1972 350SL Surging.

[Mikesmerc]

My car when idiling surges and also noticable when driving she seems to be short of fuel for a second or two then the power kicks in again. This is not very pronounced as it may only drop 1mph at 60 and back up again a sec or two later. It seems as if you have to press the accelerator that extra bit to overcome the dead spot. She revs freely when in nuetral.
Any ideas?

Regards
Mike

 

[A190]

Hi

I worked on Merc's like yours when new. The problem you decribe was very often caused by a vacuum leak. I suggest you carefully check the condition of your small bore vacuum pipes (the black rubber ones)

It may be useful to isolate the vacuum door lock system and check to see if the idling improves

 

[Andy]

Hi

After you have checked A190 recommendations. I would then check the cold start idle valve more than likely causing the idling problem especially when cold. Also check the air pressure sensor. This can cause running problems. Usually to weak or rich. Also the base contacts in the distributor situated under the base plate of the distributor plate. There should the an electrical connection to it on the side of the distributor. You will need to remove the distributor to check. Clean with petrol & blow out after. Usually covered in oil or residue. Make SURE YOU KNOW WHAT YOU ARE DOING. Otherwise you can do more damage which will be expensive!! You have been warned.


Regards


Andy@ www.mercedesservicing.com

 

[Mikesmerc]

Hi all
had a look at it today as she was really surging at idle. Removed the air cleaner and started it. Ran fine. Put back on the cleaner again, back surging again. I found that it was the sensor at the inlet of the air cleaner causing all the problems. when I disconnected it she ran fine, when I plugged it back in she surged. What is this sensor, looks like a temp. sensor but cant be sure.
Going to buy a new one but I dont know what its called or what its for.
Thanks for the advice.
Mike

 

[Andy]

Hi

I will be suprised if this is your problem. The switch is an air temp switch. The vehicle cannot be run without this connected. I think you may find that the problem will still be there with a new one fitted. I don't think I have ever seen or heard of one requiring replacment. But there is always a first time.



Regards

Andy@ www.mercedesservicing.com

 

[Mikesmerc]

Hi Andy
just recently discovered that a 116 3.5 had been fitted as a replacement which has a different fuel injection system comprising of two fuel headers over each bank of cylinders and each injector wired seperately. Maybe this is why this model will run OK disconnected. Will buy a new one and see how I get on. Drastic improvement when disconnected.
Regards
Mike

 

[Mikesmerc]

Looks like your right about temp sender. Just replace it (50euro ) +vat. Reconnected wire and the same thing. Excuse my ignorance but where is the air pressure sensor located? The distributer plate has been earthed using a bit of wire on to the securing screw of the points out through the cap and eathed to engine block.(not by me).
I dont have any central locking on my model.
Will have another look at vacuum pipes.
How could recconecting a temp sensor ont the air cleanr have such an effect of surging. Have it disconnect at the moment and drives alot better.
Any ideas?
Thanks
Mike

 

[Andy]

Hi

Removing the air temp sensor just richens up the mixture (I think). The air pressure sensor is a silver roundish unit on the ns flitch with a braded air pipe coming off it.

I would get some body who knows about your model & has a emissions tester to check your vehicle. Without it your changing things unecessarily.


Regards


Andy@ www.mercedesservicing.com

 

[Mikesmerc]

Thanks for repling Andy. Found the air pressure sensor (about half the size of a bean can?) which has a 4 pin connector and a light braided hose. Connected everything up again and she started surging, the only test I could perform was to unplug the presuure sender which made her cut out. Then I reconnected and squeezed the hose by hand which rose the revs with no apparent surge.Is there anything else I could do?
You will have to believe me when I say service in my area for such a vehicle is disasterous. The Main Dealer in Cork more or less laughed at me when I asked him to do a diagnostic on my 1990 560sec imagine if I mentioned 72. Need an old school merc wizard.
Going to disconnect every vacuum pipe from the engine just to eliminate the possibility of leaks and see if she improves(except pressure sender hose, brake vacuum hose and dizzy advance/retard hose)
Cleaned the idle air valve with WD and tested it using hot water from a kettle , like you would do with a thermostat and seems to open and close ok but not sure if it is closing enough as the ide screw adjacent is closed fully.
Know your a busy man Andy and will not pester you any further.
Many thanks
Mike
Excuse spelling-oily fingers dont you know

 

[guydewdney]

i sthe 350 a D-jetronic? by the comment of pressure sensor being a small can on wing i guess it is.

theres loads of generic info on the web for this ystem

http://members.rennlist.com/pbanders/DJetParts.htm
http://persweb.direct.ca/aschwenk/DescriptionofBoschDandLJetronicSystems.htm
etc

 

[Mikesmerc]

That time of year again, dragged the SL out of hiding for the summer and bought the following for her..

New dizzy cap, HT leads, plugs, rotor , condenser and points. Swapped out ignition coil and wired up the resistor to the coil. Got a second hand idle valve which turned out to be in worse condition than my own, so I stuck my own back on.

Cleaned the trigger contacts at the base of the dizzy which looked in pretty good nic anyway.

Same old problem, intermittant surge when at idle when warm which sometimes increases till she cuts out. Slow drivings a pain, just wont pull smoothly. Give her welly and she runs sweet until you go back to cruzing. I find myself driving like an idoit, booting it from traffic lights to traffic lights just so as not to put up with it.
Cant find any vac leaks anywhere and I disconnected the idle valve while she was surging, opened the idle screw and plugged the valve with my finger just to eliminate this item.
Opened the top of the air pressure sensor and put it back as I think Im out of my depth with this one when I saw windings and such.

One more thing my coil was wired 12v permanent by someone else, so I traced a few wires around thar area that were disconnected and some of them lead to a grey box with fins on under the passenger wing just below the headlight. This item appears to be totally disconnected from the cars leckys. Now I know this isnt the cause of my recent problem as it was running fine before with this disconnected. But what is this item?

Sorry for dragging it out, but any suggestions re any of the above I would appreciate.
Thanks
Mike

 

[Myros]

I think

that grey-finned box( about the size of a filofax, giving my age away there) is probably the ECU/transistorized ignition thingy, if you have one. However, as it's not connected to anything, maybe it was an aftermarket fitting which has been since disconnected. I have one in my 85er, but it is definitely wired up.

 

[Mikesmerc]

Thanks Myros, better leave it disconnected as I have enough troubles as it is.

Cheers
Mike

 

[Mikesmerc]

Priced up an air flow meter from germany, 850euro..ouch. Have to look around for a second hand one. Have replaced timing chain, seems ok. Same old symptoms plus the occasional backfire.

Mike

 

[Mikesmerc]

Hello all, same problems, just four years later, I know Im jinxed with my old Mercs and rarley solve any of their problems, just refreshing this for any more suggestions.
Thanks
Mike