Where is that? - PeachParts Mercedes-Benz Forum

MercedesGirl · #1 09-13-2006, 01:09 PM

Does anyone know where the idle control valve is located at, on a 79' 450 SEL?
Also it's at 15 instead of TDC and when I turn it to TDC it's starts idleing really rough and just about stalls completely. Any idea's cause I done just about everything I can think of, there are no vaccum leaks.

nhdoc · #2 09-13-2006, 02:01 PM

Are you checking the timing with Vacuum line attached or not? Does the vacuum line make any difference to the timing. Some models call for timing set X DBTDC without the vacuum some are TDC with the vacuum...what does the plate in front of the engine read on your car?

250 Coupe · #3 09-13-2006, 02:28 PM

The idle speed is set with the large plastic screw near the drivers side front corner. Next to it is the aux air valve which controls the air bypass when the engine is cold.

The idle speed is controlled by changing the timing. The change over valve is on the firewall, near the center.

IIRC, the timing is set to 4BTDC with everything connected but check the plate to be sure.

If the EGR valve is stuck open or hooked up incorrectly, it won't idle well.

Michael

MercedesGirl · #4 09-13-2006, 03:21 PM

For nhdoc
It's says to check with vaccum line attached and when it's not attached it runs even worse.

For 250 coupe
Now when you say the EGR valve stuck open, what excacly is open.
I can push on the top where the nut is and it goes up and down but when it is at the top, let's say resting, there is a small gap all the way around. Should it be sitting tight with the top? If it isn't then should I get a new one. It is hooked up right it wasn't when I first got the car.

The reason I am trying to find the idle control valve is that in some forum that I read on this webcite, the persons idle control valve was gunky and dirty and after it was cleaned there was no more rough idle.

Are any on the things you mentioned like the switch over valve, on the fire wall, also known as the idle control valve? Because I can't find any refernce to it in any book for mercedes. Is it given another name.

One other thing, the suction hose to aspirator/ check valve, in running a quick test my book says to pull that hose off at the air cleaner that leads to the suc hose and that I should feel a vaccum and hear a sucking noise I don't feel any suction at all on ether end. The end connected to the check valve is blowing and the end connect to the air cleaner is doing nothing what does this mean is the book wrong, do I need a check valve or what I'm confused?

Thanks

250 Coupe · #5 09-13-2006, 07:00 PM

MercedesGirl,
If by idle control valve you mean idle air bypass, you don't have one. The '79 450SEL uses mechanical fuel injection and doesn't have much in the way of electronics. You should have a Lambda valve controlled by an O2 sensor but the controls control pressure in the fuel distributor rather than idle.

The things that affect idle speed on your car are the butterfly or throttle plate, the aux air valve which will be closed at operating temperature, the idle speed adjustment screw and ignition timing.

Offhand, I don't know what to look for with the EGR from the outside to check it. They do build up carbon deposits and that could hold the valve open which will dilute the intake with exhaust gases and make it run poorly. Also, if hoses are hooked up incorrectly, it will work backwards.

By aspirator/check valve, I’m thinking your looking at the air injection pump? I wouldn’t worry about that too much. It will affect NOS emissions but not idle.

Head over to www.w116.org and look for the library. The Mercedes CD is online there. I used the diagram from either a Haynes or Chiltons to get my vacuum hoses routed correctly. If I get time, I’ll see if I can find the diagram again and post it but don’t hold your breath as I’m ridiculously slow.

Michael

nhdoc · #6 09-13-2006, 07:28 PM

Quote:

Originally Posted by MercedesGirl

For nhdoc
It's says to check with vaccum line attached and when it's not attached it runs even worse.

I would watch the timing while I disconnect the vacuum line to see if the timing changes. You may just be introducing a vacuum leak and not affecting the timing by pulling the line off if the actuator is stuck. Check that and see if the timing changes when you pull it off next time, if not then the actuator is bad. Also, plug the hose when you remove it to prevent a vacuum leak.

In the early years the actuator was a vacuum retard that set timing to zero or a few degrees ATDC and I think that's what you have too, so your timing with the vacuum line off (and plugged) should be about 15D BTDC but others may confirm this as I am not familiar with the late 70's era.

MercedesGirl · #7 09-13-2006, 08:04 PM

Thanks both of you guys I'll check out what you said and see what happens.

If anyone else knows anything let me know.

250 Coupe · #8 09-13-2006, 08:55 PM

Find a copy of Haynes number 63030, old number 698, Mercedes-Benz 350 & 450. Check page 112 for a diagram of your vacuum system.

Notice that the hoses are color coded as are the valves.

There are some very hard to find hoses that connect to the throttle body. Look straight down on the drivers side opposite the throttle linkage.

The easy way to reconnect them is to use a short hose and a piece of stiff wire that is longer than the hose. Put the wire into the fitting and slide the hose down the wire.

It looks like the timing is TDC with hoses connected so you may have to low an idle speed or a vacuum leak you haven't found yet.

Michael

MercedesGirl · #9 09-18-2006, 03:37 PM

The vacuum line are all correct I have the book and used that diagram to put the in order. I'm thinking that it might have something to do with the fuel some how like not getting enough or like getting spirts of fuel instead of a consistent flow. but I'm not sure.

250 Coupe · #10 09-19-2006, 01:56 AM

MercedesGirl,

Have you checked the central locking system and the HVAC system for vacuum leaks?

There is around four thousand hoses under the dash, give or take three thousand nine hundred and seventy-five

.

The collection of hoses going through the firewall near the brake booster head off the central locking, storage tank and AC. You might try plugging them one at a time or find the source at the intake and plug it.

Also, how does it run above idle?

It is possible that someone has tinkered with the idle mixture adjustment and messed it up; you could also have some injectors that aren’t working well. When the engine is up to speed, inertia can make it seem as if it is running smoother when it's not. It takes a mechanic with a good ear or an O'scope to figure that one out.

Michael