98 E300 turbodiesel rough start when cold
 

When the car is completely cold (ie. overnight), the motor misses and blows smoke for about 20 seconds when started. It then evens out but sounds unhappy until warm. The problem only happens with the motor dead cold. Air temperature makes no difference. I recently changed the fuel filter, prefilter, airfilter, and did an oil change. The cold starting problem persists. I allow the preglow to do its thing, as I always have. The motor always fires immediately. This problem started all of a sudden, without warning. Any ideas?

colinc,

What color is the smoke?

I bet the color is a bluish white. But that is still unburned diesel fuel.

It could be a weak glow plug(s).

What oil are you using? It is supposed to be a synthetic for your 98. According to MB, it has to be CG-4 or higher so Mobile 1 actually does not meet that requirement. That leaves only a few oil options in the US and many dealers use the wrong oil (oil that does not meet the MB requirement).

I agree, it's probably glow plug related. Try letting the glow cycle continue for an extra 10-15 seconds after the light goes out.

Thanks for your interest. The smoke is greyish and smells like unburned fuel. I will try the additional preglow-time tomorrow morning. I have been using mobile 1. Should I be using something else? Could a defective coolant tenmperature sensor be a potential cause of the starting woes?

You could use Mobile 1. But in theory, your car should use an oil that carries CG-4, CH-4, or the newest CI-4 rating.
Mobile 1 only carries CF.

This is especially true if you rely on your FSS for oil change. The CG-4 or higher rated oil can suspend more soot. If you cahnge your oil every 3 - 5kmi, then any oil can probably do it but a oil with CG-4 or higher rating is better for your diesel.

loubapache is correct, CG-4 is the minimum recommended diesel oil for your vehicle. Mobil 1 is CF-4, I believe. Doubt this has anything to do with your symptom though, as your vehicle did not come from the factory with synthetic. If the glow plug suggestion helps, you most likely have a faulty plug and can test at each of the lines to determine which one needs to be replaced. Did you get the extended Starmark warranty on the vehicle. I assume the new car warranty would have expired last Fall.

Goldenbear is right that the oil you are using (Mobile 1) should not be related to your problem.

I raised the oil issue as some dealer may have used dino 20W-50 in the car and I do not know where you are. If you are like me, in Michigan, then 20W-50 in winter could make any car cough on startup.

Like you, I doubt that the motor oil has anything to do with the problem. Oil change didn't coincide with onset of problem. Suggestions regarding best oil to use are greatly appreciated, however. Starmark warranty expired 6k ago at 100k miles. I'll let you know how things go with extended preglow test tomorrow AM. Thanks.

There are really only a few choices you have for synthetic oil that carry CG-4 or higher rating.

Mobile Delvac 1 synthetic 5W-40
Chevron Delo Synthetic 5W-40
Shell Rotella T Synthetic 5W-40
Petro Canada Duron 5W-40 and 0W-30

Then the Amsoil and Redline.

It's definately the glow plugs. I know of several instances - including my own car - in which the OM606 required early replacement of glow plugs. My '98 required it just about a year ago. Symptoms were just a touch of roughness and smoke, only when starting stone cold - first thing in the morning. I ignored it for a while, but eventually the check engine light came on. Dealer replaced all six glowplugs, and the relay for good measure. Problem has been fixed ever since. Suggest you head for the dealer if the car is still under warranty.

Latest development: I couldn't wait until tomorrow to try the extended preglow. Here's what happened: I waited about fifteen seconds after the preglow went out and guess what happened, it went back on, and stayed on, even after starting. I drove to my destination with the preglow light on. After about 90 minutes, I returned to the car and the preglow turned off after the normal few second interval. About five seconds later, not only did the preglow light come on, but the check engine light lit up, and the digital display said: check engine, engine electronics. I think the car needs to be plugged into the diagnostics. What do you think is going on. I need some ammunition for the dealer. Thanks.

car is out of warranty. can i do it myself?

Any luck this morning? Your problem is definately glow plug related.

Problem is simple - you have one or more burnt out glowplugs. The car will still start on 3 or 4 glowplugs firing but the cylinders where the plugs are burnt out will not fire until the heat in them builds up to the point to ignite the fuel resulting in a miss and unburned diesel in the form of blueish smoke out the exhaust until the cylinders warm up. If one or more are gone - replace them all as the others will follow soon.