Hello,

I'm trying to help my old man diagnose a surging problem with his 1992 silverado. It has about 170K miles and a 5.7 engine.

It surges while driving about 35-40 mph, under very little load, and barely any throttle. It does NOT surge at idle. It runs great under acceleration with no surging or loss of power. He's noticed a decrease in fuel economy. The surging is getting worse. It has dual exhaust - I mention this because I'm pretty sure the catalytic converter is gone, i.e. not plugged.

Here's whats been done so far:
1) Fuel pressure on weak side - new fuel pump.
2) New fuel filter
3) New plugs and plug wires
4) New dist. cap and rotor

I was thinking Idle Air Control valve, MAP sensor, vacuum leak, or t-stat sensor/plug?? Any thoughts would be great - thanks.

 

i would check the resistance of the TPS,map sensor and coolant temp sensor, did the fuel pump bing the fuel pressure back in spec? you will need a small hand vacuum pump to check the map sensor at different vacuum levels

 

Thanks - I'll let him know.