450sl - idle runs high and low - PeachParts Mercedes-Benz Forum

blueshield11 · #1 06-16-2009, 02:36 PM

my 450sl from 1974 has a constant changing idle. First high then low and again and again. Problem solves when I put it in drive. Put it in idle again and it changes again. Looks like it is getting too much air.

Does somebody recognizes this problem ?

GaryF · #2 06-16-2009, 02:41 PM

All of a sudden reving up and down when cold

meltedpanda · #3 06-17-2009, 08:27 AM

must be in the air - haha

GaryF · #4 06-17-2009, 02:29 PM

LOL, I now officially declare this "Hunting Idle" week. Maybe next year we can organise D-Jet towed floats in a parade with kids dressed as giant AAV's etc. Plenty of fire crews on standby too !!!

blueshield11 · #5 06-18-2009, 02:28 PM

Thanks GaryF, I have read the link you mentioned and before I wanted to adjust the idle screw I checked all the lines to the injectors and tightened some clips who seems to be a bit loose. And the problem was solved.

Thanks for the input.

blueshield11 · #6 06-20-2009, 05:19 PM

... problem is not solved. When I checked it and thought it was okay it seems the engine was not cold enough.
I have started the car this morning and the problem was still there.
Idle was constantly changing, 1 sec high, 1 sec low etc etc.
When the engine is warm, I do not have that problem anymore.

The adjustment screw is all closed.

What can I check next ?

GaryF · #7 06-20-2009, 05:55 PM

Hi,

What is you idle speed (RPM) hot and cold?
If its still running when hot and your idle screw is all the way in then you must have a large vacumn leak or you AAV is not closing enough. Search the forum you'll find loads of good information on this common problem.

Gary

meltedpanda · #8 06-22-2009, 09:50 AM

no way the adjustment screw should be all the way in. A little adjustment makes a significant change, Agree with Gary , look for vacuum leak, may be worth pulling air valve off, screw out, clean it?

Jo1175 · #9 06-22-2009, 08:18 PM

Could it be the trottle position sensor giving false readings??

GaryF · #10 06-23-2009, 06:36 AM

Quote:

Originally Posted by Jo1175

Could it be the trottle position sensor giving false readings??

I reckon it's working fine, the ECU is seeing the throttle closed and going into idle mode which causes the hunting idle if the idle speed is over 1,500rpm.

Gurunutkins · #11 06-23-2009, 09:54 AM

working from memory the ecu circuit on idle in our cars has a high idle cut out at 15 - 1600 rpm and will cut the fuel so the car will appear to die and then rev back up again to 1500rpm for a second or 2 then die down to 4 or 500rpm etc. the usual reason for the over reving is too much air leaking in either through the AAV or just vacuum leaks just as gary and Melted have said. pull the AAV when its cold and stick it in water to see if it closes down to just a slit. if it does then its probably working OK and you need to go around the intake manifold with a propane torch open or some starting fluid and check for vacuum leaks (remember if its a big leak and the engine is running the starting fluid will stop it running it will only speed up if its a small leak).

I have included pictures of the AAv and the valve all the way open (you can see the glint off the springs as its difficult to take a photo down the air inlet) and after boiling water nearly closed (I didnt want to burn myself so the water wasn't scalding)
cheers
barri

the slider valve open (difficult to see but you can see the top of the valve and the coils of the top spring)

nearly fully closed