1992 300CE Hard Starts and Stumbles
 

The car now has around 153k miles. I've searched and have learned a lot, but most of the good threads with a lot of information come up blank when the original poster doesn't follow up.

Starting my beast is becoming an effort and is progressively worsening. Car has a full tune up: oil change, coolant temp sensor, everything related to cooling, plugs, wires, cap, rotor, coil, fuel filter, both fuel pumps, oxygen sensor, OVP Relay, and fuel/air mixture has been Mercedes master technician tuned with precision. No vacuum leaks anywhere. Fuel pressure is not a problem. I even engage the starter I click the key to on position and let fuel pumps build pressure. My fuel economy is right where is should be. Around 18-20 in the city and 23-26 on the highway. No stumbling at highway speeds.

So the problems:

Starting cold takes two tries. First crank fires up stumbles and dies. Second try takes about 5-7 seconds of cranking to get it firing regularly and it still stumbles and eventually evens out. When I step on gas stumbles all the way to 2k rpm or so.

When the car is up to operating temp and I'm driving along it stumbles and feels like it wants to stall. If I pull up to a stop and roll through and step on gas it stumbles badly. Stumbles just about any time the RPMs are under 2k. Doesn't happen all the time, but happens a lot.

Also, when at operating temperature and the car is parked and shut off for approximately 5 to 10 minutes it takes 6-8 seconds of cranking to start.

I don't know where else to look. Is the CPS (Crank Position Sensor) causing this stumble and poor starting?

Thanks a lot everyone.

The earlier symptom of a faulty CrankPS is difficulty starting when the engine is hot. But later, whether the engine is hot or cold, it just won't start with CPS problem. So I think you are right, go for the CPS first, and update us please :D

Sounds like a fuel issue to me, of course low secondary ignition voltages can cause the same conditions.

Did you use Bosch cap & rotor? Has you tech had the car on a SCOPE?

Fuel bleed-off is the most common cause of long cranking times.

I do all my own work. Dealer works on my car for free. :D They had a car on a scope to tune fuel management I believe. They took the car for a few test drives with some computer in their hands after checking codes and clearing the computer in the shop. I don't know what they did, but the car improved fuel economy ten-fold and responds much better when it doesn't stumble.

All parts have been replaced with new Bosch components as well.

I believe your WOT switch is bad or your fuel mixture is too lean. If your fuel pump and check valve and filter are good.

You've replaced almost all of the fuel and ignition systems, so you must be getting close. I didn't see any mention of EHA (controls fuel adaptations) or fuel accumulator or fuel pressure regulator, all of which can affect hard starts. Even a failing fuel pump relay might be doing hinky electrical things. Although you've replaced the OVP relay, you probably know that OVP is nearly always the first suspect in such cases. That alone would justify checking it again to be sure it's a good one.

Previous owner posted on this forum a few times "StarGazer". Owner prior to him did EVERYTHING for the car. Super detailed records from 93-07. Then 2008 was blank. I've restarted the records on 2009 (my ownership).

From reading his posts, he's replaced the fuel accumulator. My fuel pressure is good. The car retains most of its pressure after being shut down for about an hour. I've checked the OVP and replaced with 2 known working ones, so I've killed the OVP being the problem. Injectors and seals have been taken care of in 2007. I'm left with a crank position sensor. My ex-mechanic father finally decided to help me out and he's thinking crank sensor as well. So I guess I might as well replace as it's a variable I don't feel like playing around with. The car is getting significantly worse as time progresses, so it may just be the sensor.

Crankshaft Position Sensor fixed the problem! The car runs and takes off like a bat outta hell. Life is good again. :o