Engine knock

[SailorJ]

Hello I'm new to the group and I need to pick your brains! I have a 1964 GMC 3/4 ton with the 305 V6. I have been refurbishing this truck since last summer and finally have it all back together with the exception of a few minor things. About a month or so ago, I was idling the truck in the driveway to try and get my lights going as I didn't want to do it on the battery alone since my battery was old and dying out. On about the 5th time idling the truck I heard the rpm getting slower like it was going to die so I gave it a bit of throttle and BAM! It back fired and died. I let it cool down thinking maybe I had overheated it and attempted to start it the next day. It started but was rough. I asked my dad what he thought it might be and he said maybe since the battery was no good that it didn't have enough juice for the coil to provide strong spark. So I bought a new battery. Problem was still there. Then I noticed two plug leads were swapped to the wrong cylinders so I fixed that. (Don't know how because I never messed with them.) Opened brand new distributor cap that I had replaced and adjusted rotor tang and cleaned all arc areas. Truck fires great! My parents came to visit about 2-3 weeks later and I wanted to start it up for my dad so he could see it run. Once I started it, it idled for a minute and then started knocking! That was surprising as it had never done that before! So my Dad and I proceeded to remove the oil pan to check internals as I thought it might be a rod. Boy that was a chore! I had to cut my exhaust crossover off just to slip the pan out! I couldn't see ANY problems in side the bottom half. Rods, crank, cylinders, pistons all looked fine from what I could see. I took the plate off of the bottom of the oil pump to check if internals were good and it looked fine as well. The oil pump is pretty stout so I couldn't imagine it breaking! I have run the motor with oil pan off also for a minute to possibly hear something but that was a no go. I waited for two days for the new gasket to come in and I installed the pan and poured in new oil. The knock is still there but it only seems to happen once the engine starts to warm up. I ran the motor with the valve covers off and everything is moving and is intact. I have run the motor while using a large screwdriver or metal broom handle as a stethoscope listening in all areas. NOTHING! Also, I have taken the dust cover off of the bottom of the tranny to check the flex plate and torque converter bolts as I was told they could come loose. I can't find anything!!! Please give me some pointers if you have any idea of what this could be! Thank you!

[BarryGMC]

Welcome. Its nice to see another Washington guy. This is a strange deal you have there. First it ran okay with 2 wires crossed? Second is the knock deep sounding like two boards banging or more clackety like a hammer hitting iron. Also has the truck been sitting a while. Its sounds a bit like a stuck valve and bent push rod. But its hard to say.

[705jmguy]

hi

had knock in my 305, did not appear sudden like yours

turned out to be piston slap, and had to bore it out and have new pistons made for it

[SailorJ]

Thanks for the reply! It actually sounds like a tack, tack, tack and as the rpm increases so does the noise until you can't hear it. Rpm drops...obvious tack, tack, tack is back. The push rods look straight from under the valve covers. I shined a light down the holes with engine running and push rod are straight and ratcheting while working. As far as the valves I have no idea. I guess a compression check would be a good idea! It has been sitting but I always start it and even gave it a once around the neighborhood with just the cab and radiator support multiple times. The problem didn't start until after that loud back fire(after fire) but no, it ran rough with the leads crossed. I just couldn't figure it out for a bit because I don't remember messing with the leads. The sound actually sounds and keeps me focused at the very back of the motor. Almost to the point where I'm looking at the bell mouth of the tranny. I'm lost.

@ 705jmguy - I hope that's not the case. I'm trying to get any info before taking heads off as it really sounds lower than that.

Oh, I'm actually a Florida guy stationed here in the Navy but I'll be here 3 more years.

[705jmguy]

yes from what you are saying it came about way to sudden to be piston slap

[BarryGMC]

Check the bell housing for a crack. I have seen this quite a few times over the years. The other thing is mabey a piston cracked. It will make a knock sound. But the v6 stuff is pretty stout. I have seen this on a lot of straight 6 engines. A couple of years ago i bought an izusu npr with a 350- 4l80 that the seller said had a bad engine. It knocked like crazy. The trans was cracked 3/4 of the way around right near the engine bolts. I welded it up and no knock. The sound was just like a bad rod sound.

[SailorJ]

Ok thanks! I did a compression check today just to make sure the valves aren't stuck. Everything was within 5-10 psi. Also, I took the dust cover off of the tranny again and grabbed the torque converter to see if I could wiggle it but it was tight. I'm not sure what or where it's coming from but it seems to drive fine. I'll definitely be keeping my eyes open!! Thanks again!

[David R Leifheit]

Take no chances.
I was using my truck, rev'd it up and it started making a tap tap tap noise. I figured it was something minor.
When the rod cap came off at 55 and locked up the engine (the force caused the piston rod to spread out, instead of a "U" shape to cup the crank it was almost a "T" shape. Couldn't get the piston out the top or the bottom) and when I took the heads off I found the imprint of the valves in the top of the piston. It tap-tap-tapped for months before it came apart, what was happening is that I must have spun the bearing out or something, but the rod cap started coming loose and the tapping was the piston smacking into the head.
I was not very knowledgeable about engines back then. Learned a lot since.

You mentioned, I think, taking the pan off and looking... well, take it off again and see if any of the rod caps are loose... check the nuts to be sure they aren't "loose" even a little.

Although, if nothing on the bottom looks bad, and the valves don't look bad either... then you are left with the pin... possibly.

These old trucks will take a lot of abuse before giving up. One I bought ran rough, but ran well enough to load onto my carhauler. Drove it right up the ramps. When I took the heads off, it was literally running on 2 cylinders. On one side there was no head gasket between two of the cylinders and the oil gallery. On the other side the head gasket had a "tunnel" between two of the cylinders and was just starting to make a bigger connection.
To be honest, I was surprised it even ran, let alone had power to climb the ramps up onto my hauler truck.

These are hearty trucks with strong engines. But if the engine is talking to you "tap tap tap" then listen and find out what it is trying to tell you before it gives up and just fails.

[turbobill]

Have you checked the valve adjustment? Excessive valve lash on one valve/rocker arm will make enough noise to make an untrained ear think something more major is going on. Putting pressure on each rocker arm as the engine is tapping may change the sound on the loose one.

Before pulling the pan again, with the engine tapping, remove one spark plug wire at a time, see if taking a cylinder off line will change the sound at all. If it does, then go looking at that particular cylinder for a more serious problem (bearing, piston/pin).

Another possibility is a weak fuel pump spring, allowing the fuel pump arm to lift off the cam and slam back onto it making a loud lifter/valve adjustment type noise. Remove the pump and start the engine (it will run for a short time on the fuel in the carburetor bowl).

[Liltfy]

Here's a long shot....I had similar issue with my old pal. Just getting it up and running. After much work and cleaning it still had a hard knock. Two suggestions: If your pan is off any way, take all your plugs out and roll the engine over by hand. It should not have any noticeable interference when rolling over. If there is, it could be something foreign like a screw or a something fell into the plug hole when a plug was removed. So if you feel interference, take the heads off and check for carnage. Hope it's something easy!!