Anybody here familiar with the innards of the Mercedes MBE4000 12.8? We have one in a 2004 Freightliner that's developed an audible knock at idle RPM. A knock like this makes one straightaway thing of failing connecting rod bearing. But here's where it gets weird, rev it up and the knock goes away. Typically, and again I say typically, when a rod bearing fails or is failing the knock gets louder and speeds up as rpm increases, rarely does it go away. Another weird thing is cranking the engine to start. You typically hear the started working against each compression stroke in a smooth even manner, like, ruh ruh ruh ruh, etc. What this thing is doing is ruh ruh ruhhhh, ruh, ruh ruh. One piston is causing a temporary stall of the starter until it powers through it to crank the other pistons. I've never seen an engine do that and that's the thing that has me puzzled the most. I pulled the valve covers to make sure there were no bent pushrods and that the valves weren't out of whack and working properly, nothing notable there. With the age of the truck, a half a million miles, and the cost of tearing into an engine now days they ain't going there, but I'm just curious if any of you engine guys have any clue what would be causing this knock and cranking problem.
Piston skirt and or wrist pin, Both can quiet down with slight raise in rpm.
And both of those would require tear down to confirm. They ain't gonna go there. Too bad, damn thing runs great and pulls like a mule.
But that still doesn't explain the one stroke that stalls the starter when cranking the engine to start. That has me puzzled.
If it has a scuffed skirt, it creates drag. It could be loosing a crank pin journal/bearing but you would definitely see a drop in oil psi. Cut the filter open, or if it's a cartridge type being a Merc--that will be easy to check.
Oil pressure hasn't changed, if I get a chance I'll pull the oil cartridge filter. Though I myself haven't seen it before, in theory I can see how a scuffed or missing piston skirt could possibly cause the cranking problem. If skirt was missing, when the starter is bringing it up on that compression stroke the connecting rod would be at some angle and the piston would get jacked up in the cylinder until the starter powered through it. If that's the case, not good.
Normally it's just a scuffed piston skirt, if it had a fractured skirt it would have caused a catastrophic failure in 98% of cases.
Got it.
Broken piston springs maybe
I call that the poor mans compression test. Crank engine with ignition off and listen if the compressions are even. But yes unfortunately i agree, she sounds like it has picked up a piston skirt. The cylinder will be scored. When you rev engine, you will hear it rattle on deceleration.
Correct----once on the way up and twice on the down.
Yes, like if you have a piston with broken compression rings, you'll hear one stroke speed up the cranking speed when that piston comes around. This is the opposite, one piston is putting a serious load on the starter when it hits that stroke, haven't heard that before myself. Unfortunately, I'm thinking it's tossed a skirt. My best guess, we have a hired on truck driver that's only drove autos, never drove a manual tranny. Let me tell you, that was like listening to fingernails on a chalkboard teaching how to drive a crash box, and he still ain't that good at it even after several months now. He took the Freightliner out one afternoon to pick up a telehandler and bring it back, it was making this knock the next day. I think he missed a gear and stabbed the throttle and over-revved the engine. That's my best guess.
Lugged it with throttle WFO, excessive cylinder temp.
Should be able to stab the throttle all you want and not over rev anything. That is what the computer/gov is for. In these mountains, WOT to the governor when downshifting to match RPM for the lower gear is par for the course to maintain as much speed as possible up the hill.
That's true to some extent, except for descending a grade. It's easy to go past red line if not using correct procedure downshifting with jake engaged on a AMT or even a manual. Really the worst over speeds come from downshifting. But in this case I think it was lugged.
Yes - You know this, if you are downshifting already on a downgrade you are in a world of trouble anyway. Always a very bad situation to get yourself in. But anyway, I could see that as a rod exiting the block or something, but not 1 cylinder getting sticky.
With AMT's in today's trucks it happens all the time, We've had Detroit send E-mail warnings to dispatch on over speeds. It ain't the 90's anymore. Detroit DD15 is maxed at 2400 rpm, Have seen readouts of 2900.
I think you nailed it. Driver is notorious for doing this. I've been shotgun with him on trips, like going to Chatanooga. Crossing Whites Mtn he dragged the Cummins X15 in our Pete down to ridiculous rpm grading that hill. Don't know what the rpm was 'cause I couldn't see the tach, but I've been driving these trucks since the 80's and I know it was below 1000 just by the sound and the vibration of the engine. My head was about to explode! I know why he does it, being new to crash boxes he's afraid to drop a gear on a grade fearing he'll miss. Even though I tried my damdest to teach how and when to drop a gear on a grade. He just can't get it and don't know that he ever will. I really think that's what has happened here.
Wonder if one dropping the oil pan and sticking a micro cam up there will help to see the damage to cylinder walls. Found scuffing on this recent C15 that way with erratic oil pressure drop.
Spud ---a MB 4000 isn't worth fixing, as a matter of fact you would be hard pressed to find a Freightliner dealer that would touch it---the reply would be, we are really backed up and two mechanics short right now it will probably be a minimum of two months before we could look at it.
I know of several long grades with a stop sign half way down, Mary's Hill on Hyw 97 is one where it crosses Hyw 14. I had two on each trip hauling grain this summer. Having to gear down on a grade is pretty common--only place it isn't is Interstate.
Along with a quote of 50 thousand dollars.
Yeah that's about right too.
I was trying to refrain from bashing the Mercedes engines in Freightliners, but I guess the Cat is out of the bag now.
Well I don't see any reason for political correctness when it comes to items such as engines. The 4000 was a turd in every sense. Some just had a little better luck than others.