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Rod knock?

M

miltowndiesel

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I bought my truck not running and took it to the local deisel shop to have them check it out. the OPS was bad and once it was running they said it had a lower end knock. Assuming it needed a motor i towed it home. I put a new OPS in it just so i can move it around without pushing it.

Now that is running i can hear the knock myself. It doesnt seem like a rod knock to me. I think it just needs injectors. Upon start up it puffs a little white as if you had some bad glow plugs but not as bad and continues to slightly puff white during idle.

Also the oil pressure is a little above 40 at idle. This seems odd to me. Ive never seen an engine with a rod knock hold that much oil pressure at idle. Could it be that i may only injectors? There is also a cylinder imbalance on number 3,4, and 7. What do you guys think?
 
I've only experienced an injector knock once, and it scared the crap outta me! Sounded like a rod knock from hell! Something stuck on the pintle, and it wasn't closing completely. By the time I got out from under the hood and over to the driver's door, the knock had stopped and the engine was running smoothly (and quietly).

The best bet for troubleshooting noises is a mechanic's stethoscope. I bought mine at Sears. Gotta tell ya, you can't beat it for locating noises. It'll tell you whether you're hearing a lifter click or an exhaust leak at the manifold. You can also quickly find the specific location of any bad news noise.
 
Do compression test on all cylinders. Or at least the mentioned ones.

When an injector gets stuck open, that cylinder will fire too early, giving it a severe knock sound.


If compression is fine, then i'd replace injectors and then diagnose it from there. Worst case scenario if it is lower end failure, i'd drive it till it poops out. Could run that way for years.
 
The shop i took it to is a very reputable one and works on all diesels not just gms. I trust their judgement but im real skeptical on this one. It does seem like a possible stuck open injector.

I dont know if you will ba able to hear the knock in a video. How do i go about posting one?
 
miltowndiesel said:
The shop i took it to is a very reputable one and works on all diesels not just gms. I trust their judgement but im real skeptical on this one. It does seem like a possible stuck open injector.

I dont know if you will ba able to hear the knock in a video. How do i go about posting one?
Click to expand...

Get your signature figured out so we know what type of truck and how many miles, etc.
 
All diesels are not like the 6.5, so just be weary of a mechanics judgement, and best to get your diagnosis on this site and then just tell the mechanic what to do.
 
The shop is the number one diesel shop in milwaukee i dont know if im allowed to say the name or not so i wont. These guys only work on diesels and have been that way since the 60s or 70s.

Sorry i didnt fill out my sig right away. The truck has 305k miles on it but who knows if its the original motor
 
could be a stuck injector,would explain the persistent white smoke. crack injectors one by one, if the knock goes away. you found the bad one. If that is the case I would replace all 8.
 
If you pull glow plugs for compression test, look for wet ones, they usually are wet when you have a bad injector. Yes, they can sound awful. Does the exhaust smell like raw fuel.

Also, a stuck open injector will heat up alot faster, on a cold engine. Do the finger test or temp gun. Crack that injector and see if the knock goes away, you also may see some compression gasses exscape with line cracked(kinda looks like steam).
 
bison said:
could be a stuck injector,would explain the persistent white smoke. crack injectors one by one, if the knock goes away. you found the bad one. If that is the case I would replace all 8.
Click to expand...

I would have suggested this also, but a stuck injector in my experience only knocks on acceleration, not at idle. The smoke however, will go away, assuming it smokes at idle.
 
midniteplowboyy said:
If you pull glow plugs for compression test, look for wet ones, they usually are wet when you have a bad injector. Yes, they can sound awful. Does the exhaust smell like raw fuel.

Also, a stuck open injector will heat up alot faster, on a cold engine. Do the finger test or temp gun. Crack that injector and see if the knock goes away, you also may see some compression gasses exscape with line cracked(kinda looks like steam).
Click to expand...


This may or may not be true, and may just be a cooincedence that my stuck injector was 7, the back one closest to the exhaust. BUT all my injectors/lines were cool to the touch, except for the culprit injector which was EXTREMEMLY WARM compared to the others.

Do ha hand check and see if one or more injectors is greatly warmer that the others.
 
Matt Bachand said:
I would have suggested this also, but a stuck injector in my experience only knocks on acceleration, not at idle. The smoke however, will go away, assuming it smokes at idle.
Click to expand...
Depends wether the injector is stuck open or closed. If stuck open you get insufficient burn [white smoke]and the raw fuel could mimick slight hydralock symptoms and advanced ignition creating the knock
If stuck closed,the knock is generated by the sudden expansion of the fuelline,[kinda like water hammer]cause there is nowhere to go for the fuel. that will heat the line up.In both cases the knock is there at idle as well, but less so in the stuck open position.Thats my experience.
 
bison said:
Depends wether the injector is stuck open or closed. If stuck open you get insufficient burn [white smoke]and the raw fuel could mimick slight hydralock symptoms and advanced ignition creating the knock
If stuck closed,the knock is generated by the sudden expansion of the fuelline,[kinda like water hammer]cause there is nowhere to go for the fuel. that will heat the line up.In both cases the knock is there at idle as well, but less so in the stuck open position.Thats my experience.
Click to expand...

great info. Other peoples experiences are what make this all worth while.
 
Any update Miltown? Did you get it figured out and resolved?

Since you have a 94, the GMTDScan is very helpful, it would allow you to turn off injection to specific cylinders by a click of the pointer. And a lot of other cool stuff.
 
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