Re: <E-28> Cold starting problem

[Jim Cash <j.cash@xxxxxxxxxxxx>]

Chris Rucci asks "a common question"

> Out of the blue Monday morning my 1987 535is had trouble starting! It has
> always fired within a second of turning the key, but this time it took 8-10
> seconds of cranking to get it started. After a short drive, I shut it off to
> run into a store. Came out and it cranked up like old times. Throughout the
> week I've found that when the engine has cooled down (ie. overnight or several
> hours sitting) it's hard to start. When warm, no problem. Using my amazing
> powers of deduction, I figure it's a 'cold start' valve problem. So, what
> exactly am I looking for, and how difficult is a DIY fix? By the way, I live
> in NC, so when the engine is 'cold' it's still 90 degrees!
> Thanks for any advice. Please cc: my personal e-mail.


Chris

You are probably correct in identifying the " 'cold start' valve ".  
The symptoms you describe are "usually" associated with the "cold start
system".

By cold start they mean any time the car has cooled from operating
temperature ( I can not remember the specific temperature when the
system activates. But it works this way.

On all cars you have to make the fuel mixture extra rich to get a "cold
engine" to start.  In any "injected" engine that is normally done using
a separate "cold start injector" - which sprays extra fuel into the
intake manifold for a few seconds when you first start cranking.

This "injector valve" is controlled by a "thermo-time" switch.  on the
E28 it is a round sensor unit that screws into the thermostat housing -
just to the right of the upper rad hose connection. It has a large brass
nut base.  

That switch performs 2 functions. 

When you turn the ignition to "start", battery is supplied to this
switch.  If the "coolant' temperature is below the specified point, then
the switch is operated and passes the battery to the injector - which
operates.

If the coolant temperature is above the specified point, the injector
does not fire - because you do not need a richer mixture at that point.
That is the "thermo" part of the thermo-time switch.


The "time" portion comes into play by the length of time you are
cranking. You do not want to flood the engine with fuel, so that switch
has a little heater unit inside it - which operates during cranking.
After 6-8 seconds of cranking that heater unit raises the temperature of
the switch to the point that it now thinks the coolant is hot, and shuts
off further operation of the cold start injector.  If you wait a while
(till the switch cools again0 the injector will once again fire when
cranking.



Sooooooo - the question is why is yours not working.

Well it could be the injector itself not firing, or clogged. That can be
tested by applying power directly to the unit, listen for it to click
open/closed.

But the problem is usually the switch. This is a mercury filled contact
switch - and all mercury switches are known to become a bit flaky at
times - they get a residue build up on the contacts which prevents the
flow of electricity.


Easy test - as soon as you realise the car is cranking longer than
normal, get out and strike the base of the switch unit lightly with a
small hammer (about 4-5 taps). This will help dislodge the contamination
residue on the contacts.  If your car now starts right away you have
found the fault.

The switch should now be OK for anywhere from a few days to a few
months.  It is you choice to keep striking it occasionally or changing
the unit.

Actually there is a better way of cleaning the contacts - apply 9-12
volts to the unit and keep switching the polarity about every 2 seconds
for about a minute. I would do this a couple of times a year and I never
did replace my switch.
Depending on who else drives you car you may wish to just put in a new
switch.



Now the interesting part of this item  - an bets on how many times this
same question will be asked between Sept 15 and Nov 15 - as the fall
weather sets in ?  My guess will be in excess of 8.


Cheers

Jim Cash
London, Ontario, Canada
BMW CCA 102929

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