Bad ECM???

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Oct 30, 2005
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Location
South Dakota
1994 GMC K1500, 5.7 engine. Just put a new motor in it (less than 300 miles). It has a new fuel pump, new distributor (plugs, wires, rotor, cap) and too many other new things to mention. Also just replaced the Coolant Temp Sensor. The throttle body was replaced with a used one.

It runs rough and wants to stall for a couple of minutes when first started.Once it warms up, it runs great. It doesn’t throw any codes.

Wondering if it could be the ECM? Any thoughts????
 
How did what would cause the engine to stall for a couple of minutes when first started., Once it warms up, it runs great. That is where to start diagnosing..
 
Did it ever run right?

I suspect a vacuum leak that gets corrected when it goes into closed loop. Spray some brake fluid around the intake manifold while it's running poorly.
 
Did it ever run right?

I suspect a vacuum leak that gets corrected when it goes into closed loop. Spray some brake fluid around the intake manifold while it's running poorly.
No, it wasn’t running right before the engine swap.
 
Your point being? The old engine didn't run right with the "new" one either.

I would still swap back and try the old one again just on a whim. If that doesn't help, start troubleshooting from zero. This is not a complicated system and most of the sensors can be tested with a volt-ohm meter. Also where you can, test from the ECU plug all the way through the harness to the sensor.
 
Ok, start by looking at what sensors are in play upon start up, before full warm up occurs. Check the O2 sensors....not sure how many this engine has. Also depending on how many miles the vehicle has, I would also look at the catalytic converter.

Some converters have been known to cause excessive back pressure until the full "light off" and will need to be replaced. On a cold start, if the exhaust emits a really, really sulphur smell (rotten eggs) then the cat might have "light off issues". I would also do what others have suggested about checking for vacuum or coolant sensor issues or vacuum leaks first.

Lastly stick a vacuum guage on it in non controlled port (read manifold vacuum) and see what the gauge indicates. A manifold or other vacuum leak should show up stronger when the engine is cold vs hot.
 
Most sensors for GM vehicle needs to be ACDelco OEM with some few exception.

Did your replace with ACDelco parts?
 
The Magnetic Pick-up Coil is a sore spot on TBI engines, I've seen countless "New" Distributers cause identical symptoms as the old wore out OE Distributor did.

I was using new GM Performance Parts Distributors made for RamJet Crate Engines, But the supply has all but dried up.

A good Magnetic Pick-up/Pole Piece will produce at least .70 Volts AC at healthy cranking RPM.

These trucks have a decent Data Stream for a OBD 1 system.....One thing I've noticed with faulty Magnetic Pick-ups....
*The Accel Enrich PID will toggle from "Yes" to "No" at idle which it shouldn't do.
*The RPM PID will be erratic/unstable.

Reading GM OBD 1 fuel trims via scan data....
In the old GM OBD 1 world, There were the fuel control PIDs called Block Learn and Integrator. GM used a sliding scale between 0 and 256 with 128 being the center point. If the number was greater than 128, it reflected the amount of enrichment that the ECM commanded to compensate for a lean condition. If the Integrator read 139, the ECM had sensed the engine was 11 percent lean and made the appropriate correction. In addition, if the Integrator PID in the scan tool’s data stream read 120, it is reflecting the ECM sensing the exhaust is too rich and it made the appropriate correction to subtract 8 percent fuel to lean out or bring the Integrator back closer to 128 or the zero mark. The Integrator revealed the ECM's quick response where the Block Learn was the learned value that accumulated over time and learned the trends or cells for the Integrator corrections.

1. Check fuel trims in closed loop, should be 128, plus or minus 8.
2. If fuel trim numbers are high, Look at fuel pressure, Vacuum leaks. Add alternate fuel during the poor run condition to verify a lean condition.
3. If fuel trims numbers are low, Check the Exhaust Gas Recirculation (EGR) valve not seated.

People like to say a Scan Tool is worthless on TBI GM vehicles.....I wholeheartedly disagree! The GM Tech 1, Tech 2, And Snap-on MT2500 (Red Brick) are the best I've used for this system.

Also check the ECM block grounds at the Thermostat Housing, Engine to Body ground strap between the right cylinder head & the Body under the heater hose connections. Along with the Battery to Frame & Battery to Body Grounds.
 
Also check the ECM block grounds at the Thermostat Housing, Engine to Body ground strap between the right cylinder head & the Body under the heater hose connections. Along with the Battery to Frame & Battery to Body Grounds.
Yes that's a real common mistake on any R&R engine job-- forgetting a ground wire or not getting it tight.
 
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