Possible Bent Push Rod or Intake Valve on M923A2

[brasco]

Uh Oh.... I think I may have a bent push rod or valve on a M923A2 with 6CTA8.3 engine. I was checking out my latest purchase yesterday and noticed a strange noise coming out of the intake tube. I had actually noticed it during the recovery a couple of weeks ago but didn't have time to deal with it then.

The engine starts easily and sounds normal with no excessive smoke. However, there is a very clear puh, puh, puh, puh, puh, puh, sound coming out of the air intake tube. I've done some research and I've determined that this sound may be caused by a bent push rod or valve.

I've checked the TMs and pulling the valve cover doesn't look too difficult. Maybe I should do a compression test first.

Has anyone here had a similar experience? If so, what did you find and do you have any words of wisdom? I'm not a diesel mechanic but I don't have any worries about digging into this engine with the TMs handy. It may be fun father - son activity once the weather warms up. One of the reasons I enjoy MVs is working on them and learning about stuff like this.

 

[73m819]

Before you tear into the engine, that sound could be air because that is where the air vent ends up.

 

[NDT]

If you are loosing compression out of an intake valve, that cylinder would not contribute and power would be down 1/6 th. In a gutless 5 ton, this would be very noticeable. More likely the compressed air system is venting for some reason.

 

[73m819]

If you are loosing compression out of an intake valve, that cylinder would not contribute and power would be down 1/6 th. In a gutless 5 ton, this would be very noticeable. More likely the compressed air system is venting for some reason.

PLUS unburned fuel smoke out of the exhaust

 

[spicergear]

I've had an intake noise on two other engines- no experience with it on the 6CTA though. Both were due to a cylinder not being able to be evacuated; one due to a bad exhaust lobe on the cam and the other a broken rocker and bent pushrod. The intake could open but the cylinder charge from the last cycle was still in there because the exhaust valve didn't open to release the cylinder pressure. As soon as the intake opens the pressure bursts into the intake manifold. The cam one would allow it to idle okay but start acting up with revs. Hopefully you have something simple and won't have to dig too deep.

 

[brasco]

Thanks for the quick feedback guys. I'll put the air compressor on the list of things to check out. Does a compressor problem still make the puh, puh, puh sound? I don't know if I can upload it here but I'll take a video today and try to get the sound recorded.

I do think this truck has less power that my others but it I chalked it up to needing the fuel turned up when I drove it home. I was still able to get it to 60+ mph. Acceleration was just so, so but I wasn't taking any data at the time.

 

[dawico]

One thing to try would be to open all the drain valves and/ or emergency glad hands and see if the noise still comes out of the intake stack. More or less run the truck with no air pressure. This should tell you if it is the air system or not.

Good luck and keep us informed.

 

[WillWagner]

A tappit/cam failure will make a chuffing sound in the intake or exhaust. A loose or tight overhead set will cause the same symptom. Easiest thing to do is pull the lid and take a peek at things. See if you can spin the pushtubes on the cylinders with closed valves, this will show a bent pushtube. Bar the engine around to check all of the cylinders. While you are there give the rocker levers a wiggle. The lash is .012 intake and .024 exh. If you come across one that has a mess of clearance, there's your problem. Look at the adjusting screw on that cylinder. If it looks like it is taller than the other intake or exhaust adjusters, someone botched a tune up or there was a piece of debris in the tappit when the pushtube was installed. If the clearance is excessive and the adjuster is not high, you have bigger issues.

 

[dmetalmiki]

I would go with the rocker cover take off, and check the valve lash first.

 

[74M35A2]

I'd crack each injector line open one by one and see if the idle speed drops when doing so before busting any gaskets open.

 

[Suprman]

I had that back pressure sound from the intake it was a bad ring on one piston. Caused by GL liberally applying ether on a cold day. Could also be a rocket arm slipped off and valve not opening.

 

[brasco]

I made videos near the inlet and exhaust. See files below. Any opinions on these? I can somewhat hear the noise in the exhaust but it is more pronounced on the inlet side. If it weren't for hearing the noise on the inlet side I would say everything else sounds normal.

I don't see excessive smoke, at least not more than on others I've seen.


View attachment M923A2 Intake Sound.mov

View attachment M923A2 Exhaust Sound.mov

I also let the air out of the system and it made no difference in the sound so I don't know if the air compressor has anything to do with it.

 

[brasco]

I had that back pressure sound from the intake it was a bad ring on one piston. Caused by GL liberally applying ether on a cold day. Could also be a rocket arm slipped off and valve not opening.

The use of ether by the GL guys is one possibility that concerns me the most. I picked it up during during one the cold spells we've been having.

 

[Suprman]

Is it weak on power? If you are down one cylinder it will be a noticeable loss.