Author Topic: Engine Knock  (Read 3158 times)

Offline tc197

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Engine Knock
« on: September 02, 2017, 10:03:21 PM »
Hey guys, I was driving my 87 GMC V2500 (350 SM465) on the highway earlier this week at about 65-70 mph when all of a sudden it started knocking real bad and losing power. I pulled over and had the truck towed home and now I'm trying to figure out what went wrong.

What I know:
-It starts right up, but has a knock immediately that doesn't go away.
-The knock changes when i push the clutch, I get 3 different sounds when it's not pressed, half pressed, and fully depressed (still knocks for all of them just different pitches/sounds).
-The clutch still works, I can shift in and out of gear fine.
-It seems to rev smooth and doesn't hesitate or stutter, from what I can tell the knock gets drowned out at higher rpms and comes back at idle.
-I haven't driven the truck since it happened, just driveway tests.
-The noise seems to come from bottom rear of the motor/clutch area.
-I checked the oil after it happened and it was on the lower end of the dipstick range, I added a quart to put the oil level back in the middle.
-My coolant was a little low but the truck never smoked or overheated, no smoke coming out of the exhaust.
-I dropped the trans/tcase like 9-12 months ago and replaced the clutch, throwout bearing, flywheel, flex plate, ...maybe more im forgetting and it was my first time doing this specific type of work.

Not knowing what I should do next, my plan was to drop the trans/tcase and everything again and start there. Is there anything I should check before doing this? Any thoughts would be greatly appreciated.

Offline FlatBlack77

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Re: Engine Knock
« Reply #1 on: September 03, 2017, 06:10:42 AM »
How high was the engine revving that those speeds?

Use a mechanics stethoscope to find the source of the knock. if you dont have one used a screw driver. put the end of the handle to your ear and put the tip against everything till you find the source. or you cab use your hand too just feel around 
"When you are a hammer, everything looks like a nail"
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Offline tc197

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Re: Engine Knock
« Reply #2 on: September 03, 2017, 10:42:15 PM »
Unfortunately I don't have a tach so I can't say for sure. I also don't have a mechanics stethoscope but did use a piece of pvc to try and hear a difference. It was loudest between the trans and motor section and I could see/feel the knocking most noticeably around there.

Offline tc197

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Re: Engine Knock
« Reply #3 on: September 23, 2017, 04:55:45 PM »
Update:
I was finally able to drop the trans today. I was hoping to find something obviously broken but I'm not seeing anything out of the ordinary. All the bolts are tight, nothings cracked, im not sure what to do next. One thing I did notice is that when I grab the flywheel I can rock it slightly up down. Should there be any play?
« Last Edit: September 23, 2017, 05:01:15 PM by tc197 »

Offline VileZambonie

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Re: Engine Knock
« Reply #4 on: September 23, 2017, 05:37:07 PM »
If you can move your flywheel like that by hand it is either loose, broken or your crankshaft is about to launch into space.
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74 GMC, 75 K5, 84 GMC, 85 K20, 86 k20, 79 K10

Offline tc197

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Re: Engine Knock
« Reply #5 on: September 23, 2017, 08:56:06 PM »
Well that narrows the list nicely. It's looking like it might be item number 3 however, which isn't ideal. I'll do some researching and see what I can find as far as what happens next. Thank you for the help

Offline tc197

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Re: Engine Knock
« Reply #6 on: September 24, 2017, 06:53:36 PM »
Confirmed today that all the bolts on the flywheel were still tight and that there are no cracks. I've taken it apart down to the flywheel at this point and I haven't found anything obviously wrong yet. Everything I did 9-12 months ago appears to be in place and nothing was loose, cracked or missing during disassembly. What would be the process to determine if my crankshaft is about to explode? Everything internal is new to me and I have no personal experience with it. Is it pretty much guaranteed that the engine needs to come out at this point to be rebuilt/replaced?

Offline VileZambonie

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Re: Engine Knock
« Reply #7 on: September 24, 2017, 07:04:44 PM »
So start the engine and run it since you have everything attached to the back end removed and see where the noise is coming from.
,                           ___ 
                         /  _ _ _\_
              ⌠ŻŻŻŻŻ'   [☼===☼]
              `()_);-;()_)--o--)_)

74 GMC, 75 K5, 84 GMC, 85 K20, 86 k20, 79 K10

Offline tc197

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Re: Engine Knock
« Reply #8 on: September 24, 2017, 07:15:09 PM »
I hadn't really considered this...for some reason I didn't think I could. Thank you for the tip. Next chance I have I'll see what I can come up with.

Online bd

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Re: Engine Knock
« Reply #9 on: September 24, 2017, 10:04:57 PM »
Be sure to support the back of the engine if you run it without the transmission in place, so the distributor doesn't hit the firewall and the fan doesn't gobble the shroud.  You can insert a short 2 X 6 wood block between the oil pan and a floor jack.  Make sure the wood block is situated an adequate distance from the ring gear so as not to interfere with its rotation.  Raise the jack just enough to center the fan in the shroud.
Rich
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In other words... if people learn by making mistakes, by now I should know just about everything!!!
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Offline tc197

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Re: Engine Knock
« Reply #10 on: September 24, 2017, 10:14:57 PM »
Thanks bd! That's pretty my current setup right now. I needed to support the engine when removing the trans anyways but I will have to raise it back up so the fan doesn't attack everything like you mentioned. Hopefully I'll get a chance to try this early this week, worst case it will be this weekend.

Offline tc197

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Re: Engine Knock
« Reply #11 on: October 01, 2017, 03:38:54 PM »
So I ran it as suggested and I'm still having trouble determining the noise location. From up top looking under the hood I hear a pinging sound like someone is hitting a metal pot. From underneath it sounds like a deep blender. Its definitely louder underneath but I'm not understanding how to dial it in more than that. My best guess right now is the noise is coming from behind the flywheel down low