louder and eventually will never go away.

Thanks to all for your replies...

Unfortunatley, my Blackberry's media card slot quit so I can't take a video right now but will send one shortly.
I hope this is not what I (we) think it is.

Be back soon...

Thanks again.

Hi,
No milky oil and oil level has been at the line for a few months now.
I had one of the local mechanics (reputable) listen to it. He says its too fant to make a determaination, however, the engine was not fully warm, about 150 degrees. This knock only occurs at idle when the engine is at or about 190 degrees (I have a new DELCO OEM 195 degree thermostat in it now). When cold, there is no knock whatsoever. also, the knock is not audible when I raise the idle (say about 1200 RPM), then it comes back at idle. I'm going to guess: I need to change the plug wires and plugs (all original), I have read there is piton slap in these motors and wondering if piston slap gets louder if a cylinder is not getting efficient spark/combustion (my best guess), hoping its not a bearing or rod. Still working on a video/sound recording. Also i have been running 93, I'm going to drop down to 89.

Thanks for the replies!

Finally! I upgraded my Blackberry.... Ok, thanks again for all of your replies....

This "knock" has not been any louder since I posted a few months ago, I cam hear it @ idle, then when I raise the idle past 1000RPM, I can't hear it any longer as you will note in the video. The oil is clean (I'm using 10w x 30 Royal Purple w/Wix filter).
The engine runs very smooth, fuel: between 90 and 93 Octane. However, when I'm next to a vehicle or a retaining wall, I think I hear pinging when I accelerate, it almost sounds like I'm running way too low octane (which obviously I am not).. By the way, the "knock" is not noticible when the engine is cold.

Have you removed the drive belt and checked all the pulleys for any play in them, or tightness? I'm wondering if you have a bearing going in something (besides the rods). Could be the A/C or waterpump.

Do you have any way to get a mech. oil pressure gauge on it to see what you are running? Maybe you're oil pressure is getting low when its fully hot and letting it knock a bit, then when you raise the RPMs it goes up and the knock subsides. Just throwin ideas out there.

Ok, could really use some help here....
We took a few stethoscopes to the are we beleive the sound was coming from. The sound is loudest at the oil pan where the motor meets the bell housing. We took off the inspection plate, checked the flywheel, all bolts tight, we discovered that a new torque converter had been installed by the previous owner. Thats said, it appears that the "knock" is more centralized in the oil pan. We installed a mechanical oil pressure gauge and the oil pressure is normal @ idle and above 30 - 40 psi.

We are running out of ideas, but we were thinking of dropping the oil pan. Has anyone here dropped a pan on a 2003 Grand Prix GT here? Would we need to support the motor?


Any help, greatly appreciated.

I've dropped the pan on my 00 GT and it's not fun but can be done. Yes, you need to remove the motor mount (pass. side) and the dogbones up top to raise the motor up. I stuck a 2x4 on the balancer and gently jacked it up in order to clear the mount and get to the pan bolts. You have to remove the oil level sender from the front of the pan as well otherwise it may get hung up on the oil pickup tube inside the pan (don't ask, lol). You should then be able to access all the pan bolts and get it off the motor.

I didn't support the motor other than leaving the 2x4 under the balancer and being extremely careful with how hard I was wrenching on it.

could have added gear oil that may have stopped the knocking for a little while, maybe?