Loud Knocking Noise Coming From Rear of Engine...

[9dawgs]

Hi Guys, been a while since I posted... Anyhow, my van has developed a loud knocking noise near where the tranny & the engine mate up. A mech friend of mine suggested the flywheel was cracked. The noise is sometimes loud and sometimes soft. Sometimes it even goes away completly when the van has been driven a bit. I'm trying not to drive it though.

My first thought was something catastrophic had happened but the engine is running well. Anyone think the oil pickup may have come loose in the oil pan and is hitting on the crank? I'd much rather drop the pan than the tranny. Any suggestions? HELP!

[SoCalli V8]

As an automotive technician I have seen flex-plates crack around the mounting bolts that secure the plate to the crankshaft, but only on 5.7 chevy motors never yet on a 4.3 V6, but it is a Chevy trait.

I would recommend getting the van up on a hoist and listen with a mechanics stethoscope, do you have the cover over the torque convertor? if so remove it first so that you can listen up in there. It will be a very metallic sounding noise.

Here is my modified mechanics stethoscope, it began as a Craftsman unit then I fitted a length of 3/8 pipe and it works very well, great for diagnosing noises.

[dunedog]

rear main bearing
How many miles on the engine?(140,000)

[GEJ]

Working as a engine tester yrs ago at a Chevy plant we had what was called a sounding bar.(kind of like what Calli was talking about.)It was a long pc of rod with a wooden handle on one end.We would "sound-out" a engine on a test stand.They are cheap and easy to make.

[kings-x59]

My former father in law used a piece of wood dowel rod 1/4 or 3/8". put one end on the engine, put the other end against the bone behind your ear, transmits sound very well.

[dunedog]

Jimbo the suspense is killin' me

[9dawgs]

Jimbo the suspense is killin' me

Sorry man, I haven't had time to look at it yet. I'll be climbing back under there hopefully this weekend though and let everyone know what I find. Heck I may even take it to my friend to see if he can diagnose it. I'm just wrapped around the axle at work lately... That's why I haven't been posting much.

It's frustrating too because now that the weather is getting cooler here in the swamp, I want to get down to the river to camp and canoe

[9dawgs]

rear main bearing
How many miles on the engine?(140,000)

yup...142,487 to be exact... So does this mean since I've gone past 140,000 then it's NOT the rear main but something simpler?? woohoo?!

No, I really am kidding myself.... Just wishful thinking I believe... replacing the rear main would absolutley be tragic at this time of year because humm, lets see, there is hunting, fishing, camping, fishing, hunting, camping, hunting...did I leave anything out??? Oh, and hunting.

[dunedog]

Well.......it's just that ,,,,,,I had the engine on mine go @ 136,000 a year ago this weekend. (but it climbed Mt Washington and got me home from Maine) and the high top sitting in my yard with a rapping motor has almost exactly the sames miles on it too![ '99 and '97 respectively]

[kc5gxl]

My my my!

All of the answers so far may or may not be the culprit. It may even be a bad lifter or rocker arm. Get a stethoscope and listen to the engine/trans to isolate where the noise is coming from. The hearing device can be as simple as a piece of vacuum hose, a screw driver, wooden dowel, or as ornate as an engine ear. The noise could be a multitude of things; from the flexplate being cracked, to a rear main brg, to a torque converter attaching bolt, a broken crankshaft, to . . . yada, yada, yada.

What is the oil pressure? Is it leaking oil out of the rear main seal, or into the bell housing? If you are going to have the van running, it might be a bit safer to have it up on a lift. You can listen to the entire underside of the vehicle that way. Do the simple things first. It doesn't take long to remove the inspection cover to check the flexplate area.

Good luck, and let us know what you find.

Troubleshoot safely,

Dan Harriman 91 Safari shorty
Orange, Texas

[astro355]

I just fixed this problem on my van. Cracked flexplate. Only thing is that a have a 355 in mine, not a 4.3L. But yeah, its was really shitty.

[Moreforles]

check to see if the dust cover is in place..... had a full sized that made a noise like the one you were discribing, and found on closer inspection that the dust cover had lost a couple bolts and was flapping and bouncing around some of the time.... replaced the bolts and all better.

I've always been told to start with the cheapist fix and work your way up.... doesnt get cheaper than a couple bolts.

[9dawgs]

Yeah Kc5gxl, that is one thing that's puzzling me...if the rear main spun then it would probably take out the rear main seal, right? The back of the engine is dry, no oil what so ever, inside the inspection cover is dry as well.

Hey Astro355 I checked the flexplate and flywheel as best I could with a flashlight but I think I just need to get under there and look really good with a small mirror or something. How did you come to the conclusion that yours was cracked? And when you removed it was it a hairline or something you could see easy. Did it start from the center of the flywheel ? If that's it it will be super hard to see from the small inspection hole.

Thanks for all the input here. I'll get under there this weekend with a "listening" device to try to pin point the location. I'll let you guys know what I find.

[9dawgs]

I've always been told to start with the cheapist fix and work your way up.... doesnt get cheaper than a couple bolts.

Amen to that brother! I'm all about the cheaper way to go

[astro355]

Hey Astro355 I checked the flexplate and flywheel as best I could with a flashlight but I think I just need to get under there and look really good with a small mirror or something. How did you come to the conclusion that yours was cracked? And when you removed it was it a hairline or something you could see easy. Did it start from the center of the flywheel ? If that's it it will be super hard to see from the small inspection hole.

I figured it was the flexplate by removing the torque convertor bolts and running the engine. The noise was gone. I had to drop the trans to get to the flexplate. Made sure the flexplate bolts were really tight before I removed it. If the flexplate or torque convertor bolts are loose, it will make the same kind of noise. The hair line crack was towards the center and I could not see it until after I removed it. And not trying to be anal, but automatics have flexplates, manuals have flywheels.