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.

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.

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.

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?

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.

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?

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?

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.

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!

If your power brakes work, you have a working pump

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.

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.

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.

Thanks for the link!

Do you happen to know what the valve (or whatever) is that is labeled "Vac Out?" Haven't been able to turn up anything with Google yet. Just confused about why I have two dangling vac line with only one unoccupied fitting in the area? I am going to look into simplifying the vac system at some point, to minimize these types of issues, but would prefer to get it working in stock form before I start screwing around with things...

Otherwise, totally loving the car. It's not even close to the performance of my Bimmer, but it's satisfying to drive in it's own way, and has been a pretty fun project so far to tinker on.

Well you should have 3 vac out solenoids 2 up front and that one. It's been a while for me to recall which is which but you can trace the connections. One is for the inlet flapper that closes and the exhaust recirculating valve opens the 3rd one likely the one you are asking about should be turbo vac control. The vac schematic is on rail by the passanger headlight hard to read but it's there.

Sweet, thanks for the start.

OK, based on the part number on the part on pic #1 above, it is an EGR pressure converter valve...but that still doesn't tell me which lines are supposed to be hooked up to it. It seems wrong to have a line from the turbo just dangling loose, but at the same time, it seems wrong to have one end of a 3-way fitting not used. Can anyone provide some clarification here? I have 2 free lines, and only one fitting to which either might attach to...

OK, last update unless/until someone has a definitive answer to prove me wrong: I removed the line with the red restrictor in it from the vacuum fitting, and then I cut a small piece off of the turbo line to eliminate the tear so that it would fit snugly on the vacuum fitting. After going for a test drive, the car feels more punchy with throttle application, and when I got home, it even seemed like the engine shut down more quickly. Of course, this could be all (or partly) placebo, but it seems to make more sense to me to have a single restricted port of a multi-port line be open-ended than have a dangling hose coming from the turbo.

Shift quality seemed the same with both set ups, so I don't know what to make of that. Honestly, it seems unlikely that these lines and fittings are affecting shifts, since they are on the opposite side of the engine compartment as the line going to the transmission. Maybe the shifts were helped by snugging up those other lines, which I also did. Or maybe my wife is right, and the car "fixed itself," however unlikely.

For now, I'm going to leave it as is, but the EGR delete & wastegate conversion is looking more attractive, if only to eliminate the uncertainty of the current arrangement.

Engine shutting off more quickly solved another vac leak and it does not really matter where the leak originates from many had a climate control vac pod cause lots of problems. The 3 way connector close on top of the vac pump one goes to egr vac CCT including boost, second trans, third climate control. Door locks are on a another vac pressure pump located under the rear seat. Acceleration time?

Thanks for the roadmap.

What do the climate control vac pods do - just open and close to route airflow depending on ambient cabin temp and temp setting? I'm just thinking, if I find a leak on one of the lines heading to the climate control system, and that leak is affecting other things, can I just plug it in the engine compartment and call it a day? I mean, I know I'd lose the downstream vacuum pods (if they even work), but I definitely don't want to pull the dash and start replacing vac pods if I don't have to, at least unless/until I fix the evap leak (AC doesn't even blow cold right now anyway).

Accel time has not been clocked yet. I generally don't put much stock in 0-60 times, as there are so many confounding variables involved (wheel and tire size, tire brand, but probably esp. driver technique) as to make comparison meaningless. I guess the intended use is to compare my own times before and after making changes, in which case it seems more reliable than the old butt dyno, at least. Will try to establish some baseline runs this weekend, once I settle on a methodology.

You have the vac system nailed now your assumptions are right on.

O-60 in around 10-12 sec is nice for the 124 602 slower than 16 suspect boost problems I don't think any decimal point accuracy matters.

Did a few 0-60 pulls, and seemed to be averaging around 12.5s, so not too shabby! I haven't even touched the ALDA yet, so I'm okay with those numbers. Even my worst pull (uphill) was just under 15s, so I'm not too concerned about any serious power-draining issues at this point.

My main gripe about DIY 0-60 pulls is the lack of standardization. Elevation changes, turns, and launch all have a tangible impact on results. I did several pulls in several locations, and the 12.5s was pretty repeatable within several tenths, so I think it's pretty good as a ballpark. What would be ideal is an app that used the phones accelerometer to take start and stop control away from the driver, and then one could make several pulls (an even number) back and forth down a straight road (to average out any effect of elevation).

I know, I know - I'm totally overthinking this. :rolleyes:

Plenty of acceleration apps that use accelerometer and gps for timing such things.

I figured, I just haven't looked around too much. I did briefly test one out, but it told me I completed a 0-60 pull in 3.5s, lol. So I reverted to the old-fashioned stopwatch method. I'm sure I could do more research and play around with the right app to get better results.