350TBI 1500RPM engine knock

[someotherguy]

I'll "third" the flexplate.. those other 2 dudes beat me to it.

If you have the old style stamped steel inspection cover, you can remove it, then use your choice of tools (channel locks, vise grips, or an actual flywheel tool) to turn the flywheel around and remove the torque converter bolts, and push the converter back towards the transmission. That *might* give you enough gap so that the converter doesn't interfere with you ringing the flywheel with a wrench to see if it actually makes a "ring"-ish noise, or sounds dull and dead. A broken one will generally not "ring"...

They just about always crack completely around the crank bolt flange so it's very difficult if not impossible to SEE the crack without removing the transmission.

Mind you it's been probably 20 years since I battled one so my memory could be really fuzzy on all of the above details, so perhaps others could clarify a bit for you.

Richard

 

[Andrew Barker]

I'll "third" the flexplate.. those other 2 dudes beat me to it.

If you have the old style stamped steel inspection cover, you can remove it, then use your choice of tools (channel locks, vise grips, or an actual flywheel tool) to turn the flywheel around and remove the torque converter bolts, and push the converter back towards the transmission. That *might* give you enough gap so that the converter doesn't interfere with you ringing the flywheel with a wrench to see if it actually makes a "ring"-ish noise, or sounds dull and dead. A broken one will generally not "ring"...

They just about always crack completely around the crank bolt flange so it's very difficult if not impossible to SEE the crack without removing the transmission.

Mind you it's been probably 20 years since I battled one so my memory could be really fuzzy on all of the above details, so perhaps others could clarify a bit for you.

Richard

It would makes sense that it’s the flywheel, because I don’t remember hearing the noise until the shop had the car and rebuilt the transmission. Does it makes sense that it makes just at that rpm?

 

[someotherguy]

Tough call but it's worth a look. In my experience, which hasn't been many of them, they tended to knock at cold start and had me fooled thinking they were a rod knock. Once the engine was up to normal idle they were quiet, and once warm, the gap must have been closed enough that it didn't knock again until the next cold start. Went to investigate and found the flexplate had cracked literally all the way around and came off in two pieces - crank flange, and the remainder.

Richard

 

[Andrew Barker]

Tough call but it's worth a look. In my experience, which hasn't been many of them, they tended to knock at cold start and had me fooled thinking they were a rod knock. Once the engine was up to normal idle they were quiet, and once warm, the gap must have been closed enough that it didn't knock again until the next cold start. Went to investigate and found the flexplate had cracked literally all the way around and came off in two pieces - crank flange, and the remainder.

Richard

Okay. I’m going to out the car on a lift and see if I can figure it out asap

 

[thinger2]

I also had a wierd one several years ago that I though was a random rod knock.
It turned out to be one leg of the starter bendix fork had broken off from being installed backwards and the bendix spring was pushing the gear out so it bounced off of the flywheel.
You might want to start with a cheapo mechanics stethoscope and try to narrow down if the noise is from one specific area or throughout the whole block.
Since then ive seen many starters with that fork in backwards.
I always look at them first because it can take a long time for that fork to break, sometimes they just crack and bend

 

[thinger2]

Dug the old part out of The aftermarket hall of shame.
This solenoid actually bound up in a way where it would still work but it let the bendix ocasionally bang off of the ring gear.
It sounded just like a rod knock but it didnt make much sense because of how and when it was happening.
I went at it thinking for sure it was flexplate but I found this instead

You must be registered for see images attach

 

[Andrew Barker]

Dug the old part out of The aftermarket hall of shame.
This solenoid actually bound up in a way where it would still work but it let the bendix ocasionally bang off of the ring gear.
It sounded just like a rod knock but it didnt make much sense because of how and when it was happening.
I went at it thinking for sure it was flexplate but I found this instead

You must be registered for see images attach

Dug the old part out of The aftermarket hall of shame.
This solenoid actually bound up in a way where it would still work but it let the bendix ocasionally bang off of the ring gear.
It sounded just like a rod knock but it didnt make much sense because of how and when it was happening.
I went at it thinking for sure it was flexplate but I found this instead

You must be registered for see images attach

Where would I find that piece located

 

[Andrew Barker]

I also had a wierd one several years ago that I though was a random rod knock.
It turned out to be one leg of the starter bendix fork had broken off from being installed backwards and the bendix spring was pushing the gear out so it bounced off of the flywheel.
You might want to start with a cheapo mechanics stethoscope and try to narrow down if the noise is from one specific area or throughout the whole block.
Since then ive seen many starters with that fork in backwards.
I always look at them first because it can take a long time for that fork to break, sometimes they just crack and bend

Also, I can’t use a mechanics stethoscope just because it only makes the noise while driving the car

 

[Schurkey]

Where would I find that piece located

Inside the starter motor assembly.

 

[Supercharged111]

How about a lifter? You said the oil was black. Can you hear it at an idle too? A rod knock doesn't get better with a warm engine so that seems unlikely IMO.