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Old 08-07-2006, 11:59 AM
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Join Date: Aug 2006
Location: NYC
Posts: 4
vacuum issues (and others) w123

first thanks for all the help this forum has provided me with. i have saved a lot of time and money as a result of your advice. AWESOME. now my 1983 300dt problems...i know that i have a vacuum issue and am in the process of replacing the pump and all old connectors, check valves and oil-soaked tubing. there are however some symptoms that i want to make sure aren't representative of greater issues. along with the vacuum problems listed below i also experience non-shutdown and one-shot only door locking.

#1 power brake loss. there is no problem on wide open roads where braking is infrequent, but in traffic at idle speeds after repeat depression all assist is gone. it often doesn't return until the car is shut down (when this occurs shut down is manual) and restarted later. is this consistent with a bad vacuum pump or should i be worried about the booster or reservoir as well?

#2 inoperative 4th gear. the car (auto of course) will shift into 4th around town at low rpms, but 4th will not engage on the freeway. is this consistent with a bad vacuum pump? is it because the engine draws less vacuum at higher rpms and combined with a leak somewhere or a bad pump there isn't enough vacuum to modulate the shift into 4th? it would be nice to cruise at 75 sans 3700rpm factor.

#3 black box on valve cover. i have the box, but the plug and lines that i know are supposed to be there are long gone. what is this plug and how many lines are there and where are they supposed to go? could the absence of this plug contribute to a leak or other problems? any good diagrams out there? all the ones i have come across are poor at best.

#4 dead ac compressor. it is dead (and loud) and i do not plan to ressurect it. my common sense tells me that i can just eliminate the belt and let the unit sit there, but am i missing something? will this effect the heater in any way? seems like i would just get a few extra hp!

#5 tie rod replacement. where can i source or rent a seperator tool (what do they look like?) in the manhattan area? what are the torque specs on these bolts? what should an allignment cost me and any reccs on a good joint to do this in manhattan area, or close by?

#6 brake job. i am doing the brakes, pads and rotors. have done many brake jobs, but not on mbz. any pictorials out there on this diy project?

thanks in advance for the help. i'm sure i'll be back.

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Old 08-08-2006, 09:46 AM
Join Date: Dec 2005
Location: CA... No. of S.F.
Posts: 890
Right off your replacing a $300+ Vacuum Pump...

seems a bit impulsive... that is unless you have left some preliminary diagnostic steps out of your introduction to this THREAD.
Q1 - How did you reach this decision?
Q2 - Do you own a hand vacuum pump/gage... and a selection of adapters, caps/plugs etc ?

I'd like to help but need to know more about your thinking and approach to this very common problem... you know like your neighbor would have the benefit of knowing from being there !

Q3 - Do you have the car's correct vacuum diagram?
I think the diagram at this link is for your car:

Model "300DT" makes no sense... it's either: " 300D Turbo " or " 300TD Turbo " ?

Q4 - Do you know how many miles are on this engine... & separately the same question regarding the chassis/vehicle?

You have a lot of issue to address and it will take some time. Thus it will be best if they are approached in a logical order... starting with the all IMPORTANT power brakes!!!.

Retired Safety Engineer !
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Old 08-08-2006, 11:01 AM
Registered User
Join Date: Aug 2006
Location: NYC
Posts: 4
thanks for getting back to me. first off the car is a 1983 300td with 179k on the engine and chassis. i have the luxury of having a best friend that owns/operates a wrecking yard, so parts costs are not an issue for me, hence my decision to replace the pump. also, when i tested te pump with a vacuum gauge it was pulling less than 15hg @idle and was sporadic at best. my biggest concern, as is yours, are the power brakes. i have done a leak down test on all of the lines and connectors within the engine compartment. none there. i have not tested the door/trunk system, and don't know if i really want to. how do you block that system off? i have also done the "pedal test" to check the booster, but is there a more accurate way to test the booster for leaks? thank you for the vacuum diagram. i have seen that one, but i can never get it blown up enough to read the labeling and really understand it. do you have an exploded version of this? i can't htink of anything else to tell you, but please let me know what other info you might need to help me diagnose my issue. thanks again.
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Old 08-08-2006, 11:14 AM
Registered User
Join Date: Jun 2004
Location: West of Ft. Worth. TX
Posts: 4,186
Do you own a Mightyvac?

As many will attest to here, you need a Mightyvac guage/pump to determine the condition of the vacuum systems. Your problem with the brakes and shut down does sound like a weak vacuum pump but a rebuild kit is the best/less expensive route to go. However, without the guage you are spending money and hoping for results.

The black box on the valve cover is for vacuum switching of the EGR valve. More than likely this has been bypassed/blocked/eliminated, etc... Most of the EGR valves on these cars have been blocked for "testing purposes".

4th gear can be a fluid level issue. These trannys are very sensitive to that and need to have the fluid level checked when fully warmed up.

Brake rebuild is mostly like any other car with the exception of the parking brake. Which is kind of a "mini drum" set-up for the rear wheels. (source & type of tie rod removal tools that you will need) A "pickle fork" can be used but their drawback is that they damage the rubber boots and the part is useless afterwards. The type in the link allows you to reuse things in the future projects that are sure to come.

(BTW, you need to go to "User CP" at the left end of the blue bar, and enter the year/model of your vehicle and location in your signature. It helps folks answer your questions.)


84 300SD 350K+ miles ( Blue Belle )
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