Rough Idle or High Idle : Nissan Skyline GT-R : RB26

An RB26 should idle smoothly at around 1000 rpm on the tach. There should really be little vibration, or noise, other than what a 25,27 year old solid lifter engine makes. Which in reality is pretty loud with the hood open.

R32 GT-R Speedometer in miles per hour

Most of the time, if you hear a noise, that sounds like a really bad lifter, it is normally the bottom end on an RB. However, troubleshooting that is a different post.

Troubleshooting a Rough or High Idle Nissan Skyline GT-R

Observation is key

It is important to have a look at what a car is doing. Open the hood, look for anything obvious.

Once you have had a cursory look under the, hood, make sure you have gasoline( had this happen to me recently), make sure the fuel pump seems like it is running, no crazy amount of smoke out the exhaust, onto the next step

1/2 a tank of gasoline

Are they plugged in?
Do the hoses look OK?
Give them a bit of a tap with your hand, does the idle rpm change?

If yes, then proceed to MAF troubleshooting page.

MAF with HKS air filters

Vacuum leak

There are lots of spots to have a vacuum/boost leak.
Large intercooler hoses

There are the top intercooler hoses that are visible, plus the ones that run though the fenders to the intercooler. If these come off the car will idle rough and spit black smoke out of the exhaust.

Around the fuel injectors

The lower injector bushings can get crispy and shrink over the years. They are rubber and get hot. Sometimes just giving the injectors a twist will reveal an issue.
Otherwise a can of brake parts cleaner or the like, sprayed at the injectors, and throttle bodies can reveal a vacuum leak.
A smoke test, or pressure test will not always show leaks depending on what side of the throttle bodies its on, and what you plug/disconnect.

Throttle plate adjustment

Three sets of two throttle bodies
Individual adjustment

TPS adjustment

If the car doesn't know it is at idle, you can have some idle instability.
It should be appropriately 0.5 v at idle
It should be approximately 4 v at WOT

IAC adjustment

Idle Air Control

AAC adjustment

See below.

DO NOT CLEAN OEM THROTTLE BODIES, unless you have a sealer like Tomei Throttle coat to ensure sealing. If you clean the sticky mess off the inside, once you get it all back together, the idle will be high, with no real way to get it to idle lower. That is without taking it all apart and adding something to seal up the blade edges.

The OEM paper gaskets are thin, and leak fairly easily. We like the Tomei gaskets, they are metal, and will not fail like the stock gaskets. There is one long gasket against the head, and one set of two for each side of the throttle plates, or 6 of them.

14035-05U10 - Long gasket head to balance chamber (these tear if you look at them wrong)

16175-05U70 - Gasket throttle bodies x 6. (one each side of throttle bodies)

We have used this Molykote Dry Film Lubricant to help seal and lubricate the throttle blades. It comes in a spray can.