I think that anecdotal evidence points to the left front as the worst offender.
Trev is right - I'd monitor the portals on any UNIMOG that is a highway cruiser.
I have the later, two piece axles, and while they are 'less likely' to have the problem, I don't assume anything. I have not had a problem yet, but I don't think I'm immune.
The phenomenon has been discussed quite a bit in North America, and in Europe, and the 'Burpers" or portal vents look to be a viable solution.
Franz Murr was one of the first to offer a manufactured solution to tackle the problem. There are threads on these things, but I can post plenty of photos of the pieces if you can't find them.
One thing I wonder about is the whole cycle involved - Without the Burpers, the oil can cavitate, climb up the vent tube and migrate into the axle tube and differential. Once it makes that trip, it no longer settles back into the portal, so damage can (and does) result, because the portals are either running dry, or at least "less wet".
So, if you check the oil level in the hubs, and it is down, you know the oil has gone somewhere, so you top it up and work on getting a solution.
So you buy the Burpers, and now, when you are done with your drive, you check the oil level, and it hasn't lost anything, so all is good, right?
I wonder if that is the whole story ?
These systems have a Fat part of the vent tube (Von's) or a can (Murr and others) which serves to break the vacuum and keep the oil where it belongs. The proof is in the monitoring of the oil level and having it show no loss.
My question is, what is to say that the oil does not continue to cavitate and travel up the vent tube, leaving the portal box, but it only makes it as far as the can or the fat tube part of the system ? It would not get past that part, but it might not be staying in the portal box, either.
When you park and let it settle, the oil would flow back into the portal, your oil level would be fine, and you would read this as 'Problem Solved'.
But, would it not be possible that the oil that you really want to be at full capacity in the portal box while driving is along for the ride, but it is riding along upstream in the can ? Yes, it settles, but during your trip, your portals might be running in a 'less wet' condition.
I don't know if this happens, but I don't see how just monitoring the oil levels before and after the trip can absolutely eliminate the possibility. I understand the theory, that the vent systems break the vacuum and keep the oil where it is supposed to be, but you can't see that happening from the driver's seat. The various vent systems keep the oil from disappearing into the axle tube, that much we know. Do we know that they absolutely keep it all in the portal box when the truck is at highway speed ?
It seems like we'd need a few hours of GoPro footage, with clear oil lines in the vent system from the banjo fittings to the cans, to see if the oil makes the trip up and out of the portal, or stays put like we all want it to.
I'm just wondering. It might be nothing, as I don't recall people with the modified vent systems subsequently having portal failures, and maybe somebody has done some experiments with clear tubing and a camera. I don't know.
The reservoirs that Lost in the World has installed look to be a well thought-out and innovative solution. They address the initial problem - a design that did not allow for an adequate amount of oil in the portal box to begin with. Mercedes did increase the oil capacity in the later models, but the 3-piece axles as seen in the U1300L UNIMOGs have the problem in spades, and there are probably more of them on the highway as campers than any other single model of UNIMOG.
I think those add-on reservoirs look great, and I think they are well tucked away, and pretty well protected from damage. It all depends on how and where you go.If you do a lot of Rock Garden climbing around, you could probably ding them.
If you drive through a lot of Willows or other stout sticks, you might well snag the Burper vent systems and yank them off as well. I think for 'Normal' off-road travel adventures, they'd be fine. If you do ding them, you could remove them, and keep going. You'd still have the vent system modification.
The add on reservoirs might be a little sleeker if the screws for the plate were countersunk, but that could be a problem as well. Exposed heads, if damaged, can at least be grabbed with small vice grips or something. If you smear the lid with countersunk fasteners, you are looking at drills and E-Z outs. E-Z outs with fasteners of that small scale, I would not want to try, especially out in the dirt.
These add-on reservoirs are not made to fit my axles, and I doubt they will be.
If there have been experiments that prove the oil never leaves the portal box at all, I'd be interested in seeing them. I'm not saying there is a problem with the modified vent systems, I'm just wondering what the oil actually does.
The portal gears are still churning it up as they always have, it would still want to climb up the tube. Do the vent systems keep it from climbing, or do they just prevent it from making it to the axle tube, and allow it to settle back when the driving is done?
Last edited by TRUKTOR; 06-21-2019 at 02:44 PM.