5.7 coldstart

[twostroke]

I have an old reliable '94 k1500 5.7l that has issues with idling,surging idle and stalling on cold startup.Over time I have discovered that it automatically clears up when the coolant temp reaches about 110 degrees,or whatever the magic number is that also allows the trans to shift into o.d. I have checked iac,tps,egr,base timing,ignition and tune.The thing I havnt addressed is oxygen sensor because if I'm not mistaken the ecm doesn't use o2 readings when cold.That could be a mistake on my part.Over the course of t-shooting this issue I have been driving slowly and truck is spitting,sputtering,surging,then as the temp gauge starts to move a tick above 100 the old boy instantly smoothes out perfectly and runs and drives normally.Im not sure what would cause this.

[B. W.]

it's running poorly in open loop (when cold). when it goes into closed loop it runs fine. In closed loop the PCM "reads" the sensors & makes adjustments. Is it running poorly only when idling or at all throttle positions/ speeds? I would pull codes & check for vacuum leaks first.

[68gmsee]

Other possibilities:
Coolant temperature sensor
IAC
MAF sensor (clean with CRC spray)

[twostroke]

At start-up,regardless of ambient temp,until coolant temp gets just north of 100 it acts as if the tps is malfunctioning or the fuel pressure is low.If I start driving it coughs and stumbles.I have replaced the iac,tps,coolant temp sensor and checked for vacuum leaks during the course of tshooting.No codes.Im thinking the pcm has developed a gremlin.

[B. W.]

I'd check fuel pressure, it's kind of a pain because there's no schrader valve to screw a gauge on. The manuals tell you to unscrew the fuel line at the throttle body but the easiest way is to remove the fuel filter & install an adapter to check pressure. Pressure should be around 11 or 12 psi. Fuel pressure regulators are a known issue with GM TBI. A quick way to check for a lean condition is to have someone you trust load the engine (power brake) to make it stumble (while in open loop) cover the throttle body with a gloved hand (I use a welding glove). Try to choke the engine out, if it runs better you have a lean condition. Fuel issues wont always set codes & vacuum leaks may not either but you can watch the O2 sensor on the scanner to see if it is running rich or lean. If you have a bad PCM or ignition module the problem will not go away in closed loop. I know you already checked but I'd double / triple check that there are no vacuum leaks. (I spray it down with carb cleaner or something really flammable - it can make a dull day exciting!) When the engine is cold it needs more fuel, the PCM is not reading the O2 sensor in open loop and a vacuum leak will cause it to run lean. Once it goes into closed loop the PCM reads the O2 sensor & adjusts for the lean condition.

[cadillac_al]

Aren't you supposed to get a code 12 when the ECM is working right?

[twostroke]

thanks B.W. for the info.Going out of town for a few days,but will pick up there when I return.