[DML] Re: Thermotime Switch Question

["therealdmcvegas" <dmcvegas@xxxxxxx>]

Before you go any further, there is something that you need to take
notice of. In your post, you said:


<SNIP> If I give it a bit of gas while it fires, the car will idle
> perfectly.
<SNIP>

You have probably answered your own question here. Per the the
DeLorean owner's Handbook, you >MUST< press and hold the gas pedal
down ¼ of the way when cranking the engine! If not, the car may not
fire. So it sounds to me like your car is working just fine.

The only short explanation that I can give will make me sound like Joe
Dirt. "How's Posi-Traction work? Why does a DeLorean engine fire when
I hold the gas pedal down durring cranking? Man, it just does!" And
that explanation is both short-winded, and acceptable.

Now if you do want to read a bit, here comes the science!

Cold engines need a rich fuel mixture to start. The reason is because
the walls of the cylinders are cold, and will cause the gasoline mist
to condensate on the cylnder walls. Just like when you have a cold can
of soda pop outside in the summer time. This causes the mixture to
lean out (because the liquid fuel won't burn as efficiently). So to
compensate for the fuel that has been rendered less flamable, the
air/fuel mixture has to be richened. Or as it's commonly refered to,
the motor has to be "choked". This sends in more atomized fuel, or
fuel mist to make up for the condensated fuels place, thus achieve a
successful ignition, and make the motor run. Now it should be noted
that this only has to occur until the motor has warmed up. Which won't
take very long. Otherwise, once that initial shot of fuel stops
condinsating, the fuel mixture will become too rich. And now you know
what the function of the bi-metal arm in the Control Pressure Reguator
does!

Why do you have to hold the gas pedal down when cranking a cold motor?
I honestly do not have a 100% certain answer. But, I do have a theory.
But to really understand it, you have to know the differance between
carbs, CIS, and EFI.

Carburetors, and CIS systems like the K-Jetronic all rely on fuel
pressure of some sort to increase the volume of fuel delivered. Carbs
will increase vacuum on the venturi to phyisicly suck more gasoline
out. K-Jetronic decreases fuel pressure in the lower chambers of the
fuel distributor, so the diaphrams inside will bend down, and allow
more fuel flow out to the injectors. And it does this by purging the
excess fuel in the lower chambers into the return line to the fuel
tank.

But EFI is a different creature. It's fuel pressure stays at a
specific constant. And the injectors don't stay open. They will always
spray the exact same amount when they are open, albeit intermitantly.
So if you need to increase fuel flow, the injectors simply open and
close more often, or as it's referred as, they "cycle" more often.
They do NOT physicly increase their fuel flow which passed thru them!
And any excess fuel not consumed by the injectors is simply returned
to the tank by the fuel pressure regulator. DeLorean owners are
already familiar with EFI, because it's a large EFI injector that the
LAMBDA system uses to control fuel pressures, using this same duty
cycle. We just simply call it a "Frequency Valve".

*WHEW!* Now that that's out of the way, lets talk about how these
systems perform their choke duties on cold engines.

Carbs are simple. You close the choke plate, and engine vacuum is then
taken away from the air intake, and applied to the venturi jets. EFI
is even simpler. You just increase the duty cycle on the injectors.
K-Jetronic forgoes all of this, and simply has a 7th EFI fuel injector
installed, call the Cold Start Injector. And it's job is to spray out
the addition fuel needed. Now the theory is that this injector has to
 cope with the etreme pressures of CIS, But as a result it has a
higher flow rate, and is dumping out too much fuel. If you trip a
thermal probe on EFI, or close the choke on a carb, the motors just
run crappy. BUT, if you WILL flood the motor on a D if you open the
Cold Start Injector when it's running. To prevent you from flooding
the motor when cranking, the thermo time switch has a bi-metal arm
intergrated inside to close the CSI after a few seconds if power is
continuously applied. Although this is also listed as a safty feature
to prevent the Catalytic Convertor from overheating. But still, it
shows that someone was thinking ahead. So more than likely, you've got
to hold that gas pedal down to to feed extra air into the combustion
chambers to keep the air/fuel ratio balanced out, to keep it from
becomming too rich durring choke, and stalling the motor out when it
floods. Jumping the cold start injector durring cranking will start
you car when it's hot, yes. Because you'll diesel start it, and that's
bad. Aside from possible detonation damage, you can wash out your
rings, and overheat your catalytic convertor.

-Robert
vin 6585 "X"




To address comments privately to the moderating team, please address:
moderators@xxxxxxxxxxx

For more info on the list, tech articles, cars for sale see www.dmcnews.com

To search the archives or view files, log in at http://groups.yahoo.com/group/dmcnews 
Yahoo! Groups Links

<*> To visit your group on the web, go to:
     http://groups.yahoo.com/group/dmcnews/

<*> To unsubscribe from this group, send an email to:
     dmcnews-unsubscribe@xxxxxxxxxxxxxxx

<*> Your use of Yahoo! Groups is subject to:
     http://docs.yahoo.com/info/terms/