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Pressure Sensor, Air Slide

Started by brettj, 10 October 2007, 07:56 PM

brettj

My 450SE seems to take at least 5 minutes to kick down from a cold start. The motor also seems to shake a bit when stopped at a traffic light and periodically will stall when backing up or again at a stop light. I have been told that the vacuum pressure sensor needs replacement as well as the air slide. Can anyone tell me where these parts are located in the engine compartment? I called MBUSA Classics Center to purchase a Pressure Sensor and was quoted a price of $2200 US dollars! I need to hit up a junk yard to pick  one up and save some money. Just want to make sure I am buying the correct parts.  Thanks

john skene

Take it to a MB specialist and get the problem diagnosed properly, otherwise you could be spending big bucks but not fixing the problem. It could be as simple as the cold start injector, or dirty injectors which would cost way less than $2000 to fix with new parts , or even as low as $100!.

Big_Richard

Quote from: brettj on 10 October 2007, 07:56 PM
My 450SE seems to take at least 5 minutes to kick down from a cold start. The motor also seems to shake a bit when stopped at a traffic light and periodically will stall when backing up or again at a stop light. I have been told that the vacuum pressure sensor needs replacement as well as the air slide. Can anyone tell me where these parts are located in the engine compartment? I called MBUSA Classics Center to purchase a Pressure Sensor and was quoted a price of $2200 US dollars! I need to hit up a junk yard to pick  one up and save some money. Just want to make sure I am buying the correct parts.  Thanks

By vacuum pressure sensor, I'm assuming you mean manifold absolute pressure sensor, which will make your car an early model with D-jet electronic ignition. Yes, they are bloody expensive new and most likely the cause of that particular problem.

Some smart arse out there in cyber space is yet to successfully modify an El cheapo plastic off the shelf generic MAP sensor to work on these cars, I know it CAN be done its just a case of who can be bothered doing it...

koan

Quote from: Patrick Bateman on 10 October 2007, 11:24 PM

Some smart arse out there in cyber space is yet to successfully modify an El cheapo plastic off the shelf generic MAP sensor to work on these cars, I know it CAN be done its just a case of who can be bothered doing it...


But there is someone out there with a D-Jet website that covers MAP sensor problems and fixing them, including doing a job on the sensor. I just did a quick search and can't find it, maybe one of the D-Jet aficionados knows it.

koan

Boogity, Boogity, Boogity, Amen!

oscar

Hi Brett

I recently removed my air slide off.  Soaked in WD40 overnight, then put it thru a number of heating and cooling cycles using boiling and cold water.  I also hammered the top down by 5mm approx by using a hammer on wood.  It works so much better.  Hold off on the hammering bit but give it a good soak with a penetrating oil first and see if it improves.  You'll see the slide move inside as it heats and cools

Re the MPS.  Have a look at my old thread below which has got pics of my faulty MPS pulled apart and two links in the first post.  The second link I reckon is the one that Koan refers too by Paul Anders.  A 2nd hand MPS cost me $150AUD and it's stil going strong no probs.  Check the vacuum line to the MPS too.  If that's leaking it'll lead to a choppy idle.  The picture below shows where the air slide (left) and MPS (right) is located on my RHD 350.  Your air slide will be in the same spot, not sure about the MPS though I reckon it would still be there too for an LHD w116.  At least you'll know what to look for. 

Almost forgot.- to test the MPS, pull the vac line off it, bend over the fender, suck on the suction port and see if holds a vacuum.  Serious! Or you can use a vac tester if you have one but sucking on the port will soon let you know if the MPS is faulty.

http://forum.w116.org/mechanicals/manifold-pressure-sensor-d-jetronic-repair-attempt/0/


1973 350SE, my first & fave

koan


There are some 450 D-Jet parts on ebay AUS at the moment.

Items numbers are 160166744527 and 160166744503.

koan
Boogity, Boogity, Boogity, Amen!

UT_Tech

Is the pressure sensor a 002 542 2817 or 003 542 7817?  As I recall there's a Porsch part number that will work and it's a lot less expensive.  Also, is the air valve a 000 141 0325?  If so I think there's an electrically heated VW version that was used in the 80s and that is very inexpensive.  Let me know if I have the correct part number.

brettj

Thanks to all for the replies! Unfortunately, the part number has worn off the pressure sensor so I'm not sure what model it is. I also cannot see the air slide number very well but when I get that part number I'll let you know. Is it possible that the electric cable attached to the pressure sensor could be faulty making it appear that the sensor unit itself is malfunctioning?

Oscar, thanks for the info and photo. It confirmed my thought as to where the pressure sensor was located and really helped with the air slide as I had no idea where that part was in the motor! Thanks

oscar

Quote from: brettj on 11 October 2007, 06:26 PM
Is it possible that the electric cable attached to the pressure sensor could be faulty making it appear that the sensor unit itself is malfunctioning?

It is possible but rare though you could check the wires for continuity.  From what I've read, a faulty MPS is almost 100% of the time caused by a cracked diaphragm in the MPS and that was my issue too.  Suction test will confirm.   Besides, if you pull that electrical lead slowly to try and break contact with one or two contacts, you'll soon find that the engine stalls.  There's been posts before about guys walking amongst the w116 wrecks at dismantlers and sucking on the pressure ports of MPS's to find the good ones.  At $2200 for a newbie, it's no wonder.

If the MPS is faulty you'll find that there is no smooth transition between idle and part throttle and full throttle.  For me, it seemed like idle was ok and full throttle was good but I had boggy performance whilst trying to maintan speed limits around residential areas.

BUt, re reading your first post.  How does the engine behave when driving whilst the engine is cold and warming up?  Does it still suffer from the stutter and stall?

It leads me to think the air slide may be your main issue.  I'm thinking that what is happening with yours is that when your idle revs have been set during a service, your revs may have been running away due to the air slide staying open or the revs have been set when the engine is cold.  So the mechanic or whoever has compensated by screwing down the idle screw (pictured to the right of the air slide, screw with spring around it).  Your air slide sounds like it sticks or operates too slowly perhaps but by the time you get to your first stop at traffic lights or whatever, the slide has closed and the because the idle screw is adjusted way down, the engine is starved for air at idle with the throttle plate closed and the idle circuit (air slide and idle air screw) has been restricted.





1973 350SE, my first & fave

UT_Tech

I earlier offered to dig up the VW part number for the electrically actuated slide valve but I didn't stop to ask if this was really needed.  Is it?  Switching to the VW part would involve the hassle of a non-standard part with different tubing connections and a block-off plate for the cooling water hole.  Is it worth the work?  Is the OEM slide valve repairable/available?

BTW, Oscar that's a fantastic photo you took of the air valve.  I wish Mercedes service docs were this clear.  The lighting is fantastic.  What did you take that photo with?

Sincerely,
Steve

oscar

Hi Steve,

Got a pentax digital slr istD model.  Nothing too expensive.  Was about $1000AUD a year or more ago.  The sun is high in the sky on a cloudy day.  So plenty of diffused light, I just zoomed and clicked.  Sometimes the auto function does get it wrong and a part is either under or over exposed but I agree, I wish official manuals etc had clear full colour photos.

FWIW, my part numbers (73 350se) to compare: Don't know if they'll be the same
MPS - 0280 100 012
AirValve - 0280 014 0017

Apart from lubricating the slide valve and hammereing the top down to limit it's movement and make sure it fully closes, I haven't read about anyone taking an air slide apart to repair.  Apparently new ones are becoming scarce too, so the idea of another method sounds attractive though I can't imagine it would be easy to retain an original look.  Though sometimes in the past, I wouldn't have cared if the result was a smooth idle and a well behaved engine.


1973 350SE, my first & fave

UT_Tech

Oscar,

I'll check out these part #s.  I have a USA only subscription to the Mercedes EPC so I'm not surprised that my numbers didn't match.  I'll post with what I find.


brettj

Oscar and UT Tech,

I'm leaning on the air slide being the main problem here as well. When the motor is cold, especially when you first start it in the morning the motor will race like mad at idle. It will not stop racing for at least 5 minutes and will calm down after the car  is driven. Then only periodically will I get the stalling and choppiness of the motor at stop lights and so forth.

New MB Air Slides are available for $400 US dollars. A bit expensive but better than the $2200 for the Pressure Sensor. I think I will try to replace the Air Slide myself to see if it solves these problems and have been looking for a used one before spending  extra money on a new one. Is it easy to place the Air Slide? Will I need any special tools etc to do so?

Thanks,
Brett

UT_Tech

The air slide is very easy to replace.  Just two bolts to remove it and remove two air hoses.  I'm working on a possible VW replacement for this part which costs a lot less than $40.

UT_Tech

I think I found it.  This is a VW part.  This is an air slide valve for a 1980 to 8 Jetta.  The tubing fittings are a bit smaller than the mercedes but it will fit.  This part is electrically heated, not heated by cooling water jacket.  If you want to try this you'll need to fabricate a block-off plate to cover the hole left when the OEM part is removed.

Here's the part:

http://www.discountvdubparts.com/VW_Auxiliary~Air~Valve_list.html