Cold idle good - then bad - then good

[homepagz]

My '92 300D with 200k miles on it starts up with just a touch of the key. Given a good glow plug cycle of 15-20 secs the car roars to life and idles wonderful - for about 3-5 secs.

After the first 3-5 secs it sounds like one of the cylinders is missing and it coughs a bit - spitting out white/gray smoke.

After about a min or so of this the engine purrs like a kitten again and all is good.

This only happens when the engine is cold. Glow plugs were just replaced and tested (resistance readings were within spec).

Any idea what this might be?

Thanks,

-John

[chazola]

If all plugs and their wires test ok, have you tested for 12v output at all pins of the glow relay? Also could be an air leak somewhere into the fuel system, my old E300D used to do this on a cold start, it'd smooth out once I gave it a bit more gas and the air had bled out.

[compress ignite]

1. What Brand ? (Big differences between brands)
2. 'Could one of the Plugs NOT be getting the Voltage of "Afterglow" ?
[Your "Pre-Glow" relay does provide current to the plugs AFTER
Engine Start.]
A 12 Volt test light held with the probe on the individual G.P.s
DURING Glow and AfterGlow and recorded with a stopwatch
will tell the Tale.[Other end of the Test Light's cable is of
course Grounded (Negative).]
3. Some kind of Weird Coolant Leakage into one of the Cylinders...
only during startups ?['Throwing uncooked Pasta on the Wall.]
4. What are the Compression Test figures?
If you've a Weak Cylinder...Coldness will make it SEEM WEAKER...
And If that Cylinder DOESN'T get Full AfterGlow...Culprit.

Only REAL GP test is with them OUT of Head and Visually inspected
for REDNESS/GLOW whilst 12 Volts applied with Jumper Cables!
CAUTION !!! Active GPs WILL BRAND YOU LIKE A Heifer.
(ANYTHING over 1100 degrees "F" is not FIRE, it's the SUN.)

What's your GP Light on the Dash Doing ?
It'll tell you of a BAD G.P.

[homepagz]

They were Bosch glow plugs. I have tested the voltage at the plugs during pre-glow and all is as expected (batt voltage).

I have *not* tested the voltage AFTER the engine starts. I wasn't aware the glow plugs should continue to get voltage after the engine starts.

I'll check that.

Thanks,

-John

[Diesel911]

White smoke is either coolant or atomized unburned Fuel.

What does the Smoke smell like?

Is there evidence of combustion gasses getting into the Coolant?

[homepagz]

The clouds of white/grey smoke smells like diesel fuel.

Nothing sweet about it.

[barry12345]

As much as a pain it is. I would do a compression check. Then I would know what the engine condition is . If the compression is good on all cylinders then I would look other places for the issue.

I guess you could hold a small mirror just past the tailpipe. If the exhaust condenses on the surface the white smoke is coolant. If not it is fuel.

[bustedbenz]

Originally Posted by homepagz

My '92 300D with 200k miles on it starts up with just a touch of the key. Given a good glow plug cycle of 15-20 secs the car roars to life and idles wonderful - for about 3-5 secs.

After the first 3-5 secs it sounds like one of the cylinders is missing and it coughs a bit - spitting out white/gray smoke.

After about a min or so of this the engine purrs like a kitten again and all is good.

This only happens when the engine is cold. Glow plugs were just replaced and tested (resistance readings were within spec).

Any idea what this might be?

Thanks,

-John

For 200,000 miles... to me, this sounds normal. I may be the minority on that but it makes sense to me.

As mileage increases, compression decreases. The thing's not "worn out" but it's partially worn. How well a diesel runs is strictly a function of whether the vapor in the cylinders is hot enough to burn cleanly or not. When you first crank it up, there is plenty of heat for clean combustion because the glowplugs have just been heating things up. Once the glowplugs turn off, the surrounding metal is still so cold once a few explosions have happened in each cylinder that the engine starts to burn uncleanly, because there is simply not enough heat in the cylinders to burn cleanly. The gradual loss of compression due to age/mileage (mostly mileage, if I understand correctly) simply means that there is less heat being produced by each stroke of the engine than there was when it was new and started more smoothly.

Once the thing's been running a bit, the internal temperature rises high enough to burn clean again.

I'd guess that the cylinders that "miss" when cold are the ones that would read the lowest in a compression test, if you could measure to that degree of consistency and accuracy. As a practical matter... this is "old, cold diesel" behavior and isn't anything to worry about in my book. That book may not be read by many other forum members, but... there's my $0.02.

My 603 behaves similarly. The glowplugs give these 60x engines a whole lot of heat, but on the coldest temperature days, that heat dissipates more rapidly after the engine is running (and pulling cold air in through the intake) than it is replaced by combustion heat, for the first twenty or thirty seconds.

[homepagz]

Great news. I tried pulling the purple wire from the glow plug relay - the one that is connected to the ignition switch and tells the plugs to go off at start.

Now I have "post glow" and the car starts without a puff of smoke.

Sounds like one of the cylinders has low compression and the "post startup glow plug" is assisting during cold starts.

Any long term issues with me keeping this wire disconnected? I'm assuming my glow plugs will go bad sooner - but I wonder how much sooner.

-John