Low Vacuum Reading

[Tom]

Group:

I have historically had a low vacuum reading at normal (800rpm) idle.  I generally get a steady 13-14 on the vacuum gauge, and I suspect this should be more like 17-18.  I have excellent compression and leak down stats (175 across all cylinders and 8% leakdown) so I suspect a vacuum leak.  I tested the brake booster-no leak.  I turned the idle air screw all the way in and could not choke off the car-in other words, the car kept running at a very low rpm with no idle air.  This sounds like a vacuum leak somewhere in the intake manifold area, but I am not sure.

Any ideas about how I can isolate the vacuum leak?  Any other ideas as to what could cause the low vacuum readings?  I have ordered a new intake manifold gasket and nuts in case I go so far as to replace that gasket, but this may be more of a winter project.

Thanks,

Tom

[Naj ✝︎]

Tom,
Removing the manifolds with the head in-situ can be a ball aching task. Mind you, with left hand steering you've got more room to work,what with the brake booster being out of the way.
I would check the air valve under the thermostat on the fuel inj pump. It may not be shutting off the air fully when warmed up.
To check this, warm up the engine and then remove the small hose on the F.I.pump that goes to the cold start valve and block it with a bolt or similar.Restart and check vac.
Could be sticky valve or thermostat that does not extend fully....
naj

'Kloines Scheisserle'
65 230SL
68 280SL

[Tom]

Naj,

Thanks for the post.  When you refer to the air valve under the FI thermostat, I take it you mean the air filter.  I have checked this recently and found that the strong vacuum that exists at a cold start completely goes away after warm up.  However, I will check it one more time.  The FI pump was rebuilt about 18 months ago, but it could be the problem.  If there is another valve other than FI pump air filter that I a not thinking about, please let me know.

Also, please keep the ideas coming-especially the easier items-I sure would like to avoid taking the manifold off!  Along the easier potential fixes, I am going to check the venturi valve to ensure that it is closing all the way at idle stop.

Best,

Tom

[rwmastel]

Tom,
First, I think Naj is talking about the valve at the air filter.  Bob recently wrote a post about cleaning/lubricating the valve that opens/closes the air passage at the FI Pump air filter.  Make sure that yours is closing and not letting extra air in through the filter and into the intake manifold.

Second, check the line that runs to the transmission.  I assume your car is an automatic?  Joe Alexander has mentioned that the vac line runs from the intake manifold to a plate on the transmission.  Under this plate is a rubber component (part of a switch?) that reacts to the intake manifold vacuum and thereby creates some input to the operation of the transmission.  This rubber component can fail over time.  Plug this line (don't drive!) at the intake manifold to see if this effects your vacuum reading.

Third, there is a vacuum line running to your distributor.  I'm not sure how you could test the vacuum in this line while the car is running.  Maybe you could install a T-fitting and measure off that?  Maybe you could use a felxible line as a stethescope(?) and listen for leaks along this line?

Rodd
Powell, Ohio, USA
1966 230SL, Euro, Auto, Leather, both  tops
1994 E420

[Tom]

Rodd,

Thanks for the additional ideas.  Looks like the line to the tranny is the next easiest check point and I will test that line this weekend (ooooh-a weekend project!).

I tested the line to the distributor and it equals the vacuum to the brake booster.  I also switched out the lines to the distributor and tested again and got the same reading.

About a year ago, my wife noticed a crack in the brake booster vacuum line.  I fixed that and wow, what a difference!!  So that was one of the first places I looked and it checked out ok.

Another idea that comes to mind is the breather hose at the top of the valve cover.  Is the top end of the head under vacuum and could that be a source of my low vacuum readings?

Thanks

[Cees Klumper]

Tom - I think we need an expert to step in here. Have you asked your wife's opinion? I don't think the valve cover is under vacuum pressure. Maybe check the connections of the large flexible rubber hose between the intake manifold and the air filter. Does the plastic return air valve that is in the brake booster hose function properly?

Cees Klumper in Amsterdam
'69 white 280 SL automatic