Re: Hesitation off the line when cold.

[Charlie Smith <charlie@elektro.cmhnet.org>]

Earlier, Jeff Goldberg wrote:
> 
> When my engine is cold, I get a short lived hesitation when coming off the
> line ('86 4KCSQ). First start of the day is the worst, but the car runs
> great as soon as it warms up. I find that over riching the mixture avoids
> the hesitation. Any ideas or BTDT?

There's a control pressure regulator (for the FI) low on the driver's 
side of the block.  This has a bimetalic gizmo in it that adjusts
the FI control pressure based on engine temperature.  This is probably
causing the system to run a bit too lean before the engine completely
warms up.  Or maybe a lot too lean.

This control pressure regulator (CPR) is about 1.5"x3" and maybe 1.5"
thick, it has two fuel lines connected to it, and (I think) one of
the multiple-wire electrical connectors.  This CPR is used on all
of the air flow controlled FI systems that have the flapper plate
connected to the fuel distributor, on Audi, VW, Porsche, and I don't
what all else. 

The fact that the problem goes away when things warm up probably
means that the heater for the bimetalic element is working OK.

Not to dispair, this non-adjustable (sealed?) unit can be adjusted.
This is more of difficult to explain than to do it ... the pressure 
adjustment mechanism inside that's modulated by the bimetalic element
is fastened to an aluminum pin, or post, that's pressed into the body 
of the CPR.  The end of this aluminum post is about 3/16" or 1/4" in 
diameter, and you can see it on the outside of the CPR, what you are 
seeing the end that's been pressed into the CPR housing.  The end of 
this post is seen on the surface of the CPR that's parallel to the
block - near where the fuel lines connect.

To adjust one of these CPR's that are running too lean, you can tap on
the visible end of the aluminum post wtih a punch and hammer  :-)
As the aluminum post get's pushed into the CPR, the CPR runs a richer
mixture when cold.  Easy, right?   Well, sort of.  You just drive the
aluminum post A LITTLE BIT into the CPR, until your hesitation goes
away and cold running is just right.  Really.

Now, about that calibrated punch and hammer ...  the aluminum post
usually doesn't want to move right off, so as you turn up the 
calibration of the hammer sooner or later the post will move into
the CPR body.  You probably want to do this when the engine is 
cold, so that you can start it and watch the hesitation diminish
as the post gets driven into the CPR.

So, the first three times you used the lightly adjusted calibrated
hammer and punch assembly, you started the engine and you didn't
notice anything changing ...  as you hit the punch with more vigor
it's not too hard to drive it too far into the CPR.  Resluting in
not only curing the hesitation but making the cold running a bit too
rich.  If you do this, take the fuel lines and stuff off the CPR,
take it off the block and take it apart.  You can drive the aluminum 
post back the other way if you have to, then put it all back together
and try again from then outside with the punch and hammer.  At least
this time the aluminum post is freed up a bit and slight movements
are easier to accomplish.

When you get it straightened out, let us know about the fun you had!

    - Charlie


 Charlie Smith   charlie@elektro.cmhnet.org  614-471-1418  
 http://elektro.cmhnet.org/~charlie/  Columbus Ohio   USA

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