Okay so I've had this issue with my 04/2002 MCS since I got it at 32k miles(currently at 63k). I've got a K&N filter and the one-ball mod but stock other than that. Every once in a while when the car is idling it will produce a weird wub-wub-wub-wub sound where the idle will bounce between 800-950rpm and the car gets a bunch of vibration and harshness. If I rev it a little, it'll usually go away but sometimes comes back after a few seconds.

I'm not reading any codes and I've replaced the 02 sensors, fuel pump, spark plugs every oil change(usually properly gapped coppers though I just put in iridiums and had the same issue).I seafoamed the gas tank & through the PCV... I'm running out of things to replace/clean and while I don't think it's damaging anything, I know it's not normal. I've got an OCC I'm working on installing and I was going to do the bypass modification of which the name is escaping me right now.

Any input would be great, Thanks in advance!

 

It is a somwhat common issue with the 1st Gen MCS's. It is my understanding that it's a software issue as it struggles to re-regulate idle after being driven, due to our superchargers.

My 03 had this issue often enough. After upgrading to a set of JCW 380 injectors 12 months ago, it has only done it once or twice since.....couldn't precisely explain why though.

 

I've been considering getting a pulley/tune, but I assume you've gotten that and it still does it?

 

The problem is with the ECU idle circuit. My guess is the fuel trim is set too low. A tune will only edit the wide-open throttle (WOT) ECU side, which occurs when you press your accelerator beyond 80%. My old '02 MCS did the same thing ever since the last recall for an ECU upgrade back in November '05. The dealership informed me they could not go backwards on it. Even after I had Way tune it at MOTD 2009, I still had the idle problem.

 

gotcha.... so no shops have gotten their hands on an older OEM tune? That's a shame but at least I know it's not something messed up! Thanks guys

 

Yeah, it is messed up. I would suggest getting an OEM tune for the '05-'06 models. I think the only change is the teflon-coated rotors in the supercharge. I'm not sure what else is different.

 

It can also be caused by low grade gas and injectors being dirty.

 

Yes, low-grade gas is certainly a possibility. Besides Seafoam, you can get STP fuel injector cleaner (black bottle), then use an STP fuel treatment every 3,000 miles or so (red bottle). I previously had an issue with cold starts. 1st attempt to start up, engine would quit. Immediately turn the key, engine would start. I eventually attested the issue to using Hess gas. A couple of other local club members had the same issue.

You will more than likely continue to have the problem. The condition will occur either more in summer or more in winter (I cannot recall which).