Engine rattles around 2500-3500 rpm under load or cruising

[Ceeker]

Update: characteristic of conn rod bearing rattle


I'm sure this issue has surfaced in the past but I have not found any definitive answers

Note: sound was present pre and post rebuilt head installation.

The motor has approx 380k km on the bottom end. I run 91/94 octane fuel with a chip.

the sound started last fall before storing the car. taking out this spring it began again but seems to be present once the car warms up.

Thinking it was timing belt tensioner issue I had replaced those and the secondary oil pump gear with the updated one.

The top end has been rebuilt. New reground 284/272 camshaft; resurfaced head, reground rockers etc.

Symptoms: under reving in neutral the sound is more or less present. between 2500-3000 rpm for a short time.
no sound under deceleration.

while driving: the sound can be heard under load or hard acceleration again between 2500-3500rpm but tends to lessen or disappear - not sure as the engine sound maybe drowning out the noise.

up until last fall the engine ran great with no noise no loss of power no nothing...

Your competent help would be greatly appreciated.

Many thanks..

[Tzantushka]

I know it sounds odd...
But if you have a stock exhaust try tapping along the length of the exhaust with something to see if it replicates the sound.
I was chasing a rattling sound, thinking it was loose belts or bad ancillary bearings.
And it turned out to be merely a heat shield attached to the exhaust pipe between the cat and manifold.
One of the welds had cracked and was rattling away.

[Ceeker]

Ive confirmed the ooccasional knock noise under idle to be just below the head around cyl no. 2 or 3. Upon further investigation ive concluded its the conn rod bearings.

[Tzantushka]

Yuck - sorry to hear that.

[ADEN]

Ive confirmed the ooccasional knock noise under idle to be just below the head around cyl no. 2 or 3. Upon further investigation ive concluded its the conn rod bearings.

I have the same noise after my diy attempt to service my m20 new bearings, rings, new sleeves and used pistons and rods my mech told me it is probably a piston pin out of specs (noise is coming from piston 5) i know my bad not checking them...But switching from 10W40 to 20W50 almost killed the noise i rarely hear it now.

[Ceeker]

I have the same noise after my diy attempt to service my m20 new bearings, rings, new sleeves and used pistons and rods my mech told me it is probably a piston pin out of specs (noise is coming from piston 5) i know my bad not checking them...But switching from 10W40 to 20W50 almost killed the noise i rarely hear it now.

if you didn't get the rods spec'd it possible one of the rod journals is out of round. Wrist pins aren't common in these motors. But what do I know...? lol it's also good to have the bearing surfaces plastic gauged just incase you need oversized. The other thing to consider is whether you torqued the caps properly. But if the oil is helping and you aren't beating on it on a daily basis it could last another 20 years..lol

I am running 20w50 for a long time. It's just wear and tear and most probably abuse over the years. I will be replacing the conn rod bearings in car; it's about a 6 hour job

[ForcedFirebird]

Do an oil pressure test.

A knock should come from the pan if it's a rod.

We had an m20 that we used for racing. Stock pistons, fresh rings/bearings, fresh head. It always had a tick in it, but never let us down (oil pressure was good). Blew a HG in Atlanta, put a new head on it, and it still ticked. That motor probably saw 6-8 14hr races before it gave up the ghost. It was a team error and we never checked the oil level during the race, 13th hour, a rod came through the side of the block on a long seeping turn. It had 4 quarts in it, we usually ran 6 to help prevent starvation (which ultimately killed it).

One time I had a ticking in the head, turned out one of the bosses for the rocker shafts was oval-ed and the shaft was resonating at certain RPM. Found it with a plastic hammer and tapping on the rocker shaft.

[Ceeker]

If I showed the condition of these rod bearings you'd be amazed the crank journals weren't damaged. remarkable..I just caught in time to prevent catastrophic failure...lol.

[ForcedFirebird]

Bearing knock comes from the crank to bearing clearance being too large. If all the bearings looks perfect, I can assure you that was NOT the noise. Bearing knock means that bearing was wayyyyy to far gone to salvage.

[Ceeker]

Bearing knock comes from the crank to bearing clearance being too large. If all the bearings looks perfect, I can assure you that was NOT the noise. Bearing knock means that bearing was wayyyyy to far gone to salvage.

I worded my findings poorly...I meant to say I was amazed at the damaged bearings and even with that my journals were not damaged in anyway. it was definitely confirmed bearing failure and oil starvation due to a partially clogged oil pump pickup filer.

[Ceeker]

well folks...the con rod bearings were the source of my rattle..I swapped them out and ran the car and she's good. not a job I'd not recommend..lol..doing it on the car on your driveway isn't that glorious...lol