Date:         Mon, 4 Mar 96 18:16:36 EST
Sender:       Vanagon Mailing List <vanagon@vanagon.com>
From:         jag@cs.rochester.edu
Subject:      re: '82 Diesel Smoke


sw@smus.bc.ca writes:
> I didn't read it carefully the 1st time - yes, you're right, it is 
> normal for *some* smoke on a cold start (ie: 0C/32F or less).

Yes, when starting a diesel in cold weather it is normal with some 
(read little) whiteish smoke. This comes from vaporized, but unburnt fuel, 
and potentially a little water vapor. The original poster talks about
"a large cloud of blue smoke", which is definitely not normal.

Blue, or blueish-black smoke is usually associated with burning the
lubricating oil. (Caused by for instance: worn rings, valveguides, oil 
being sucked to the intake by the crankcase ventilation). However I don't 
understand why the oil burning would stop when the engine is warmed up.
My intuition tells me that warm oil flows easier, and should get into the
combustion chamber easier.

Black smoke ("dryish" if one can put that characteristic on smoke)
consisting of carbon particles comes from incomplete combustion.
Possible problems:

The engine misses when cold (this is consistent with the "rough
running" the orig poster menttions). The root of the problem is
that not enough pressure and/or temperature is created during
compression to ignite the air-fuel mixture. When eventually on
a later compression stroke the fuel is ignited black smoke is caused
by burning of fuel left from previous misses. Causes of this problem: 
Too low temperature: non functional glow plugs, non functional glow
plug relay, or broken electrical circuit. Too low pressure:
check comression. Should be around 450 psi. If better "wet" (ie
compression after a *little* lubricating oil has been squirted in)
rings are most likely worn, otherwise a bad valve or some other
head problem can be the cause.

Shawn wrote about the pain of testing the glow plugs behind the
injection pump. I leave the glow plugs alone, and remove the
injectors instead. With the injectors removed you can peek into
the prechamber of each cylinder. The glow plug should be cherry
red when glowing if it is working correctly. Take the chance
of having the injectors tested at a diesel shop (look under
"diesel" and/or "fuel systems" in the yellow pages for an authorized
Bosch shop). Dripping injectors ruins the glow plugs, and injectors
which are bad in some way causes smoking also. (ie wrong opening
pressure, uneven spray pattern, sticking open etc.) Don't forget
to use new heat shield washers when replacing the injectors,
and don't put the heat shields in upside down. Look how they
should be put in when taking them out, or check the manual.


While on the topic of glow plugs I'll mention another useful test 
technique. The resistance of the glow plug is temperature dependent.
A hot glow plug has a higher resistance than a cold. Now this gives us
a way of actually testing individual cylinder performance giving graded 
results, other than the on-off test done by loosening the injectors.
Measure the cold resistance of the glow plugs before starting the
engine. Start the engine and run under normal load until it is
warmed up. Measure the difference in resistance between the two
cases. A cylinder which is weak will typically show a smaller difference 
in resistance. If one does not want to remove the copper rail feeding
the glow plugs (this is tedious as mentioned above because of the
injection pump), use a pyrometer and measure the exhaust manifold
temperature instead.

/Martin Jagersand