I've been trying to find answers for a rough idle at closed loop. Seems that a lot of people suffer from this without really knowing why. Here's what I noticed: My car started to have this annoying rough idle problem some time ago after a shop tried to fix a low charging problem. Charging got a bit better (around 12v) but still not what it should be. They needed to change the connector on the side of the alternator which gives it the feedback from the voltage in the system. According to them the connection was bad. The car idles fine when cold and at open loop (higher idle) but as soon as it warms up and goes to closed loop and 650 rpm it starts to idle rough. The more accessories I turn on the worse it gets and I can see the volt gage go down every time it hesitates at idle. At higher rpm's it runs perfect, no problems at all exept maybe a bit poor gas mileage.

After trying several things to cure this and after verifying the still low voltage with the datamaster I disconnected the voltage indicator wire from the alternator and the car started to idle perfectly. Obviously it wasn't gharging the battery so there was no load on it. Could the alternator be going bad and as soon as there's more load it needs to compensate it starts to put so much load on the engine that it hurts the idle? Charging goes down and fluctuates once the rough idle starts. Also the alternator was too hot to even touch after the car was running for less than 10 minutes. Am I on the right track? The alternator doesn't have more than maybe 5 thousand miles on it though. It isn't the stock alternator as we needed to alter the brackets and mounting due to higher valve covers. Any ideas?

 

I am facing somewhat of the same problem at the moment (not charging) and you are on the right track from the research I have been doing about alternators. My experience is that your alternator should not be running that hot at all! Have you checked the AC Voltage from the alternator? Measure from the large output post to the negative terminal and you should no more then 0.03VAC. I am also doing the test tomorrow myself of plugging a new pigtail into the alternator with a 470 ohm resistor and then to a 12 volt power source to see if the harness going the alternator is bad. From what I have been reading this should trigger the alternator into charging. Mine is of course a single wire alternator (96) I am not sure if yours is the same. Anyways my 2cents worth maybe it will help.

 

Thanks for the reply. I just pulled the alternator and it's only a 105 amp one. I'm quite sure the alternator is bad due to all the symptoms and also the fact that it gets almost burning hot in 10 minutes of idle. The fact that it's underrated for the car probably doesn't help either. Time to try a new one and see what happens...

 

105amp??? Should be a 140amp for the 94~97....... 93 has 124amp... Looks like you are running the wrong alternator...

FYI: The alternator should put out 13.8~14.2 volts at the battery......

 

Put a volt meter on the battery terminals at idle and it it's above 12.8 (min batt charging voltage) or so (should be a lot higher 13.5++) then you aren't looking at the alt as the cause of your rough idle.

 

Thanks for the input guys. I forgot to mention that I measured the voltage with the volt meter and it's the same as the readings from the datamaster. At cold start it's about 12.2-12.5 and after the car (and the alternator) warms up it drops to 11.3-12.0 and the car starts to idle roughly every now and then. Also after ten minutes the alternator is already too hot to touch. I found out today that the alternator is actually ment for a 2.8 V6 Camaro from 1989. A new similar one is 90 bucks so I'll try a new one until I can find valve covers that accept the stock alternator. The whole thing with this undersized alternator started with Crane Gold rockers that were too tight for stock valve covers. I'll let you know if the new alternator fixes the problem. Thanks.

 

OK, put in the new alternator and upgraded all the wires. Seems that the charge is a little bit better but the new alternator also gets really hot after a while. I guess it's just too small for the job. Also the rough idle had nothing to do with the alternator. After the car goes to closed loop, the same thing starts. Couldn't take it for a real spin as it's raining and naturally the drivers side window desided to quit working while it was down. New window motor was put in by a shop this June, I guess this car decided to really try my patience!! I got to fix this window now before I can take it for a test drive. No vacuum leaks either so I'm just ????? with this idle problem. The exhaust also smells really rich.

Could rockers that are too tight cause something like this? A few of them were ticking too much after the initial adjustment and were tightened a bit later. Seems that there are also some fumes coming out from the open element breather. Nothing major but some. Ideas??

 

Just checked the Datamaster files again. As I'm quite new to this computer tuning etc. it takes time to digest all the info on this site. On the left side the long term counts settled to 118 which means super rich, right? The right side stayed quite close to 128 which should be ideal? So everything was ok at open loop but as soon as it hit closed loop the left side went nuts. Leaky injector maybe causing this? I had the fuel pressure gauge installed and it looked ok when the engine was running but all the pressure from the system went to 0 over night. The pressure should stay high longer than that? I'm thinking now that the idle problem is either a leaky injector(s) or the FPR. The charging problem is probably just the undersized alternator.

 

Just for s@#$'s and giggles have you tried testing the coil, maybe it's putting a huge load on your alternator and zapping it of all it's amps?

I recently replaced my coil and noticed my volt gauge go up quite a bit, and my DataMaster logs are showing higher voltage since the swap..

 

Thanks, you might have a point there. I haven't checked the coil.

 

After changing a leaking header gasket (first tried a copper Mr. Gasket, stay away if you have Hooker shorties. It will not work!) the problem still remained. Decided to bite the bullet and change the spark plug wires. Thanks to the shop that decided to "kindly" check the plugs during the last oil change they messed with the wires so that 2 wires got burnt on the headers and caused the misfire. Why it started on closed loop, I have no idea. That cured the rough idle but it still is charging low according to the dash volt gauge even though I got a new 200 amp alternator just installed. If I measure from the battery terminals it is about 13 at idle and about 14 at 800 rpm. So the real figures are now ok but what can affect the factory gauge to show so much wrong? It is around 10 volts at idle now versus the 13 real. The new alternator also gets really hot just after a few minutes. I normally wouldn't worry about the stock gauge showing wierd numbers but it used to be dead on correct before all this messing started. Maybe I should start a new thread for this... Oh, and I also changed the coil so it's not that either. And the datamaster seems to show the same as the stock volt gage.