Off-idle stumble resulting from ported EGR

John T Stein JSTEIN at
Thu Jun 1 15:26:34 GMT 1995

> lusky at (Jonathan R. Lusky) Wrote:
> | 
> | 
> | Steve=Ravet%Prj=Eng%PCPD=Hou at writes:
> | > I've seen this exact problem in two '86 4x4 S-10 blazers (3.8 
> | v6?) and an '88 
> | > corsica (also 3.8 v6?).  I never thought it was related, but 
> | now I wonder.  
> | > This probably belongs on, but since someone else 
> | brought it up, 
> | > I thought I'd offer my piece.  Anyway, the symptom is 
> | hesitation just off 
> | > idle.  Give it a little more throttle and it is cured, but it 
> | persists as long 
> | > as the the throttle is held barely open.  It is a feature on 
> | the corsica 
> | > (had the problem since new), may be a bug or feature on the 
> | blazers, since 
> | > they were used by the time I drove them.
> | 
> | Hmmm, 86 & 88 GM 2.8 V6?  Don't those have the infamous GM MAF disease?
> | If so, you need to the update PROM that converts the EFI over to speed
> | density.
> | 
> | -- 
> | Jonathan R. Lusky                        lusky at
> |                 (615) 726-8700

I don't think the problem was MAF-related, or even unique to FI 
engines, the '83 2.8 L V6 on which I first saw the problem had a 
carburator (remember those??).  

Also, the stumble appeared to be a transient issue, if one held the throttle 
static just "off idle" the engine would stumble but eventually get its act together.  
It was almost as if the EGR valve "overshot", over-diluting the 
fuel / air mixture  when confronted with the sudden 
application of vacuum when the throttle plate first uncovered the 
EGR. port and eventually settled to a stable point which did not 
over-dilute the mixture. 

Does this mechanism  sound reasonable?  If so, anyone have suggestions to 
correct it, short of putting the EGR under ECM control.  A restriction 
in the EGR vacuum feed (a pneumatic low-pass) is not a contender as 
the EGR valve has an internal vacuum bleed.



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