W202 C220D Poor Vacuum/Bad Autobox Changes-Help!!

[merc-mark]

Hi All,
As a long time avid Merc enthusiast (W124 Coupes are my "thing") I watch this forum regularly, however I thought I'd better register sooner or later!
In any case the trouble I have is with my daily run around. Its a 94 W202 C220D Elegance auto. (its done 150K with full MB SH) Just lately, which I assume is related to the vacuum issue, the autobox changes have been getting jerky,holding onto the 2nd-3rd change too long and the 3rd-4th change is also quite abrupt, however the syptom is only apparent under low engine loads (under heavy engine loads, eg going up a steep hill or hard acceleration all changes are as smooth as silk) Also some of the vacuum operated devices, e.g headlamp levelling etc have packed up all together. I know on the diesel the vacuum is provided by an auxilary pump, however I'm not sure whether this is the cause, as the brake servo, which is also operated by this pump works fine - the brakes are powerfull as they should be.
I'd really appreciate any suggestions of where to look for the possible cause, and how to test for "sufficient" vacuum at tickover, without any specialist tools. I had a quick peek under the bonnet of my W124 E220 Coupe, which has a very strong vacuum on tickover (finger over end of feed pipe) however that's a petrol engine, and takes its feed from the inlet manifold.

Thanks in advance,
Mark.

 

[merc-mark]

Problem Sorted!!

Hi Guys,
After much disconnecting of vacuum pipes, armed with a syringe and a length of vac hose, I traced the bad vacuum down to a leak where one of the hoses passed through a plastic sheath at the front of the cylinder head.
At some stage in the cars life someone had zip-tied this sheath which also carries another pipe and a couple of wires to the hydraulic V-belt tensioner on the head, and had pulled the zip tie too tight and cracked one of the plastic vacuum hoses.
I fixed the breakage with a length of new pipe, re-routed the sheath and hey-ho! the car is transformed; smooth autobox changes, air reculation function of heater/vent system works & the headlight levelling works again...don't you just love it when a solving an apparently small fault has major benefits! :grin: At one stage I got to thinking the autobox was goosed, and needed a rebuild, so you can imagine my relief when it turned out to be only a few pence worth of vacuum hose!
Also, its worth mentioning for future reference, if you want to test the vaccum,without specialist tools, a length of rubber vac hose, attached to a syringe (like the ones you get to give young kids/babies medicine) makes an excellent vacuum detector or tester - just remember not to use it on the kids afterward!
Hope this helps someone else in the future.

Cheers,
Mark.

 

[kth286]

merc-mark

Could you please go into a bit more detail of how exactly, and each step, you took to find the lack of vacuum.
Also, perhaps a photo of the syringe.

Seems like an excellent bit of diagnosis - well done.

Regards

 

[merc-mark]

Bad Vacuum Diagnosis

Hi David,
As requested, here's how I did the diagnosis:

1/Take a syringe, like the one pictured below, and connect a piece of vacuum pipe to it (the rubber variety is better so you can then in turn connect a piece of the MB plastic hose to it if required )

2/Start the engine (obviously!)

3/Start at the vacuum source; The vacuum pump (on a diesel) or at the inlet manifold (on a petrol) Watch your fingers when working on diesels here, as on my W202 C22D the vacuum pump is right behind one of the V-Belts! :!:

4/Systematically disconnect each hose, one at a time, each time replacing the disconnected hose with the syringe

5/The syringe will "suck" its sellf in when you find the offending circuit.

...from then on you may be lucky like me, and find it in a relatively easy place to get to, or you may have to remove a few components, guards etc to find the offending line. Also bear in mind that a vaccum compent acuator may be at fault. Its easy to test these, just connect a larger syringe to the component and suck the air out, and see if it works. If you don't have a bigger capacity syringe like me, simply suck on the pipe with your mouth, but be carfefull to check there's nothing loose in the pipe before you suck, as you might end up swallowing it!

Hope this explains it ok.

Mark.

 

[dogsbody]

W202 c220D poor vacuum

Congratulations for an elegant vacuum tester!

 

[mercedes13156]

Hi there, I had a C180 which had really poor ventilation and the headlights wouldn't go up or down. There was also a loud hissing noise from the back of the headlight switch when I backed the engine off.

I now have a C220D which also has crap ventilation, but the headlights adjust OK. This is of great interest to me and I'd like to try to fix it myself. Has anyone got a picture of what I should be looking at?

Additionally, several Websites say I should be grtting about 35 - 53 mpg. I'm getting about 43 mpg on average. I drive 100 miles every day with 30% on quiet country roads and the rest on Motorway / dual carriageway at about 65 on cruise. A long motorway run at 70 - 75 average gave 44mpg. Am I expecting too much?