'88 300SEL stalls when accelerating - PeachParts Mercedes-Benz Forum

maheath · #1 10-17-2004, 07:39 PM

I'm not sure if I have just one problem, or if it's a combination of problems. The first symptom was the car stalling at idle. I checked the distributor cap and cleaned the rotor and that problem seemed to disappear. The cap and rotor (and plugs and wires) have about 10,000 miles on them. The fuel injectors, seals, and holders are about the same age also.

The next (and current) symptom is the car bucking (missing?) and stalling on acceleration. I changed the fuel filter and fuel pump and the problem is a lot less pronounced, but is still there. The car doesn't buck as much, but it does still stall on acceleration most of the time. I drained and replaced the gas also. I thought maybe I had some bad gas.

I tried to attach my fuel pressure gauge, but I don't seem to have enough clearance at the lower chamber plug. The smaller fuel line from the distributer to the fuel pressure regulator is in the way.

I don't know if this makes any difference, but when I had the air cleaner off and tried to start the car, white smoke was coming out of the breather hose that connects the valve cover to the air cleaner. Not a lot, just a little.

I pulled all of the plugs and none of them were fouled. They also didn't look tan like the old set of plugs. They weren't wet either. They seemed to be a cross between tan and black. They are Bosch H9DC0. I'm thinking about replacing the plugs, but I don't think that is the cause of the problem.

At this point, I'm somewhat at a loss. I can't seem to connect the fuel pressure gauge to help diagnose and I'm not sure what else to do. Any ideas?

Thanks

stevebfl · #2 10-17-2004, 07:49 PM

The first thing to do is to verify that you have closed loop control. From this you will also know your basic mixture and be able to see if fuel control is continuous.

maheath · #3 10-31-2004, 02:57 PM

OK, I made a harness for the EHA and decided to try it out on my 560SL first, to see what good readings would look like. I don't know which pin is neutral and which is hot, but I put red on pin 2 and black on pin 1.

I'm using the Sears 82139 digital multimeter (with duty cycle). I've got it on the mA setting and I'm reading 0 mA at all times, from startup through about a 5 mile drive. If I switch it over to VDC, I get constant change in the voltage, but I can't seem to get a current reading. The 560SL is not hard to start. It needs new engine mounts and has a noisy lifter when cold, but is otherwise running fine.

Why would I not get a current reading?

stevebfl · #4 10-31-2004, 03:30 PM

It is my best guess that you don't have your meter set up properly. Most meters don't read amps and volts with the wires plugged to the same holes.

As far as current goes polarity matters for point of view but not quantity. You should get 20ma with the key on and engine off. Also, to measure current the meter must be in series with the circuit. Voltage readings are read parallel to the circuit. This means that to read EHA current you must remove the two pin connection. The ammeter will then be connected from one terminal of the harness to one terminal of the EHA. The other terminal then must be jumpered from the EHA to the other harness pin. In this configuration current flows through one leg of the harness through the instrument, through the EHA, through the jumper into the orginal harness and back to the controller.

maheath · #5 10-31-2004, 03:52 PM

Thanks, I didn't have it in series. I'll correct that right now.

maheath · #6 10-31-2004, 05:10 PM

OK, I'm now in series with pin #1 on the EHA. Key on, engine off, I'm getting 1.12 mA. Driving around, I consistently get from 0.64 mA to 0.72 mA when at idle or giving throttle. When I release throttle, I get ~ -1.50 mA until I return to idle again. At 2000 rpm, I'm getting 0.71 or 0.72 mA.

Does this look reasonable? Is there a problem with the difference at Key on Engine off?

This is all on the 560SL, which appears to be running fine.