[Gmecm] 1227747 ACSU promID Idle Problems

Tue Dec 5 05:12:23 UTC 2006

Remove the grounds from the engine, clean them, and make sure they are not producing too much resistance.  Also make certain that the ground strap was re-attached when the engine was installed.   EFI systems like well established grounds with low resistance.

  Is the new O2 sensor by chance a Bosch?   If so I would get my hands on a KNOWN GOOD AC DELCO unit.   Many of us on the DIY PROM board at Thirdgen.org have found that Bosch O2 sensor do not work well with the O2 sensor constants programmed into the GM ecms and the Proportional Gains that are meant to cause the Air/Fuel Ratio to cycle in closed loop is thrown out of whack.   That could even be severe enough to cause random stalling.   Does the problem occur in Open Loop or Closed Loop?

Dave Thomas <Dave.Thomas at us.festo.com> wrote:

I have a 1987 GMC V1500 4WD truck with a 700R4 transmission. The Prom ID
is ACSU. I have replaced the original engine with a GM Factory Crate
Replacement for VIN K engines. All stock except for an Edelbrock EDL-3704
manifold. I am having some problems with the idling of the engine and
stalling when put into gear. I have noticed on a cold start that
everything seems to be fine with the exception of BLM of ~154. As the unit
warms up to temperature, I notice the engine will start to reduce the idle
speed trying for a idle speed of ~625 rpm. At this point the IAC counts
are hovering around 40 counts. When I put the truck in gear, the IAC
counts start reducing along with the rpms until the rpm reaches ~525. At
this point, the IAC counts are usually at 0. When the vehicle is shifted
back into park or neutral, the idle will raise back up but the IAC counts
will remain at 0. When the vehicle is put into gear the second time, the
engine will stall and die. I have tried setting the minimum Idle on the
throttle body to allow the IAC to remain open, however 3-5 counts is the
best I have ever achieved but this did not correct the stalling. Also, I
have tried the service procedure for checking minimum idle air at 425 rpms
in gear with the IAC unplugged. I cannot achieve this number without the
vehicle stalling. I have sprayed TBI cleaner around the intake manifold,
tbi gakset, all vacuum connections and I have not seen the idle raise
showing a vacuum leak. Also, after stalling the vehicle is very hard to
start and keep running unless you rev the engine up above 1500 rpm and let
of the throttle very slowly. Also, occasionally the engine will surge up
and down (50-100 rpms) but not too extreme.

We replaced the rotor, cap, wires, spark plugs, knock sensor, O2 sensor,
new EGR valve, PCV valve, cleaned the throttle body, and all new gaskets
during the engine swap. The MAP sensor, throttle position sensor, ignition
module, coil, EGR switch, are from the old engine. I have also swapped
the IAC motor with one that is known to be good with no change. The
original engine was gunked up and not allowing the oil to flow back from
the head, after some time of running, oil would enter the exhaust valve
guides on the number 7 and 8 cylinders causing the spark plugs to foul. I
haven't checked the back pressure from the exhaust system/ cat but it is
suspect. I also plan to check the Fuel pressure to see if the unit is
functioning within specifications (9-13 psi). I have ordered an updated
prom with the bin file AMUR which seems to be the latest eprom however I
don't really expect this will fix the problem.

TPS = 0.66 volts at idle
Map at idle was in spec.

Everything seems to be working fine but obviously something is not.

Any information greatly appreciated,

David Thomas
Food & Packaging SBD Engineer
Festo Corporation
Phone: (704) 527-1427
Fax: (704) 527-7935
email: dave.thomas at us.festo.com

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