JoeH

Sounds like you have a bent pushrod. I'd be checking valve lash on everyone. Common problem on these engines is the carbide faces on the lifters breaking and then what remains shaves the lobe right off the camshaft. Berry Cams I think can weld and grind a new lobe onto the cam for you if you don't have a spare cam. You'd have to come up with a new lifter tappet, and replace the bad pushrod. Truck could have weak valve springs that allowed the pushrod to come loose and get bent in a high rev situation. I'd drop ol pan, look for tappet face it there and get eyeballs on the cam from underneath. Valve covers and oil pans show you a lot of what's going on in there.

Sideways torque arm? Rubber doesn't have as much resistance to keep spring packs straight, so your axles will tend to wander more. We have a truck with 38k rubber bushed camelback and the tires will wander as much as 6 inches out of alignment backing up in turns. Truck doesn't take long to hit this point after putting new bushings in.

Gas engines have valve seals because when you slam the carb shut it creates vacuum pressure that can suck oil down passed through the valve guide. Diesels are fully open intake all the time, so no jump on vacuum pressure when you get off throttle. People more familiar with these engines can chime in, but it's my understanding diesels don't really need valve seals.

Did the truck have wedge brakes originally?

Lol I was reading service upside down, it looked blurry to me. Somehow I got DOT AV30 out of it... I'm going to take another stab and say Holding Valve.

Looks like it says DOT AV30 on it, so google provides an air volume control. Where was it mounted? Did it have lines going anywhere? What was this truck before?

Tractor protection valve?...

We bought a 1/4 cow last year and put it in a chest freezer. Was about $5/lb hanging weight, we dealt with the farmer who worked with a local butcher. Ground beef at the grocery store is cheaper, but where you save the money is on the roasts and steaks. Local small butcher shops I expect could put you in touch with a local farmer, but maybe the butcher environment is a little different here in PA.

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Our 79 r686 has about 6 inches of wander. It can get bad enough the slack adjusters can hit the frame rails...

There should be a sideways torque arm from the right frame rail to the rear differential. The eyes on these wear out, but if the truck is acting this way from new I'd wonder if someone forgot to install it... Reread, you stated it's missing the transverse torque arm. There's your culprit. I assume all your other trucks have it. For shits, go to your dealer parts dept, and run the trucks VIN and ask for that torque arm. If it comes up as a part on the truck then I'd tell the dealer to file a warranty claim and install it on Macks dime, citing that it never had it from new so they must have forgotten it at the factory.

You'd blow up the motor at 14,000 rpm! Change your fuel filters (2 I expect) when they clog they become a restriction point and act like a governor.

I think you're overthinking it. Its thin wall metal, and gets softer the more you heat it up. I'd think the pipe would give before anything else. My '79 endt676 is rigid pipe off turbo, down along transmission, and below frame. It then has flex pipe before heading to the muffler. Pyro says downshift at 1125 degrees, and it's factory rigid.

Its not an expansion joint, it's a flex pipe. You're fine on rigid pipe until you make the transition to the frame mounted muffler stand. That transition will need to be flex pipe. Never used the bellows type. The engine and transmission can rock sideways in the frame a good bit under load. Especially if your transmission bolts work loose.... 🙄

Heck, I'd be fertilizing my pants on those inclines! My truck runs a V box type body that is about 11'6" overall height at 73,280 lbs so I get nervous at any kind of angle.

Drove a Peterbilt once that had an air piston to raise/lower the passenger window via a switch on the dash. I thought that was slick. Driver side was hand crank. On those steep inclines I'd be a bit nervous on light springs. I'd want heavy springs to a) create less lean angle between frame and wheel alignment, and b) more safety factor to offset overload fatigue on the downhill side.

I'd love to see the rig you're hauling around at 30 degrees! That's a great gauge to have in the cab!

That's good work for one of these trucks. Camelbacks have phenomenal articulation over rough terrain as I'm sure you already know.

Reading into your post a little more thoroughly, what are you using the trucks for? How many miles are on them? The older camelback suspension lacked a sideways torque arm, so the axles tend to wander. You'll want your steering and suspension components nice and tight to help stay in a straight line going down the road. King pins, drag link, tie rod ends, spring u bolts all tight up front, and trunnion bushings, elephant pads and torque arms in the rear. You'll also want to keep an eye out for vertical frame cracks under the cab where the frame starts splaying out to go around the engine and transmission. This is a single frame issue, haven't heard of a double frame cracking there. We've cracked/replaced both frame rails at different times on our '79 R686ST.

Best thing to do is keep everything in the same engine family. That way one spare engine can supply the fleet, or if a truck goes down it still has Parts value.

The real driving force behind whether or not you'll see them in another 10 years is a) they aren't building them any more b) they're so old that it's unrealistic to expect Mack to keep making parts for them and c) the repair industry is so much further down the road simply because of a) and b). There's no real reason you shouldn't expect one of these to perform admirably for you. Nothing's bulletproof, new or old. On these, if you're prepared to dig for spare parts and fix it in house then I'd say go for it. Just remember, Mack engineered their whole engine/drive line to work together "as engineered." Dont go ramping up the HP, Mack already built their engines to their limits.

If it happens again I've got 11 more good lifters in the spare engine, and I'll send the currently bum spare cam out to have a new lobe welded on while the good engine gets ready for cam swap.

*shrug* last time we lost a lifter the truck got the cam and a lifter out of our spare engine.

These motors will run until you break the carbide face on a valve lifter and wipe that lobe off the camshaft. Then you're borrowing the cam and a lifter from a spare engine. Could happen tomorrow, could happen in 20 years.

Yea, modern diesel is "bio" and grows stuff pretty quick. They make an algicide fuel treatment that will kill stuff, but won't break it down. Our front end loader sat for 6 months once while we pulled the motor to deal with oil leaks. When we got it back together it kept clogging the strainer in the electric primer pump.