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R/C107 Moderator
1986 560SL: '84 500SL: '84 280SL 5 speed: other 107s
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31,877 Posts
Discussion Starter #1
What, other than a leak, causes low engine vacuum. I have tried and read every thread. The engine is rebuilt. No vacuum leaks I can find, even with propane. Timing is correct and changing it doesn't make a difference. I have two ECUs and no difference. Replaced the electronic ignition with the original. Replaced temp senders. Cleaned injectors. Checked cold start valve for leaking and cleaned it. Ran the valves 3 times. Oh yeah, heads were milled .008 due to warping.

Car is running rich and has a miss at idle and through rpm range. Hard to start when warmed up. Vacuum is running less than 10. Can one or two bad injectors cause this? Both cams are lined up at the mark at TDC. Thought I might have read somewhere that one should be advanced or retarded 2 degrees or something.

Before I put it on a trailer and take it to an Indy in Tulsa, Any suggestions?:bowdown::bowdown::bowdown:
 

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R/C107 Moderator
1986 560SL: '84 500SL: '84 280SL 5 speed: other 107s
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31,877 Posts
Discussion Starter #4 (Edited)
Thanks Supercharger. Any thoughts are appreciated.

Compression is very even, 155-160 lbs. This is the early d-jet system.

Anyone know if the MAP sensor should hold vacuum with a vacuum pump? I have two on hand. One holds vacuum (on the car), the other does not.

The cam timing is what I am thinking. It sounds like the miss in in the left cylinders, like 5, 6, or 7.
 

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82 380SL 96 SL500 03 SL500
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6,154 Posts
On the chance there is still a vacuum leak causing the issues, did you clamp off the non-engine related vacuum circuits? ie. cruise control, locking system, climate control to see if that made a difference.

Also, I have a vague memory of someone having an unfindable vacuum leak which they eventually found only by taking to someone with a smoke generator, and that it was cracks in some type of plastic manifold that was pretty well hidden on the engine.

And, away from vacuum leaks, I'll second asking if you checked engine compression?
 

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R/C107 Moderator
1986 560SL: '84 500SL: '84 280SL 5 speed: other 107s
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31,877 Posts
Discussion Starter #6
Sealed off all external vacuum including the one going to the trans. No cruise or climate control on this '73. The only vacuum I left connected is MAP and vac advance. Tested both of them, including valve and all held vacuum.
 

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Premium Member
2010 Mercury Milan Hybrid, 1993 BMW 325i convertible
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6,077 Posts
What, other than a leak, causes low engine vacuum.
Besides leaks, another culprit would be a badly blocked exhaust, but you should feel that on (lack of) acceleration. It can be caused by the catalytic converter or the muffler, or a crunched and restricted exhaust pipe.
 

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450SLC 5.6 Twin-turbo
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350 Posts
the MAP sensor is not going to hold any vacuum is just a realtime sensor. but i think you must have at least 14-16 in at idle if there are proper combustion and so on. eather if you are haveing a faulty trigger contacts at the lower section in the distributor will still cause bad vacuum like a missfire. first everything must be in perfect order and fire as it should. you will allso got lower vacuum when you put in D-drive but thats normal cause the ignition timing is not change but if it did you will see slightly higher numbers.
 

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R/C107 Moderator
1986 560SL: '84 500SL: '84 280SL 5 speed: other 107s
Joined
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31,877 Posts
Discussion Starter #9
Exhaust is new from headers back. Stainless from Timevalve. No Cats and eliminated the center resonator. I held back on this post for 2 months hoping I could find the problem on my own. That is why I have been doing the body work on the 280 I just bought. Instead of posting I want to be driving!!!!!

Just want to be driving this summer.

Jim
 

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R/C107 Moderator
1986 560SL: '84 500SL: '84 280SL 5 speed: other 107s
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31,877 Posts
Discussion Starter #10
Not funny from my perspective but this whole thing stared with the piece of shit Haynes manual. The car was running great but I heard the fatal timing chain noise. Haynes indicated that the timing cover can be removed without pulling the engine. I followed the manual and put a nice big crack in the timing cover. Luckily I had a '72 for parts. One thing led to another and the engine was out being rebuilt.

Never would be here except for Haynes. I keep it in the barn now for TP.
 

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R/C107 Moderator
1986 560SL: '84 500SL: '84 280SL 5 speed: other 107s
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31,877 Posts
Discussion Starter #12
I have used both distributors with no change. So probably not the lower contact points. Today I tried the other MAP sensor, still no change. Still leaning toward bad cam timing, anyone know a secret here? Or maybe a leaking intake manifold. I did use the intake off the '72 as I saw a crack in the '73. Different numbers on the intake but the difference was obvious. Replaced plenum seals while I was there.
 

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1981 380SL - Lorinser bodykit w/16x7 LOs - AKA "Frau Blau"(Lady Blue)
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7,563 Posts
Are you sure it didn't slip a notch off the timing gear?
It sounds like a timing issue since it's missing across the RPM range...
 

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R/C107 Moderator
1986 560SL: '84 500SL: '84 280SL 5 speed: other 107s
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31,877 Posts
Discussion Starter #16
Are you sure it didn't slip a notch off the timing gear?
It sounds like a timing issue since it's missing across the RPM range...
By this do you mean when installing the dist? Or when replacing the chain and gears?:confused:

And are both cams to be aligned at TDC? I have read conflicting info on this.
 

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1981 380SL - Lorinser bodykit w/16x7 LOs - AKA "Frau Blau"(Lady Blue)
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7,563 Posts

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Registered
1981 380SL - Lorinser bodykit w/16x7 LOs - AKA "Frau Blau"(Lady Blue)
Joined
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7,563 Posts
I read a wiki thing on replacing the cam chain and I believe it had the alignment procedure.
 

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R/C107 Moderator
1986 560SL: '84 500SL: '84 280SL 5 speed: other 107s
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31,877 Posts
Discussion Starter #19
cam chain and gear.
I will pull the small cover off the dist housing and note the gear position at TDC and let you know. I lined it up as best I could at the time. The mark on the block on the early 4.5 is just a small dot, not a pointer like the later models.:thumbsup:
 
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