1987 GMC R2500 V.S. Parts Cannon

[Andrew57]

I own a 1987 GMC R2500 with a 350 TBI and finally got it running but will not have any power past half throttle, My suspicion is that the ECM is stuck in limp mode but I have no idea. Everything I replaced- MAP Sensor, O2 Sensor, Temp Sensor, Throttle Position sensor, EGR, Distributor, Fuel Pressure Regulator, Fuel Tank with new fuel pump, new soft lines and new inline fuel filter. The spray on the injectors looks normal and consistent and idle is fine. I Found almost any ground I could and sanded them and I’m practically out of ideas.

 

[Schurkey]

Replacing parts without testing is kinda futile, ESPECIALLY in a world where most replacement parts are outsourced to Communist China. The replacement parts are often not as good as the thirty-year-old OEM stuff you're removing.

TEST BEFORE REPLACING. There are very few exceptions to this general rule. O2 sensors may be one of those exceptions.

The Usual Three:

1. Verify fuel pressure at prime, at idle, and under load. Most fuel pressure gauge assemblies have a push-button pressure release connected to crappy vinyl tubing. Route the tubing so it empties into a drain pan, then push the button while the engine is running. This simulates higher fuel demand if you can't check fuel pressure on the highway. Fuel pressure should remain steady even with fuel flowing down the pressure-relief tubing. How old is the fuel filter? Have you ever dumped a bottle of Chevron Techron Complete Fuel System Cleaner into the gas tank? (Recommended at every oil change.)
   
2. How old are the usual “tune-up” parts and procedures? Inspect/replace distributor cap, rotor, plug wires, spark plugs. Make sure the ignition coil will reliably fire a spark-tester calibrated for HEI when the coil is fully warm, and misted with water from a squirt-bottle. Cranking compression test of all cylinders while the spark plugs are removed. Verify EGR, PCV, EVAP, and Heated Air Intake (if used) systems for proper operation. Verify proper initial timing (TBI) and electronic spark advance (TBI and Vortec). Replace old O2 sensors unless you can PROVE that they're working properly—old O2 sensors get lazy, they don't provide accurate data, but they do provide “data” that fools people into thinking they're “working”.
   
3. Connect a scan tool (NOT a crappy “code reader”) and look for “codes”. More important, look at the data stream to verify EVERY sensor and computer output. Verify fuel trims during the time that the vehicle is not running properly. Look for misfire counts for each cylinder (OBD2 only.) “Codes” have official diagnostic procedures that will be found in the service manual set for your vehicle. The service manual set can be downloaded from the links in the Sticky thread section of the Engine forum on this web site.