Gen 4 Idle Hunt / Fluctuation

[Rocket]

Thanks in advance for your thoughts. At times my idle will "hunt" / fluctuate between 500-900 rpms. Normal idle about 750 rpm. Details - 2009 Hennessey 700r Package which includes headwork, intake work, full exhaust with a single high flow cat, and the Mopar pcm. Here's a link to a video (wish you could hear it - no obvious misses though) and it runs normal. No error codes btw.


https://youtu.be/0DpSuUGuITw

[Martyb]

Only happens once in a while or is it constant? Does it happen when cold or warmed up, or both?

[fuggles]

Hunting idle usually means vacuum leak.

[Rocket]

Just does it every once in a while - not constant. Cycling ac on/off does not seem to effect it. Have not noticed it when cold or first starting - so happens at full running temp.

[fuggles]

Maybe I should explain more. Most likely there is un-metered air getting in the engine caused by a vacuum leak. You can look for the leak yourself with a smoke machine or a good cigar. Some people use starter fluid. The idea is spray the fluid near suspected leaks. if the engine surges you have found the leak. Of course use with caution. Starter fluid is highly flammable. Personally I use the cigar method. If none of this sounds appealing take to your favorite mechanic for diagnosis.

[nautoncall]

Every highly modified car I have had, does that some. Just got my 03 back from Aaron at Nth Moto and it still does it some. That was not the reason that he had my car but it's a Heffner TT that Aaron refreshed and installed an Infinity .

[Steve M]

There are two things that control the idle speed:

1. Spark (fast response)
2. Throttle body opening (slow response)

The PCM pulls and adds spark to keep the idle around the target RPMs...it is the preferred solution for maintaining a stable idle because the response time is very quick compared to moving the throttle blades to compensate. As the PCM notices trends, it will adjust the throttle bodies to keep everything happy...swings like that look like the PCM is having a hard time keeping everything where it wants it. That could be one of two things:

1. You are out of the dynamic range of the idle adaptives, and it can't compensate enough with the tools it has available
2. You have a vacuum leak (maybe small) that is compounding the problem

These PCMs will usually throw a code if the vacuum leak is big enough, but I can't quantify how big that would be. I'd go over all of your connections to the intake manifold with a fine toothed comb and go from there. Worst case: your PCM might be in the process of shitting the bed, but it would be a very remote possibility IMO.

[Rocket]

After a little more research, going to remove and clean the Idle Air Controller. Possible sticking of this valve may cause the symptoms.

[Steve M]

After a little more research, going to remove and clean the Idle Air Controller. Possible sticking of this valve may cause the symptoms.

There is no idle air controller on a Gen 4/5 car...when they moved to drive-by-wire throttle bodies, the need for an idle air controller went away. That is now controlled by the PCM moving the throttle blades themselves.

[quickster2]

I would run some Techron and Startron (50/50 mix) through the fuel as well.

[01sapphirebob]

My car does this as well and has done it since i had headers/exhaust/ MOPAR (now ARROW) PCM installed. Doesnt do it to the degree yours does but still does some hunting once in a while.

[Rocket]

Thanks for confirming I'm not crazy as I took a look this morning before going out for a cruise and did not see anything that remotely resembled an IAC :p


There is no idle air controller on a Gen 4/5 car...when they moved to drive-by-wire throttle bodies, the need for an idle air controller went away. That is now controlled by the PCM moving the throttle blades themselves.

Will give this a try and also consider cleaning the throttle bodies.


I would run some Techron and Startron (50/50 mix) through the fuel as well.

Good to know. Mine seems to do it most often in slower, stop and go traffic. Maybe there is something to the DSE heat shield even for the Gen 4?


My car does this as well and has done it since i had headers/exhaust/ MOPAR (now ARROW) PCM installed. Doesnt do it to the degree yours does but still does some hunting once in a while.