Chevy Pickup Rod Knock

majorxlr8nFebruary 7, 2007

1996 Chevrolet Silverado C1500 with a 305 (5.0 liter) and only 56k miles. I need some input on a rod knock that has surfaced recently on my Chevy pickup. Actually, I've had this for a few years, but it never was as bad as it currently is. Its moderately loud when the engine is cold, but will disappear after a few minutes of running. Its more noticable during colder temps, and on a warmer day 40+ degrees, its only there for a few seconds.

Now, I'm sure its a single rod knock because it only happens under load & is rhythmic with engine speed. Oil pressure is excellent: 60 psi @ cold idle, 20 psi hot idle. I know what multiple knocking sounds like from poor oil pressure when cold, and its definitely ONE rod.

I think the pan could be dropped & the rod bearing replaced with the engine in the truck. If I go this route, I'm thinking of pulling a spark plug each time at cold restarts to isolate the bad rod, which may take eight attempts.

What would I expect to find when I disassemble? A scored bearing or a broken/split bearing? Could this solitary bearing simply be not getting enough oil when its cold?

Thanks for any and all input!


Thank you for reporting this comment. Undo

How is the oil pressure? An sloppy bearing may cause a drop of oil pressure. You may be able to extend engine life by installing a high capacity oil pump. But first, you need to find out exactly if it is a loose rod and which end of the rod.

Piston slap can cause the same sound you have described. The difference is that piston slap does not affect oil pressure.

A little rattle on startup can be caused by the oil filter! If the design expects an anti-drain back valve in the oil filter and it has none or one that isn't working, then oil can drain out of the system after shut down. ( I had this experience years ago with a slant 6 Valiant engine. The Fram filters were ok, Autolite were not. Back then the dealer installed a secret fix. They installed an anti-drain back tube in the top of the oil pump.)

If you do drop the pan and discover a loose rod bearing, be sure to measure the crank throw with a micrometer in two perpendicular directions to check for out-of-roundness. If the crank throw is too far out of round, a new bearing will not fix the problem for long.

Question: Are you certain that the engine has never been oversped? Overspeeding can open up rod bearing and stretch rods.

    Bookmark   February 7, 2007 at 5:38PM
Thank you for reporting this comment. Undo

Without hearing the noise it's hard to guess. My past experience has been that a rod knock or wrist pin problem doesn't go on for years. You can try pulling off plug wires one at a time to see if it changes the sound. IF there is a rod bearing problem bad enough that you can hear, I wouldn't goof around with trying to drop the pan and change the bearing. Too many other things are wrong if the bearing is bad. Rods and crank throws get out of round on a good engine. Odds are a fix consisting of replacing a bad bearing alone in a bad engine wouldn't help. My observation in the past has been that a bad rod bearing doesn't really affect oil pressure much if any. Way bad main bearings and cam bearings do. Sounds like your oil pressure is good anyway.

Without hearing the noise, odds are it's a lifter or rocker arm issue. Could be a piece of carbon bouncing around on a piston too. If it were mine, I'd start by pulling plug wires one at a time to see if the noise changes on any one in particular. If not, I think I might give a can of carbon blaster a try, though that might be hard to do on your engine. I don't know how it's configured. The next thing I'd do is pull the valve covers, just one if you know which side's the problem, start the engine and push down on each rocker on the push rod end. You'll find it if that's the problem. Also look at the push rods to see if any are bent. Tip, if you do this, first buy the little clip things that fit over the rockers and plug the oil holes so oil doesn't squirt everywhere. If you find the source of the noise that way, then you can decide how much trouble it's worth to you. Replacing the lifter would require removing the intake. Some GM engines in the past have had a tendency to wear at the point that the rocker arm touches the valve stem. A notch can wear into the rocker at that point and can be noisy as it hits the high spot and slips off. The fix for that problem is replace that rocker.

In the days before fuel injection, a bad fuel pump could make a pretty big banging sound too. I'm sure yours is new enough for that not to be it.

Those are my brain storming ideas. Good luck.

    Bookmark   February 7, 2007 at 11:41PM
Thank you for reporting this comment. Undo

There are no easy answers for this repair. I can just see all of the arm chair mechanics saying "If he knew what he was doing".

Years ago, shops/mechanics would try to diagnose and slip in bearings. History showed that success was random at best, with engines surviving that really should not have, and engines failing when they by all visual means should have survived.

The techs that had high success rates, pulled the engine. They removed the crankshaft, and all of the connecting rods so that complete proper inspections could be performed. A connecting rod that is losing a bearing usually needs resized, in otherwords it will not properly crush the new bearings if you simply try and swap them. Plus most of the time a connecting rod fails, the source of the problem is actually the main bearing associated with it, which needs to feed oil to the crankshaft, which then feeds the rod bearing. As mentioned by one of the other responders, the crankshaft also needs carefully inspected. In fact it needs checked in six points, not just two, and that's difficult to do in the car, because you can only get the connecting rod just so far away from the crank. That's why they say check two points, it gives you a 30% chance of confirming damage. It does NOT confirm the crank is not damaged.

So that brings us back to the arm chair mechanics. One will say you should do what your thinking, just try and replace the bearing. Who knows, you might just be lucky and it works. Another would say you should just get a reground crank and fitted bearings. That would be true but you could be ignoring the blocks line bore, and the connecting rods sizing. In the end this all boils down to the fact that it is going to take a machinist to clean, test, and service all of the parts for you to do this right. Just to have some arm chair mechanic say "you didn't need to spend all of that money, they are ripping you off you could have just slipped a bearing in it".

In the shop, we rarely do this level of work anymore. Too many experts out there looking over our shoulders, too many things that can go wrong, and not enough profits for us to be able to aford to stand behind our work. I have a car in the shop right now, the block is warped .004 across the cylinder head deck. I don't have deep enough pockets to risk pulling it out, tearing this down, sending it out to the machine shop and then rebuilding it and give the customer a "fair" warranty. In otherwords, if I do it and it's perfect, everyones happy. If I do it and one thing goes wrong, I can't afford to make it right, so I have to refuse the repair. The customer on his side, does not want to spend the money to have a remanufactured engine installed. A reman engine comes with a warranty from the rebuilder, that takes the risk off of me. It's also $4000 installed.

So that leaves this 40,000 mile car staring at getting an 80,000+ mile used engine installed, with a 90 day part only warranty. It's getting towed to an auto wrecker, and their "mechanic" is going to do it.

    Bookmark   February 8, 2007 at 5:07AM
Thank you for reporting this comment. Undo

Hi John. While you are here i have a situation that you might have an answer for or anyone out there. Wife's 94
3.8 taurus has had 3 engines in it in 2 years. I've rebuilt them all. First engine was a complete burn out
when the head gaskets went. first time in my life i've ever
seen a crank burnt purple. Both mains and rods. 404,000
K.M. original before i pulled it. ( bought used ) I sent out the block. Hot tanked, checked for warpage and line bore. O.K. " NEW " crank kit. NEW cam bearings and NEW counter balance bearings. NEW timing chain kit and tentioner kit. NEW rings " pistons were fine " rods resized. Valves done, Heads checked ok. NEW oil pump. No ware on the lifters ( roller cam ). Yes i put the lifters back in the same direction. The engine was assembled with
rebuilding lub and the oil system was spun up and pre primed. Knocked instantly on start up but quietened down.
Started knocking on startup about 1 year later. Pulled it again. #3 rod hammered out. Again NEW crank kit and rechecked rods. Wrist pins and pistons fine. 4 months later knocking again but knocks only when it feels like it. May knock for a week in a row or just one day and not
knock again for 5 months or more. 80 oil pressure when cold
60 hot running, 45 hot idle. There is no end slap on the
crank. What do you figure ? OH. man, i just realized i
hijacked someone's tread. Sorry about that.

    Bookmark   February 8, 2007 at 4:46PM
Thank you for reporting this comment. Undo

Guys - I really appreciate all the input here!

I beared the bitter cold & wind chill today & did some inspecting. This noise that sounds like a rod knock may not be so. The sound was not that loud when I was underneath the truck, but was louder near the front at topside. I'm thinking maybe a bad serpentine belt or some accessory (even the balancer) connected to the belt.

If the weather cooperates Friday, I'll remove the belt & report my findings...

Again, MUCH thanks!

    Bookmark   February 8, 2007 at 11:44PM
Thank you for reporting this comment. Undo

Hi kalining

My first question is, when you assembeled the engine, did you plastigage every bearing to measure clearance? Some re-ground cranks have bearings of different sizes, and occasionally the machinist does not send a .011 or a .012 for the odd small journal. The next thing to consider as you are building the short block. The crankshaft, and it's bearing and seals alone will spin with almost no force at all. As each piston and rod are installed, the torque required to turn the crank a full revolution climbs. A seasoned tech will "feel" if one of the rods when installed suddenly raises the required torque an uncommon amount. I always advise someone without a lot of experience to actually measure the torque required to turn, and keep turning the crankshaft at a steady speed. Some of the really dumb things that can happen and ruin a rebuild is dirt/debris on the back side of a bearing insert. Everyone knows to keep the front (crank) side of the bearing clean, they often forget about the back (rod) side. I once was building a 2.8l V-6 that when I installed one of the rods, the crank became almost impossible to turn. The problem was some kind of varnish that is on the back side of the bearing insert had a bubble in it and caused a tight spot. By paying attention to the build, this was simply a nuicense, and not the potential cause of a complete rebuild failure. BTW, as a tech, there are no rewards for not getting burned by a suprise like this, only punishments if you get caught in it's trap.

Now you say, wrist pins and pistons fine. I'll say, objectively, how do you "know that for sure"? You could easily have a collapsed skirt, and be getting some piston rock, which will sound a lot like a rod bearing. Piston rock (slap) normally only occurs once every other crank rotation, but there are engines that make their own rules.

With engine noises I have to really trust my hearing, and the ability to tell what direction a sound came from. Locating the noise, left VS right, forward or back, high or low is critical before teardown.

    Bookmark   February 10, 2007 at 9:40AM
Thank you for reporting this comment. Undo

kalining, I gotta ask, why are we rebuilding an extremely high miler '94 Taurus? For fun? In my area they're not worth much when running well. Engine or more often transmission trouble and they're scrap iron. Just curious.

If that happened to me, after the 1st rebuild didn't last, I'm afraid I'd have to take a sledge hammer to the whole car just to make me feel better. LOL!

    Bookmark   February 10, 2007 at 12:11PM
Thank you for reporting this comment. Undo

Actually, Gary, didn't have much choice. Wife bought it off of a so called friend at work after her car was written off. Body was immaculate and perfect interior. Still is. She couldn't afford anything else that actually
had a body left. John. Pistons and the entire engine was sent to the machine shop. They assembled the rings and all
bearings. All they do is rebuild engines and race cars.
The only thing i can think of is that engine has hydrolic
motor mounts and if one started leaking after the rebuild it will sound like a knock. The bore is at the factory
max with original pistons. Probably piston slap. I probably have to do the trans. this summer so i'll recheck the mounts. I can do an entire trans for $500.00 Canadian with converter. I've got most of the parts here anyway so no biggy. Have too much money into this so can't stop now. I used to work at a rebuilder so this is something i've never seen or heard before. You wouldn't believe the slop work and short cuts i've seen. They never had an engine come back. Well, not to my knowlage anyway. Thanks for the input guys. I'll look again this summer. That might be on a sunday so hopefully i don't have a hangover and sleep through it. P.S. crank and pistons turned at 20 lbs.
Assembled engine with heads broke loose at 50 lbs. and turned at 35.

    Bookmark   February 10, 2007 at 1:30PM
Thank you for reporting this comment. Undo

kalining it sounds like you may need a rev limiter the little woman may be getting frisky out on the road and over revving that bad boy.

    Bookmark   February 19, 2007 at 8:41PM
Thank you for reporting this comment. Undo

What you are descrbing is piston slap and not a rod knock. As the engine warms up, the piston will expand and fit tighter in the bore which your noise will disappear. if it was a rod nock, the noise might be loud when first started becuase of lack of oil on the bearing but would probly turn quiet after a few seconds as the oil pressure rises. then the rod knock would get more noticeable as the engine heats to due to the oil thinning out.

    Bookmark   February 20, 2007 at 9:01AM
Thank you for reporting this comment. Undo

Posted by blue_fastback (My Page) on Tue, Feb 20, 07 at 9:01

What you are descrbing is piston slap and not a rod knock. As the engine warms up, the piston will expand and fit tighter in the bore which your noise will disappear. if it was a rod nock, the noise might be loud when first started becuase of lack of oil on the bearing but would probly turn quiet after a few seconds as the oil pressure rises. then the rod knock would get more noticeable as the engine heats to due to the oil thinning out.


WOW ,You can tell all that from a few paragraphs ?! PLEEEEEZE, Anyone that responds the way you have certainly DOES NOT KNOW.

    Bookmark   February 22, 2007 at 8:04PM
Thank you for reporting this comment. Undo

What makes you far more superior then me candy1965? Why is it that the way I responded certaintainly DOES NOT KNOW? I sorry you find my OPINION so wrong. Did you listen to the Chevy pick up? I didnt. I heard symptoms. I dont see any post from you you on this subject except on slamming me! Please tell me more.

    Bookmark   February 23, 2007 at 5:45PM
Thank you for reporting this comment. Undo

Posted by blue_fastback (My Page) on Fri, Feb 23, 07 at 17:45

What makes you far more superior then me candy1965? Why is it that the way I responded certaintainly DOES NOT KNOW? I sorry you find my OPINION so wrong. Did you listen to the Chevy pick up? I didnt. I heard symptoms. I dont see any post from you you on this subject except on slamming me! Please tell me more.


The reason I said you didn't know is because you act as though you know EXACTLY what the problem is based on some written info and having NEVER HEARD THE ENGINE ! My experience has always been that someone with your kind of"I KNOW WHATS WRONG & THIS WILL FIX IT" attitude, is almost ALWAYS WRONG ! Did I say that I heard the Engine ? NO, YOU STUPID PHUCK ! Now STFU !!!

    Bookmark   February 27, 2007 at 9:21PM
Thank you for reporting this comment. Undo

Blue Fastback - I think you nailed it...

I finally removed the serpentine belt to not only quiet down the engine compartment, but to also see if any peripherals were the cause. The noise was still there.

I did some research & apparently the 1990's Chevy V8's are notorious for piston slap.

Looks like I'll be "short blocking" this at some point down the road when/if it grenades.

I thank you all for the input!

    Bookmark   February 28, 2007 at 1:55AM
Thank you for reporting this comment. Undo

Hey Marty,

I dont think the engine will grenade but again I dont know how loud your noise is. Piston slap from GM's standard is "normal". I myself hate noise and would fix it but unless that piston skirt is broke, its just going to rattle until the engine warms up. This is what I would do. First losen all the spark plug wires from the spark plugs before starting the engine. Then start the engine and remove the plug wires one by one and see if the noise deminishes so you can try and locate the offending cylinder. I myself would after finding the noisey piston is pull the head off and remove the oil pan and pull out the piston and replace it. With the low mileage you have and the good oil pressure you speak off, the engine is worth saving. It would take a Saturday to do and $100 in materials and maybe a twelve pack of beer. If the motor was high mileage and used oil with noisey lifters then I would think twice about just putting in a new piston. I dont know how handy you are but this is what I would do. Honestly, its hard to get a good reman engine now a days. Let me know if I can be of help.


    Bookmark   February 28, 2007 at 7:57PM
Thank you for reporting this comment. Undo

Posted by blue_fastback (My Page) on Wed, Feb 28, 07 at 19:57

""Piston slap from GM's standard is "normal".""

PISTON SLAP- NORMAL ? Oh sure, didn't EVERYONE know that all good engines have thier fair share of Piston slap ? Where did you learn that crap, at "GOMER NIPPLEDICKS SCHOOL OF DIPSHIT ENGINEERING & CAR FIXIN".


"I myself hate noise and would fix it"



"It would take a Saturday to do and and maybe a twelve pack of beer."


    Bookmark   March 2, 2007 at 3:22AM
Thank you for reporting this comment. Undo

**""Piston slap from GM's standard is "normal".""


Blue fastback is correct. Though I wouldn't use the term normal, 'not unusual' would be a better choice of words since they don't all do it. A higher number than one would expect of late model gm engines have a knock. There are a couple (out of many) company trucks at my work that do this. One employee I know of purchased a brand new Suburban a few years back that does this. They have knocked practically from the time they were brand new. These vehicles are much newer than the posters truck so they may have different issues. I don't know. The work trucks have quite a few miles on them by now and have been run hard. So far whatever is making the noise hasn't led to any catostrophic failures. It would be annoying if one purchased a vehicle new and it did this even if the engine life wasn't affected. What happens if you want to sell it? You say, runs great and it does. Perspective buyer asks what's that noise? You say it's nothing. Perspective buyer says like you '65, uh huh, sure.

If you want to troll or flame please go somewhere else. The regular posters on this board aren't that way.

    Bookmark   March 2, 2007 at 11:57AM
Thank you for reporting this comment. Undo

Here is a link that I thought that might be of interest. This does not pertain to the 5.7 engine but newer GM vehicles that G.M. says is "normal". I myself did not say that piston slap was "normal". Read this if you own a late model V6 or V8 engine.

Here is a TSB that GM had on the 7.4 "normal" noise.

Bulletin 77-61-21


"Some owners may comment about a light tick or knock noise that resembles either a valve lifter tick/rattle or a piston or piston pin slap. The condition will be most noticeable on cold start. This noise may be present upon engine decel after initial starting flare and may not be consistantly repeatable. This condition has no assignable cause at this time and as a result no repairs should be attempted. Although the noise may sound unusual, there is no indication that this will in any way affect the durability or longevity of the engine. Etc,etc ,etc"

This is another TSB that GM put ont on certain engines.

Engine - Cold Knock, Replace Oil Filter/Bearings/PROM
File In Section: 6 - Engine

Bulletin No.: 37-61-XXX

Date: October, 1995

Cold Engine Knock (Replace Oil Filter/Bearings/PROM)

1990-97 Chevrolet and GMC Truck C/K, R/V, S/T, M/L, G, P Models
1991-92 Oldsmobile Bravada
with 4.3L (VIN Z - RPO LB4), 5.7L (VIN K - RPO L05),
7.4 (VIN N - RPO L19) Engine

This bulletin is being revised to add the 1997 model year information.


Some late model truck engines have been reported to exhibit "cold knock" on start up. "Cold Knock" usually occurs after the vehicle has been completely warmed up, then parked for 8 or more hours in ambient temperatures of 35F or less. "Cold knock" can be separated into three distinct categories.

1. Short Duration - Harsh, deep metallic knock that usually lasts from 1 to 10 seconds. Generally classified as a bearing or rod knock.

  1. Valve Train - Light clatter, tick or click that may last up to 1 minute.
  2. Piston Slap - Metallic knock that occurs only under load. Piston slap may last as long as 5 minutes.

The above is a reprint from G.M. their definitions on engine noise.

G.M. has also came out with a T.S.B. to recalibrate the pcm (on some models) check with your G.M. dealer
Updated March, 1999 #77-61-21 The recalibration is to reduce or retard the timing which will take some of the pressure off the pistons so they wont slap in the bore and knock. They figure reprograming the PCM is a lot cheaper then new pistons or a new short block in 1000's of vehicles.

Do you always get this excited over a post on the internet? You still have provided no useful information on this subject and continue to slam me. You know nothing about me or my knowledge about cars. All this is a friendly discussion on what might be, based on given symptoms.Instead "Mr. Helper" decides to chime in and give us all his opinion from GOMERS point of veiw.

    Bookmark   March 2, 2007 at 4:19PM
Thank you for reporting this comment. Undo

Hey, you can't have all of this fun without me!! VBG

Seriously some of what is being said about piston slap is true. Piston slap alone is NOT a reason to tear an engine down and do a repair. Piston slap is in fact normal on a cold engine because aluminum expands faster than the cast iron engine block. if the piston is tight enough cold, it will be too tight when warmed up. Piston slap should a repair actually be indicated actually does not even mean that a piston needs to be replaced! If the problem is a collapsed skirt, it can in many cases be "knurled" and reshaped to take up the slightly excessive clearance. A knurled piston is one that has many tiny marks made into it's surface, that cause the outer edges of the mark to rise up. Sort of like the way a center punch makes a "crater" in a piece of metal. These two links give both a description of knurling, and an actual application.

Now the one big problem I have with this thread is the idea that $100 in parts, a saturday, and a twelve pack of beer is all that is required to make the noise go away. I do understand the perception that to the right person pulling and reinstalling a piston isn't difficult to do but the reality is to do it right is in fact much different. This is a job where a very simple mistake can cause a major engine failure after the attempt. Plus if it's 4WD just getting the oil pan off and back on will take up most of your staurday, and that's if you have a hoist. People have been successful simply replacing a piston(s) in chassis but not always successful at eliminating piston noise or slap. That's because the piston is only half of the equation, cylinder wear or taper could easily be coming into play, and I would leave those checks/measurements up to a qualified machinist. Plus we still have the little issue of pressing the piston pin through the rod. Very few shops have the equipment to do that, let alone DIY's. There are many traps lurking in this repair that can make the whole attempt go south in a hurry. One common trap is as simple as not torquing the intake manifold in the correct order! An incorrect torque sequence distorts the engine block, and that changes the line bore of the engine resulting in main bearing/camshaft bearing wear. The $64,000 question from there is, does the resulting wear end up causing the engine to actually fail? Sometimes it does, and sometimes it does not.

Lastly, stroke length plays a role in piston slap issues. The longer the stroke, the more often a piston skirt has to be shortened for clearance issues. Cylinder block deck height and other design issues can also impact piston design. The longer that the skirts can be, the less likely it is that piston slap is likely to occur. Although think about it, the longer the skirts, the greater the piston mass and you start impacting engine efficiency due to inertial, and frictional losses. Plus you can impact RPM and horsepower. It's all an intricate dance that even the manufactures are not going to be perfect at every time. That's why we have tolerances, and some things (noises) are allowable even if they are not completely desireable.

    Bookmark   March 3, 2007 at 5:12AM
Thank you for reporting this comment. Undo

LOL I just stumbled across this thread and it gave me a good laugh. That (blue_fastback) fella is quite a character. I mean cmon, a 12 pack of beer on a sat. ??? No thanks, I'll fix it myself

    Bookmark   May 1, 2007 at 10:43PM
Sign Up to comment
More Discussions
Tire Question
I have a 2007 Honda Ridgeline with 13,000 miles (yes,...
Hey All
This Is Sophie.I Am New To This Forum.
Honda Accord 2014 or 2013 navigation
I'm looking at a 2014 Accord and wondering whether...
pump keeps stopping when trying to fill gas tank
Hi! Can someone give me a clue what might be suddenly...
1992 Chevy Truck 1500 running BAD!
About 2 1/2 years ago my truck was running bad, had...
People viewed this after searching for:
© 2015 Houzz Inc. Houzz® The new way to design your home™