W116.org

Gentleman, I am seeking some advice from yourselves due to my lack of knowledge and experience.

When starting the engine my car has no problems but after a little while (once warmed) begins to chugs and lacks power.  The engine slowly chugs more and when i press the accelerator pedal it wants to cut out and on occassions does cut out.  I have been advised to change the air slide, rotar arm and distributor cap.
I have a few questions regarding this.  Firstly, does this sound like the problem can be resolved by replacing these parts?  Is it an easy job? Could the solution to the problem be different?

In particular, the mechanic said that the air slide, when pressed down,  should pop back up rather quickly. When he showed me mine, it was rather slow coming back up.  There was something below the slide that is supposed to push the air slide up but it seemed rather slow and there was very little resistance.

Many thanks in advance

Samiul

I reckon if the air/slide moves too fast there's a potential for the mixtures and emissions to be all over the place...

My 6.9s air slide neither moves fast nor slow, but seems the same as on every other 6.9 I've looked at.

What is your idle speed when standing still, fully warm, in D, with your headlights and a/c on? 

I.

The auxiliary air valve on K-Jet cars often sticks, you could try lubricating it with penetrating oil. The main problem seems to be with the valve not closing properly which normally results in a fast idle, this might be due to the integral thermostat not functioning properly.
It sounds like the Idle adjustment screw has been closed all the way to compensate. You should try turning the screw anticlockwise when the engine starts misbehaving until the idle reaches the specified speed in Park when full operating temperature has been reached.

Tim

Gentleman , thank you for the responses so far.

The idle speed is not particularly high (in my estimation) at start.  After 5 mins when the car has warmed , the idle speed drops to 700rpm or there abouts.  Once placed into D the idle speed tends to drops further and eventually cuts out.  Not immediately - it struggles to cut off!

Tim, you mention the integral thermostat possibly not functioning properly.  Is this replaceable? can it be fixed?

Prior to this problem occurring two weeks ago, i had no problem with the car cutting out.  The idle speed was a fairly constant 700rpm

thanks

Sounds like a vacuum leak.

Or a stuck EGR vave if you have one.

If the AUX air valve has failed, you should get a very high idle speed rather than a low one.  The integral thermostat is part of the AUX air valve.  It's job is to supply more air when the engine is cold and close off the valve as the engine warms up.  They seem to fail wide open which will give you an idle of around 1500 RPM when warmed up.

On my car, when I push the air plate down, it moves smoothly and against a slight but noticable resistance.  When released, it will follow my finger back up but seems rather lax about it.  It will then have resistance again when pushed down.  DO NOT push it down with the key on, the fuel pump will run and you will flood the engine.

Michael

Hi Michael

when you refer to a vacuum leak are you talking about the loss of resistance when pushing down the air plate?

If i push the air plate down the and then release , i can't really feel it follow back up with very much push.

However, from your advice it does not appear to be the aux air valve as the idle speed comes down after the engines warms up.

If this is a vacuum leak, what does one need to do to rectify this?  How can i be certain that it is a vacuum leak? How easy a job is this?

Thanks for your help so far.

Samiul

By vacuum leak, I mean that air is getting into the engine at some point after the air plate.

When all of the air is measured by the air plate, the engine runs at or near stoichiometric http://en.wikipedia.org/wiki/Air-fuel_ratio

If any air leaks in to the intake after the air plate, it will dilute the air/fuel ratio and cause problems with idle.  Most of the time, it shows up as a engine that will idle acceptably in neutral but will die when put in gear.  It may run better when first started due to the warm up regulator adding fuel.

Some folks use carb cleaner and others use WD40 to find leaks.  Just spray around the intake and vacuum hoses while listening for a change in idle speed.  Keep in mind that these things are very flammable and you are spraying them on a hot engine.

Fixing leaks can be as easy as  reconnecting a hose thats fallen off it's fitting or as difficult as replacing the intake gasket.  One is free and the other might cost $400 or more if you have a shop do it.

It sounds like your air plate is reacting much as mine does when pressed down.  I don't think it's part of the problem.

If you have a EGR valve, it will dilute the air in the intake with exhaust gases and cause a poor idle.

Replacing the distributor cap, rotor, spark plugs and maybe the spark plug wires, are relatively cheap and easy things to do.  A Haynes or Chilton's manual will guide you through.  Also make sure that the firing order is correct.  This means the the spark plug wires go to the correct spark plugs.  The engine will run poorly with two wires crossed and may clear up a bit at higher RPMs, if you can get it to accelerate at all.

Michael

Unfortunately the thermostat in the auxiliary air valve is not known to be serviceable. Basically, it limits the travel of the plunger in the valve. This means that it will remain slightly open when it should be closed, as when the engine has reached full operating temperature. This is normally compensated for by closing the idle speed adjusting screw towards the fully closed position. Unfortunately this is a big compromise, as occasional, after a hot soak or extended idling in high ambient temperatures, the valve will actually fully close. When this occurs, the engine will not idle without manual assistance.

Perhaps your aux air valve is functioning correctly, but the screw is still adjusted to the nearly closed position. The idle speed slows when you shift into drive... perhaps it is slowing too much. Next time the problem occurs, I would have a screwdriver nearby and open up the screw. Otherwise, it would be a piece of cake for any mechanic to check this.

Tim

Hi Michael/Tim

Your advice is much appreciated.  Having gone through the points you raise, i have to agree that it does not sound like the air plate.  I am not sure what the EGR valve is or where i'd locate this but a pointer will again be appreciated!


I have already changed the spark plugs and am yet to change the rotor and distributor cap.  Is this easy enough for a layman to deal with or should i employ the services of a mechanic?  I would rather do this myself but i suppose i dont want to create unecessary problems due to my lack of experience in these matters!

Tim, I need to find the speed adjusting screw and do as you say.  What is the correct or most desirable idle speed?

With all of your helpful advice the resolution to these problems seem somewhat straightforward and not as 'difficult' as i had first imagined. Fingers crossed!

thanks

Samiul

You are very welcome Samiul.

Changing the cap and rotor is a very easy task.  You MUST be very anal about putting the wires back exactly where you pulled them from.

From the W116.org main page, click library, click service manual library under the heading "handbook", then scroll down to the "open service manual" link.  You will find a copy of the factory service manuals for the W116 cars.

Poke around looking for how to check and set the idle and etc...

It's not searchable but try it out and if you need a link let me know.

I do recommend that you pick up a Haynes, Chilton's or whatever is available where you live.  It will detail takes in your comfort zone without bogging you down with the finer details.

Michael

Michael

Thanks for the pionters.

I am already in the process of ordering the Haynes Manual but i notice it applies to US model.  I don't think there is mouch difference to the European models but i am sure this will help me loads.

As a matter of curiosity, what is the EGR valve?

The other things I need to ask is that the idle speed of the car should be around 700-750 rpm but if i need to adjust this, the screw is placed under the air filter? Sorry if the question is silly and obvious, but i'd rather ask a silly question that create unecessary problems for me!


I've checked out the library from the homepage but it wasn't so clear to me.  I'll wait till i get hold of the Haynes manual.

Thank you once again and i shall certainly keep you updated.

Samiul

Good morning Samiul,

The EGV valve recirculates exhaust gases into the intake which helps reduce Oxides of nitrus or something like that.  It would be mounted to the front right (as you are look at the engine from the front) cylinder head, kinda in the way of the spark plug.

Here is a link to setting the idle.  I found it by selecting 1976 450SEL, then engine manual, then Mechanically controlled gasoline injection system, then selecting Adjustment of idle speed.

http://handbook.w116.org/Engine%5C107%5CM117_45%5C073-100.pdf

The arrow pointing out the idle screw is a bit hard to see, it shows up better when printed.  The screw is black in the picture but is natural nylon on my car which means it is a cream color.  Being plastic, it is fragile.  It may be all the way in right now so don't force it, try taking it all the way out and lubing the threads with a silicone grease.  Try to get it back to it's original setting before starting the engine otherwise the engine will over rev and thats not good.

Michael

The Haynes manual is for a US car but the only real differences are types of pollution controls and whether you drive on the wrong side of the road

Hi Michael

You truly have been most helpful.  I'll give it a go and certainly hope that this resolves the problem.  In this way I can begin enjoying the car again!

Regards


Samiul