First daily start idle quality - PeachParts Mercedes-Benz Forum

NoSparkNeeded · #1 07-03-2008, 10:42 AM

300TD 85. First start in the morning always requires a little throttle input to get it running smooth. After 1 or 2 seconds it then idles just fine. My wife's 84 300SD will start and idle fine without touching the throttle on first daily start. She has 200,000 miles old nozzles. I have only 105,000 miles with brand new nozzles I just
put in. In every other way the TD runs superbly. Yeah I know it's really nit
picky, but what could I try to get my car to start cold and idle fine without touching the throttle at all? After the first start even if it only runs a minute or two, it will start and idle fine with no throttle input.

Diesel911 · #2 07-03-2008, 10:51 AM

What is the idle speed on your TD? Idle speed set to low would be my first thought.
The next would be IP timing.
Are all of your glow plugs working?

NoSparkNeeded · #3 07-03-2008, 11:14 AM

Idle speed is 750 when fully warmed up.

NoSparkNeeded · #4 07-03-2008, 12:10 PM

Just tested glow plugs with an ohm meter. A-OK

lkchris · #5 07-03-2008, 12:41 PM

Since this is not an "electronic" diesel, there's no way for the fuel system to know the engine isn't warmed up and there is no other adjustment for cold running. (Glow plugs are only for starting.)

Your best bet is for the engine to be like new, i.e. compression, fuel system, etc.

ConnClark · #6 07-03-2008, 02:12 PM

I have the same problem, however I know my problem is my glow plugs.

A diesel needs 3 things to run. Air, Fuel, and Compression. You might have a problem with your compression on a couple of cylinders when its cold.

Jeremy5848 · #7 07-03-2008, 02:40 PM

My '85 300D-T (ex-CA) idles more smoothly when warmed up than it does when cold but it doesn't need any throttle to keep running unless it's really freezing cold.

The '85 does have an electronic EGR control; it's possible that yours has malfunctioned (another good reason to disable EGR). EGR is not supposed to function until the engine has warmed up a little; an open EGR valve on a cold engine might cause the effect you are seeing.

As a possible fix or work-around, I suggest a homemade "afterglow" modification that costs nothing. This is done by cutting the violet wire to the glow plug relay. The relay normally turns off when the ignition switch is moved to the "start" position. By cutting the violet wire (it is violet, isn't it, experts?), the glow plugs stay on until the relay times out (about 30 seconds). The hot glow plugs will assist the engine in running smoothly and not stalling until it warms up. There is a slight associated cost in that the glow plugs will not last quite as long. I have not tried this myself but others have reported good results.

rrgrassi · #8 07-03-2008, 03:07 PM

Valve adjustment done recently?

NoSparkNeeded · #9 07-03-2008, 03:27 PM

The "violet wire" fix sounds intriguing but this is just a minor annoyance
at the moment. I like things to work as they are supposed to. Most
times in less than 2 seconds the idle will smooth out with a little throttle
input, then I can lift my foot off and all is well, smooth and solid.

Funny you should mention the valve adjustment. I haven't had a chance
to make the wrenches yet, so I haven't done it. I have no idea if it's ever
been done or not. There are only records going back 4 years, and no mention of a valve adjust. I think I can hear one clicking. Soon as it comes back from the paint shop I'll check that. Thanks Guys.