Hello, for about 4 months now i have been trying to figure out my idleing problem. I have it finnaly narrowed down to when it happens. Once the radiator fan kicks on the idle becomes rough and almost sounds like it is misfiring. I know that it isnt misfiring though because once i rev up it has no problems. I also just put a new throttle body onto the car and it didnt help, even though thats what the shop said it was. Has anyone had any kind of low voltage problems that caused rough idleing? I am going to get my alternator tested today and battery. Tell me what you think could be the problem thanks.

 

That same exact thing (rough idle and misfiring sound) happens to me when my car is warmed up but idling. Even when I come to a red light sometimes. It goes away when my AC is on and my idle is higher. I have no idea what that sound is, if this is a normal phenomena, and if not what's wrong with it. Can anybody chime in?

I have an 03 6mt with 20k on it.

 

disclaimer: I am not a mechanic or familiar with the VQ motors.

If it was a low voltage issue, I think that you would see the same symptoms when you load the electrical system with lights, HVAC fan and rolling the windows up. Try all those things simultaneously when the engine is cold.

As you know, the radiator fans turn on when the engine reaches a preset temperature. Question is, do you get the same symptoms when the engine is colder and you force the fans on by turing on the A/C?

If none of the above causes the engine to idle rough, then I think you can start looking at the ECU and other engine management system parts (this is the specific VQ area I'm not familiar with) It would seem that when the ECU sees that the engine is getting warm/hot and it's compensation for this (fuel, timing, throttle plate position) is not happening correctly or that it's overcompensating. Or, maybe it's trying to properly compensate and something in the engine management is not working properly.

 

I got my battery and alternator both tested and they were both good. I have load tested the battery with everything on and there was no dip until the radiator fan kicked on. Even when i go to put the ac on there is no problem until the otherfan kicks on.

 

Is the check engine light on? If so diagnose that. If not do an idle relearn.

 

No i do not have any lights on. Also i just got a new thrttole body and already did the relearn. But I did just try un plugging one of the fans once it started to idle hard and it caused the engine to even back out and not have a bad idle. I think that the fans are pulling to much electricity making ether the coil packs or the full pump not work correctly.

 

Find someone who has an ammeter and do amperage draw. Or swap someone elses fan. The LH and RH fans are different, can't swap left to right. If it is the fan, eventually the resistance will be too high and the fuse will blow.

Which is why I doubt its the fan, the fans are the only thing on that circuit and each has it's own 40 amp fuse. And the ECM should recognize the higher load and compensate with more throttle angle/idle speed.

What mods do you have?

 

I only have a cold air intake. But i did test the fans. Once my car started to idle rough i unplugged the right fan and vise versa. With one of the fans once i unplugged it the idle came back up to normal. Like i said i did the same with the other fan but it did not make any change to the idle and it was still rough. Now that i tested the fan i switched the wires to make sure it wasnt the wiring harness which did not make a change the same fan once plugged in still caused the rough idle. From these results i went ahead and ordered a new fan assembly and should be here wednesday. I will keep yall updated, but feel free to give me other ideas of what it could be.

 

I just got done installing the radiator fan and it is running great! I think that it was a bad fan that was just over drawing voltage. Thanks for all the help forum!

 

That was some impressive troubleshooting, no one would have ever guessed that it was your cooling fans causing the problem.

Thanks for updating your thread!