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The same old 450 SEL idle problem..

Started by skatinblind815, 26 April 2011, 07:26 PM

skatinblind815

Hello everybody.
I am back with the same problem. I've had my '78 450 SeL since summer, 2010. Ever since then, I have been studying this website trying to find out everything I can. I've fixed a lot of my old cars problems, but a few things will not go away. I have had an interesting relationship with this automobile. Here it goes, I'll just lay it all out  :-\

I go out to start my car on a regular day, and a few things happen. The engine starts every time, but as soon as it starts, the idle plummets down to almost nothing. The engine never dies at that point, and after a few seconds it struggles back up to about 1200-1400 rpm's. Usually this is when I take off for school. As the car warms up in park, the idle gets higher as the the engine temperature goes up. At the highest, the rpm's are about 1800-1900 and engine temp is normal. Except for the very first start in the morning, the engine seems to idle very smooth, not rough, just really fast.

I have taken a lot of advice on this problem from the good folks at this website and here is what I have done or tried.

Replaced all the old vacuum lines in the engine bay with new custom silicone lines. Re installed them as they were. Although I would be interested to see pictures of a perfect 450 SEL vacuum line set-up.

Adjusted the idle speed on top of the engine but it doesn't make a big difference.

Checked for any throttle linkage hang ups, there doesn't seems to be any. Cruise control doesn't work..

Checked the auxiliary air valve in hot and cold water and it opened fully and closed almost all the way.

Removed and took apart what I believe to be the warm up regulator, and found it to be extremely clean on the inside. I was looking for small screens that I was told to check, but didn't see them.

Other things: New fuel pump, oil change, new spark plug wires and spark plugs. Recently had a radiator leak that I fixed with BarsLeak. What else... new tires.

But it still has the idle problem. I'm Just trying to give you all the variables. You guys are my last hope before I take it to a Mercedes mechanic!! I'm a student trying to fix his beloved 450.

I would really super appreciate anybody out there who could give me some pointers on how to cease this problem or at least take a step in the right direction. I'm hoping to get some of this websites guru's in on this!

Thank you
-Nick from Washington State.

jbrasile

Nick, when you said the aux air valve closed "almost" all the way, that tells me part of your problem might be right there. Unfortunately the valve is only available new from MB and it is quite expensive so I would try a used one from a junk yard if possible to see if things improve. You should be able to drop your idle speed to 750-800 rpm using the idle speed adjusting screw.

With regards to your warm-up regulator, the screen you are looking for is under the fitting where the fuel line from the fuel pressure damper connects to the WUR clean it with carb cleaner or compressed air if dirtyor clogged.

Tks,

Joe


skatinblind815

Hey thanks for reading my post! I will look for a different Air valve asap . But with the Idle adjust screw, turning it either way only raises or lowers the idle very little.

jbrasile

If the aux air valve is partially open after the car is warm, turning the screw will not do  much. Try to replace the valve first and then re-adjust to see if you notice a significant change in rpm.

Tks,

Joe

WGB

Basically if it idles too fast with the throttle closed you have an air leak.

The obvious first start is the auxillary air valve

Here is an earlier link which contains a method of repair (I have no knowledge if it works)

AAV link

If it is closed the leak must be elswhere in the inlet system.

Check all hoses and manifold connections.

Bill

M110COUPE

Im not familiar with the 450, but I am with auxilary air valves as used in d jetronic 250ce's & m110 's 280 as used in the w116.

A quick check to see if this valve is the culprit is too simply clamp this line, ie, once the car is warmed up block off the line so no extra air can enter the system, ie the only air feeding the inlet manifold is via the throttle body. This quickly shows if the unit is at fault or not.
Auxilary air valves in my opinion are a constant source of trouble, as i have come across sticky or seized up units on a number of old mercs.You can pull them apart give them a clean up with a steel wool, and reassemble with a synthetic high temp lube, I also make a little gauze filter up to stick to the inlet of the breather to stop any debri entering the valve.The valve is similiar to a thermostat valve, but instead moves an alloy piston in an alloy bore, which seizes very easily with corrosion.
they are sealed with a press fit from memory with a kink punched in to the perimeter to lock it in place,they not designed for refurbishment but can be if a new replacement is not readily available.
Anyway thats my experience with aux air valves and it may not apply to the 450, so others may confirm this to be similiar or not.

john6007

Thanks Bill and the rest, I think your advice and links could solve my 450's (ex TJ450's) hot start issues, IE starts fine cold, needs encouragement when hot and does idle on the high side. My unfounded guess was an air leak but I've been too busy to tinker. :)
79 - 450SEL 6.9 #5628 &  #6466, 92 - 300CE-24, 98 - MX-5, 89 - Ducati 906 Paso

hans moleman

While tinkering with the WUR I also found that a very rich cold mixture raises your cold idle a considerable amount. My 450 sel would idle at 1900 rpm with cold control presure at something like .8 bar. Once the cold control pressure was brought into spec the idle would only go up to about 1200-1400rpm then hunker down at about 750.

peter newman

I live in the UK and I look after a very good friends 1979 450 sel and he had the same problem as you have and I had the warm up regulator over hauled in the Uk and it totally cured the problems that you have, The engine always fired up instantly but it then ran very regged for about two minutes and then it ran ok ish but with the over hauled warm up regulator it's like a brand new car with a MASSIVE improvment in performance. It's on the top front of the engine and it's held on with two allen bolts and it has one large and one small fuel pipe union and an electrical cable. The phone number in the UK is 01462 682432, This guy gave me a 100% and 10/10 service, Peter Newman

skatinblind815

Hello all. I have dissembled the warm up regulator. Looking on the inside, underneath the section where the fuel lines connect, there were four tiny silver screws that held down a metal circle piece. (I dont know what to call it :-\ ) Does anybody know if those screws are supposed to have a certain torque when you tighten them back down? ???

I wanted to ask because after I first disassembled the WUR to clean it, I got it re-installed and the old 450 wont start. Trying to figure out where I went wrong.

Also, I was checking spark plugs, and I have found out that the car is running very rich. Spark plugs slightly covered in black stuff. I think this is because the fuel/air mixture screw is set way off what it should be. Does anybody know how you get it set in the right spot?

Thanks for the help guys.

((I'm going to check the auxiliary air valve now...again))

skatinblind815

I just tested the auxiliary air valve in ice water as well as boiling water. In the boiling water (220 degrees F), the valve closed ALMOST all the way. I could still blow air through it when it was at its most closed point. As far as my high idle, this is a problem, right??? After a few minutes in cold water, it was fully open. I read that I can possibly open the auxiliary air valve and lube it up and it should work. Anybody have an idea on how to disassemble the Auxiliary Air Valve? It doesn't look like its meant to be taken apart. KOAN said "The fix is to take a hammer and a block of wood and tap the top bit down until the center piece is flush with the outer piece. The idle speed will probably need adjusting after this.t

I'm not sure I understand what that could mean! someone help me!

Thanks again.

TJ 450

The four screws just need to be 'reasonably tight'. I assume you put the regulator back together correctly.

With the Aux Air Valve, taking it apart is not really possible. The top section is pressed in. Koan is talking about pressing it in further to close the gap, but this is only possible if it isn't maxed out already.

If the valve almost closes, I think your fast/high idle is probably caused by something else.

If the car is running rich, then you will have a fast idle anyway...

Tim
1976 450SEL 6.9 1432
1969 300SEL 6.3 1394
2003 ML500

koan

Don't play with too many things at once, good way to get in a mess. If it started before and it doesn't after playing with the WUR most likely its the WUR.

Is it rich and stinking of fuel when cranking or almost starting and not firing up?

On the WUR did you get the rod into the upside down top hat on the spring and the other end into actual regulating piece in the middle of the plate with 4 screws?

The four screws had locking compound on them, put some loctite on them and do them reasonably tight, no need to go super human on them.

The AAV should be fully closed at around 75 to 80 deg, shouldn't have to have a boiling hot 100 C engine to fully close it.

Have a good look at the AAV, you should see how the top is pressed into the body, if its even with the body not much can be done, but if the top edge (not the cast raised part) is higher than the body put a block of wood on the AAV in place and give it a few taps with a hammer. Read the above in conjunction with my crude ASCII diagrams.

koan

Boogity, Boogity, Boogity, Amen!

skatinblind815

Hey Dudes. So, I got the WUR back together its working great. I had the little springs on the inside put together wrong. Now the car starts right up . I tried to lube and clean the AAV and I got it to close a little more, but still not fully. Luckily though. since it was closing a little more, it DID bring down the idle a bit. About 1500 rpms steady.If the AAV closed fully, how much more could it possibly lower?

But , my car IS running pretty rich because the air/fuel mix screw is all out of whack to lower the idle.  And It actually has a pretty good cold start. Doesn't reek like gas or fumes upon starting. I haven't started it in a couple days though because I'm trying to locate a new AAV.

Also, these cars have a electronic diagnostic plug-in in the engine bay. If a shop had the right equipment, what type of things can you find out from these?

Thanks!!!

jbrasile

skatinblind815,

Good to hear your are on the right track.

A fully operational AAV should bring your idle down close to spec, anything but fully will increase rpm's by quite a bit.

With regards to the diagnostic plug, it was supposed to be used to set the timing without a timing light. I have never seen a shop use it, not even a dealership.

Once you get the new AAV and the idle goes down close to spec, it will be time to set the correct air/fuel mixture. You can do it pretty close by ear but an exhaust analyzer will make sure it's spot on.

Joe