Engine braking????

[Mazda Ebrahimi]

talltom wrote:
> 
> A friend of mine has a 96 chev 3/4t 4x4(with a 350) and just got back
> from
> a hunting trip. This guy was a GM line mechanic for 20 years and
> has owned his own shop for over 15 years, and he insists that his
> new truck does something so that once the brake is tapped going downhill
> it will not under any condition gain speed. Neither he or the other
> mech can figuire out why, but both swear to it.
>   My question is is it possible that the guys at ac-delco have rigged
> things to retard timing to the point that the compression stroke
> would be infringed up on as necessary for deceleration??? Makes me
> wonder if some wise ass has done a little abs system interfacing here????
> 
>   I realize that some of the company guys have non-disclosure agreements
> to deal with, but I'd appreciate as much insight into this as is
> pheasible!


Hi Talltom,
That is strange.  My company installs aftermarket alternative fuel systems on vehicles, 
and the Chevy P/U is a popular vehicle.  We have a fair amount of experience with that 
truck, since we certified the aftermarket system for EPA on the 1996 MY.  I have logged 
about 3,000 miles behind the wheel of about a dozen of these trucks with 4.3L, 
5.0L, and 5.7L engines (mostly C-1500 and a couple of C-2500s), and haven't noticed 
anything special about deceleration.  In fact, two things stick out in my memory that 
may or may not help you: At speeds lower than approx. 45 MPH, briefly after entering 
decel, engine RPM drops to about 1100 RPM, so there can't be a whole lot of engine 
braking going on.  The second thing is that if the engine is warmed up, and you are in 
closed loop, the ECM conducts EGR system tests, in which it cycles the EGR valve and 
looks for changes in MAP to verify its operation (required by OBD-II).  This goes on at 
other engine operating points also, but its fairly strong at decel close to idle.
Hope this helps,

Mazda