Hard starting and rough running 1991 500sl w129 - PeachParts Mercedes-Benz Forum

devincie · #1 06-19-2007, 09:50 PM

I was hoping someone here could help. I've had my car in the shop for the last three weeks (waiting for parts and diag), and the mechanic cannot find the problem.

Symptoms:

Vehicle hard to start, at all times except up to 15-20 minutes after its driven, and first thing in the morning.

running very rich, spark plugs are black, all of them, black soot on pavement from exhaust pipe.

running rough, it does not matter weather the car is warmed up or not, but it seems to get worse if its hot.

items replaced:
spark plugs, wires, distributor and caps. fuel pressure regulator, EHA valve(was leaking) coolant temperature sensor.

There is no codes in the system, apparently they tried adjusting the fuel pressure (hex key connector on the fuel distributor), this did help the power a bit, but still no luck with the starting problem, and the miss is still there...

Any help would be greatly appreciated.

saumil · #2 06-19-2007, 11:45 PM

Cold start valve ? Check the actuator resistance, it should be low, between 10 and 30 ohms. When trying to start, the voltage on the actuator should bearound 10V. If you remove the start valve and try to start the engine, the valve should spray gas. If voltage and resistance are o.k. and the gas does not spray, replace cold start valve.

Is the black smoke occuring after repeated attempts to start. Because this may mean that you are accumulating gas and may actually point to something other than the cold start valve.

Could also be the crankshaft position sensor. But this means you will not have spark on any of the spark plugs. Can be checked with a simple and inexpensive spark tester from local auto store.

Playing with the fuel distributor can cause a whole lot of problems, hopefully it is not there.

david s poole · #3 06-20-2007, 10:14 AM

i think it's the fuel distributor sending unequal amounts of fuel and leaking down when engine off.

devincie · #4 06-20-2007, 09:46 PM

I've checked the cold start injector, the resistance is almost 20,000 ohms, voltage at plug is just over 10volts, when the injector is removed, and the engine is cranks, there is no fuel coming out of the injector....

just to be sure I loosend the fuel line to the injector, fuel started hissing out, so I have fuel pressure there too.

I will replace this and see what happens, but it looks like we have the culprit.

Thanks for all the hints.

devincie · #5 06-20-2007, 09:48 PM

Sorry, I misread the meter, its 20ohms for the resistance.

saumil · #6 06-20-2007, 11:00 PM

Yeah, 20K sounds almost like an open circuit, 20 ohms seems reasonable.

devincie · #7 07-13-2007, 06:57 PM

Thanks guys, it looks like I was a victim of multiple issues piled on top of each other. Every part I change makes a bit of a difference and changes the symptoms, I leaned out the mixture to a bit, hesitation at low rpm is almost gone, idle much better, leaned it out some more, no difference, and still the car takes a long time to start. We had a really hot day here today, and it took twice as long as usuall to start. I will perform the test that Saumil describes was the fuel distributor this weekend, and report the results. Thanks again. Greg.

saumil · #8 07-13-2007, 11:30 PM

I noticed that you have replaced the EHA, have you tested if the current is flowing thru it ? When the switch is set to run (right before start), 10 - 20 ma current should flow thru the EHA. The current will drop at idle. But if you blip the gas pedal, you should get currents larger than 15ma for each blip. Do this test when the engine is cold. Also, the current measurements must be made via a series circuit with the EHA (not parallel like voltage measurements).

devincie · #9 07-15-2007, 07:39 PM

Wow, this is weird, I am getting 74.5 mA.... I've checked three times, this seems awfully lot. I did get a WIS manual, but its taking me a while to learn how to use it. I've retested it throughly. The fuel distributor is holding pressure very well, equal amount of fuel from all lines when plate depressed, the plate is holding pressure when closed.

devincie · #10 07-19-2007, 08:08 PM

Well, I have tested over and over and my results don't make sense. WIth the engine off, key on run, the current at at eha is 74.5mA, once started the current drops to around 9, then slowly drops to 5mA. When I blip the throttle when the car is running, there is a drop to between 1-1.5mA I have repeated these tests numeroust times, with engine cold. I also replaced the overload protection relay, and now have the check engine light on (this could of been caused by the action of replacing the relay as the manual states) This being a california car, I am able to read off the codes, I get 9 blinks on the led, I have not looked them up jet.

saumil · #11 07-19-2007, 10:03 PM

Yes these EHA currents do look wierd atleast for my 87, 260E and a lot of other W124 models. Is it possible that they are different for your model ? Though I dont understand why the EHA current does not change dramatically (i.e > 15 ma or so) when you blip the accelerator. That defeats the whole purpose of the EHA. As you must have already noticed, the car will run without the EHA because the mechanical fuel injection system is adequate for that purpose, but during acceleration, the extra fuel is rapidly provided by the EHA. It is not required to mantain a constant speed, the mechanical system (i.e. the throttle, air sensor and the fuel distributor) is good enough for that. The ECU is controlling the current thru the EHA, and it determines the current based on various sensor signals. Either one of the sensors (air flow sensor and coolant temp sensor are the most likely candidates) is bad or the problem is in the ECU. The fact that the current is so high just in the run position of the ignition switch makes the ECU a very likely possibility.

devincie · #12 07-24-2007, 07:34 PM

I have dug some more through Mitchell, at my friends shop, the spacs for the EHA looh dead on, for a 1991 500sl. So I guess there is quite a bit of difference in the models.

I did more probing around, and I found that #1 injector is not getting any fuel in. Funny, when I started this, all of them were getting fuel, including #1. I checked the fuel distrubutor and nothing is coming out of there as well at #1. I guess were back to the distributor, I just can't figure out how an 8 cylinder engine would not start on 7 cylinders...

Is it possible that fuel is metered before this happens, so the end result is more fuel to the remaining 7 injectors, resulting in the overfueling? I'm really starting to dislike this Bosh solution, reminds me of the old Lucas injection problems....

saumil · #13 07-24-2007, 07:59 PM

Back to what David was suspecting from the very beginning, unequal fuel to different cylinders. If you dont get any fuel on one of the outputs of the fuel distributor, there is no other possibility.

devincie · #14 07-25-2007, 12:27 AM

Thanks guys for all your hints and time, I've learned a lot through this forum. With all these issues, this is still the most enjoyable car to drive and own, hopefully when I get the whole thing up to snuff it will return the favor.