89 Chev 5.7L TBI won't idle with "Set Timing Connector" Plugged in

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I'm working on an 89 Chev Suburban 5.7L TBI automatic trans 2 wheel drive. I'm not sure if it is a 700R4 or 4L60E trans. (truck is not here to check)

Symptom - With the "set timing connector" connected, the truck starts right up but then stalls out. I can keep it running by throttling around 1/4 throttle although it "pops" in the exhaust and backfires through the TBI. The timing appears to be approx 36 BTDC and stable when "set timing connector" connected. It sounds like the IAC is opened all the way because there is a significant air flow sound through the TBI.

It came to me with codes 42 - 45 set. I found the "set timing connector" disconnected and the base timing set to approx 44 degrees BTDC so the truck could be driven without ECM timing control. This With the base timing set to 0 BTDC (spec) and the "set timing connector" disconnected, the truck starts and idles fine but doesn't run real strong. When I connect the timing connector the symptom occurs.

The truck has a recent history of various issues surrounding ECM codes (owner doesn't have details) and subsequent replacement of MAP sensor, IAC motor, EGR. Then it overheated and blew a headgasket. This symptom appeared after the headgasket was replaced. The owner replaced the control module in the distributor without a change in sysmptom.

I have been unsuccesful finding any details on how the ECM functions or what sensors/controls are impacted with the "set timing connector" connected. With the "set timing connector" disconnected both male and female connectors measure 120mv and measure 5v when connected. Not sure if the signal is an input to the ECM or output from it.

Any help that you can provide is greatly appreciated. Let me know if there is any additional info that would help isolate this issue. Thank You.

posted by  tblentrprz

I'm not familiar with the SPROUT connector. The "set timing connector" is a single tan/blk stripe wire that when disconnected disables the electronic spark control (EST) from the Engine Control Module (ECM). Any help would be greatly appreciated.

posted by  tblentrprz

When the head gasket on that engine is replaced the intake manifold is removed, therefore so is the distributor. More than likely the distributor was not re-installed correctly ( they didn't find tdc and get it on the right tooth etc), when the timing connector is hooked up the computer is trying to compensate for this fact, but can only adjust so far.
What that connector does is allow the computer to change the timing to match current demand, without it hooked you are running on just the base time ( or mechanical time setting), which gives the computer a starting point or reference if you will. You may also want to find out if the timing gears are lined up, depending on what all has been done to this engine.

posted by  fido

I had that problem with my 89 K5 TBI and it was the valve adjustments for overheating. The order of the spark plug wires was reversed. The truck ran rough with no power. Also, make sure the thermostat is the correct one for the computer to close the loop to operate the system. Last the EGR solenoid can start to go out and cause idle problems. Change that and the O2 sensor.

Bill   31 Oct 2012 23:33

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