R107 500SL: New Fuel Distributor, car stalls when warm

[Bondavi]

Because I was having some shuddering upon hard acceleration, I decided to replace the FD in my car with a new, rebuilt one. But not having much luck with this new FD.

What happens is the car, when cold, starts right up, fast idle no problem, but then when it warms up it gets more rougher running. The vacuum gauge will move 1/3 way and the car stalls. If I keep my foot on the gas it will rev up and I can drive as is but as I let my foot off and let the revs drop to idle it stalls 50% of the time, or barely stays running.

I'm wondering here...is there an adjustment to be made, either mixture control or the fuel pressure regulator pin at the back of the FD assembly, should I look into these?

I thought the FD base gasket was bad (it was new) so I re-replaced to ensure there was no vacuum leak between the FD and base, but no change so obviously the vacuum loss is coming from something else. The old FD did not have this problem.

Or is this a sign of a faulty FD?

[Bondavi]

I was told each time a FD is replaced, is to readjust the mixture screw.

Because of the vacuum loss when the car is warmed up, I'm guessing the metering flap is letting too much air through when idle. Would CCW on the screw bring the flap up shut? Or is it the opposite? The car isn't running rich as far as I can see/smell.

Other option is the FD is leaking through the plunger, but I wouldn't think so if vacuum was being lost, hence the flap adjustment.

[romansek]

I have a similar problem as yours. I changed the FD and it did not help. When I checked the fuel pressure it was 130PSI because my return fuel line was blocked and the pump was shutting down which caused the engine to die. After unblocking the fuel return line the car runs perfectly. My 280SL has not been driven for 5 years and this caused the problem.
Good luck.

[miles]

I have been here with this problem also!!! 1990 500SL (r129).
It was a fuel pump problem, where both pumps were operating but outside the specifications required. Both pumps replaced - problem fixed. Pumps can be found under the car at the back, infront of the tank and under a cover. Very simple to remove.

[t walgamuth]

i was going to say that the original problem was probably not the fd and you still have it.

is it acting the same way as before?

tom w

[Bondavi]

Actually the car behaved better and did not stall at all when warm with the original FD but when accelerating hard, I would see some vibration as if some cylinders were not getting as much fuel. So I decided to swap out the FD with a new one.

Think the pressure regulator in the back of the FD would be the cause? Or just re-adjust the mixture and go from there?

[ctaylor738]

You definitely need to adjust the mixture. The problem with gray market cars is how. Do you have a cat converter and an oxygen sensor? Do you have an aftermarket lambda setup connected to the diagnostic socket?

[Bondavi]

I have brand new cats on the car (less than 500 miles), but no lambda system hooked up. It was removed from the car but the oxygen sensor is there (new) but no lambda system. So I would have to adjust the mixture using the O2 sensor readings, that the best way?

What about the pressure regulator in the back of the FD, would that be a culprit too?

[ctaylor738]

Then I would adjust the mixture with the sensor - somewhere around .5V at hot idle, maybe a little higher if it isn't running right at .5V.

You could test the fuel pressure, but it sounds more like volume-related to me. Try a volume test. Basically you jump 30 to 87 on the FP relay with the return line run into a bottle. I think around .5 liters in 30 seconds is the standard. There are numerous posts on this.

[Earlybird]

Sounds like a big vacuum leak to me. Low vacuum meter at idle, runs fast at start with extra fuel from the inrichment injector and then when hot it runs lean and trys to die.

[Bondavi]

The car runs much better now! Richening the fuel mixture adjustment seem to have done the trick...no more stalling. Vacuum gauge stays put at zero.

However, I'm still not able to get a steady reading out of my DMM out of the O2 sensor. At one time I had it at 1.0v and then richening got it down to .4v, then I adjusted to back off a little...but DMM didn't get the right reading.

Even though the car runs better now I'm still tempted to just have a Mercedes shop do the adjustment to get it just right for CO and NOx levels for the upcoming smog test.

Happy to say the least! This is a great group...

[ctaylor738]

You did go clockwise to richen? The fluctuations in the mixture may be driving your DMM crazy. Otherwise your readings don't make sense.

There is no benchmark for the CO setting at the tailpipe. With a new cat you are probably going to get zero or very close to zero at the tailpipe. You cannot just adjust it to 1.25%, because then you would probably be way to rich.

If you take it to a shop, have them remove the sensor and replace it with a fitting that can be opened, like a brake bleeder, and have them connect a CO meter there. A Gastester with a tube would work.

Or if the car is running well, then just have the CO and HC checked to make sure you are OK and get it inspected. With that out of the way, you can tinker with it, check the plugs (you can try Colortune plugs) and get it running perfectly.

[garymand]

Congrats, and keep the O2 sensor, The meter is you problem. Fallow my advice below and try again. Check in three places: idle, 2500 and full throttle. make adjustments first without driving the car. Then when close take the car out to verify and fine tune.

The sensor is great. Its a good sensor for a portable gas analyzer. It tells you how about your mixture and which way to adjust your mixture at any rpm. You just need to figure out what to adjust and alwys take baby steps and repeat. Newbies always make mistakes so measure twice and cut once.

The mixture is stoic when the O2 sensor reads .5V, but it won't just sit at .5V. That is where it is most sensitive and the slightest change in mixture, or mixture need, will change the reading so fast a DMM can't respond quick enough. This is why you don't see digital tachs. They all have old fashion needles.

You need a MM, a plain old multimeter which will have a needle. Radio shack has cheap "old school" multimeters. try to get one with a 0 to 1 Volt scale as opposed to something like 0 to 5V for best resolution. That is the range of the O2 sensor output.

The sensor puts out 0 to 1volt only when hot. 1 volt is way rich. 0 is way lean, bad sensor, or cold sensor. It has to warm up. Benz uses heated sensors (it has two extra wires for the heater) to get to good readings fast.

You eye will naturally follow the movement of the needle and see the average of where the needle is and which direction its moving. The objective is to get (stoic) 0.5V +-.1 always, no mater what the rpms are. Carbs don't do that well and fuely now has computers and multiple sensors to do it.

Search some articles on O2 sensor try patrick_mb mixture adjustment, a good one. You just watch it to see how good your mix is at idle, midrange and full throttle. Make adjustments and try again.

I have added O2 sensors to my old cars TO have a portable 1 gas engine analyzer to road tune and check my fuel. jmo

[Bondavi]

THUMBS UP Gary for an excellent write up! (Ctaylor too!)

Using an analog MM sounds like the way to go. I was able to "reset" my DMM reading by increasing the revs and getting an "average" around .6V, but I think I had better re-check with analog.

Thanks again!