Scary engine knock

[david85]

Not sure how much I should be worried about this, but my Dad's 1993 GMC 6.5 turbo (non ECU) is knocking a little and has been for about a year. Recently its gotten a little worse and is most noticable on deceleration or engine braking around 1500 RPM.

The key symptom here is that the knock is louder on deceleration than applying power.

It starts, runs and drives fine with no smoke other than the initial startup puff.

 

[Smokey73]

Rod Knocker Baby!!!!!

 

[david85]

Ouch, I think you might be right, but.....can I get a second opinion from anyone?

 

[cm1hedge]

I say wrist pin. Still needs to come apart to see.

 

[Smokey73]

Knock, Knock, Knockin' on heaven's door
I think i'd have to agree, more than likely a wrist pin, but possibly a fuel knock. I don't have a lot of wrench time with those engines, but I have seen a few with bad injectors that sounded like gremlins trying to beat thier way out with a 10lb hammer

 

[bab029]

Had a buddy have this happen to a junkyard diesel suburban last year. didnt bother fixing it, just ripped it out and put a tbi 350 in it. But that's probably not the direction you wnat to go with it....

In my estimation, the problem is probably directly linked with the red lettering on the grille. IMO, of course. ;-)

 

[david85]

Definately not going with a gasser. Most likely end up rebuilding the engine if its really that bad.

I tried to use the broom stick method to listen for the sound and got nothing off the oil pan. The rear passenger side injector line did give me that specific sound though. I guess it could be that injector (no idea if the injectors are factory original), or maybe a wrist pin knock is simply being felt up the steel body of the injector.

I did notice some times on shut down, that noise can still be heard. Once the key is off, there shouldn't be enough fuel pressure to pop the injectors anymore right?

Any more thoughts?

 

[towcat]

if you go with a newer 6.5, do consider the gen2 longblock from r-j performance. they're selling them for $4300 and they're up in Canada too.
what's the big deal in the gen2? they now have piston cooling jets like everyone else been doing for years

 

[david85]

No cooling jets in the 6.2 and early 6.5? thats pretty dumb. Even the little 2.3 4 popper diesel in the ranger had bolt down cooling jets and it was only intended for compact japanese cars and pickups.

I've been told by a former GM tech that this is a rare 6.5 because there is no EGR, computer or wastegate pressure line on the turbo so who knows what is in this truck. The PO was also a GM dealer lead mechanic who didn't really want to sell the truck. We suspect the dealer pressured him into upgrading.

For $4300, I would sooner just rebuild the engine myself. My Dad can drive one of our other trucks in the mean time. Being in canada would be nice since there wouldn't be any brokerage (AKA free trade surcharge), but I just don't trust any reman engine outfits after all the horror stories I've heard over the years.

I'm wondering if this could be an injector, but would an injector knock be louder on deceleration?

 

[towcat]

dude-
it's a brand new motor.
gen2 shortblocks was recently released to keep up with the humvee meltdowns. uncle sugar was paying $10k/ea. for just the shortblocks.
with the cooling jets, they are now as reliable as the IH idi's

 

[towcat]

I've been told by a former GM tech that this is a rare 6.5 because there is no EGR, computer or wastegate pressure line on the turbo so who knows what is in this truck. The PO was also a GM dealer lead mechanic who didn't really want to sell the truck. We suspect the dealer pressured him into upgrading.

you have a "F" series motor in the truck
I don't know what the emissions rules are for up north, but the 1/2t and light 3/4t trucks got the "S" motors. iirc most of the "F" stuff is a bolt-on, so that's a good thing, there might be a difference in the cam profile, but I don't have confirmation yet.

 

[NJKen]

How about a cracked flex plate? Noise only on decelration could be because the flexplate gets torqued in the other direction? Just a thought
Ken

 

[cranknrods]

had this happen to me when i had a 6.5, it ended up being the harmonic balancer.
it wouldnt make any noise under acceleration but decel and high idle made a lot of racket that sounded just like a rode nockin. it makes alot of noise cause the metal ring slips and nocks against the inner pulley. really hard to explain but if you get under the truck and look at it youll see what i mean.
if it is the balancer dont buy one from GM its like $400
look around and you should find a new one for about 75 bucks from another manufact..

my second thought would be the plastic retaining clips on the rockers--(yeah plastic ***?) pull the valve covers and take a look. if thats not it then i would start worring about wrist pins and injectors

hope this helps

 

[david85]

Well a little update here.

The truck was never taken off the road and was driven all this time.

A few days ago I wanted to try and swap injectors between two cylinders to see if the knock followed the injector from the offending hole (rear, passenger side cylinder). Turns out I didn't have a socket of the correct size so all I was able to do was disconnect the return lines and the steel high pressure lines from those two injectors, then put them back on after realizing that pulling the injectors was impossible.

Fired it back up to purge the air and after it settled into a stable idle I noticed the knock was gone. Not reduced.....GONE!:***:

I don't know whats going on here, but my best guess is that one of the injectors had a problem that was temporarily knocked loose by ingesting air during the restart. Been running for a few days since I messed with it and the noise still hasn't returned. I have no idea how old the injectors are, but they are bosch and I don't think they are OEM. No clue on the injector pump either. Could be factory for all I know.

Safe bet that based on this, the problem is not in the bottom end?

Maybe the Diesel Gods are smiling on me.....or laughing themselves silly

 

[david85]

you have a "F" series motor in the truck

I take it thats a good thing? I know next to nothing about the history and back round of the 6.2/6.5 series.