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91 S10 Surging/Bad idle

Discussion in 'Chevy S10 Forum (GMC Sonoma)' started by xJCross, Jun 14, 2013.

  1. xJCross

    xJCross New Member

    I'm running out of my own ideas. So, I've come here hoping to find some answers.

    I have a 1991 Chevy S10 with the 4.3L V6 TBI. When the problem first began, it felt like just bad gas. It would surge some, and the gas pedal felt intermittent. It got so bad that it would cut out at traffic lights. The first thing I did (when I managed to limp it back home) was change out my fuel filter, but that didn't fix the problem. Really hoping it wasn't my fuel pump, I had a friend come check it out for me.

    After some fiddling around, he pulled my MAP sensor and the truck acted normally, the fuel injectors worked fine and it ruled out the fuel pump. It did die once on the way to get a new MAP sensor at a light, but it wasn't surging like before. I went ahead and got a new PCV valve too just to be sure. I put them both on in the parking lot, and attempted to drive home. It was still surging and cutting out like before.

    The next day, this same friend of mine checked all my vacuum hoses for leaks. There wasn't any. I tested the TPS. It checked out fine on my multimeter. He pulled the EGR and gave it a good cleaning. Still no luck. Just to be sure, we replaced the O2 sensor, but it's still surging and ultimately dieing out especially when its put in gear.

    What kind of magic do I need to perform on this thing?

    Thanks for any input,
    Joseph
     
  2. xJCross

    xJCross New Member

    I guess everyone's as stumped as I am haha. We pulled apart and cleaned my throttle body and IAC valve yesterday and determined I need a new IAC. I'll post back if that fixes my problem finally.
     
  3. xJCross

    xJCross New Member

    Update: Cleaned out the throttle body, it wasn't too bad inside. Replaced the gasket and put in a new IAC last week. Still had idling issues.

    So, today we took a pressure gauge and checked the fuel pump. It read a steady 10 psi. We were looking to see if it would dip when the engine began to surge, but the truck was idling better than it had been the week before. We're pretty much stumped because we didn't change anything today.

    Does anyone know if it takes the computer a certain amount of time to learn new sensors?
     
  4. Gramps43

    Gramps43 Member 1 Year

    xJcross.....Did you solve your problem, if so what was solution? Gramps
     
  5. xJCross

    xJCross New Member

    Hopefully, if anyone has this same problem they'll find this and know what to check for. I've been meaning to come give some feedback about my situation and it slipped my mind.

    So, the truck was still acting terrible at idle. After keying it on, it'd go through it's 2 second warm up RPMs then promptly drop to a very low RPM, surging for a balance till it would ultimately die within just a few seconds sometimes longer.

    I've since replaced the spark plugs and wires, distributor cap and rotor button, and alternator. Still nothing. (It needed those parts replaced anyway because they were OEM.) I discovered that once I got the truck moving it would keep going so long as I didn't slow down too much or stop. With nothing left to replace now except the fuel pump, I took off the bed and replaced the pump and sock filter. The assembly looked fine. Now, everything seems to be much better. It hasn't died since the new pump, but it still isn't acting "normal" either. It still idles lower than I remember, but it isn't choking out. My theory is that with a bad alternator, the system will run at a higher idle to compensate for the lack of power. Now that there's a new alternator, it doesn't have to run as high.

    My recommendation, if you're having a rough idle and can get it going like I talked about, chances are it's the fuel system. Again, my theory is that once the vehicle is at speed, the higher speed puts less stress on the pump because of the vacuum-like effect of the fuel being drawn out by the engine. My pump has never been audible when I turned my key. So, I couldn't check for it that way, but now with a newer pump in it I can definitely hear it pressure the lines.
     
  6. Gramps43

    Gramps43 Member 1 Year

    Did you happen to look at the EGR valve. My "94" had a similar problem, Idle was very lumpy, once up to running rpm it ran pretty good but sucked up gas like crazy. $50 bucks later I had the code in hand "EGR". mech. said valve not repairable new valve $180, ouch, but he didn't say it couldn't be cleaned. I took it off and sure nuff there was a chunk of carbon holding it open, got it out and flushed with acetone and put it back on. After resetting the code all is well. It's worth a try, cost $00.00, time about 20 minutes._______________Gramps
     
  7. xJCross

    xJCross New Member

    I did, yes. Should have mentioned that with the throttle body post. That was cleaned when I took off my TBI. Thanks for checking though.
     
  8. obrienkims

    obrienkims New Member

    I had this problem but it wasn't as serious. I also have the V-6 X engine in a 2003 sonoma. Turned out to be the battery. I discovered it after getting a jump after leaving it sit for a week with out using it. Got my jump than traveled about forty miles plus about half an hour in traffic. Battery wouldn't turn over the engine despite plenty of recharging time. Called triple A, got a new battery and surging at idle disappeared as a free "extra".

    I never saw the battery voltage gauge surge though. Only the oil pressure one. I think you could test for this by swapping batteries or by placing another car battery in parallel in our out of another car or truck like a jump start. If the surging stops than you know its the battery.

    - - - Updated - - -

    I also noticed that after say letting the battery discharge a bit by say leaving the lights on for about 2 to 5 minutes or running the radio in accessory mode that after start up the voltage gauge reads a little bit high and gradually drops just a bit. The only reason I can think this happens is because the battery voltage is measured at the alternator and not the battery. If you were to measure voltage at the battery, battery voltage would rise as the battery reaches a fully charged state instead of dropping.
     

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