HELP!!! Please - my brother has replaced everything we can think of. '87-300E 182,000 miles, had it a few months ran fine until about a month ago. Started losing power at high speeds, low speeds, at idle anytime you name. We replaced all the norms, plugs, filter, distributor all bosch products. Still was stalling. We then replaced OVP relay (thank you to folks on this site for that little tidbit). Car ran fine for one week - what I did notice during that great week was that it wouldn't stall, but it would surge to real high speeds and then check back. Finally after a week of this the car idle dropped extremely low (5rpms) and then you guessed it it started the same problem again stalling when it felt like it and then not restarting until the car sat for 20-30minutes - in rush hour traffic blocking the slow lane. So we replaced the fuel pump relay twice and now just recently replaced the ovp again as well as the Klima relay - now I'm being told to replace the ECU which I can find on ebay for about $100. Help. Every mechanic that has tried to recreate the problem can't and Mercedes wants too much money to look at it and suggested I just start replacing relays. Otherwise car is great. FYI the cruise control does not work and the blower for the heat went beserk one day and wouldn't turn off so the Transmission folks that had my car at that time disconnect the blower. I do know that when I first purchased the car it would start and then ref real high and then you would push on the gas and the car would do nothing and you would coast to the side of the road stop and turn the car off and then restart it. The tranmission place I took it to said that there was a "cord" that was overstretched and they replaced that and the car ran fine after that. Could this cord be overstretched again? Do I need a new fuel pump? I've heard mixed stories about whether or not you need to hear the fuel pump running. I do hear when I turn the key the OVP run really loud for a few seconds and then I start the car.

Can you elaborate on this? What does it sound like? How did you identify the OVP as the source of the sound?

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2012 E350
2006 Callaway SC560

because when the OVP was bad and you had the key on you could jiggle it and then it would make this noise like a buzzer and then it would stop buzzing after a few seconds maybe like 3 seconds.

I just found something out - I took the car out for a spin up the highway and drove about 20 minutes. Got to the Auto Parts store came out and sure enough the car wouldn't start. I lifted the hood removed the new OVP relay, the new Klima relay and the new fuel pump relay and kept them out and the hood up for about 5 minutes. Put them all back in and closed the hood and the car started. Proceeded to drive back and when I came to a light I would keep my foot on the brake and gas so that the idle would stay at 10rpm so it wouldn't shut off. Unfortunately got the last light before my job and I didn't brake correctly and the car shut off. I got pushed to the side of the road. Proceeded to lift the hood and took everything out (Klima, FPR, OVP) for 5 minutes with hood open - reinstalled everything closed hood and car drove the few miles back to my job, but kept surging most of the way. Got in the parking lot shut the car off and it started perfectly. I did this 3 times and the car still started. Could it be that removing the relays is resetting something? maybe the ECU or could it be the Voltage Regulator. As long as you keep the car going and don't let the idle drop to 5 when you brake or are at a light then the car will keep moving. Any thoughts?

Try replacing the voltage regulator. It is fairly inexpensive and an easy job.

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1987 300E

I would suspect the fuel pump relay except that you already replaced it twice.

When the car won't start, can your hear the fuel pump run after you turn the key on but before you turn it to start?

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2012 E350
2006 Callaway SC560

Yes we can hear the fuel pump run. We also just found out yesterday we did a test. We let the car run in drive until it stalled - pulled spark plug that's on the side stuck in screw driver and car would start. So we know spark is getting out. We ran it in drive again until it stalled and then pulled the spark plug that runs to the distributor cap and the car would not start - so we know no spark is getting in. We then pulled another spark plug that is on the side - put in screwdriver and car started. Could it be the distributor, distributor cap or coil?

Okay I guess no one has anymore suggestions for me. I have searched all the threads and have tried everything everyone else suggested for their problem. These are the new symptoms that we found out. We can let the car run with the hood closed and then after about 5-15 minutes the car will shut out once temperature goes above 80. We open the hood remove the OVP relay (this is the 2nd one I've installed brand new from Mercedes) and keep it out for 2 minutes and then replace it and the car starts right up. We close the hood let the car run for 5 minutes it shuts out again and then we remove the OVP for a few minutes and replace it and it starts up. Could the ECU be bad and we need to replace that? what does the ECU do? I do noticed that when my brother shakes the the wires and the "bed" that the OVP sits in the Antilock light comes on and the 02 Sensor light comes on. We also found that there was no spark going in the cap, but there is spark coming out. I took the coil to autozone and they tested it and it is fine. That leaves the CPS, but I had one of those replaced on my jeep and when that went it just went. My brother seems to think it is with the ECU - maybe the wires are bad where the OVP is or maybe they are exposed and heating up? I do know that when I replaced the OVP the first time the car ran perfectly fine for 1 week, but was having surging issues. Help!!!

The car should still run even with no ECU.

The car really has mechanical fuel injection. The ECU electronics are there only to fine-tune the fuel injection. It is not like "real" electronic fuel injection where the injectors open and close. The injectors just spray all the time with the amount of fuel going thru the injectors determined by the amount of fuel pressure.

You might attach a fuel pressure gauge and see if the fuel pressure drops and makes the engine quit. If pressure stays up, look to the ignition system.

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2012 E350
2006 Callaway SC560

If the car starts and runs for 10 minutes, and if it runs fine, like smooth and responds to gas pedal, and at idle if the rpm can be raised to 6000 or so without misfiring or hesitation or excessive white smoke at the exhaust, it cannot be the ECU. It is also not the OVP relay, the job of this relay is to cut off the electrical circuits in case of over voltage, which if the RPM are raised should occur at idle even in the first 10 minutes. If it does cut off on high RPMs at idle in the first 10 minutes, then the alternator is sending a high voltage, chenge the voltage regulator. It sound to me like there is a temperature related problem, either there is some non-metered air going into the ait intake or your idle valve is bad or you have a bad fuel injection temperature sensor, the temperature sensor was definitely a problem in my car (87 260E) at one point and gave the same symptoms.

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Saumil S. Patel

We have changed the voltage regulator a week ago and that changed nothing. I do note now that the Antilock light is staying on so definitely my OVP is bad now. Something is causing me to blow this thing. I tried to start it yesterday and it wouldn't stay running and then every time I put my foot on the gas it would just cut out. I will definitely have to replace the OVP because the Antilock light stayed on until we replaced it the last time. I will definitely have my brother check the temperature sensor. I will also let him know it is definitely not the ECU because he kept thinking this was the problem and I don't want to pay for that. Unfortunately I may have to take it to the "stealership" as everyone says. Could this be the CPS?

As others have also said, it is highly unlikely to be the ECU, and if you have the urge to replace it for whatever reason, get one from ebay, I dont think this is the problem. OVP relays dont blow at the rate you are talking about, the antilock brake light can come on for various reasons, one of them is an abnormal voltage from the alternator, but the problem cannot be fixed by changing the OVP, it is in the alternator or the voltage regulator, which you have already changed. Measure the voltage at the battery when the car is idling, it should be no higher than about 14V. If the car idles fine and it is shutting off when you depress the pedal, it could be a vaccum leak also. Check vaccum at idle, it should be 20 (I am not absolutely sure but can check) or so at idle and then drops when you depress the pedal. If the vaccum is low at idle, the problem is either a leak in the vaccum lines OR a clogged exhaust system. Without descent levels of vaccum, nothing will function right including the throttle mechanism (i.e. your gas pedal). The catalytic converter will be the first place that such clog will develop in the exhaust system. If you are a person who knows little about the operations of the fuel system in this car, I would take it to the dealer atleast for diagnostics.

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Saumil S. Patel

I think you are right and that we are going to have to take it to a mechanic to fix. This all started with it stalling out intermittently and then to stop and go traffic and now it just stalls out when it wants, low idle (5rpm), starts when it wants (whether it be hot or cold start - hot or cold outside). It restarts as soon as we remove the OVP Relay and replace it back in the car like it is resetting something. Yesterday it just decided not to stay running and just is running like it is ready to blow up. I think before we start the chain again of replacing things it's just best to spend the money to have it looked at professionally. Thanks anyway everyone.

Sorry I have no ideas.
Sounds like you have tried all the obvious stuff.
I suggest a good Independant Mercedes Mechanic, NOT the dealership.
Look at the top of the page for good shops.
Good luck and when you figure it out come back and enlighten us!

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2009 Mercedes ML320 Bluetec

Have you considered the fuel filter?

Michael