High tickover. 1971 3.5 M116 Mercedes

[W109Driver]

In neutral and park.
Tick over is at about 1200rpm.
I've checked timing and tried winding in air idle screw.
1200 is as low as i can get it.
If I squeeze hose from idle valve.
Revs drop but still ticks over.
Does this mean itd getting air from a vacuum leak?
Ive had a really good look but can't see anything.
Any other ideas?

[tyl604]

Sorry. What do you mean by tick over?

[BWhitmore]

Idle speed?

[Frank Reiner]

Originally Posted by W109Driver

In neutral and park.
Tick over is at about 1200rpm.
I've checked timing and 1) tried winding in air idle screw.
1200 is as low as i can get it.
2) If I squeeze hose from idle valve.
Revs drop but still ticks over.
3) Does this mean it's getting air from a vacuum leak?
Ive had a really good look but can't see anything.
Any other ideas?

W109:
1) It is possible that the idle screw is worn enough that even when turned all the way in there is still significant air passing. Look to the throttle plate & the bore of the throttle body as another source of air; when the throttle bore and the plate are clean the stop screw should be adjusted to just keep the plate from sticking in the bore when closed. Sometimes the stop screw has been erroneously used to adjust tickover (idle speed in the colonies).
2) Tends to confirm the possibility of the throttle plate and the air screw leaking.
3) Try pulling the hose from the manifold pressure sensor and plugging it. If that hose is leaking there will not be a change.

The auxiliary air valve (AAV) may not be closing completely; cleaning often remedies that problem.

If none of these have an effect, there may be leakage at the seals between the plenum and the runners.

[Tomguy]

The plenum seals may be leaking, or the "Lower" injector seals (Uppers aren't actually seals, even though they're labeled such). MAP hose may be a source, as well. Central locking system, if in disrepair, would also cause it. Is the engine running rich? If so, it's got a vacuum leak almost 100% guaranteed.

[fonzi]

AAV stuck open?

Plus vacuum leaks.

[W109Driver]

Okay, had poke round today. Couldn't find any split hoses.
Throttle body seemed to be closing nicely.
Switched in a AAV that I thought was good used.
Made no difference, so guess was stuck open as well.

So question is how do I go about cleaning the AAV.
It looks a sealed/press fit part.
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[W109Driver]

To previous point.
I've squeezed rubber hose next to AAV and the revs drop down. So this makes me think it is definitely AAV fault.

Have not ever put it on a COmeter, but how would you be able to adjust the mixture, lean it off if needed?

[Graham]

Originally Posted by W109Driver

To previous point.
I've squeezed rubber hose next to AAV and the revs drop down. So this makes me think it is definitely AAV fault.

Have not ever put it on a COmeter, but how would you be able to adjust the mixture, lean it off if needed?

You can try soaking the AAV in both solvent and detergent. Then use a heat gun and cold water and try and cycle the valve from hot to cold and back several times. It should close completely at about 150F.

However, the piston does get rough from grit entering with the air. It is possible to dismantle the AAV and then clean and polish the working parts properly. Covered in this thread (read whole thing )

If you can find a reliable CO meter (or a wideband air-fuel ratio meter) then you can adjust mixture. There are two adjustments. Main adjustment is a small screw on the Manifold Pressure Sensor. Very sensitive and should not be done unless someone has been there before and removed the seal on the screw. There is a knob on the ECU that only adjusts idle mixture. You can mess with that to your hearts content Set in middle of range and move to each side of that to achieve best idle.

Other thing that affects mixture, is fuel pressure. Higher than 2bar standard pressure, richer mixture. Lower pressure, likely hot start problems.

[W109Driver]

Thankyou for that nicely written reply.
Has anyone got pics of the two adjusting points mentioned. Cheers.

[fonzi]

Originally Posted by Graham

Then use a heat gun and cold water and try and cycle the valve from hot to cold and back several times. It should close completely at about 150F.

Just curious... is there anything wrong with putting it in a pot of water on the stovetop and watching the temp with an IR temp sensor (or any old thermometer)?

[Graham]

Originally Posted by fonzi

Just curious... is there anything wrong with putting it in a pot of water on the stovetop and watching the temp with an IR temp sensor (or any old thermometer)?

There is nothing wrong with using two pots of water - hot and cold. But I found it quicker to heat the bulb with a heat gun.
When checking if valve closes at 150F, it is hard to see the valve when submerged in water and not easy to get water to desired temperature, below boiling. But results are same, so use whatever method works for you.

W109Driver - You should locate all the key Djet parts and understand what they do before attempting to tune engine. There are many on-line write-ups on Djetronic Injection. I will add a link to one below. I don't have a W109, but I suspect the ECU (the computer) may be RH side front of engine bay? MPS (Manifold Pressure Sensor is likely on LH side of engine bay just behind ignition coil.

Early Mercedes Bosch D-Jetronic Training manual - Mercedes-Benz Forum

[W109Driver]

Took my car for a run today.
With the switched in used AAV.
It did work.
But only because I was parked ,engine running, as I took photos.
As temperature gauge got above 85* I noticed revs reducing. At 90* it stalled and wouldn't restart.
Until I remembered and backed off the idle screw half a turn.
So the AAV does work but not under normal driving.
As in normal driving I've never seen temp gauge get above 80*.

Some one earlier mentioned altering it by squeezing sensor.
Anyone done this? How and how much?
Thanks.

Thinking. ...... even if you have a good working AAV , it may be worth squinting a bit of WD40 in every now and then.

[W109Driver]

I've cleaned out AAV.
Got it moving up and down with hot air gun.
It is shutting at too high a temperature.
The sensor is crimped but I guess I need to crimp it more.
Anyone done this?

[fonzi]

I believe the bulb is filled with wax. Maybe some has leaked out. I would warm it up to the temp at which it should be closed, and give it a little squeeze, but not too much. Then do a full cycle test.

I've never done this, but may need to soon. I've got one I think is stuck closed.