1990 5.7 Suburban Runs Lean In Closed Loop

[John Boelte]

I'm about to pull my hair out over this. 1990 Suburban with 5.7 TBI. Runs great when it's cold. When it warms up and goes into closed loop there's a stumble or miss (like it's running out of gas) when mildly accelerating. If I floor it, it seems to run pretty good (goes into open loop at WOT?). When cold, if I romp the gas pedal (in park) it revs smoothly and quickly, when warm, it stumbles a bit.

 

I've replaced the O2 sensor, temp sensor, and fuel filter. I also swapped in a MAP sensor from an unknown year/model GM car (same p/n 866 or something like that). I haven't checked fuel pressure yet (cause it hard to hook up a pressure gage). The fact that it only runs poorly in closed loop leads me to believe that there is a bad sensor.

 

Help! Let me know if I need to provide more information!

 

Thanks!

[John Boelte]

Wow! Hey , is there anyone here that has any insight to my problem?

[TEE]

How does the pattern look on your injectors at idle ?

Fine spary ? Any drips ?

[90GMCZ71]

I fought a miss at idle on my 90 GMC Z71 5.7 when warm. For me, it was 2 vacuum leaks. One at the PCV valve gromet (it was hard as a rock) and the other was the vacuum hose to the MAP. One of the rubber ends was loose and it would not hold a vacuum.

 

Good luck and look hard at all vacuum lines.

 

//90GMCZ71