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Old 08-05-2004, 09:58 PM
sjcruiser's Avatar
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Join Date: Nov 2003
Location: San Jose
Posts: 88
transmission vacuum on gasoline engines...

Hello all,

I've read a few documents regarding about the tranny vacuum check/adjustment; but all of them have been for diesels. Do we have any article for gasoline engines as well ?

One of the prong of a three(3) way vacuum connector that goes to the vacuum check valve (mounted on the engine firewall) is quite loose on mine (rubber harden, and does not grab tightly). Another prong has a line that connects to the rear of the engine, where appears to be a metal orifice(sp?); and on the bottom of this orifice there is an vacuum output that goes to the transmission modulator.

My mechanic reassured not to worry about the looseness at the check valve since vacuum sucks in anyway. However, when going up hill, or under heavy load, my economy gauge shows very little vacuum, given that I do not have a heavy foot. In short, the tranny shift very smooth at times; but also very harsh at other times also.

Any tips are greatly appreciated (as I'm very itched to correct this problem) !!!

Thanks for keeping till this point,

'86 560SEL | Silver Ext. / Blue Int. | 201K miles (Apr. 05) | 204K miles (Feb. 06)
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Old 08-06-2004, 06:46 PM
Registered User
Join Date: Feb 2004
Location: Northern California
Posts: 31

Sorry, but I don't have a cure for your engine. I have similar economy gauge indications as yours ("84 380sl):
@ idle..all the way left
@ cruise..nearly full left (1/8 deflection or so)
@ slight accel (barely depressing the pedal) least halfway
@ normal accel from stop..nearly full right

The engine has a very noticable loss of power compared to previous performance. I hope someone will be able to point us in the right direction. Thanks in advance.
'84 380SL
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Old 08-06-2004, 09:31 PM
Join Date: Mar 2000
Location: Gainesville FL
Posts: 6,844
The reason for an issue with diesel vacuum is they don't have vacuum naturally so the system is created to mimic gas cars. Well gas cars don't need to mimic gas cars......except...

Gas engines have the most vacuum at every load when they are running best. An engine running poorly will have poor vacuum and the trans will shift bad accordingly.
Steve Brotherton
Continental Imports
Gainesville FL
Bosch Master, ASE Master, L1
33 years MB technician
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Old 03-29-2005, 05:12 PM
sjcruiser's Avatar
Registered User
Join Date: Nov 2003
Location: San Jose
Posts: 88
(another) vacuum leak identified...shifts much better now

Haunted by Steve's post...on my never ending quest on improving my old girl...

Found another vacuum leak last month...the connection hose between the idle air distribution hoses has a large crack on it. Since these hoses are pretty brittle (very old) and might break if I try to remove them, I just put a dab of RTV clear goop on the leak. Let it cured for a while and took the car out for a test drive. My economic gauge no longer bottoms out on moderate accelerations (shows more vacuum in general); and the transmission shifts much better.

Have been playing with the transmission/throttle linkage and adjusting the vacuum modulator since - very very smooth shiftings.

Just wanna share my findings...

'86 560SEL | Silver Ext. / Blue Int. | 201K miles (Apr. 05) | 204K miles (Feb. 06)
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