The Stovebolt Site Forums The Shop Area Engine Shop Knocking Noise

I've got a knocking noise in my 270 GMC engine. I've dropped the pan and checked the rod bearings and the rist pins, they all look good. I've pulled the plug wires one at a time with the engine running to see if the noise changes, but it doesn't. It sounds just like a rod knocking. I hear it most of the time but there are times when it stops for a few seconds. What else can I check?

Thanks, Tray

Sounds like you have possibly injested something & it is sitting on top the piston hitting against the head. Some times a screw comes out of the carb. Use a mechanics stetrascope or hold a large screwdriver to the area the sound is comeing from & see if you can pinpoint the cylinder. Sometimes you can run the engine with the spark plug out & blow it out.

Could be a noisy fuel pump.

I've got the same thing on my 235..Runs great, but have a little knocking sound at the front of the engine once in awhile..I thought it was the water pump, didn't know a fuel pump could do that..Same area

Had a similar / same problem with my 216 engine. Turned out to be the cam gear (fiber). It had lost some teeth and it was working on short "nubs". It did not lock up and I drove it to the shop after going through the same process of "knock analysis". Replaced the gear and all is well. BW

I had this problem last year ... 3 different (pro) mechanics told me I needed a valve job. They were wrong ... somehow I knew that ... then I finally found a "real" mechanic who's into the old-school stuff. He came over the house, put a large screwdriver against my fuel pump and said, "Listen to this."

It was the fuel pump! A few bucks and some 30 minutes later, I was driving around with no more knocking sound and a huge sh*t-eatin' grin on my face.

sometimes a harmonic balancer can make a noise if it has or trying to work off the shaft.Fred

Could also be loose flywheel bolts

Well, replaced the fuel pump. Didn't fix the noise. I removed the fan belt and started the engine. The noise stopped. I guess the next thing to do is place a v-belt on the crank and generator. If the noise is still quiet, I can assume the water pump is the problem. Does anyone know what the torque specs are for the harmonic balancer on the 270 GMC engine?

Mine went away after I tightened up the fan belt on my 235.

Train driver. they will make a noise, wont they?Fred

Piston slap is always a possibilty. A slighly collapsed skirt will do it. I think thats the problem with my 250. I don't know what the pistons have for a pin offset either. No offset will make them noisy by design.

trayw before you put that belt back on grab your fan and see if you can wobble it front to back. If you can your water pump is toast. If not well get your self a Long Screw drive and start listening. If your water pump isn't bad ( I think it is) then my wild a** guess is that the fuel pump cam is lose! If it aint that then it is most likely the third from the front offset (not to be confused with recessed) muffler bearing.

Good luck and I hope you find your knock soon

This gets crazier by the minute. I loosened, not tightened, the fan belt and the noise got quieter. It's still there, but not as loud.

Water Pump is getting bad. Period. You have proven it by loosening the belt which take pressure off and less force on pully. Ben

popcorn The Truth is Out There (Take belt off and try running it.)

http://www.stoveboltengineco.com/howto/lift.htm
Applies to hydrolics on 235 & 261 but may help someone with the same sort of phantom.

can you hear a noise better with a screw-driver, I always used a wooden stick/handle? Fred

I had no idea that a fuel pump could make that kind of noise..Mine is pretty old, and was gonna take the longest thing I could find and stick it on there to listen...Then, I looked up and saw my head was planning to get a haircut on the fan...Guess I got to look around..I have a shop manual, is the fuel pump like the sbc and has a push rod, or does it have a lever? Hate to pull it out and have the rod fall into the oil pan..I really think it sounds like the water pump though..Any help, Steve

Hey Joker i use a stethascope want one got the inside to a few. Oh I also repair them also. Popcorn The Truth is Out There (Have used a piece of garden hose also) Ben

Just talked to an 80yr old guy at the auto parts store..bought some stuff from him before..said he used to have two 54's..I took him a 8X10 pic of mine last time I went..Anyway, he told me to bring it on down..We both need a ride today, so it's off to do some shopping..planning on changin all the trans and rearend lube anyway..Hope my A/C is working (cowl vent)..Later, Steve

I replaced my fuel pump a few yrs ago,, dont recall it being like a sbc with the rod deal. just the lever,,no big deal to change,,yeh, you have to watch it when messing around the motor with it running,, going to go to sears in mine after I get thru with my child-care duties,98 degrees, and boy do you feel that extra heat from those fentons when you stop at a light, but hey, it is fun........Fred

Trayw,, what part of Tx do you live?Fred

Joker, I live just south of Dallas in Duncanville.

hey, we are almost neighbors,, just a nice little drive for my 52, bought it 7 plus yrs ago in Grand Parrie, I always screw up that name,,,sometime you need to hook up with a small group of us,,we attend some of the meets, cletis, Tonynweatherford, Hardy-restorer,, 6cylinder over Tx, truck-rodder,and others, cletis and I usually will make most of the local type,,,,glad to have you aboard,,,,Fred

I live in Mesquite

I had that prob on an 87, 2.9L Bronco. I redlined it and it stopped. lol..... It may have been a lifter acting up.

Knocking noise?...Sometimes it's just the simple things, or I just got lucky..I flushed my entire cooling system three times, warming it up between each one..Did it until it got to be clear water..After final drain, New Prestone and good clear water...No more knocking noise..I guess there could have been some junk in the water pump..Don't know, but not complaining either..Figured I share, just in case, Steve

Ok, I've checked rod bearings and replaced the fuel pump and the water pump. Still have the knocking noise. Could it be CAM bearings?

you have checked your alt/gen...they can get real loud also..you mentioned that you had losen the belt, have you tried running it without the belt on for a few seconds(dont let it get hot) just to see what happens to the noise,,,,,,,,,,,Fred

Problem Resolved! Well, after $60, 4 sets of eyes and much aggrevation, we've narrowed it down to the Harmonic Balancer. It has the smallest movement in the pulley you can imagine. Evidently it's enough to create the noise. The Harmonic Balancer is the problem...Thanks for all your input.

Tray

Hey. I got one right,, mine use to make some noise and I could tighten up the belt or replaced the belt and it would quit, this went on for a few yrs, I could move the balancer with my hand but this method worked till the balancer came off while exiting off the hi-way, did not hurt anything other then I had to be out $70 bucks for the tow,,a few months ago at a local swap meet cletis noticed that it may be moving around again,anyway I am glad you got it fixed...........Fred

Thanks for the report.

Glad you finally nailed it! And, we all learned something, I think.