Low RPM roughness

[Rich Vandervoort]

Someone once mentioned that the 4.3 TBIs sometimes have two different sized 
injectors.  I think the edlebrock intake has a separate plenum for each 
throttle blade except for a passage at the top of the plenum.  Fueling each 
bank differently could cause interesting drivability problems.

Rich V.

>Date: Mon, 28 Aug 2000 14:44:06 -0400
>From: Reid Bishop <rbish at ibm.net>
>Subject: Low RPM roughness

>I recently completed a Jeep conversion, placing an 89 4.3 TBI & 700R4 >into 
>a
>Jeep Cherokee.  I used a Howell Harness, a '7747 ECM, and stock 4.3 >EPROM
>image (AKDX2088).  This engine was completely rebuilt, bored .030 >over, 
>and
>I installed an Edlebrock Performer manifold, Edlebrock Performer CAM 
> >(fairly
>mild roller cam, but definitely not stock), Edlebrock Roller timing >chain,
>and roller rockers.  O2 is mounted on the Y-Pipe, just below the >drivers
>side manifold collector (stock location on an S10 I believe...), and >stock
>S10 manifolds were used.  Exhaust is 2.5 inch all the way back, with >a
>high-flow cat and a FlowMaster muffler.

>After about 800 miles, the engine runs fairly good, but still has a >few
>problems.  It idles very smooth, but does tend to hunt about 100 RPM >when 
>it
>is warmed up to 195F.  It also runs very rough just off idle- >anywhere
>between 750RPM and 1800RPM it just can't seem to keep from stumbling >all
>over the place.  ECM never sets a code, and I have been having >trouble
>getting stable ALDL data when the motor is running.  Timing is set to >6
>degrees BTC with the ECM control disconnected.  The ECM goes into
>Closed-Loop after 1-2 minutes after a cold start.

>Any idea what to look for when hunting this problem down?  Fuel >Pressure 
>is
>dead nuts at 13PSI after I switched my fuel return line to a 3/8ths >line.  
>I
>don't detect any vacuum leaks, but I wonder what the stock vacuum >reading
>should be on this engine?  Again, it idles great aside from a minimal 
> >hunt,
>but immediately off idle is where it starts to get unhappy.

>Also, this combo gets terrible mileage right now- I didn't expect >good
>mileage, but I am running 14.5MPG right now, no matter how I drive >it.  I
>have 4.55 gears, with 32 inch tires, with a corrected speedo gear- >this
>'appears' as something like a 4.11 ratio.

>I don't have the Edlebrock chip yet for this manifold/cam combo- so >maybe
>that's the problem, but I would not expect to see it have problems at 
> >these
>low RPM's.

>Any techies have a suggestion?
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