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scia63 02-06-2008 05:36 PM

350 SL engine warm up

New to MB cars and this forum. Just accqired a 1972 350 SL I quess really an early 450 SL.

Is it normal, during engine warm up, for the engine to rhythmically surge in rpms until the engine reaches operating temp?????? Then all is well. Nice smooth idle @750 rpms

The car starts without hesitation and runs great.

I disconnect the battery when I park the car. So does the ECU need to go through a "learn porcedure" when I reconnect the battery and restart causeing the sing song in rpms ?????

Thanks in advance for learnin me

350SL4spd 02-06-2008 06:37 PM

ECU isn't that intelligent.

I'd say that you have either a vac leak or a bad wire in the cold start circuit somewhere. Possibly in the temp gauge that controls it.

Of course it's D-Jet and therefore very very touchy about vac leaks and dirty points etc.

If you plan on doing much of any work on the car yourself you need to familiarize yourself w/ the D-Jet. I recommend "How to Tune & Modify Bosch Fuel Injection" by Ben Watson as well as the M117 FSM.

Good Luck.

Edit: Check the temp sensor in the snorkel on the air cleaner. It might be unplugged. If it is, plug it in & test. If it's plugged in, unplug it and test. Some of them run better w/o it. I'm sure there's a resistance test out there for that sensor books aren't in front of me right now.

Edit 2: Also check the hard plastic hoses around the cold start and the ICV. These tend to crack w/ age and can play merry hell w/ your idle.

scia63 02-06-2008 07:08 PM

Thanks 350SL4sd:
Is there an "O" ring or rubber gasket that fits under the air cleaner on whitch it sits to make it air tight connect with engine intake??

What is ICV?

I'm reading about D-Jet

Frank Sciannameo

350SL4spd 02-06-2008 08:52 PM


No O-Ring on the bottom of the air cleaner (well, not on mine or my friends '75 450SL anyway).

ICV is the idle control valve (3). Look for a big 'ol flat-head screw looking thing towards the front of the intake mani near the dizzy. There you should see the afore-mentioned idle screw (2), the hard rubber hoses I mentioned earlier (coming out of the ICV), and the cold start valve (1). (See attached image.)

You should definitely also check the engine temp sensor (4). My Bosch FI book states: "Should the sensor become shorted or the wiring harness grounded, the effects may not be noticeable at all when the engine is warmed up. The symptoms would be more like a carb w/ the choke stuck open: rough or erratic idle, stalling/hesitation when the engine is cold and progressively running better as the engine warms up."

Usually when the switch fails it fails 'open', causing the car to run super-rich through the whole cycle, so I'd look for a short in the switch or the wiring.

Just where I'd look first. D-Jet idle problems can be really hard to track down...

scia63 02-06-2008 09:42 PM

Thanks again 4spd: I have D-jet book, saids to check hose between auxiliary air regulator and intake air distributor. But where are these items ?

Is it me? I have an Etype and finding the names and location of parts is so much easier. The jag has a parts book that gives you all the info. of every part.

Seems all the MB service manuals I currently have are elusive in giving you basic info. Seems you got to search hard. I have the MB maint, engine,chassis & body with numerous MB reference books but I' still finding it difficult to find the info. I'm looking for.

My current questions are a reflection on that difficulty.

Is there one source you can go to for that would ID parts and their location in the engnie bay??


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