Rod knock or Weak Motor Mount ?

[Landyacht]

I have been dealing with what I thought was a detonation issue up until recently when the noise got louder. My motor would seem to make a knocking sound at around 2500 RPM and then go away as engine speed increased or decreased so I didn't suspect it to be a rod knock. I tried playing with timing and did some carb tuning to try to get rid of the problem but was unsuccessful . I eventually switched over to fuel injection which made the car run way better but didn't cure that knock I heard at 2500 RPM. A couple of days ago I noticed the sound getting much worse. It can now be heard at all engine speeds. It started sounding more and more like a rod knock. Today I got the car up on jack stands and dropped the pan to take a look and at first glance everything seemed to look normal. No metal shavings in the oil pan , no discoloration caused by heat on the piston rods. I could still see the hone marks on the cylinder walls. I checked the torque convertor bolts to make sure they were tight. So far I have only opened up one of the piston rods and the bearing looked to be in good shape. the motor has less than ten thousand miles on it and everything still looks clean under there. One thing I did observe is that the motor seemed to shake a lot when I revved it trying to locate the source of the sound. I'm wondering If my problem could be related to weak motor mounts.

 

[goodneighbour]

Broken flex plate/starter ring gear?

 

[furyus 67]

Put it on jacke stands,block the tires, put a medium screwdriver tip to the bottom of the oil pan and your ear on the other end(handle) while its running. If it's a rod you will hear it loud and clear!!! Or go to harbor freight and get auto stethoscope to use in place of the screw driver.

 

[furyus 67]

Never mind. I should have finished reading your post. I see your pan is already off.

 

[Landyacht]

Never mind. I should have finished reading your post. I see your pan is already off.

Yeah, It was a ***** dropping the pan but I have too much money tied up in this motor to take any chances. Have a spare set of bearings on hand. Not quite sure if I'm gonna check the old ones with plastigage to see if their good or just replace em .

 

[furyus 67]

When you say weak motor mount, what do you mean? I mean I think bad mount and engine twisting and causing the fan to contact the fan shroud. How else could a weak motor mount cause a knock?

 

[Landyacht]

When you say weak motor mount, what do you mean? I mean I think bad mount and engine twisting and causing the fan to contact the fan shroud. How else could a weak motor mount cause a knock?

I'm not quite sure. It sounds internal . The motor shakes like its out of balance. The motor has a fairly big cam in it and it has always caused the entire car to shake at idle.

 

[Landyacht]

Broken flex plate/starter ring gear?

Its possible something could be out of balance. I did check the torque convertor bolts and looked over the flex plate and the vibration damper on the front of the motor but it wouldn't hurt to check it a second time.

 

[commando1]

I'd give a million dollars to hear what you're hearing.

I'm throwing in bent pushrod.

 

[1970cat]

cam lobe? bad valve?

 

[70bigblockdodge]

What year engine, external balance?

 

[Landyacht]

What year engine, external balance?

The Block is a 73 , 400. I am running a stroker kit from 440 Source. Forged crank internally balanced. I am using a vibration damper that is meant for internal balance motors which I also purchased from 440 Source. The torque convertor has no balance weights.

 

[70bigblockdodge]

Probably should just pull the rod caps to make sure since pan is off. Could be a piston pin or cracked skirt, assuming you know its not a valve train noise hard to tell without hearing it.

 

[Landyacht]

Probably should just pull the rod caps to make sure since pan is off. Could be a piston pin or cracked skirt, assuming you know its not a valve train noise hard to tell without hearing it.

Sounds like a game plan. I'm going to a local speed shop today and picking up some plastigage. I'm also gonna see about getting some polyurethane mounts since I have to raise the motor to get the pan back in. Might as well throw some new motor mounts in to eliminate the possibility . If that doesn't work then the next step will be to pull the intake and valve covers and inspect the valvetrain .

 

[70bigblockdodge]

If it is a rod bearing you will most likely be able to pick it out from the others will be well into the copper or through it. This is most likely the result of poor QA in the machining process and the journal is not straight across causing uneven bearing load making it wear prematurely. I will leave it there you can insert your own story on how Jaun Dong Li does not like his communist provided job and does not dress his grinding wheel on the machine often enough.
Good luck sorry for the bad story.

 

[bandsdean]

I had a bad motor mount and it gouged up the upper fan shroud.

 

[70bigblockdodge]

I had a bad motor mount and it put a dent in my hood above the carb stud, did not sound like a rod knock

 

[Landyacht]

If it is a rod bearing you will most likely be able to pick it out from the others will be well into the copper or through it. This is most likely the result of poor QA in the machining process and the journal is not straight across causing uneven bearing load making it wear prematurely. I will leave it there you can insert your own story on how Jaun Dong Li does not like his communist provided job and does not dress his grinding wheel on the machine often enough.

Bad rod bearing is confirmed. I was pulling the bearing caps and got to the number 4 piston rod it was definitely shot without a doubt. The journal has a small scratch in it. Im not sure if emory cloth will clean it up or not. I guess for now ill just throw in a new bearing and hope it gets me through the next few months. I definitely plan on pulling the motor when winter comes around and having the crank checked out.

 

[commando1]

Any chance of a pic of the bad bearing before it goes to the trash?

 

[Landyacht]

The Chinese crank may have failed but the Mexican made rods and piston seem to be holding up pretty good.