Sc400 Engine Knock Diagnosis

[budster1]

My 1995 SC400 recently developed an odd sound. It is a knock that occurs only when the car is in drive, at rest with full brake on. As the RPM is raised to about 800-850 with full brake on, this sound goes away. The knock is not heard while idling in park or neutral, or while driving, even under load conditions. It is not very loud (the surrounding area must be quiet to hear this), although it is definitely present. I have changed oil and filter using Toyota parts and flushed trans using T-4 fluid (not knowing where the sound is coming from). The sound seems to be more noticeable on the passenger side of the engine and seems "deep". The vehicle has 125K and is driven mildly. All oil changes at 3K. Being that these engines are supposed to be close to bullet proof, I am rather baffled.

Please help :cries: .

[Sadistic]

Could be a Rod Bearing starting to make noise. I don't think I would call it a Knock. Does it make any odd noise during startup?

[budster1]

Could be a Rod Bearing starting to make noise.  I don't think I would call it a Knock.  Does it make any odd noise during startup?

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Thank you for your reply. There is no noise on startup, or while the engine is warming up. The noise is random and does not always occur under the same circumstances. I do not hear it unless the car has been driven for a few and is completely warm. It does not sound like valve clearance noise as the sound is too much of a knock, not a tap. However, I'm not familar with what a failed lifter would sound like, nor a failed camshaft, or rod bearing or main bearing ... except under very conditions. It disappears when the RPM is raised slightly above idle, in drive, brake on and car not moving. Left off the brake and the car moves forward ... the sound goes away. I have not deterimined if it does it in reverse as well in 1st. I assume it will as the loads on the engine should be similar. I will verify this.

[Sadistic]

Well, from my experience under different load situations, a bearing will or won't make noise. When my Rod Bearing started to make noise, it would do it at idle, but as soon as you put the Engine under load, then it wouldn't make the noise until a certain RPM, which was about 3,400 or so.

Eventually it got bad enough that it would do it all the time. I also had noise at startup, which was a different tone. I'm assuming that was a Main Bearing at startup, and a Rod Bearing at idle, but I really can't be sure about that. Again, not fact, just my experience with a 2JZ-GE.

[budster1]

Well, from my experience under different load situations, a bearing will or won't make noise.  When my Rod Bearing started to make noise, it would do it at idle, but as soon as you put the Engine under load, then it wouldn't make the noise until a certain RPM, which was about 3,400 or so.

Eventually it got bad enough that it would do it all the time.  I also had noise at startup, which was a different tone.  I'm assuming that was a Main Bearing at startup, and a Rod Bearing at idle, but I really can't be sure about that.  Again, not fact, just my experience with a 2JZ-GE.

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This rod bearing you speak of ... is there any way to determine which cylinder is causing the problem? Can one unplug the spark plugs one at a time to establish the affected rod? Once one finds the culprit, can you fix that rod bearing (or is it actually a bushing?) without have to tear everything apart? I.E. can you remove the pan, disconnect the rod, push the rod up the cylinder a notch to gain clearance and replace the bad components? Or, does the head need to come off and the piston completely out?

BTW, any thoughts as to why this would occur in the 1st place as the engine only has 125K and has been driven mildly. I thought these engines were supposed to last a heck of alot longer than 125K before they started to fail mechanically. What went wrong?