Hey guys I have a 1977 300D with a bit of an odd problem Every now and then it tends to burp the transmission fluid out of the dipstick tube while at speed to the point where it will start to slip. Keep a gallon of Trans fluid in the trunk and usually just throw it in neutral and do the normal hot fill procedure. It dosen't do it all the time..and dosen't matter if its hot or cold. This is something I want to figure out pretty quick as having tranny fluid burping out onto a hot exhaust manifold dosen't sit well with me

A bit of history on this car

I bought it from the son of a man that had a repair shop back in the 1980s and around 1989-90 they put a new engine in this car, customer decided not to pay it and instead brought him the title...and so the shop owner drove it home and parked it in his back yard where is sat untouched until 2013 where I bought it for my dad...[whom wouldn't leave me alone about my W115 300D I had at the time] he drove it for a year and a half like that until I ended up with it Engine is super strong and runs perfectly, the shift into 3-4 is a little early but I know thats adjustable and it needs shifter bushings which I have coming in the mail and should be installed next weekend along with a trans filter kit. I know he's never changed the filter in the trans...and it did sit for 23 years. Any info would be helpful! This transmission is not vacuum controlled and has the bowden rod setup I have dealt with quite a few of the old w123-W126 diesels back when I worked in a junk yard so I do know a bit about them..just never ran across this before....I do know that the rubber head on the dipstick is pretty rotten and I don't recall seeing the type of head its got on it so I did order a new dipstick...just in case the guy that put the engine in put the wrong dipstick in it....

Vent blocked ?

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Hmm Could be I'd have to check it. I've only been under the car enough to check linkage bushings and put shocks center link and dampner on it.

A few things.

Make sure you have the correct dipstick so you are not overfilling.

Make sure you have flow through the trans cooler.

Something is aerating the fluid, this can be a front pump ( torque converter ) bushing that has too large clearance or something loose on the valve body allowing fluid to spray internally.

Frothy ATF on a hot exhaust manifold will catch fire. Some manufacturers went to a locking dip stick because of this.

I have ordered a new Dipstick from a MB Dealer, As I know for a fact that Mercedes did use a locking dipstick. I have run across transmission fluid foaming but this one is still nice and red and not aerated as that tends to look like Pepto Bismol with the air in it. At least thats what I seen from my days in the junkyard business

Mercedes used a locking dipstick on the later cars up to I think 95? Then they switched to no dipstick..... Dunno about the earlier ones...and the stick on the old 77' is pretty rough looking.

You're not by any chance over filling the transmission are you?

(In other words - do you check the fluid level in accordance with the maintenance manual)

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Don't leave that there - I'll take it to bits!

My guess would be blocked vent (or should I say, I would hope it is a blocked vent). OP stated until it slipped, indicating to me that even if he did overfill, it wouldn't keep pouring out when the level of fluid was low.

Another thing to check would be the hoses. If a hose is expanding (most likely due to blockage) then suddenly releases a bit of trans fluid because the hose returned to normal size, I could see it burping transfluid out the dipstick tube.

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I'm not sure if he was overfilling it or not. I haven't had to add trans fluid to it yet since I got the car from my dad. Though it did blow trans fluid out the first day I drove it, but only after I gave it a bit of a thrashing just to see what sort of shape the trans was actually in got it up to about 85 mph seemed fine to me aside the blowing fluid out. I hope to have a better idea of whats going on this weekend when I service the trans and replace the shift linkage bushings and the new OEM dipstick as I don't trust the one on it right now. Will keep you guys posted!

Well the new dipstick came in and it looks like a redesign of the origonal..same stick but it has a heavy duty locking mechanism on the end of the stick and locks into place nice and tight so its not blowing any more fluid out!

I say the vent is probably clogged.

No it won't burp fluid out the tube if a locking dipstick is in place, BUT it was burping fluid for a reason and now with no outlet it's going to blow a gasket out next time it needs to burp. Fix the pressure issue or else. It could be a pan gasket, it could be a front pump gasket. Don't roll the dice.

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I also dropped the pan gasket and replaced the filter, also as a side note if it was a clogged vent tube wouldn't it blow the fluid out every time I fill it up? This is an intermittent problem Sometimes it would blow the fluid out and sometimes it wouldn't. As for inside the pan it was spotless and there was no signs of excessive wear. Also from my time in the junkyard I've seen more than my fare share of transmissions with the vent tube completely sealed from gunk sitting on top of the transmissions and they had no signs of blowing fluid.

The point of changing the dipstick is to make sure you are filling it to the correct point. The fact it has a lock suggests to me you may have been sent the wrong part.

The cars of that generation did not have locking dipsticks, they came in sometime post-1980. The only reason that one being supplied now for an old w123 would have the lock would be if it was a mandated emissions item and put onto the replacement dipsticks at some point in time.

Did you find the part number for this dipstick, or was it a dealer parts staffer? Was the parts search done using your car's VIN?

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2002 e320 4matic estate│1985 300d│1980 300td
Previous: 1979 & 1982 & 1983 300sd │ 1982 240d

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The part was located with the vin and the part number from the origonal stick and oart was ordered from a local MB Dealership And looking through the owners manual it did have the origonal dipstick its just that the rubber head was in very bad shape. I also measured the sticks and both are identical with the exception of the head of it, the new one being locking type. Its not blowing fluid like it was, and I've driven it quite a bit although I'm not getting the car to shift right just yet and will try to check the vent tube when I have the car up on jacks this weekend to inspect the bowden rods unfortunately the vent is going to be extremely hard to get at as its way on top between the top bellhousing bolt "eyebrows" And as close as it is to the transmission well I'd either have to pull the trans to check it or take everything off the firewall and hope I can get at it from there. Will keep you guys posted