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Re: Low RPM roughness
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@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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