Chevrolet Repair: 1995 GMC Sonoma 2.2L, fuel pressure regulator, vacuum leaks

Question
QUESTION: This is a pick up, same as an S-10. This truck passed emmissions 4 years ago but since then it won't. Under light throttle there is an obvious miss, and it fails on hydrocarbons. It has been in 2 repair shops and no fix yet. Compression tested fine. Reported that #3 cylinder is the issue. Second shop said the injector was bad. I replaced it, same problem. I have replaced 3 of 4 injectors, in tank fuel pump, ignition module and both coils, MAP, TPS and 2 O2 sensors, plugs, wires, air filter and fuel filter. 1500 to 2500 rpms you can definitely hear and feel the miss and if you romp the throttle hard from idle the engine bucks and jumps. I thought the EGR was surely the problem but I have replaced it before and today I made a shim to block it off completely with no improvement. Now I'm thinking fuel pressure regulator? Help, I can get an exemption for all the money I've spent on repairs but I want this thing to operate properly.

ANSWER: Hey,

 Check the intake manifold for vacuum leaks.  Although you might think it is running rich causing the missfire it is probably running lean and creating a lean missfire which gives you your elevated HC reading at the tailpipe.  Can you give me the emissions results including CO and Nox?  I can better calculate Lambda with those readings.

WAyne


---------- FOLLOW-UP ----------

QUESTION: When I replaced the injectors I did replace the upper intake gasket, but not lower. Emissions results are, failed HC-3.8771gpm(1.8000gpm state standard), passed CO-24.5832gpm(60.0000gpm standard) and barely passed NOx-2.5749gpm(2.6000gpm standard). Previous tests failed HC & NOx, this one squeaked by...What's really puzzling me is the weird surging and bucking when I push it hard...even in gear under load it will buck and kick at high rpms..3500- or above at times. Also, when I start it cold, it runs smoothly for 15 seconds or so. The missing starts right around there and doesn't go away.

Answer
Its running too lean.  Have you checked fuel pressure at all?  There is a pressure regulator on the fuel rail that might be diverting fuel back to the tank.  You should hook up a fuel pressure gauge and drive it to see if it fades away under load.

w