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Old 12-02-2001, 10:21 AM
Posts: n/a
Unhappy Temp/Vacuum Switch 300SE

I need help identifying a broken vacuum switch on the head of my '89 300SE. It's located just to the left of the dipstick and looks like a temp switch screwed into the head with two vacuum nipples protruding from the top. The problem is one nipple is broken. I need to identify the part to order from Fastlane tomorrow am (along with other parts).

Also, I "jumpered" the two vacuum lines together bypassing the vacuum switch. Haven't started the car yet, came in to determine what systems I would be effecting.

I most likely broke the nipple removing the idle valve for cleaning. The car "hunts" while at idle (fluctuates 0-50 rpm). Also there is a noticeable hesitation from a standing stop, especially when cold. Very similar to a inoperative accelerator pump on a carburated car. Have replaced the O2 sensor, injectors, cap, rotor, and plugs. Took resistance readings on Water Temp Sensor, at 20C it was 204k ohms, at 80C it was 505 ohms.

By the way, while removing the idle valve for cleaning, I understand now the previous posts I've read regarding replacement of the idle valve hoses. I'm going to order those tomorrow and replace them but am not looking forward to doing it.

Also have a bearing noise from the belt guide pulley, but that's another story.

Any help on the vacuum switch would be appreciated. Thanks in advance.

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Old 12-03-2001, 10:03 AM
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Join Date: May 2001
Posts: 356
I am not certain but I believe the darn thing is called a Thermo Vaccum Valve.
Also I am pasting something from a recent post of mine regarding the Idle Valve breather hoses -
if you need more info about replacing them I could give you more details. ---

"Thanks Peter (psfred) for your help with trying to cure the rough Idle on my 1987 300SE/Euro. I finally got to replace the two breather hoses on the Idle Valve. Gladly I must say that Idling has never been better. There is only a hint of an occational shudder when the car comes to a stop. It is Almost MB perfect.
This was the last chapter, I hope , after replacing fuel pump & filter, plugs ,wires , cap , rotor and clean flushing the Idle valve itself.
Like you said the hoses were a bear to replace , especially the one to the Manifold. I did manage to do it myself though, both from the top and bottom . Had to remove the cold start valve though (reached from above) , in order to fit the hose into position (pushed from below).
In order to reach and replace the second one(going) to the fuel distributor you really need to remove the Idle Valve itself

I was wondering if members here would believe that the changing seasons could have something to do with better Idling as well. It is colder and perhaps the Hoses and fittings and valves do not expand as much..any thoughts on that ?"
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Old 12-03-2001, 10:30 AM
Posts: n/a
Thanks gidpor,

Do you know what system the Thermo Vacuum Valve is in? I lose the vacuum line once it enters in the left fender well.

Also thanks for the pointers on the idle control valve hose replacement. We'll see how that goes later this week when the parts arrive.

Thanks again, Jeff
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Old 12-04-2001, 02:32 AM
Registered User
Join Date: May 2001
Posts: 356
Not sure where it goes...I'll look at the car tonight.maybe mine is better marked.
Also, re the Idle valve hoses, I forgot to mention the wire harness -
it too needs to be moved out of the way before work can be done - no big deal .
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