adjusting the air-gas ration and reasons hunting engine (saw effect)

Hello from Istanbul

Greetings to all community members.

Hope the pandemic did not cause any problems to the community members.

I would like to address two issues regarding my 1972 - 350 SLC V8 - C 107 - D- Jet.

My car is running on a rich mixture which I can smell during idling. How can I adjust the mixture to reduce the excessive gasoline ?

My second problem is the persistent sawing effect of the engine when the gas pedal is not pressed.

Depending on the mood of the engine the hunting happens when the engine is cold (not always).
After warming up ususally the hunting disappears.
However after slowing down from a long high speed journey (ca. 30 min with an average speed of 90-100 km/h a slight hunting appears again.

Today I have experienced hunting of the engine when I slightly depressed the gas pedal. When pressed harder it disappeared.

Sometimes when the hunting starts I press the gas pedal one or two times and the idle returns to normal

The AAV (auxiliary air valve) was refurbished last year. The result was lower rpm when the engine is cold. Before replacement the cold idle was above 1800 rpm. now it is ard 1100-1200 rpm.

What do you recommed I should check?

Best Regards,
Juan

Hi Juan,

best regards back to Istanbul! Unfortunately we are banned from travelling there this year.

Fuel mixture is normally set by MAP sensor, all other sensors and ECU. Most effect have MAP sensor and engine temp sensor.

What I suspect from your sawing and high fuel consumption: It looks like you have a problem with vacuum leakage. Can you measure your vacuum that MAP sensor sees in idle on warm engine. Use a T-connector and a vacuum meter. If that value is too low, MAP sensor will enrich mixture. Even better would be to insert smoke into manifold and verify whether it leaks somewhere. You will find German instructions on that in "Nebel des Grauens" when you search here.

Even that AAV is overhauled and that cold engine rpm seem okay, please make that test with closing hose to manifold on warm engine to verify that it is fully closed. rpm must not change when doing on hot engine. Just to be on the safe side and it is an easy test.

Hi Volker,

Greetings from a hot and humid Istanbul nite to all


thank you for the explanations.
I have watched the Nebel des Grauens.
the parts you mention exist (ie the fog generator) in Turkey but I believe it should be the last resort to build one

we also have a steam machine to clean the couches and the grout lines on the ceramic floors. I think it is a type of pressure steamer
Can we try this as well?
Later I have watched again how to test the SDF.
'Einfaches Prüfen eines Saugrohrdruckfühler'
Measuring resistance is easy I can do this on my own.
Testing the SDF with a manometer should be possible.

Now I wrote to my mechanics to check whether there is a steam machine to check the manifold leaks.

I have read in Bosch DJet workshop manual that by turning the knob(potentiometer) on the ecu the air/gas ration can be adjusted.
while adjusting the engine I remember measuring the exhaust gas and the mechanic kept the CO level high to be on the safe side which I understand results in rich mix
would also this have an effect?

Merhaba Juan,

CO rheostat/potentiometer ONLY affects idle-run CO. To be more specific it only matters as long as the idle switch in throttle switch is closed. If that is open, it is simply ignored. So let your mechanic verify idle-switch to only close in idle and then set your CO to something like 2.5 - 3.5%. It will not affect anything else than idle.

The rest is determined by MAP sensor and other sensors.

I do not know about your steam machine. Water steam could condense on manifold and not show any effect. However you can use any kind of smoke as long as you do not use too high pressure. Let it go gently inside.