1975, MBz 110 engine. I have 6+ sources of info on the D-Jet, including MBz service manual and the Bosch yellow book, none of which will tell me the lowest intake manifold vacuum required for the MPC to operate correctly at engine idle. 10in, 15in, 20in,?? Surely Benz engineers, knowing that the vacuum would change as the engine aged, designed for a broad range. Anyone know what it is?? I think the poor idle on this car is due to rings and/or valves which are causing too low a vacuum for the MPC to operate correctly. Any info appreciated.

I don't know either, but in general it's 18-22 inches of vacuum at running idle. Cranking vacuum should be around 5" or more.

Not that you couldn't have ring or valve seal issues, but I think they would manifest themselves through oil fouling too. I would do a compression check for those items. Low manifold vacuum could come from other sources such as air leaks.

__________________
Prost!

Have you checked the archives here?
- There's a great website that I was just at this morning with D-jet theory and operations data (I'm on the wrong computer or I'd past in the URL).
- Definitely check carefully for vacuum leaks. Also make sure your ignition is exactly to spec. Don't settle for "close enough".

-CTH

__________________
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http://www.111lakeside.com/

Fuel Injection

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http://members.rennlist.com/pbanders/djetfund.htm

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http://www.vectorbd.com/bosch.html

Bosch Fuel Injection and Engine Management
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Bosch K-Jetronic
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Bosch D Jetronic and K Jetronic Troubleshooting
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Thanks to all who responded.

As a general answer, it'll run with 0" vacuum, but be pig rich, cold. It'll stall. It depends what the MAP is set to as to what the lowest it can tolerate is. Worn rings affect compression much more than vacuum, but worn valves can cause vacuum to be much lower - I'd suspect that as a cause first and foremost (especially worn guides or misadjusted ball studs causing valves to not seat).

__________________
Current:
2021 Charger Scat Pack Widebody "Sinabee"
2018 Durango R/T

Previous:
1972 280SE 4.5
2014 Jeep Grand Cherokee Limited "Hefe", 1992 Jeep Cherokee Laredo "Jeepy", 2006 Charger R/T "Hemi"
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I am not an expert but have lived with a D-Jet for 20 years, so will try!

Regarding the vacuum. It varies as you adjust the timing. If you hook up a vacuum gauge to the vacuum line that goes to the MPS (use a temporary tee) and then rotate the distributor while at idle, you should see a wide variation in vacuum - maybe 5-20" Hg. Note: If you measure the vacuum elsewhere, it will likely be lower.

You should see maximum vacuum near to max advance that you can get - this turns out to be about 30deg BTDC on our M117 engine. I have found that my car runs at about 16-17" Hg when timing is at 27deg BTDC (where I happen to have set it). With less advance, you might only see 10-12" or so which is not what you want.

If you can't get a decent vacuum, your Aux air valve may not be closing. it is supposed to close at 150F. So try removing the hose and plug the inlet to the idle air valve.

If you still can't get proper vacuum, check that all other vac hoses and connections are not leaking - on throttle body for vac retard, brake booster, connection to transmission and to locking/climate systems all at back of engine.

If all hose connections are good and AAV is closed, then you need to look at hoses that connect upper and lower parts of intake manifold as well as intake gaskets.

Finally, you get a valve job, but it's hard to see you not getting pretty good vacuum unless valves are very bad. Poor valves more likely cause variations in vacuum and if this is a problem, you may see the gauge needle jumping around when you do the first test. It should be quite steady.

Good luck and let us know how it goes.

__________________
Graham
85 300D,72 350SL, 98 E320, Outback 2.5

I have always regarded a steady 15" at idle as the basic requirement for vacuum.

Do you have any other signs of wear issues like blow-by, smoke at start-up, idle, take-off, or acceleration?

__________________
Chuck Taylor
Falls Church VA
'66 200, '66 230SL, '96 SL500. Sold: '81 380SL, '86 300E, '72 250C, '95 C220, 3 '84 280SL's '90 420SEL, '72 280SE, '73 280C, '78 280SE, '70 280SL, '77 450SL, '85 380SL, '87 560SL, '85 380SL, '72 350SL, '96 S500 Coupe