Sounds like bearing knock, but good oil pressure

[vetteboy86]

So what gives. Everyone that has heard the car run says yes it sounds like low end noise. It sounds just like vettefans car when his bearings went south. However with his car, he had very low oil pressure. Mine is still very good.

Cold oil on start up fast idle the oil pressure was 70-72 psi. It dropped very little, the coolant was 167 when I shut it off, and the oil pressure was still 63 or so.

What do you all think?

Craig

 

[Rufus1050]

Craig, if you have an auto transmission...it could be the flywheel. My 94 Ford developed a bad knock and you'd swear it was a rod. Crawled underneath and use the long screwdriver technique and sounded the block, pan and transmission. It was the flywheel that had cracked all around where it bolts to the crank. Check it out.

 

[vetteboy86]

Thanks for the suggestion. I bought one of those mechanics stethoscopes. Where are the best areas under the car to check. I have noticed that some noises travel through motor.

Craig

 

[AdvancedAutoCC]

A rod knock will be a sharp "flat" sounding metallic sound as opposed to lifters and or pushrods that will "tick". A main knocking (rare on L98) will have a deeper tone than a rod. A broken flywheel will sound very similar to a rod knocking.

To determine if indeed it is a rod, you need to short each individual cylinder while the engine is running. If the sound stops when a particular cylinder is shorted, you've found the offending rod. How you short cylinders... either pull off a plug wire (at the distributor end - not really recommended), or take a good test light (Snap-On preferably...not a Pep Boys or Auto Zone cheapie..they're not insulated) and pierce the plug wire end, again at the distributor.
The best way is to get a cylinder shorting "engine analyzer" (tach/dwell meter...real old school stuff) and short them that way. You can usually find one of these on eBay. I have a Mac ET985 that I purchased new back in '85..it's a good all around unit to have if you have an older car.

Mechanics stethescopes are good tools to have, but hard to pin down rod knocks with, as the noise travels through the engine block. Try placing it on the rocker covers to make certain it's not in the valvetrain. Otherwise place it in various spots around on the block, but as I've said..it's hard to pin down a rod knock in this manner.

good luck to you regardless...

 

[tkrphxr]

I think I ran across a harmonic balancer working loose or coming apart sounding like a con rod.:confused

 

[AdvancedAutoCC]

I think I ran across a harmonic balancer working loose or coming apart sounding like a con rod.:confused

Close.... if the crank pulley bolts were loose, you'd get a knocking noise...

 

[bobmook]

on a jaguar xjs I had I had what I thought was a knock. Some of the transmission bolts had work loose and the transmission and engine were slapping to-gether.

 

[maxrevs85]

A main doesnt knock , a rod knocks!!.....bad mains will show low oil pressure a rod wont .

 

[G Winter]

With that small of an oil pressure drop it makes me believe it is not a bearing.
Has the knock gotten any louder? Flexplate brakage on the automatics is quite common in GM cars. I have seen many of them over the yrs. Usually at first they sound like a lifter tick and progress to the knock. Heres hoping your lucky and that is what it is.

 

[vetteboy86]

It is louder at start up. Once the car runs a bit it will get quieter, but I can still here it.

Gwinter,
How do I check for this?

 

[AdvancedAutoCC]

A main doesnt knock , a rod knocks!!.....bad mains will show low oil pressure a rod wont .

Oh? I've personally heard (and verified by disassembly) two in my time in the trade (20+ years)..coincidentally, they were both Fords As I said, extremely rare, but it does occur. You are correct about oil pressure.


As to how to verify flexplate breakage, you have to get the car where you can get at the transmission, remove the torque converter dust shield and with a light visually inspect the flexplate for cracks. You can use a prybar to roll the engine over, but a flywheel turner is the best tool to use.

 

[DAVZKAR]

I had a flex plate crack like that on my 77 TA after It was highly modified. I was sitting at a light next to a Chevette and thought "Man his car is nocking" I was wrong it wqs me. But like you I had great oil pressure. My Pontiac buddy at work was stumped . But one of the ole timers said sounds like a cracked flex plate. He was right on it was cracked through all the crank shaft holes but one.

 

[vetteboy86]

And this noise sounded just like a bearing knock?

If I hold the brake and put the engine under a load, it really knocks bad.

 

[DAVZKAR]

Actually more metallic it was louder sounding. Hold a common screw driver to the oil pan with engine running trans in park and listen through the handle you should be able to tell if it is in the engine versus just tellographing trough the pan. Plus noise should change with engine running in park versus at idle in drive. Good oil pressure is a good sign. Sometimes if a flex plate is improperly torqued or flexed during engine installation it may crack soon after.

 

[DAVZKAR]

It is a little harder to change the flex plate in a C4 than in my old TA. But it can be done you just do not have as much room to move the tranny back. All you need is enough room to get a wrench to take the bolts out.

 

[Mad-Mic]

might wanna check the waterpump also. this is a common thing once the bearings start to seize up. sounds just like rod knock.

in your case though since you put the car under load and it gets louder i am almost willing to bet it's a cracked flywheel. get it checked out. dont want that puppy detatching itself and launching up thru your dash. (it can happen) i've cracked one or should i say shattered one in 3 places. when my broke it really BROKE and blew the trans pump this was in my 82 Z/28.

i seriously doubt it's a rod knock at 50k in miles. start with the process of elimination. start with the smaller stuff first and check those out before you jump to conclussions that your motor is letting loose.

or you can do what i would and go flog on it till it does break then you will know what needs to really be fixed :gap

 

[C4Tom]

or you can do what i would and go flog on it till it does break then you will know what needs to really be fixed :gap

Wow... you do that too???

Jason, I would have to agree with the others, I had loose flexplate bolts on a car once that sounded just like rod knock.

 

[Ken]

I had loose flexplate bolts on a car once that sounded just like rod knock.

Just as I once thought my pre-cats, which were going bad, sounded like a rod knock to me when they rattled. It sure didn't sound like what it actually was.