72 w109 4.5 running rich

[MANIATIKO]

Hello. My 109 some times will start to load up .and run rough . And the plugs get black .I have a second set of plugs that I clean up I install them and runs fine for a while then it starts to load up again. Any suggestions?
Also on the side of the computer it has a dial what does it do?. Thank you!

[meltedpanda]

just depends on how far down the rabbit hole you want to go but Id look first at the trigger points as the issue

and read this
my journey here
https://www.peachparts.com/shopforum/mercedes-benz-sl-discussion-forum/297036-back-infamous-rich-djet-operation.html

the knob is the rich lean adjustment from the ECU, but it only compensates , does not cure - many items on these old DJets that are interconnected

[Frank Reiner]

The adjustment on the side of the ECU is a mixture adjustment for idle ONLY. Once the closed throttle contacts are open it is inoperative.

The overall richness may be caused by an air leak into the manifold-to-pressure sensor hose. More pressure at the sensor = more fuel.

[cth350]

Hunt down vacuum leaks. Any, all of them.

Hunt down threads, recent and not-so-recent about d-jet here.

-CTH

[Tony H]

Seems like running rich is a much more common issue than lean. I wonder if the engineers designed the FI to run rich as a safeguard since it is a closed loop system with no oxygen sensor.

[Idle]

A word on vac hoses....

I usually just cut off the ends since those are the parts that get all the heat and harden up. Then just replace them to their barb.

Of course replacing the entire hose is best, but cutting off the tips can help you to find a problem faster.

And replacing worn hoses is never a bad thing to do.

There is also the starter spray solution. Just shoot a bit of starter spray at the end of a vac hose and if the engine RPM jumps up then you have found a leak. Of course this is somewhat dangerous as you are pumping either into a hot environment. If it flashes you can burn down the car and your house.

[cth350]

A 100$ smoke machine from amazon does wonders for finding leaks. I picked one up while working on my 500sel. Found them all really fast. -cth

[MANIATIKO]

Hello guys .I figured out the issue .
Since I have a w116 will the same engine I tested sensors . And found the Air temperature sensor in the air filter housing is bad. Now I have scoured for a replacement and it has been no available. Is someone know of a good one used or new for sale please let me know. Thanks in advance

[Graham]

Originally Posted by MANIATIKO

Hello guys .I figured out the issue .
Since I have a w116 will the same engine I tested sensors . And found the Air temperature sensor in the air filter housing is bad. Now I have scoured for a replacement and it has been no available. Is someone know of a good one used or new for sale please let me know. Thanks in advance

Good that you found that. Rich running can be caused by many things. Air leak may be one, but so can anything that affects the mixture.
- Air leak can be AAV not fully closing
- As Meltedpanda suggested, worn trigger point rubbing blocks once car has done about 100k miles can cause rich mixture.
- Both the air and coolant temperature sensors provide input to the ECU for mixture control. Easy to test (as you have done)
- Incorrect fuel pressure (should be in the 29-30.5 psig range
- Signs that someone has tampered with the MPS.

As Frank said, the ECU knob only fine tunes mixture at idle (throttle fully closed)

Those air temperature sensors are used on many cars. The part numbers are Mercedes A0015428817 and Bosch 0280130006.

I did a search and found several for sale at a very wide range of prices! Search for the Bosch number.

https://www.oreillyauto.com/detail/c/bosch/fuel---emissions/fuel-injection/efi-temperature-sensor/b9537ee001bf/bosch-multi-purpose-sensor/bos0/0280130006

The engine in your car is similar to the one in my 72 SL. You might find tonk.ca useful - choose the 450SL. Your Djet (EFI) engine is covered.
This link is also informative: https://www.tonk.ca/misc/djet-efi-manual.pdf