brake Posted November 26, 2011 Report Share Posted November 26, 2011 My '96 3100 has a bit of a rough idle. The spark plugs are new, air filter is fine, pcv valve is also new, not that it matters but the water pump and tensioner are also new. IAC and throttle body/intake has been cleaned, EGR valve and ports have been cleaned. MAF is clean. I couldn't find any vacuum leaks but that isn't to say they don't exist. I'll put a gauge on it the first chance I get. No codes. It's a random miss that vibrates through the whole damn car, it drives me crazy. When I bring it up just above idle (either in park or in drive w/ my foot on the brake to load it up) it is smooth and doesn't seem to miss at all. Car seems to run fine when I'm on the throttle. Sometimes when I start it, it will either idle very very low (~500 rpm..sometimes lower) and then stall or idle really low continuously - lights dim until I hit the throttle - sometimes the idle will stay up at normal speed but sometimes it will go back down to the ridiculously low idle w/ dimming lights etc. On a rare occasion it will simple crank, fire, and die and I can't get it to stay running unless I keep my foot on the throttle slightly to keep the RPMs up. Gotta keep my foot on the throttle for a few seconds longer from time to time. Other times it starts perfectly fine. other known issues: piston slap making it knock when cold. also has an increasingly noisy exhaust leak between the motor and firewall somewhere that i need to get fixed. it's stinky. Quote Link to comment Share on other sites More sharing options...
urbex Posted November 26, 2011 Report Share Posted November 26, 2011 Piston slap is common with these engines unfortunately. Quote Link to comment Share on other sites More sharing options...
brake Posted November 26, 2011 Author Report Share Posted November 26, 2011 Yeah I'm used to the noise but the funny idle is bugging me Quote Link to comment Share on other sites More sharing options...
brake Posted November 27, 2011 Author Report Share Posted November 27, 2011 is there a method of testing the IAC on these? the starting problem could be the valve not opening i'm thinking the exhaust leak before the catalyst may cause the funky idle but i don't know. Quote Link to comment Share on other sites More sharing options...
AL Posted November 27, 2011 Report Share Posted November 27, 2011 Is the exhaust leak before the front O2 sensor? Quote Link to comment Share on other sites More sharing options...
Mel87 Posted November 27, 2011 Report Share Posted November 27, 2011 Sounds like a weak alternator to me. A bad alternator, or even a bad battery WILL cause lights to dim and even have the engine stall after a few moments. Piston slap happens with every 3.1, mine did it when I bought the car with 62K miles on it and it was well maintained by the previous owner. Nothing you can do about that but let it warm up. Take a volt meter and see what voltage your putting out. If it is significantly below 13.8V could be your battery. Check the terminals on it and see if it is a tight connection. Quote Link to comment Share on other sites More sharing options...
brake Posted November 27, 2011 Author Report Share Posted November 27, 2011 Is the exhaust leak before the front O2 sensor? Don't know where the leak is, it smells and sounds pre catalyst though but that's all I can say for sure. Quote Link to comment Share on other sites More sharing options...
brake Posted November 27, 2011 Author Report Share Posted November 27, 2011 Sounds like a weak alternator to me. A bad alternator, or even a bad battery WILL cause lights to dim and even have the engine stall after a few moments. Piston slap happens with every 3.1, mine did it when I bought the car with 62K miles on it and it was well maintained by the previous owner. Nothing you can do about that but let it warm up. Take a volt meter and see what voltage your putting out. If it is significantly below 13.8V could be your battery. Check the terminals on it and see if it is a tight connection. battery is new, shoulda mentioned that. where can i test the alternator output? Quote Link to comment Share on other sites More sharing options...
AL Posted November 27, 2011 Report Share Posted November 27, 2011 At the battery when its running Quote Link to comment Share on other sites More sharing options...
Mel87 Posted November 27, 2011 Report Share Posted November 27, 2011 battery is new, shoulda mentioned that. where can i test the alternator output? You test the alternator from the battery. Take a volt meter and put them on the battery when the car is OFF. Should read about 12.6 to 12.8 volts. Then turn the car on with the meter still connected and see what it jumps to, with the car running... a strong alternator should push about 13.8V. Also would like to add, are the terminals secure on the battery? Loose terminals can cause it to do wacky stuff. Quote Link to comment Share on other sites More sharing options...
brake Posted November 27, 2011 Author Report Share Posted November 27, 2011 ah okay, I was curious because on my Ford the alternator has an output stud and it's easier to see exactly what it's putting out if you take it from there. the connections are fine but it wouldn't kill me to disconnect the battery and clean the connections up anyway. thanks! Quote Link to comment Share on other sites More sharing options...
Mel87 Posted November 27, 2011 Report Share Posted November 27, 2011 Pretty much your testing for the charge the alternator is sending to the battery. I don't believe GM has hookups for testing the alternator directly. Like a said a strong alternator should recharge the battery at about 13.8V (with the car running), I think it can even go up to 14.5V. But yes try to make sure there is no battery acid or anything in the connectors. Quote Link to comment Share on other sites More sharing options...
brake Posted December 9, 2011 Author Report Share Posted December 9, 2011 It was the IAC. Hooked up a scan tool/scope and the ECU was commanding it to open all the way (step 255), but it still couldn't hit the desired idle RPM. My alternator is charging just fine and the TPS reading is within spec Popped in a different IAC (still old/OEM but has been on a shelf for a while) and it runs just dandy. Thanks for the advice, all. Quote Link to comment Share on other sites More sharing options...
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