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  #1  
Old 07-17-2016, 10:28 AM
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Location: Central Indiana
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Correct vacuum reading for 1995 C36?

I purchased a 1995 C36 with a blown head gasket. I fixed that and the car runs fine at speed. However the idle is low and rough. I checked manifold vacuum by hooking a gauge up to the port on the top of the manifold where the 180 degree vacuum line with about a 8mm ID normally goes. The gauge read 15 inHg which sounds low. Also worth noting is that this car sat for 2 years before I got it last week. I have changed all the fluids including fuel but have not done plugs or 02 sensors since they looked fine. Compression is good on all cylinders after the head gasket.

I was able to drive the car into my garage before the head gasket job and it did not idle as rough.

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Current fleet:
1995 C36
1989 190E 2.6 5 speed
1974 450SLC
1988 5.0 swapped Supra

Last edited by MaxC107; 07-17-2016 at 11:44 AM.
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  #2  
Old 07-17-2016, 10:40 AM
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Are you sure the timing was set right? cam timing on that motor is slightly different on the exhaust cam sprocket, vacuum seams about right a little low
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  #3  
Old 07-17-2016, 10:43 AM
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Oh it is different? How so? I marked everything before taking it apart and it seemed to line up with the marks used on other M104.
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Current fleet:
1995 C36
1989 190E 2.6 5 speed
1974 450SLC
1988 5.0 swapped Supra
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  #4  
Old 07-17-2016, 10:47 AM
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Not sure of the port that you are using. A photo would help. 15in hg of manifold vac seems low for sea level or near sea level altitude (higher altitudes need correction factor).

pretty good article on vacuum based diagnostics:
AutoSpeed - Using a Vacuum Gauge for Engine Diagnostics

Have you verified correct base timing? How about mixture?
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  #5  
Old 07-17-2016, 10:55 AM
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I am at about 800 feet and have not checked the timing yet. Thanks for the link. Based on that the vacuum sounds low and I should address that first before trying to check cam timing and ignition timing is also going to be a pain since the fan bolt is stuck.

I'm not with the car right now but the port I was using is in the top of one of the runners.
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Current fleet:
1995 C36
1989 190E 2.6 5 speed
1974 450SLC
1988 5.0 swapped Supra
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  #6  
Old 07-17-2016, 12:58 PM
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Typical idle manifold vacuum varies among engines, and the two biggest factors are valve overlap and idle speed. Also, the same engine with a manual transmission idling in neutral will usually have higher idle speed and vacuum than an automatic idling in Drive, and automatics should always be measured idling in Drive.

The lowest manifold vacuum I have ever seen on a heathy production engine is 10" @ 900 on the '64-'65 Corvette 365/375(FI) horsepower 327s that have the huge "30-30 cam" that is probably the longerst duration, highest overlap cam even installed in a production engine. They're high rev screamers, but a pretty soggy below 3000 even with the aggressive OE spark advance map that gets all the WOT advance in at 2350.

In that same era the 250/300 HP 327s, which had a short duration low overlap cams for stump-pulling low end torque idled at 18-19" @ 500 in Neutral with a manual trans. (In case you rwondering those ratings were "SAE gross", which were probably overated by at least ten ten percent, so take that away and another 10-15 percent to get the modern "SAE net".)

Many Mercedes engines are high output and have fairly long duration and overlap cams, particularly the inline sixes, so they will usually show lower idle vacuum than a typical big displacement V8 that has lower specific output.

My 190E 2.6 five-speed idles at 15" @ 700 in neutral, and it has recently been reported in another thread from "James Dean" that his automatic is 12-13" @ 550 in Drive.

Given that the C36 has a high specific output engine I would expect it to show relatively low idle vacuum, but you need to specify the idle speed - in neutral if a manual and in Drive with an auto.

Reporting idle vacuum without full context as specified above doesn't mean anything.

Duke

Last edited by Duke2.6; 07-17-2016 at 01:25 PM.
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  #7  
Old 07-17-2016, 01:14 PM
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15" was in park at about 500-600rpm. When I get to it later this evening what should I see in drive? It normally holds 500 rpm in drive.
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Current fleet:
1995 C36
1989 190E 2.6 5 speed
1974 450SLC
1988 5.0 swapped Supra
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  #8  
Old 07-17-2016, 07:45 PM
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Quote:
Originally Posted by MaxC107 View Post
Oh it is different? How so? I marked everything before taking it apart and it seemed to line up with the marks used on other M104.
There is a different hole position with mark than regular 104 engines, look up previous search info, I don't have exact procedure at the moment, its been discussed before , I do have a c36 engine that soon will go into my wagon
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  #9  
Old 07-17-2016, 07:51 PM
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The exhaust cam gear gets the dowel installed at"U" position , info from c36 introduction manual, which may be retarded slightly , if memory correct 4 degrees
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  #10  
Old 07-18-2016, 04:16 PM
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a 36 wagon sounds awesome. Is that a W124?

Thanks for the info about the cams. I have verified my are set correctly. I guess I'm going to move onto plugs and maybe the O2 sensor. Too bad its not throwing a CEL since I have access to a diagnostics tool
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Current fleet:
1995 C36
1989 190E 2.6 5 speed
1974 450SLC
1988 5.0 swapped Supra
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  #11  
Old 07-18-2016, 04:26 PM
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yes 124 wagon with c 36 motor,motor out of an 95 c36 complete everthing
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  #12  
Old 07-19-2016, 10:47 PM
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Join Date: Oct 2014
Location: Central Indiana
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That's going to be super cool.

I'm still having issues with my car however. So far I can say:

- Idles rough at about 500-600 rpm and sluggish after first startup

- 15" of vacuum at idle at between 500-600 rpm

- Good compression

- plenty of top end power

- clean plugs

- cams set correctly

- swapped MAF from a known good M104

- hunts and idles rough as one would expect with the MAF unplugged

- EGR moves freely

- CEL is actually burnt out I'm thinking because I just realized its never on even on start up.

So I have a rough idling car after a head gasket job. The only thing I haven't checked is the O2 sensors Ideas on what else to try before buying parts? I might just do the plugs here soon.
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Current fleet:
1995 C36
1989 190E 2.6 5 speed
1974 450SLC
1988 5.0 swapped Supra
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  #13  
Old 07-20-2016, 06:31 AM
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Location: Norwalk, CT
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There is info on benz world about real time hfm scan software that you can get that will help eliminate the guess work you are having to do.
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  #14  
Old 07-20-2016, 09:04 AM
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Join Date: Oct 2014
Location: Central Indiana
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That only works if I have the tools and a check engine light no? Guesswork doesn't take that long and is pretty cheap.
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Current fleet:
1995 C36
1989 190E 2.6 5 speed
1974 450SLC
1988 5.0 swapped Supra
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  #15  
Old 07-29-2016, 11:22 AM
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Join Date: Oct 2014
Location: Central Indiana
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Update: The car started to have hesitation and surging issues under transient and full throttle almost like what happens when you have a carb with the float set too low. I cleaned and swapped in a known good MAF from a different M104 which helped. I then replaced the IAT sensor, cleaned the TB, and swapped in known good coils in one shotgun approach and now its doing alright. Still not perfect but its acceptable. Surprising that what was basically a dirty MAF and TB could cause such extreme issues.

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Current fleet:
1995 C36
1989 190E 2.6 5 speed
1974 450SLC
1988 5.0 swapped Supra
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