Weird noise, almost like a rod knock

[6.5 Burb ’94]

1994 k2500 Suburban with 6.5l, I’m kind of new to diesels and I thought when I got this truck it would be a great way to learn diesels, boy was I right owned 3 years and had to replace many different parts and diagnose many different things but I’m stumped now.

Problem:
I was driving down the road around 50-60mph when I came to my turn so I left off the throttle and then this G-d awful sound came from the engine bay but no abnormal vibration. Luckily there was a parking lot I was able to pull into and shut the engine off and check a few things out. Gauges were all normal, no fluid omg leaking onto the ground or anywhere I could see. Started it back up and same noise. Shut it off unhooked the PMD, and started it same noise. This is where it gets weird, started it back up and went down the road a little to the gas station and the noise went away after going above 35-40 mph and above 2000 rpm, let off the throttle noise comes back. Best way I could describe the noise is a really bad rod knock but based off my knowledge of vehicles it’s not a bent or broken rod.

What I’ve tried:
Replaced all 8 injectors, and glow plugs, checked flex plate bolts, took the belt off and ran the engine with the belt off to rule out an accessory. No change. I’m completely stumped

Could it be a valve or lifter stuck? If so, is there a way to test before tearing into the engine. Thanks for any help.

[DmaxMaverick]

Welcome aboard!

Check the balancer and the acc. drive pulley. Both are rubber isolated, and will make all kinds of racket when the rubber goes away. Not uncommon. Could also be a broken flex plate, and possibly something else (bad news) I'll save until everything else is eliminated.

[6.5 Burb ’94]

I replaced the balancer about two months ago, I did check it though just to make sure it didn’t disintegrate in that time and it’s fine. When I had the belt off I also went ahead and tried wiggling the balancer to possibly rule out a broken crank and it didn’t budge. I haven’t inspected the flex plate meticulously, just checked the bolts I’ll do that next, I've thrown enough tools for today.

[sctrailrider]

While checking things, put a socket on the alt bolt or something else, and turn the motor both ways and see if you feel or hear a bump in the rotating assembly, if something is loose you may feel it ...

[6.5 Burb ’94]

Welcome aboard!

Check the balancer and the acc. drive pulley. Both are rubber isolated, and will make all kinds of racket when the rubber goes away. Not uncommon. Could also be a broken flex plate, and possibly something else (bad news) I'll save until everything else is eliminated.

The flex plate is fine no damage to it. If it helps it only makes the noise at idle and white smoke at start up but revving and driving down the road no smoke and no noise until idle.

[DmaxMaverick]

Did you replace the injectors before the noise, or after? Even new can be bad out of the box, and it sounds like an injector issue. If it's making the noise at idle, loosen an injector line nut, one at a time, to isolate the subject cylinder. Taking fuel away should eliminate the knock (it will miss, but shouldn't knock). Once you identify the suspect cylinder, swap that injector with one on the opposite bank, and repeat the test. If the noise follows the injector, then the injector is the problem. If it stays in the cylinder, then it's a cylinder problem.

[Yukon6.2]

My 6.2 did that once on the highway.
Stopped and checked a couple things ,couldn't find anything obvious.Drove a couple miles slowly to a pull out to do a more in-depth search.
Checked everything i could think of,plus my buddy with me was a wrench and tried everything he could think of as well.Found nothing and the noice was still there.Had some Marvle Mystery Oil and dumped that in it and said fug it we are going to miss out t time.
Had about 30 miles to drive to the golf course started slowly driving as fast as i dared by the noise.Slowly i was able to drive faster and by the time we got to the golf course the truck was almost back to normal.
I never did find out what the noise was,put a few more years and thousands of miles on it.My reasoning was a lifter that stuck and the MMO unstuck it slowly and was OK after.The water pump plate started leaking last year so the truck is parked.I will probably change the engine now,i have a fresh 6.5 sitting in another truck that is destined for the crew-cab.

[6.5 Burb ’94]

Did you replace the injectors before the noise, or after? Even new can be bad out of the box, and it sounds like an injector issue. If it's making the noise at idle, loosen an injector line nut, one at a time, to isolate the subject cylinder. Taking fuel away should eliminate the knock (it will miss, but shouldn't knock). Once you identify the suspect cylinder, swap that injector with one on the opposite bank, and repeat the test. If the noise follows the injector, then the injector is the problem. If it stays in the cylinder, then it's a cylinder problem.

I replaced the injectors after the noise started thinking maybe that would fix it. It didn’t, I did end up putting Marvel Mystery Oil in it and it helped quiet the noise a little bit not by much and i just sat in my drive way letting the engine run to get the MMO through the engine to see if it helped and sat there holding it at around 1500 rpm. The noise was only noticeable from idle to about 1200 rpm and thing higher and it sounds like a normal diesel clack.

[DmaxMaverick]

OK. Yukon has a good idea, too. It's possible, and since you used MMO and it "helped", maybe stay on that track. It can take a while to work through fully. Follow the suggestion in post #6 to identify the suspect cylinder. If it is a lifter, you'll still need to know where it is.

[6.5 Burb ’94]

OK. Yukon has a good idea, too. It's possible, and since you used MMO and it "helped", maybe stay on that track. It can take a while to work through fully. Follow the suggestion in post #6 to identify the suspect cylinder. If it is a lifter, you'll still need to know where it is.

Thanks! I’ll let y’all know what I find, after work tomorrow I’m gonna give it a shot cracking the injectors to hopefully find the cylinder or cylinders that are problematic.

[6.5 Burb ’94]

Engine grenaded on me....I was testing to find which cylinder was at fault and as I was testing it, the engine made a metal grinding noise and died. Went to start it back up hoping it wasn’t toast and no compression...

[Yukon6.2]

Now its time to tear it down and see what was wrong.
Always interesting to do a postmortem on it after they make a bunch of expensive noise.

[Robyn]

Almost sounds like a broken crank...

[arveetek]

Almost sounds like a broken crank...

That's what I was thinking.....

Casey