91 G20

[ihatelaramie]

My dad has a 91 G20 Van that's been giving him fits for some time, and now he's come down to visit and he's not sure he'll make it back home. It's got a TBI 350 with a nasty surge at idle. They've chased down other problems, and to date it has a complete new distributor, plugs, wires, PCV valve, O2 sensor, MAP sensor, TPS sensor, fuel filter, catalytic converter, ECM, and EGR valve. I probably forgot something but someone also rebuilt the fuel pressure regulator and tested the injectors and they are supposedly good. They searched the entire engine top to bottom for a vacuum leak and the ones they found (and fixed) had no affect.
Anyway, the deal is that it surges something wicked at an idle, to the point of stalling. On a bad day it takes half throttle to keep it at a rough idle. What they found out was that it will run descent with the MAP sensor disconnected from vacuum to where it reads atmospheric pressure. The downside to this is that it runs so rich that it smokes worse than my truck
Does anyone have any ideas what could be causing this? My dad is at wit's end and it's a long drive home, so any help would be appreciated.

 

[sle2115]

Yep, have experienced this twice with same motor and looking at your list, I don't see where the coolant temperature sensor has been replaced. It should be front and center on the intake, close to the thermostat. Unplug it and see what happens! When they fail, they put the system in rich run mode...my dad's was dumping so much fuel it was unreal, causing a rough idle, horrible mileage, and it would literally drown itself! You would think the O2 sensor would pick up the rich run condition and lean the motor out, but the computer is so basic, it won't! I ran a scan, no codes, etc! It won't run well at anything above idle with it unplugged, but if it idles decent with it unplugged, throw a new one in it, they are like $12-15 and seem to fail regularly!

Hope that helps. I swore dad's had a major vacuum leak too, but it was much simpler.