Rich running 3.5

[HerbPhillips]

Hello all,

I have a 1971 280SE 3.5 cab that I have owned for a few years. It was a good running car until a few months ago when my fuel pump died. I replaced the pump and ever since it has run like C$(p.

The car is running SUPER rich. I have a vague comprehension of D-Jet but I'm no expert. I removed the air breather and found the following leaks

-the brake booster hose had a crack

-One of the lines nearest to the feed for the MAP was missing from the intake (open intake line)

both have been corrected.

Also I had whitchunter performance rebuild all of my injectors and I replaced all of the seals.

Installed a used Fpump from a 1972 4.5.

New fuel pressure regulator.

When I say the car was running rich, I mean it was spitting gas out of the tailpipe...

If I try to adjust the computer pot switch, which way is rich/lean. And the same for the idle air screw, which way is low-high.

I'm going to go get some new plugs today and start tinkering with it.

Additionally my MAP doesn't have an allen key, it has a large slot in one end (unless that's a cover).

Any ideas would be appreciated.

Kind Regards,

Herbert Phillips
Memphis,TN

[andersbenz]

Herb,

Sorry to get off target but were you ever able to resolve your problem with the 1993 600SEL?

[alabbasi]

It sounds like your D-Jetronic system is way out of adjustment. If it's running this rich, it can cause damage to the engine so you need to be careful.

I had very similar problems with my 280SEL 4.5. The guy who took care of it is in Atlanta GA. I can put him in touch with you if you PM me.

You may need to adjust the map sensor and ECU (ECU for idle mixture Map for running mixture).

Good luck.

[HerbPhillips]

yes my 600 is fine, the mass air wiring harness was bad, bad ox sensor, and the throttle actuators had lost their memory. Whew!~

The 3.5 has fouled plugs, I'm going to replace them and see how she behaves.

What direction do I turn the pot switch on the ecu to lean the system and can I adjust the MAP? It has a large slotted screw facing the driver's side. I might take a picture today and post it.

[alabbasi]

Herb

If it's running as rich as you say it is, then you're going to need to change the oil too as there will be a lot of gas in the oil that will make it run bad and it will skew all your tests. Expect to change the oil a few times during your diagnosis.

With my 4.5, It would run run, and then all of a sudden, turn super rich. It would then wash gas into the oil and make the car impossible to start. I ended up replacing the computer and map sensor from a junked car and the car had run beautifully since.

Good luck

[psfred]

Super rich sounds like a dead map sensor, and the only way to test it is to find an old Bosch D-Jet tester and check the opening times on the injectors (or use an oscilliscope -- about 7 ms while cranking, I think). It's an inductive device (moving slug in a set of coils) so no way to test it easily.

A couple of stuck injectors will do the same, but if all the plugs are black, time for a replacement MAP sensor -- unless the hose to it is bad.

One other thing, since this happened immediately after you replaced the fuel pump -- VERIFY the fuel pressure! 28 psi, I think -- if you put the pump from a K-Jet on there, the pressure may be way too high, and 40 psi or so will flood you out badly! A bad pressure regulator can do the same thing, so can a plugged return line.

Peter

Have you checked the fuel pressure? You should have right around 30 psi at the injectors.

If the wrong pump was sold to you(like a CIS pump for a later car) the pressure would be around 70psi, and would cause the symptoms you describe.

[ABDow]

The MAP sensor should also be checked with a vacuum gauge to make sure that it is holding a vacuum.

[HerbPhillips]

The culprit appears to be an injector that was stuck open after firing. It was puking the pressurized fuel into one cylinder. I lifted the rail up and cranked the car and noticed that it kept puking after I turned off the ignition.