Idle dip: vacuum? PCV?

Hi guys,

 

I'm trying to rectify an odd idle on my 2004 JDM GT-B. It'll drop down to 500RPM momentarily when I come off the gas, but only occasionally. My understanding is that this is a symptom a vacuum leak, but I'm wondering why it is intermittent, and frankly, it's driving me nuts.

 

Could it be a sticky PCV valve? If so, where is the PCV? Any other ideas? I recently pressurized the intake with a bike pump and sprayed soapy water on all the joints in the intake without finding any leaks, and the issue is still present. Admittedly I haven't checked the exhaust-side joints for leaks but I can't hear anything there when the car is running. Seems to boost fine and am seeing 1.4 bar on the gauge. The wideband shows acceptable a/f ratios throughout the running range, although it goes lean when I come off the gas.

 

The car has an HKS mushroom filter cold air intake pipe, and full HKS turboback exhaust (with a cat) and a tune but is otherwise stock. Plugs are fresh and it's not throwing any codes. I have a tactrix and wideband, so I'm reasonably well equipped to try diagnostic suggestions. Thanks in advance!

It could be the tune.

 

It's not the PCV. In a perfectly operating engine, all the air that's in the engine swirling around is "metered". So, when the pcv operates, it wouldn't cause the engine to run rich or lean.

 

Have you considered the vvt actuators might be acting lazy? They adjust between operating speeds and idle speeds, and if one of them is slow to react, it could cause a momentary disruption of smooth engine performance at idle. Have you datalogged Engine Roughness to see if one of the cylinders (or bank of cylinders) is misfiring?

I had a similar issue recently, and just cleaning my MAF sensor helped 90% of my off-throttle dip. It's an easy one to mark off the list...

 

Sent from my SM-G900V using Tapatalk

Kind of have to agree with ^

 

Buy a can of MAP cleaner, remove the 2 small screws that hole it in, spray up the tube for about two seconds, let it dry for a couple minutes, reinstall it.

 

It's one thing I do at least once a year, like cleaning the battery terminals, checking the grounds...

Thanks guys. The MAF was cleaned very recently so I can check that one off the list. I'll do some logging. I see from another thread I should be logging A/F trims to check vacuum leak. What parameters should I watch to check the AVCS? The banjo filters have all been removed, fwiw.

How clean is the air filter ?

Air filter is clean and lightly oiled.

If you’re located in an area where you’re experiencing change of season right now (ie: getting warmer) maybe try doing an idle relearn....Reset the ecu, then start the car and let it idle for 10 min without any accessories on, then shut down and restart allowing it to run for 5 more minutes.