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  #1  
Old 10-03-2016, 07:02 PM
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Vac Eval

Recently my 1992 300D 2.5 started shifting roughly, and feels a little underpowered off the line. I've only had the car a for a few weeks, but I feel like it just started doing this after mostly driving more or less normal (whatever normal is for these cars, I guess).

Anyway, I know I haven't given a ton of info to point to anyone system or component failure, but my first hunch is to evaluate the car's vacuum system and look for a leak or other problem, at least as something to rule out. I wouldn't mind also checking out if I'm making enough boost. The thing is, I have no idea what I'm doing - I mean, on one hand, it seems pretty straightforward: just hook up a vacuum gauge and measure away, right?

The thing is, I don't even know where to start, having never done this before. It seems like the older W123 diesels are well-supported by all kinds of websites offering DIY info aimed at the novice (DieselGiant, MerecedesSource, etc.), but I'm finding a lot less out there pertaining to my W124 OM602. I'm aware of the FSM, but I don't recall seeing anything about vacuum assessment and diagnosis.

Anyone have any suggestions or tips? Other than being trained by Mercedes back in the 80's, how are people figuring this stuff out?

Oh, and before someone tells me to "take it to a pro," get this: I emailed the guy in my area and he wrote back to tell me that he was too busy and not booking appointments! Go figure. I mean, he wasn't rude about it, and I guess good for him that he can afford to turn customers away, but it was a little weird. One of the idiosyncrasies of moving to an island, I suppose.

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  #2  
Old 10-03-2016, 07:43 PM
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On the upper left of your timing cover, find the vacuum pump (follow the brake vacuum line). There will be a small fitting on the pump, to which a 3 way branch is connected. Your problem is along one of those branches. If you dont have a mityvac, you can eyeball each run for breaks. Or block them off one by one untill you find the break.
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  #3  
Old 10-03-2016, 08:21 PM
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I believe that the '92 wastegate is vacuum-closed, if it doesn't have vacuum due to a leak or other control problem, no boost.
Do a search for vacuum wastegate and I believe you'll find some info, but your search for vacuum leaks is an excellent first step.
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Old 10-03-2016, 08:44 PM
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Gents, thanks very much for the info on a starting point. I guess I should also check the condition and version of my vacuum pump - I understand there is an older version that is prone to catastrophic failure, and worth replacing if found.

Any idea on how much vacuum I should expect to find as normal?
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  #5  
Old 10-03-2016, 11:20 PM
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Originally Posted by Bimmer-Bob View Post
Any idea on how much vacuum I should expect to find as normal?
About 20" is normal, but that also depends on where you measure. The wastegate/EGR branch has some intentional leakage, as does the transmission vacuum modulator circuit. Replacing the pump is a good idea, but it's not necessarily the cause of the current problem.

My habit when getting into a new old Merc diesel is to replace as many of the rubber connectors as possible. You can drive yourself nuts looking for a hard leak, when it's just the result of porous rubber. Get a few feet of vacuum hose and cut to size. You'll also have to count up the two way, three way and four way fittings and order what you need.
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Old 10-04-2016, 06:32 PM
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Thanks again for all the helpful replies.

Bear with me, as I'm new to diesels (and Mercs in general), but I've been sifting through as much info as I can find and have come up with the following plan of attack - I'd appreciate any opinions on whether or not I'm missing anything important, or am including anything extraneous.

So, to start, I think I might do the mechanical wastegate conversion and EGR delete. Can't hurt, might help, and at a minimum it will simplify the vacuum system somewhat and minimize potential points of failure. Next I will inspect the condition and function of the vacuum pump. If broken, or old style, will replace. Next will inspect (and ideally replace) all lines and fittings branching off the 3 lines from the VP. Since I'm having shifting problems, I will pay special attention to the VCV (is this an easy and cheap thing to replace?). After all of the above is done, I should be done in the engine bay, is that correct? Then I will move under the car and check the transmission modulator, and replace as needed.

So far, so good?
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  #7  
Old 10-05-2016, 01:04 PM
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Originally Posted by Bimmer-Bob View Post
Thanks again for all the helpful replies.

Bear with me, as I'm new to diesels (and Mercs in general), but I've been sifting through as much info as I can find and have come up with the following plan of attack - I'd appreciate any opinions on whether or not I'm missing anything important, or am including anything extraneous.

So, to start, I think I might do the mechanical wastegate conversion and EGR delete. Can't hurt, might help, and at a minimum it will simplify the vacuum system somewhat and minimize potential points of failure. Next I will inspect the condition and function of the vacuum pump. If broken, or old style, will replace. Next will inspect (and ideally replace) all lines and fittings branching off the 3 lines from the VP. Since I'm having shifting problems, I will pay special attention to the VCV (is this an easy and cheap thing to replace?). After all of the above is done, I should be done in the engine bay, is that correct? Then I will move under the car and check the transmission modulator, and replace as needed.

So far, so good?
Anyone?
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  #8  
Old 10-05-2016, 02:08 PM
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Nope. Don't do any of this. Buy a Mityvac and track down your vac leaks. Then move forward.
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  #9  
Old 10-05-2016, 03:35 PM
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Originally Posted by tyl604 View Post
Nope. Don't do any of this. Buy a Mityvac and track down your vac leaks. Then move forward.
Well, yeah - I'm just trying to outline a systematic way of doing so, by working through the system. You don't think simplifying the maze of vacuum lines by doing the EGR delete and wastegate actuator swap is worthwhile?
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Old 10-05-2016, 03:37 PM
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I agree Bob. Find your vacuum leaks, get the system tight so that you will be able to troubleshoot further. This should be the easiest and lest expensive also.

Use the mityvac also to ensure proper flow of boost air to your ALDA, and that your ALDA holds pressure.

Going to a pressure-wastegate and eliminating the EGR might become a next step if you wish, but the system works if all is in good shape.

You can temporarily run vacuum directly to the wastegate (holds it closed) to see if your boost improves (it will), but you will risk overboost.
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  #11  
Old 10-05-2016, 05:08 PM
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OK guys, I appreciate the tips. I will focus on finding the leaks first. Of course, this will necessitate testing the pump and possibly replacement of lines and fittings along the way, and will probably lead me to testing and adjusting the VCV and transmission modulator as well, but I'll try not to over complicate the process and get ahead of myself. I'll defer the EGR delete and wastegate conversion for another day. Thanks again!
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  #12  
Old 10-05-2016, 06:11 PM
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If your power brakes work, you have a working pump
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  #13  
Old 10-07-2016, 08:08 PM
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What's this thing?

Ok, so I just got home after a week of being away for work. Haven't picked up a Mityvac yet, but just to have a look around, I popped the hood on the 300D. Right away I noticed many of the 3- and 4-way fittings were not snugged up all the way, and the rubber hoses are of varying quality - some probably need to be replaced, but I'll let the Mityvac be the judge. For now I just pressed the hoses onto the fittings as tightly as possible.

Anyway, upon looking little more closely, I see this thing in front of the coolant tank that says "Vac Out" on it (pic #1) and it has two little prongs for vacuum line, only one of which is connected. The trouble is, there are two free ends of vacuum line dangling around. You can see the one with the red thing in the end, that is part of a 3-way fitting. The other is just a single line, and looks like it heads towards the turbo, maybe? The one with the red things is hard to fit because, well, of the red thing. The other line is impossible to fit because of a small tear in the end. For now I just shoved the red one onto the prong, but the more I think of it the more I think maybe the torn hose is the right one, and that it came loose because of the tear. Anyone know what I'm looking at here? I haven't been able to test drive the car yet, because my wife is out with the keys (in another vehicle, obviously).

Pic #2 is what I think is the vacuum pump, amm I right? It looks to me like the new version, with the bolt heads on the outside.

Pic #3 I just thought was interesting - it's the line that goes (I think) from the vacuum pump to the brake booster. It looks to be in decent shape, and it's definitely not original - it's labeled "Daimler Chrysler," and stamped with a November 1999 date! I wonder what the story is there...maybe the vacuum pump was replaced with the newer version at a dealer service around that time? Or maybe just the hose was replaced, who knows? I just thought it was interesting.

Vac Eval-img_0400.jpg

Vac Eval-img_0402.jpg

Vac Eval-img_0403.jpg
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  #14  
Old 10-07-2016, 10:08 PM
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Well, I'm thoroughly confused - when my wife got home, I asked her how the car had been shifting since I'd been gone. She told me she hadn't driven the Merc all week long, but did take it to work this morning, and didn't seem to notice any rough shifting. I took it for a drive, after I'd readjusted the vacuum lines as noted in my last post, and it was back to buttery-smooth, unobtrusive shifts. So I have no idea if I fixed it, or if the car somehow fixed itself (I know, makes no sense, but based on my wife's report...?).

So anyway, things seem to be back to normal, for now. I'm still planning on picking up the Mityvac and giving the whole system a once-over, but I'm really more confused than I was before I started. If anyone can make any sense of the pics I posted before, I'd certainly appreciate any insight. I've tried deciphering the engine's vacuum schematic, but it's pretty indecipherable to me at this point. For now, I'm just going to cross my fingers and hope for the best. Plans to overhaul the vacuum system remain on the short-list, though.
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  #15  
Old 10-07-2016, 10:59 PM
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Welcome BB and congrats on your purchase these are great cars for sure. Confused whats there to be confused about just a ton of vac lines. However should one become disconnected it will affect shift quality and shifting will be abrupt or harsh. Guess you solved one issue already. Not certain what pic 1 is but the red and yellow are orifice restrictions of different sizes used to limit the amount of vac in a circuit. Could be going to the vac controlled actuator. Have you tested your 0-60 mph time? Above 16 or so and you would have no boost. When I had boost problems I disconnected the MAF by passanger headlight and that restored my boost. I ran it like that for a couple years without issue. You'll find everything you need here

Model 124 Maintenance Manual Index

But do ask.

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