knocking sound in engine

[milk123]

It's an 02 Bravada with a 4200 engine. Today I went on a 2 hour drive and when i returned and parked it in the driveway, my wife went out to go to the corner store and when she went to reverse out the yard i heard the strangest sound. It sounde like two pieces of plastic hitting together( like something in the fan ) so I open the hood and the sound was gone so I just waited for a minute or so and then this loud knocking sound started coming from the engine, or at least the front of the engine.. I was dumbfounded, the sound goes away as quick as it starts. It's not there all the time,it comes and goes. The oil pressure is fine on the gage, the oil level is fine, cooling is fine.. I hope it's not a main or rod bearing which is what it sounds like. If it was, would'nt it be there all the time? I cannot make the sound happen meaning , i've been trying different things with the throttle and so on but like i said,it comes and goes. By the way , I've driven it since and there is no missfire or loss of power or anything like that. Basically,it's just making this sound. Any ideas?

[maxwedge]

Not without hearing it, maybe disconnect the belt and see what happens, but as you say it comes and goes.

[milk123]

Originally Posted by maxwedge

Not without hearing it, maybe disconnect the belt and see what happens, but as you say it comes and goes.

What did you mean? Not without hearing it.

I did take the belt off and never heard a sound for the ten or so minutes i ran it. I reinstalled the belt and fired it up and the sound came right away. I just chalked it upto coincidence.

When you look down at the crank pulley it looks like it is woobling a little. I mean the bolt deos'nt seem to be rotating a perfect axis. I don't know if that means anything or not.

I also read about a condition in that engine on the trailblazer part of this site that mentioned a cracked cylinder sleve that causes a knock.

What do you guys think?

[maxwedge]

I meant we here in this environment are obviously handicapped trying to diagnose noises, that said, the fan clutch on occasion has been known to make this type of noise, a cracked cylinder sleeve would be more consistant. BTW I own the same drivetrain in a Trailblazer. The only thing relative to this I've run into is the usual piston slap when cold.

[milk123]

Originally Posted by maxwedge

I meant we here in this environment are obviously handicapped trying to diagnose noises, that said, the fan clutch on occasion has been known to make this type of noise, a cracked cylinder sleeve would be more consistant. BTW I own the same drivetrain in a Trailblazer. The only thing relative to this I've run into is the usual piston slap when cold.

Oh, ok, thanks. I know what you mean about trying to diagnose sounds..I'm waiting for the dealer to open in a couple of days to drop it off and see what they think.

Please feel free to make any suggestions.

Should i be concerned about the crank pulley bolt?

[maxwedge]

Some slight pulley movement is normal maybe 1/16" ( due to machining tolerances), you could have them make sure there is no issue there for starters.

[ericn1300]

not sure about the 4.2 drivetrain but on the older 4.3 these kind of noises can come from broken motor mounts, you might want to check that too.

[milk123]

Six days at the dealer and no fix!! Luckily enough with a bit of patience i think i may have found it. Now that it is in it alot more it seems to be coming from the a/c compressor. When the noise starts up with the a/c turned off i just turn on the a/c and the sound stops immediately. The only thing that is puzzeling me is that it sometimes happens with the a/c off. Once i was looking under the hood with the engine running and the a/c off and the rattle started and when it did the clutchwhich was stopped, turned about half a turn like something was binding up in it. Could it be a bearing in the clutch that is making the noise where it sometimes can be heard with the a/c on or off..Besides, The shaft in the compressor shoud not be turning with the a/c off..I'm 99% sure that this is where the noise is coming from so do any body know if the bearing and or clutch are replacable?

[maxwedge]

They should be seperately replaceable normally these bearings don't knock ,but make a growling rattling type noise, careful here make sure this is the problem.

[milk123]

Originally Posted by maxwedge

They should be seperately replaceable normally these bearings don't knock ,but make a growling rattling type noise, careful here make sure this is the problem.

Your description of the noise seems alot closer than mine. It is more like a rattle. According to the GM dealer and Uap/Napa in my area, they are not sold seperately.(new or remanufactored) Dealers price is $1076.00,while Uap/Napa is $530.00 reman. Seems like alot of money to me.. Anyone know where to get one??

[maxwedge]

Salvage yard, but the risk is greater.

[milk123]

To follow up: It was the compressor, It knocked for weeks and i even had two mechanics say that it was the engine..I kept trouble shooting until the clutch finally gave up.. It got to the point that the knock stopped and the clutch started to whistle.. Anyway, it's fixed now with one from the scrapyard, $350.00 with a years warranty plus they'll pay the labor if it has to be replaced. Thanks for the help guys..