98 Knock Knock

I have a 1998 with the 5.7L and 110,000 miles. Within the last year it has developed a knock that is not a ping. The knock is very loud when accelerating when the engine is cold, but quiet at idle. When I rev the engine, it revs past the knock and is quiet. When the engine is warm there is still a faint knock while accelerating but when it shifts into high gear it goes away. I have not bothered to do the "pull the plug wire" thing yet because you cannot hear the knock at idle anyway. I have not wanted to accelerate it with a plug wire off for fear of the computer going wacko. I do have the tell tail "orange" coolant drips from the back of the engine but have yet to find water in the oil which may suggest a bearing problem and my experiance with wrist pin knocks is that they do not get quiet. Any ideas as to where to start would be appreciated.

First off water in the oil has nothing to do with a bearing going bad. When the intake gaskets leak on those lik mine did , it leaks out of the engine. Is is a knock or more of a ticking , could it be a lifter that is collapsing, and if it is more of a knock it would almost seem to be a main bearing......

First off water in the oil has nothing to do with a bearing going bad. When the intake gaskets leak on those lik mine did , it leaks out of the engine. Is is a knock or more of a ticking , could it be a lifter that is collapsing, and if it is more of a knock it would almost seem to be a main bearing......


If it is the same noise mine makes when it is cold I am beginning to think it is piston slap. Mine has done it for the last 50,000 miles and the colder it is outside when I first start the truck the loader it is. When the truck reaches operating temp it goes away so it is something to do with a clearance somewhere and it is not a bearing. I have listened with a stethascope and it comes from either the piston, wrist pin or a bad lifter. I don't think it is a lifter because that would pump up right away when the engine is running. As far as coolant not having any effect on bearings that is incorrect. Even a trace amount of anti freeze in the oil will ruin the bearing surface over a period of time and the more coolant the faster the surface will score. When I say trace amounts I mean an amount small enough that it will only show up on an oil analysis and cannot be seen by visually inspecting the oil. I guarantee if the intake manifold is leaking some coolant is getting in your oil. Once the intake starts leaking around the coolant port there is nothing stopping it from getting in the oil. This may be a minimal amount, But it will dramatically reduce the bearing life and should be fixed ASAP. As far as the noise goes mine is still running and the only thing I have noticed in the 50,000 miles since it started the engine has had to run longer and longer for the noise to go away. It used to only take a minute or two and now the engine has to get up to operating temp before it is totally gone. I plan to drive it until the noise doesn't go away and as of now I am at 196,000 miles.

Thank you very much for your replies. It is very much a "knock" like a hammer. I will get to the intake gaskets to fix the water leak soon. whats funny is that very thing I have read here about these trucks has come true to mine. Thanks again.

I dont believe that the piston slap type knock was pre 99 or 2000. I thought that it started with the new engines, 4.3 4.8 5.3 ect.... I think you have a main bearing knocking on you. Just a thought.

I dont believe that the piston slap type knock was pre 99 or 2000. I thought that it started with the new engines, 4.3 4.8 5.3 ect.... I think you have a main bearing knocking on you. Just a thought.

His very well could be a main bearing knock, But wouldn't there be a reduction in oil pressure. Mine I know isn't a main bearing because it is in the cylinder. I don't think the pre 5.3's had a problem with piston slap either, But mine didn't start until 140,000 miles so I figure something is worn. Mine is a metallic knock , But not the same noise a bearing makes and I also pulled my pan and checked the rod and main bearings and replaced the oil pump. When I had it apart the bearings all looked fine however I threw a set of standard size rod and main bearing in anyway. The sound never changed a bit and even if the crank was worn it would have quieted it down for a short time. Mine I think is either a piston slap or a wrist pin, But I don't know. It is still going after more than 50,000 miles so I just drive it. I do what until the engine is warm though. When it goes it will get a new long block.

Thunderbolt, Were you able to pinpoint the offending cylinder? Can you remove the pan without jacking up the engine? I am leaning towards the wrist pin theory as my oil pres. reads the same now as when I purchased the truck. My idea would be to replace noisy piston/wristpin but retain the rings. I would however breakout the plastiguage and check all the bearings.

Thunderbolt, Were you able to pinpoint the offending cylinder? Can you remove the pan without jacking up the engine? I am leaning towards the wrist pin theory as my oil pres. reads the same now as when I purchased the truck. My idea would be to replace noisy piston/wristpin but retain the rings. I would however breakout the plastiguage and check all the bearings.


I haven't tried pin pointing the cylinder yet, But I do know it is on the passenger side. I don't know if it would be worth tearing the engine apart to replace one wrist pin, But if you can definately pin point it it may work. You have to either jack up the engine or pull the front diff to get the pan off. I pulled the diff because it is easy to do. You will not be able to do a wrist pin without removing the engine and dissassembling it. The head has to be removed in order to remove the piston and if you replace the piston you should hone the cylinder and use new rings. By the time you do all that you are spending almost as much as a reman. engine. In fact I don't know if you can even by individual pistons.The trans and engine has to be seperated to remove the crank and even if you were able to do that removing and installing the crank from underneath would be nearly impossible. With all the work it would take I don't think I would do it. I wouldn't tear a 100,000 mile engine apart for a wrist pin or piston slap if it were mine. I would keep driving it. If it is a bearing you could replace the crank, bearings and oil pump, But you still have to remove the engine. I would drive it and save for a long block if it was mine. I now have the money set aside when mine does go (although it has been over 50k since it started). Brand new from the dealer I can get a complete long block for $2,500 and it has a 3 yr 50k warranty.

I hear ya. With all that I am having to spend on this darn thing, if I can cut a corner and make it work I will. Guess I'll start some bank.

If you wand to find out if its a rod knock the fastest way is to run the engine to a point where you can hear the noise and then remove 1 plug wire and see if the noise changes. If not replace wire and move to the next 1 . If none change is sound chances are its not a rod bearing but a main bearing.
And whom ever said water in the oil will not hurt crankshaft bearings or rod or cam bearings has NO idea what he/she is talking about.

First off water in the oil has nothing to do with a bearing going bad. When the intake gaskets leak on those lik mine did , it leaks out of the engine. Is is a knock or more of a ticking , could it be a lifter that is collapsing, and if it is more of a knock it would almost seem to be a main bearing......

:banghead: Let me phrase that the way i had intended it to be. What i meant to say is, a coolant leak, more particularly the one he is talking aobut would not cause a bearing knock. But i do agree that water in the oil could lead to that.......sometimes the typing gets ahead of the brain. Sorry for any confusion this may have caused.

All pulling wires would do is eliminate the possiblilty of a dead cylinder, if it idles fine, then you're hitting on all 8. From the sound of it, there is a part is worn past specs. I would say it's most likely not a main bearing because the crankshaft is always pushing against the main bearing caps.

More likely, it's the rod bearing or the wrist pin. Here's why:
Rod and piston go up, at TDC, the piston slides slightly past where the rod pushes it to. When the rod goes back down, the wrist pin hits the piston (at idle, it's moving slow enough at this point that the sound is very quiet, and the piston is not pushed far past where it should be), the same thing happens at BDC, but it's all reversed.

J-Ri
You are wrong , pulling a plug wire will kill that cylinder and if that rod is bad the noise WILL change because now it has no load on it. This is a very old trick to use and if done correct it will work or at least give you a good idea as where to look for the noise. This is not to find main bearing problems but for rod/ piston problems.
This was showen to me 20 years ago by a MASTER Auto Tech and when I was MASTER I mean this guy new his stuff. He could tell you what was bad with your car by just walking up to it. Now I know the cars have changed a lot over the years BUT the 4 cycles and how they work have stayed the same

The small block Chevy engines had (probably still do) a short skirted piston.
I've had them crack on the side and then knock a bit. Pulling the plug wire did find the bad ones.
I don't think this noise went away when it warmed up though.

If it is on the pasenger side, it may be a vapor canister valve making the noise. at least the Express vans have this problem. There is a service bulletin about them I believe. The symptom is that it sounds just like a spark knock.

If it is on the pasenger side, it may be a vapor canister valve making the noise. at least the Express vans have this problem. There is a service bulletin about them I believe. The symptom is that it sounds just like a spark knock.


It is actually called the Evaporator Purge Solenoid according to my Service Information. The suggested remedy is to replace the valve. Apparently the old valve makes this noise when the valve seats.

What is a vapor canister? And where is it?

Mine started out as a knock on acceleration and ended up being a blown head gasket and two cracked heads at 105,000 miles. Could also be carbon build up quieting as the engine gets hot. Pull your egr valve and check for carbon build up. I'm leaning towards cracked heads, you will need to do compression check to verify. Bear in mind, I am a TIG welder/laser operator but these are problems that I just had on my truck.
Thanks, acewelderman

This is piston slap, IMO. As piston expands, knock typically gets quieter, or goes away completely. If it was a bad rod bearing, it would knock all the time. A little psiton slap is no big deal. My 98 Silverado has some. Hell, my fresh 468 race motor has some too, although forged pistons are more susceptible to the slap than cast, as bores need to be bigger to allow for extra expansion of forger pistons. Cast pistons expand less, so bores can be tighter, but still piston slap is a fairly common thing.

Pistom slap will not be there after the engine is up to operational temps. GM piston slap is a very old problem hell the 2.2, 3.1 just to name a few had that problem and it was always on a cold (first)start up then go away as the engine warmed up.
This might sound dumb but there was some talk about bad pcv valves cause this problem and we did replace a lot under a very quite recall