My 84 L98 all of sudden started knocking when cold or hot. I've used Mobil 1 10-30 in it since I bought it 4 years ago with no problems. I went thru the shop manual procedures for checking knock conditions and found no problems with broken pullys and harmonic balancer. Using a stethoscope it appears the noise is coming from the #1 cylinder valve train. Should I pull the valve cover and check for a broken valve spring or ? The car has 85,000 miles.

 

Sure, maybe the rocker arm nut cam loose.

 

I think the 84 was an L83. Anyway, a knock usually comes from the center or lower part of the motor. use a stethescope to diagnose. Hopefully, you are hearing a tapping noise which could indicate a problem with the valvetrain; rocker arm, bad lifter, flat cam, etc. which is far better than rod or main bearings. Good luck.

 

Great point...

too often the termanology determines the diagnosis and thats where it goes south...

A "knock" is NOT a tic, tap, rattle, or vague noise that comes from the top end or when accellerating. Clatter is another confusing term...

Knocking is 1 or 2 things...

Knock can be described from low octane fuel, poor tune or bad ign parts that makes the engine suffer incomplete combustion which causes a distinct "knock" as you give it throttle or when it gets hot..

OR

the most feared of all "knock"..
that comes from a bad bearing or rod big end that is knocking from metal beating against metal in bearing surfaces or when a bearing has slipped or failed. Small end rod issues can better be described as a rapping noise. These bearing knocks will be there with every cycle, at idle and worse with rpm, and they will never go away until the worn or damaged part exits the engine somehow...


So, when someone describes a "knocking" noise...we really gotta clarify what it is they are hearing. Otherwise, the wrong info gets put out there and possibly a simply issue gets mistakenly turned into a major deal. This case, a knocking or rattling in the top end of #1 sounds more like a rocker or valve...

 

Yes it is a L83. I did some troubleshootin, took the car on the freeway and got the oil temperature up to 185 deg F and it started knocking lightly, when the temp reached 201 it was knocking pretty loud. Took it back to the garage and with a stethoscope listened to the valve train on both banks and they sounded equal in noise from the lifters, nothing loud. Reved the engine in Park to a constant 2000 RPM and the knocking was quite loud from the oil pan area. Sounds like bottom end issues, what do you think?

 

Sounds bad. Cut open the oil filter and see what's inside.

 

I think at a minimum you'll be installing new rod bearings

 

Does the "knock" get faster as rpms increase?

 

With 383 Vette

If its valvetrain it will make the noise at half the crank speed.
Sounds like trouble downstairs. Unfortunate but everything wears out eventually even with good oils.