I finally got tired of dicking with the ISC on [464]. Despite setting base idle speed according to the factory service manual (grounding out the two pins, adjusting BISS at temp, etc.), the return of cold weather to Phoenix resulted in my getting “stupid race car” cold idle and idle surge all the time.
Background
There are a pair of coolant lines feeding the throttle body on the first generation DSM and 6th generation Galant Vr4 (4G63T). Basically, once the engine is up to temp and the thermostat opens, the hot coolant melts a wax pellet or something inside the fast idle air valve (FIAV), blocking airflow past the throttle plate to deliver a reasonable idle (approximately 75orpm). Until then, FIAV allows the engine to idle at a higher speed for easier cold starting. When this piece of kit shits the bed, you get a 2500rpm (or higher!) idle until the engine warms up. That’s pretty annoying.
The idle speed control (ISC) is a stepper motor in the throttle body (TB) which the computer uses to adjust idle speed within a set range. It’s probably most active when you activate the power steering at low speeds or turn on the AC/defroster. If the ISC doesn’t burn out on you, odds are good you will do something which will turn it into a complete retard, giving you the infamous idle surge. Always nice to pull up to a stop light and be “that guy” with the engine revving up and down by 500-1000rpm.
I got tired of it.
It’s not uncommon for people to block off these bits and wash their hands of them. Nice thing about OBDI cars; this doesn’t trip a CEL.
So I took a spare TB to Keith’s shop, opened it up with an impact driver (the machine screws holding it all together have threadlocker on them and are made of butter), then cleaned it up in the parts washer. I made a block-off gasket by cutting my own blank from bulk gasket material I had in the garage. No need to get medieval with metal plates, given the low temps seen by the TB. I also removed the little metal springy-thingy inside the FIAV/ISC ports within.
With the TB ready to be installed, I got the old TB out.
- pull the vacuum lines on top of the TB
- disconnect the ISC and throttle stop switch
- disconnect TPS
- remove small Philips bracket screw atop TB
- disconnect intake hose at TB elbow, shove downward
- disconnect/plug TB coolant lines (drain some coolant or expect a leak)
- unbolt throttle cable from intake manifold (10mm)
- pull slack in throttle cable toward passenger side, slip cable from TB arm
- undo 12mm nut on CAS
- undo 12mm bolts on front of TB, remove TB/CAS bracket
- undo 12mm nuts on rear of TB, set aside
- remove TB elbow (try to save the gasket or replace)
- slide TB away from intake manifold, lift fuel line OVER base of TB
- drop TB down, rotate toward firewall, lift up and out
Here’s another picture:
Re-assembly is the reverse, but you will want to bypass those coolant lines. I found the hose that ran from the thermostat housing to the TB was plenty long enough to simply attach to the water pipe below the TB (the other line removed in the process above). You an see the coolant hose I re-used just below the CAS in the picture above.
TPS adjustment
I got it all back together only to find I got a check engine light (CEL) when I let the car idle. I swapped the TPS but forgot to adjust it! This wasn’t a hard process at all (but it would have made more sense to do this before putting everything back together). Simply connect the TPS, set the TB on the intake manifold, turn the key to the ACC position, load your favorite datalogging program (I use the old TechnoMotive), pull up the DRB2 info and adjust. In the picture above, I was holding the throttle wide open while gently adjusting the TPS to read 100%. Closed, it reads 10%. Problem solved.
Life without FIAV/ISC
The car won’t just start with a turn of the key when it’s cold now. I have to crack the throttle a bit with my right foot prior to bumping the starter and hold the revs around 1500-2000rpm for 30-60 seconds before it will stay running on its own. Small price to pay for a rock steady idle. RPMs are consistent and predictable between gears, and once warmed up, the tach sits exactly where I set it now. Very happy with this m0d.