Yikes!!! 305E V6 knocking like crazy!

[bigblockv6]

Originally Posted by David R Leifheit

I strongly second this.
I had a knock, it wasn't bad so I ignored it. At 55 mph the knock stopped, so did the truck. I had dropped a rod cap, the knocking was the piston slapping the head (had a nice imprint of the valves on it). BIG mistake to ignore any noise, or "get used to it" as I did.

Carbon build up on the piston could be a possible problem.

[AZKen]

96.4% chance it has something to do with what he did recently....or some info he has not remembered yet. I do it all the time. TJ: Jolly's site says AC4 plugs for 1964 BUT you indicate in other posts your motor may be newer. Get correct plugs and re-time even though you did not touch timing. Make sure you use the specs/parts for whatever year that motor is. Things you say are still pointing to detonation or pre-ignition or timing. But they do say that happens on Acceleration or a load. Maybe cheap/old gas, plugs too hot or as BBV6 says ...carbon. Hope it's not rod slap. Don't know if you have had this motor long enough to say it was quiet before "tune up". If it was, I don't think rod knock comes on all of a sudden. Don't think it goes away and comes back. It could change a little because of thermal expansion of parts. TJ: are you 16 years old?
EDIT: Experts say that sometimes after changing old gunky oil, with new oil, it will reveal a rod knock. Check filter for flakes/glitter.

[TJ's GMC]

Ok so I'l explain the story of this truck. So I bought it from a guy exactly 3 weeks ago. Ran like crap...misfired like crazy. Limped it home(3 miles) and checked the plug order to make sure all was well. Found out 4 and 6 were crossed. fixed that and the thing ran like new. Snappy, fired up everytime, carb is crapped out so its a little hesitant but is a great cruiser. Anyhow...skip two weeks later and I'm driving to a friend's house...thing runs great the whole way there....let it sit for a few hours....and on the way home not even half way it starts running crap...hesitates...stumbles...(idles ok) and is not all at wanting to stay at high rpm. So zoom home as quickly as it will go(mind you its 10 pm and I'm wanting to get home. (lol) Get home and park it. So a few days later I fire it up...still runs like crap. Put new plug wires on it and that helps a tad....didn't help with the stumble or hesitation...So after tinkering I decided it was a carb issue and was having a hard time getting it to idle so I pulled the choke out and brought the rpm to around 1500(ear judgment) for a few minutes while gunning some B12 carb cleaner through the carb and into the engine. Push that choke back in and she's running fine again. Ok! Now my tune up parts are in! Go to Napa and get my new parts(plug wires, and plugs, and a can of engine restorer for the heck of it), Put the new plugs in(wires were already on) and fire it up...well...you guessed it...the knock starts. SO...Skip to today.

Fire it up this morning and its still knocking. Pull ALL the new plugs out and put the Old ones back in. Didn't help a thing...stick knocking...go around with a screw driver and whatever I could find and couldn't pin point the knock. Pull both valve covers to make sure everything is moving ok....did see a little more sludge in there than I wanted to....but oil is going up there and everything seemed ok. So while tinkering and just letting the engine idle..suddenly...ALL BY ITSELF the knocking goes away....rev it...starts to knock on deacceleration.....rev it again..same thing....rev it again..no knocking...rev it again...no knocking...stop and put the new plugs back in...start it..rev it..still no knocking.....Now it knocks once in awhile.....and for the past 20 minutes of it running there has been no knocking at all. Oh yeah..so I found out I have a new coil and cap and points from my 292 I6 chevy laying around so I decided to see if those would work and walla...the points and cap fit on there like a glove. Adjusted the points(without a feeler gauge) until the engine ran fine and that's where I'm at. Drove it around the field goosing it here and there and all seems fine. She's ticking away like she did when I fixed the crossed wires. Smooth as a swiss watch. So..to be honest...I think the motor cured itself(LOL)...did talk with a friend of mine who's kind of an expert on these and he thought that a lifter was sticking or that a piece of carbon was wedged somewhere. And if that's the case it seems to have burned it out. Anyhow...Also did find out I have a leaky head gasket on the top of the engine..I can see some water seeping out the top in front of the intake port so I'm a gonna be ordering new head gaskets as well. But as of now everything is back to normal. And yes I'm 16...just turned 16 a few days ago.

[AZKen]

Good work. You have the vocabulary, skills and thinking ability of an older expert mechanic. You are going to go a long way. Good to see someone of your age interested in vintage trucks. We can learn from you. Time will tell if it was just a quirk or not. Let us know.

[705jmguy]

Will share with everyone my experience with knock in my 305, the knock was not as sharp as yours so may not be same thing, it was running smoothly, good oil pressure, compression was on the lower side on all cylinders, not smoking or using oil.
Mine had a annoying knock at idle, was told it was the harmonic balancer when I purchased it, and of course that is not true as it has no harmonic balancer on the front pulley
Pulled the engine to make sure it was not a rod bearing, thankfully it was not, so decided
to give the engine a little spruce up so to speak, but not having found the cause of the knock as of now, had crank shaft checked for specs, on the low side but still in range, still no answer for the knock cause.
Was beginning to think it was my imagination at this point, decided to pull pistons, clean up cylinder walls and put in new rings and bearings.
Had the cylinders checked and found the problem, piston slap, the old girl still was running good no smoke even with the pistons slapping away.
Had to have it bored out over sized, had new pistons made as could not find over sized.
Before I purchased it was sitting for at least five years, not started, perhaps a little rust developed on the cyl walls and a hard start caused rapid wear on the walls causing the pistons to slap due to premature wear
Just my two cents for everyone info thanks

[TJ's GMC]

Well if mine was a piston slap it must have worn enough or burned enough off the piston to quit knocking. lol I'm gonna do a compression test when I can find my darn compression tester. haha Also will be putting another video on youtube about it as proof she quit knocking.

[bigblockv6]

Back in the 70's GM had what they called " Top Engine Cleaner" that you could by at the parts department at most GM dealerships, designed to clean up the top end of the engine. I used to use this on my 68, it was thick as molasses , you would start the engine and pour it down the carb and let run for less than a minute then let it sit for about ten minutes and then start the process again. It really put out a strong smell of ammonia but really did the job of cleaning up carbon. If there is anything similar like this out there today it would be a good idea for engines that have carbon build up.

[AZKen]

According to the internet (always reliable, of course), you can blast carbon off the pistons thru the plug hole with fine walnut shells. Then clean/blow out/change oil.

[TJ's GMC]

A machinist friend of mine was saying that dripping water down the carb while the engine is around a 1500 rpm idle will cool things off enough to break the carbon build up and burn it out.....all this while being careful how much water you drip in. lol Well here's a video of how she runs now. All is ok again minus a leaky head gasket. But we'll order those from summit!
https://www.youtube.com/watch?v=0xoTKUE_7z4

[bigblockv6]

What also works good is to put water in a spray bottle that it's atomized as it's going down the carb, I've seen the results after the head was pulled an engines, the pistons were so clean they looked good as new.