1991 Chev 1500 Pick-up Other - Fuel System

Question: 5.0L engine runs rich when starting - smoking, rich smelling exhaust. When engine is <125deg the rich condition leaves but the engine starts surging between 750 and 1200 rpm. At times when the engine is fully warmed and driven the engine idles above 1500. I have replaced the CTS and IAC. Checked voltage at TPS and MAP sensor. Disconnected the MAP sensor vacuum line and installed a Mityvac testor to simulate steady manifold vacuum. I have switched fuel injectors with another vehicle. Engine seems to run fine going down the road. There are no hard codes set in the computer. Any suggestions?

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Technican: Have you checked the fuel pressure regulator?

Owner: I have not. I discounted it because it runs good at high RPM. I did swap the entire injector/regulator assembly with another vehicle, looking to eliminate both the injectors and regulator. I will check just to make sure. If this is not it, any other suggestions? I'm considering replacing the ECM. I forgot to mention I have replaced both the CalPak and the PROM.

Technican: If you have already substituted it with a known good unit it must be something else. If you have fuel pressure within specs and no vacuum leaks it would be nice to swap the ecm. Especially if your lucky enough have a donor vehicle. GM ecms have a history of weak solder joints that loosen and cause various problems. You appear to have a lot of experience, but to be safe, make sure this is black smoke on start up. Black smoke indicates a rich conditon and blue smoke is oil.

Owner: Well, found the problem. The fuel return line was pinched at the tank where it was not obvious without dropping the tank. Checked fuel pressure and had 40psi. Never considered high pressure...duh. Thanks for your time.

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