250 SL Intake Manifold Pressure at Idle Speed of 800 RPM

[Atazman]

I'm wondering if someone can check their BBB (or another reference) to find the intake manifold vacuum specification for my '67 250 SL at idle??

I have a BBB, and in Section 00-0/20 there is a table that shows the manifold vacuum range at idle for several cars; but, the 250 SL is not shown.  However, on page 00-0/21 another table seems to indicate the manifold pressure should be 300-400 mm Hg (11.8-15.7 in. Hg) for the 250 SL at idle.  I would like confirmation whether the 300-400 mm range is correct.

I have a nicely tuned car with head work done less than a year ago.  I'm having some brake issues and I pulled the hose from the brake booster to the intake manifold and installed a vacuum gauge to read manifold vacuum.  I measured 13.5 in Hg, and was quite taken back that the reading was so low.  However, this appears to be within the range I mentioned above.  The brake booster is not causing this low vacuum reading because it was isolated from the intake manifold when I ran my test.  A year or so back, I posted a similar question..... and I did not get a response as to what the specification was.

I'm thinking these low vacuum readings at idle may be because the distributor is retarded at idle (0-2 Deg. ATDC).   If the specification is indeed 300-400 mm Hg, does this explanation make any sense?  I'm used to seeing 18.5---20 in Hg vacuum on American cars.

Thanks in advance......

[jeffc280sl]

Mine is around 15 inHg at idle.  I've not read the spec but can tell you my car runs great.

[ja17]

Yes around 12-15 seems to be normal. Of course this depends on the idle speed and engine timing.  A lower reading than American cars may be a result of the vacuume retard distributor.

[Atazman]

Thank you, Jeff and Joe.  Appreciate your quick response.

[Benz Dr.]

12 - 15 inches is kind of low. 18 + would be much better. If I tested a car and saw 15 of vacuum I would start looking for a problem.
I would advance the ignition a little bit or at least to TDC at idle which might give you a bit ore vaccum. The 051 distrbutor ( the correct one for your car ) should advance 30 degrees if it's working right so you can start at TDC and not be going too far.
 Try setting your full timing advance to 30 degrees at 3000 RPM or higher. Regardless of where your idle timing is it will always be correc at speed. Don't pay any attention to anyone who says '' disconnect the vacuum line at idle while checking timiing  ''  - that doesn't give you the proper readings.  Leave everything connected the way it's going to be when you're driving the car.

[Atazman]

I appreciate your comments, good Dr.  I installed a 123 distributor a few months ago.  Timing is dead on at 3000 rpm, and 0 Deg. at idle (vacuum line attached).  The car runs great, and idles very smoothly.  But.......if higher vacuum is expected, I'll keep looking.  Thanks.........

[Benz Dr.]

Do you have any flat spots during throttle up? That can indicate a vacuum leak. Try 35 degrees total advance. As long as you don't hear any pinging you should be OK.

[Atazman]

No flat spots on acceleration.  I'll advance the timing about 5 Deg. to see what happens.  At present (0 Deg. at 800 rpm with vacuum connected), there is no "pinging" on regular grade gasoline.

Thank you for the advice.

[Benz Dr.]

I'm going to go on to say that 15'' is '' dead '' low. The last time I had this happen I had a 250SL engine running with the wrong IP from a 280SL. In an attempt to make it lean out the throttle valve was opened up and the timing was advanced but it still ran very poorly.
After I found the right pump and put that one back on I had to reset all of the throttle linkage and ignition timing. I'm not saying you have the wrong pump but all I could get was 15'' max and I had about 19'' after this repair. The engine would go up to 24'' on decel.

I also had another engine that had poor vaccum. This was a standard while the 250SL was an auto. You will notice problems with low vacuum more on an auto car because it affects transmission shifting. The car will pound going into gear even if you let off on the throttle.
In this case the cold start thermostat wasn't working and the extra air along with the rich mixture was causing a low vacuum condition. I would check the cold start system and look for vacuum leaks.

[Atazman]

You really have me thinking.... and wondering.  I'll be doing some more checks and will get back with whatever I find.  I do believe I have the right FIP, but will confirm. 

My air adjustment screw seems to be quite a ways "out", suggesting to me that there is not much air, if any, getting into the intake manifold from another source.  But, I will check all of this also.

I just installed a new brake booster, and I have not checked my vacuum and done any fine tuning since that installation.   

Please let me know if you think of anything else that might be affecting me.

[Benz Dr.]

I don't know......... you hang out around this site. I'd call that enough to affect anyone....

[Peter van Es]

Note that the Dr is one of the longest standing "hangers around" here....

[Benz Dr.]

Holly ****! Peter actually got one of my jokes without me having to explain it. There's hope after all. Not much, but there IS some...........

[Peter van Es]

I'll remind myself that you're joking everytime I read one of your posts now! 

[Atazman]

No joke!  I don't have 19" Hg vacuum.  The best I can get at idle (800 rpm) is 15" Hg.  And this is after I advanced the timing 5 Deg. .....to 5 Deg. BTDC with the vacuum line connected.

I do have the correct FIP.  Also, with the engine fully warmed up, I checked whether air is being fed into the intake manifold through the small air filter on the FIP.  The answer is........no air when warm, but lots of air during the time the engine is warming up.

When I suddenly open the throttle, the vacuum drops to 0" Hg.  After revving it up and immediately allowing the throttle to close, I notice that the vacuum does not go beyond about 19" Hg.  But, I believe this is attributed to the dashpot, which prevents the throttle from suddenly closing.

The car seems to run great at idle, no flat spots on acceleration; no backfire on decel............  Now I wish I had never measured vacuum. 

[Benz Dr.]

Sometimes it's not what you expect. Try a different vacuum gage - it could be broken.

[Benz Dr.]

I'll remind myself that you're joking everytime I read one of your posts now! 

 Not really. After all, we haven't finished with the financial statements yet.....

[66andBlue]

Not really. After all, we haven't finished with the financial statements yet.....

[http://www.sl113.org/forums/index.php?topic=13126.msg89560#msg89560  ... either knows or says nothing about the subject - I do the same thing which is why you won't hear me say anything about A/C, radios, electronics or auto tramsmissions."
Hmm - what about financial statements? I didn't know that you are not only our resident curmudgeon but also an accountant  

[Benz Dr.]

Hummm ....... don't give up your day job.
 Jay Leno isn't looking for a replacement, BTW. He's actually funny and he knows about cars too.

 I'm not an accountant. Maybe a few more questions about the annual report which may or may not include financial statements. I'll have to study it a bit more first.

[J. Huber]

Sorry to veer off topic here but doesn't the vacuum reading need to take into account the elevation? I recall you would add a point for every 1000 feet up. Thus a reading of 15 at my house (2600) is actually closer to 18... Also, I remember the wise Mr. George Davis saying that the specified vacuum reading is only a guide, its more about how the car is responding than what's on the gauge.

Now, as you were...

[Benz Dr.]

Good point James. I took that into account but I don't know where our question is coming from or where he lives.

I'm about 600 feet above sea level so my readings are always near max. I've had gages quit working or give me false readings before so I thought I'd mention that. Normally a gage will drop down to about 2 - 4 inches on heavy throttle and pull up to 24 or more on heavy decell. You won't get that by blipping the throttle in park but somewhere near that amount while driving.
 
2,600 feet shouldn't be high enough to see all that much difference plus this engine has a barometric compensator which should help to keep fuel ratios constant, at great help in keeping the vacuum as high as possible. Unless he lives in Denver the readings should be higher than that.
However, if the engine pulls hard and the mileage is within the range I wouldn't get too excited about it. As I said earlier, auto trans cars are far more sensitive to low vacuum and become difficult to drive below 15'' of vacuum.

[Atazman]

Thanks for the comments, James and Dr. Dan....   I live on on the West Coast (California), and the elevation is 350 ft.  So...  elevation is not my excuse for low vacuum readings.  By the way, I did change gauges and didn't gain anything.

I can't help but think about the "BBB", and what it says about vacuum readings.  Please look at the table on page 00-0/21.  Does this not say the  manifold pressure should be 300-400 mm Hg (11.8-15.7 in. Hg) for the 250 SL at idle?  If not, what is the table referring to?  The previous page lists manifold pressure for several cars, but not the 250 SL.

Thanks in advance......

[stickandrudderman]

The last one I had with this problem ended up needing a rebore. Compression test and leak test were all fine, but ovality was found in the cylinders nonetheless.
Engine turned out to be on max oversize and so we had to sleeve it down to first oversize, but the improvement after rebuild was dramatic.

[Benz Dr.]

Thanks rudder, I was going there next. At max compression you will have lots of vaccum. With  low compresion you will have lowered vacuum even though the engine will start and appear to run normally.
It may not use oil or smoke but is still worn out. A compression test will only tell you so much. I use a vacuum test which will show very small leaks.

[Atazman]

Gentlemen..........  My engine has been replaced with a new engine from MB; it has about 70,000 miles on it now.  I measured compression right after the installation and recorded 160-165 psig on all cylinders.  I admit, compression could have taken a "dump" since I checked it in 1984.

Although this car runs absolutely great, I can't help but wonder what it would do if the manifold vacuum was "right".  But I have to say.... I'm getting mixed feedback on what is "right".  Some people say 15" Hg is good; and others say it is "dead".

I sure wish someone would take a look at the BBB on page 00-0/21. Please tell me what you think it says about vacuum readings for the 250 SL.  Does this not say the  manifold pressure should be 300-400 mm Hg (11.8-15.7 in. Hg) at idle?  If not, what is the table referring to?  The previous page lists manifold pressure for several cars, but not the 250 SL.

Sorry to beat a dead horse, but I would sure like to see what Mercedes says idle vacuum should be.

Thanks....