mercedes 450sl fuel injection

My mechanic says that the fuel mixture is too rich. He kept the car for 2 weeks but was unable to fix the problem, says he does not have the specs for the 1975 electronic system... Help!!

[stevebfl]

The D-jet fuel system gets the bulk of its mixture correction from the input from the manifold pressure sensor. It also is the place for basic mixture correction (adjustment).

Your technician needs to know how to do this and also how to recognise whether the need for adjustment comes from the deteriation of the input from this device. This device's most common and inevitable failure (loss of sample vacuum from its evacuated bellows) causes the device to loose range and it looses it to the zero vacuum (full throttle) state.

The adjustment is a 4mm allen in the opposite end of the device from the electrical connector. It is under the black plastic cap.

To adjust ----- first let me tell you that in MB factory training on that system (yea I'm that old) they didn't hint at the availability of this adjustment they were so afraid of inexperienced corrections here for other problems - one should remove the throttle switch connector. This disconnects the idle correction adjustment on the ECU. The system should be adjusted to 1% CO with an exhaust gas analyser. Then the throttle switch should be reconnected and the idle corrected to 2-3% where ever it runs best. Then it will need to be driven to see if it still has enough range to add fuel during acceleration. After running a while it will have to be readjusted if there was a large rich to lean correction. It will wind up too much correction the first time if you make a large change.

------------------
Steve Brotherton
Owner 24 bay BSC
Bosch Master, ASE master L1
26 years MB technician

[JPMilbrand]

my 1975 450SL has been pouring out a gas smell . .which makes my wife say "just get rid of it". I have had it to several shops in town (Buffalo, NY) and nobody could figure it out. I believe that it is running to rich (as you can smell gas in the exhaust) and this post may be my long lost answer. Thoughts?

[Graham]

Originally Posted by JPMilbrand

my 1975 450SL has been pouring out a gas smell . .which makes my wife say "just get rid of it". I have had it to several shops in town (Buffalo, NY) and nobody could figure it out. I believe that it is running to rich (as you can smell gas in the exhaust) and this post may be my long lost answer. Thoughts?

It could be, but not only will you need to find a garage that knows about these cars, but they will also have to be equipped with a CO analyzer. In our area (the other end of Lake Ontario), I have not found anyone who really knows much about this car. As a result, I have tried to learn as much as I can myself. I need a CO meter, but so far have not found one at a reasonable price.

By the way, your gas smell could be due to other causes. Does it come from exhaust?? Over the years, I have had gas smell due to leaking gas cap - there is a vent under car, I think. New cap was a cheap fix! Maybe it could also be due to a bad injector? This might show up as an uneven idle? I tried to check mine by taking IR temperatures at each exhaust port, but results were inconclusive.

Good Luck

[bwostosh]

Check EVERY fuel line and the vapor recovery system.
for fuel stinking
Then do what stevebfl recommends with the MAP sensor.
But turn that hidden 4mm out to lean no more than 1/8 turn per time.
We did this on a 1,500 mile run and got it perfect (without the co2 sensor) by reading the plugs for color.

[Gurunutkins]

just remember though guys that as stevebfl says this system is controlled by your vacuum. your first step should always be to check that you have around 17 inches or more of vacuum at idle. Only if you have this should you try to adjust the MAP. if you dont have the vacuum at idle then correct the vacuum problem first!
cheers
Barri

[jamesmilbrand]

jmilbran@hodgsonruss.com Thanks