REALLY... REALLY Need HELP for W108 280SEL 3.5L Idle Problem

[ESO]

2 months ago after the body restoration work has completed for my W108 280SEL 3.5 from 1971, every time after I started the engine while it is cold, the idle speed will go crazily UP & DOWN between 800 to 2500 RPM, in which it would go on like such until the engine warms up to normal temperature, which mean that it is almost imposible to drive the car before its warm up, otherwise the foot brake won't even fold the car still.

I have previously posted this problem on some forums and most people suggest me to check on VACUUM leakage which I have and confirm NO leakage on the hoses. Some said I should slightly hammer the idle sensor on top of the engine which I also did BUT you can imagine the results....... Makes No Different At All. I have been told by my mechanic that the idle sensor might be mulfunction..... So, please if someone can help me here by telling me who on the internet can supply me all these 3 sensors for controlling the idle speed.

Thank you


Eric

W121 - 190SL (1955)
W111 - 220b (1961)
W108 - 280SEL 3.5L (1971)
W107 - 280SL (1979)
W116 - 280S (1980)

 

[Michel]

There are many suppliers... (every MB dealer is one)
The one that comes to my mind would be MB Spares in Canberra
http://www.mbspares.com.au/

Excellent service and reasonnable prices

 

[ESO]

There are many suppliers... (every MB dealer is one)
The one that comes to my mind would be MB Spares in Canberra
http://www.mbspares.com.au/

Excellent service and reasonnable prices

Thx Michel,

Well, I bought from MB Spare twice before, I agree, pretty good service but recently all emails I sent over during the last a few months have received NO reply at all....Don't know why.

Thx

ERIC

 

[John S]

This is a relatively common problem with a simple "fix" as described by Greg about 12 months ago on this thread:http://www.mbcnsw.org.au/forum/index.php/topic,1117.0.html

I would suggest that later you find a good Mercedes mechanic and have him set up the adjustment correctly for you and take out the restrictor.

 

[Ron B.]

Lot of Crap on that site...Spark plugs jammed in hoses????
Let us go back to basics for a few minutes.
The car was in a body shop. I always begin a diagnoses on a Djet by getting the ignition perfect FIRST.
Then replace the hose to the MAP sensor. if the hose IS PERFECT ,replace the MAP sensor with known good one to eliminate that as the cause of the problem.
I always remove the idle airslide and drop it in hot water. It should open COMPLETELY. Then,I pop it in the freezer and it MUST close Completely. Anything less than either of those ,buy a new one. Replace the large black rubber hoses because 40 year old hoses are a waste of time.
If the symptom persists and bearing in mind that the car has been worked on,Check the ECU earth connection on the right hand inner mudguard .It's a collection of brown wires bolted to the panel with a 13 mm bolt.
That being correct it's time to begin real checking and not guessing.
The ECU is is behind the right front panel under a metal bracket. Remove the bracket and the rubber boot. Under this is a plastic cover. Unscrew the cover and pull the large plug free from the ECU.
The terminals in the plug are numbered.
Air temp sensor ( it's in the aircleaner snorkle)
Using a multimeter check the connection between 1 and 13. the resistance must be 200ohms at ambient air temp. Higher air temp means lower resistance.
Water temp sensor.
resistance at terminal 23 must be 2000 ohms at room temp. At engine operating temp it must be around 1000 ohms.
Any difference means you must check at the sensor themselves. Replace if necessary.
Reconnect the plug and close up the ECU.
Throttle switch.
I don't know how many times I have rectified this fault on Djet cars as a result of Holden trained 'technicians' adjusting the throttle stop screw to adjust the idle.
The throttle needs to adjusted so it is closed when the engine is in idle position.
next ,a 0.4 feeler gauge is placed in the throttle stop.

Connect a volt meter to terminal 17 on the switch
. Turn on the ignition and check that there is a voltage reading .
pull the feeler gauge out and the volts should return to Zero.
If not,Loosen the two screws on the switch and rotate it until it reads a voltage with the gauge in place and no voltage with gauge out.
With the switch reconnected to it's plug,and the key on,Slowly open the throttle. You must hear the injectors click 20 times in succession from idle to full throttle. Any difference to this means the switch is faulty.
finally check the fuel pressure at the cold start valve. It must be 30 PSI.
Barring faults,high or low pressure can be adjusted at the regulator on the rear of the intake manifold.
Vacuum leaks cannot be checked effectively and the best idea is to remove the intake and replace all the seals .

 

[Michel]

Wow......

Talk about a detailed diagnostic

I enjoyed that.... even though it's 10 past midnight and I'm about to go to bed...

Thanks RonB

 

[gabs]

Ron B,
Pardon my ignorance if you have identified what the problem as I have no idea about diagnostics and I am not mechanically minded.

I had a similar problem with my w124 the difference was when the engine was surging during warm up this did not cause the rev counter to increase on the tachometer. When the surges were occuring occasionally the lights on the dash would flicker. But once the car was warmed up off it went. I also started to notice the car became a bit sluggish. Not that it was a rocket anyway.

In the end the alternator packed up Just a suggestion, but it looks like you have more of an idea than myself.

Best of luck
Gabs

 

[s class]

gabs, your w124 has a different type of injection and air metering, so the diagnostics is different.

 

[WGB]

Thanks for that RonB.

I have never had anything to do with D jet and don't want to start but to see that piece of industrial eloquence from sheer experience is marvellous.

Bill