4cyl 30L IO Engine Knock

"I was having issues with my e

"I was having issues with my engine, brought to credible mechanic. Exhaust tube replaced, heads rebuilt and restored compression from 60, 80, 85, 110, to 150 across the board. The upper half of my engine is top notch.. there is however a knock from the bottom side of the engine. Could it be a piston? The mechanics believe it is but can't say for sure and I don't have the money to continue to go for diagnostics... can anyone help or give suggestions?"

"Kevin
Was the knock there be

"Kevin
Was the knock there before the work was carried out ?
Without hearing the knock it is just about impossible to judge where it is coming from, it could be a piston, it could be a bearing, there are several possibilities.

Is the knock there on start up, does it get worse with engine temp, what is your oil pressure gauge doing, is it high or low or fluctuating with temp. When was the last time you changed your oil and filter, what grade of oil are you running
A good mechanic should be able to roughly pin point the noise for you but the only way you are going to fix it if its internal is to strip your engine and do a rebuild.
Cheers
Peter C"

thats basically what the mecha

thats basically what the mechanic said they would have to get internal to pin point it. They also said they didn't hear it until the upper half of engine was rebuilt. I didn't notice the knock before. They pressure tested the oil system... said it was good. Thanks for your reponse Peter.

"Carbon on the pistons would h

"Carbon on the pistons would have been removed by a good tech. when the head was done. Did they? Heavy carbon will also cause the engine to knock and shake like a miss. Put a broom handle to the head at the spark plug and to you ear/cheek to see if you can feel it and then isolate it. If you feel it at one piston, it could be carbon knock, piston slap, loose rod and/or bearing or worn crankshaft."

"no... they didn't get tha

"no... they didn't get that far into the engine. at 95 an hour and already at 1500 for rebuilding the head, replacing exhaust, and whatever else they did (id have to look at the invoice) they said it would be nearly another 1000 in labor to disassemble and reassemble the engine to look at the pistons and crankshaft. thanks for your response guy gaspar"

"The way I see it you have 2 c

"The way I see it you have 2 choices.

That knock undoubtedly means something serious, like bad main bearing, bad connecting rod big end or small end, spun bearing, knocking piston, etc. If not addressed, it will likely lead to a major failure where the crank will be scored and the block could be trashed if it throws a rod. In this case you would be in the market for a new short block (~$2500), since your heads are good. There will also be about $1500 in labor to switch parts over and R&R the engine.

Otherwise you can bite the bullet now and have your engine torn down and diagnosed. If only one thing is wrong that would be less to fix. If the conclusion is a major rebuild, you are likely still better off to buy a short block than pay the local labor to have yours rebuilt. I only say this because engine rebuilders are engine rebuilders; that's all they do. Your local mechanic may be quite capable of rebuilding yours, but it would likely wind up costing more to go that way.

Having said all this, GMC gas truck engines from 1999 thru 2003 (4.7, 5.3 & 6.0 L) have a notorious characteristic known as cold start knock, or CSK for short, due to the teflon coated piston skirts used during those years I beleive. This problem is well known to GM, and affects low and high mileage engines equally. They maintain that it does not affect engine life. In fact they have stood behind these engines and even offered extended warrantees on them to set customer minds at ease. CSK is disconcerting to say the least, but not life threatening. Your engine would not be knocking for the same reason.

So you have an awkward decision to make. Either way, it will involve some $'s.
How many hours on it? If more than a few hundred, a rebuild is probably where you are heading.

Rod"

"Guy
Just on the decarboning

"Guy
Just on the decarboning of the piston crowns, there are 2 schools of thought on that, the stock standard procedure is to decarbon but there are a few guys I know who like me have spent alot of time building engines and dont see it that way.
Some of them maintain that if you rebuild a head on an engine that has a fair few hours on it not to decarbon.
Personaly, I generaly have always decarboned the crowns, but I still do know a few guys who wont if the engine has alot of hours on it.
Cheers
Peter C"

That's a shame Peter et al

That's a shame Peter et al. Turning the crank by hand until the piston tops out and scraping the carbon off the piston can't be that much work. I guess it comes down to not even opening up your engine w/alot of hours or years if you can't do the work; just replace it w/a reman. and save money in the long run.

"Guy
It was not generaly done

"Guy
It was not generaly done through lazyness, the theory was that in a tired engine, you may just push it over the edge.

When you have a half worn bore and rings, the theory is that the carbon build up may be the only thing keeping the compression viable enough for the engine to run half decent.

We use to see it alot back in the bad old days particularly when a vehicle would be towed in with a blown head gasket, the practise was to machine the head if required but not to decarbon.

Hopefully that practise isnt carried out that much nowdays.

Cheers
Peter C"

Peter: thanks for the clarifi

Peter: thanks for the clarification