DeaconBlue

iTrader: (31)

In the past few weeks or so I noticed that the warm base idle speed on my '90 GXL (S5 n/a) was a bit low and somewhat erratic and "lumpy". It would hunt between 500-650 PRM. If I hold my foot lightly on the accelerator pedal, I can get it to idle steadily at any speed from about 750-800 RPM or higher. When I turn on the a/c the idle speed comes up instantly to a steady 850-900 RPM. The only modifications that I have done to the engine in the past month were the installation of the; CorkSport header, a high flow main cat and the Borla cat-back exhaust. Last fall I had installed the custom fabricated CAI from a friend's former SCCA race car. Would the installation of the high flow intake and exhaust systems necessitate the need to re-establish the base warm idle speed?

The engine also has had a minor surging issue (again), mostly at drop throttle or in the 2800-3200 RPM range with a constant light throttle. It was more noticeable in 1st or 2nd gear.

I did a bunch of searches in the forum and read a lot about resetting the TPS. So I did that again (it seems like something I need to do a couple times a year). I did the test on the 3-way connector near the air box and yes I grounded the test lead. That helped to reduce the severity of the surging, but not completely eliminate it. Then I read about back probing the TPS connector to read the actual voltage and fine tune adjusting it to be as close to 1.0 volts as possible, but not over. I double checked and mine was at 0.85-0.87 volts. So I adjusted the set screw to read in the 0.97 to 0.99 range. I double checked it several times by blipping the throttle and snapping it closed. I then rechecked the 3-way connector to make sure that the reading there had not change and it had not.

I took the car for a short ride and the surging was finally gone - hurrah! But the too low and erratic warm idle speed was still there. Now if this was one of my former Ford cars, I would have known that this was due to a weak spring in the IAC (idle air control). So I pulled the manual for the test procedure for the bypass air control (BAC). It was then that I learned that BAC is controlled via pulse frequency from the ECU - okay so much for that theory. I left the coolant hoses attached but unbolted the BAC from the intake manifold so I could more easily do the resistance check, which ended up reading 7-8 Ohms. This was below the spec of 10.7-12.3 Ohms. On the S5 n/a, the BAC has a 5mm hex head fastener at the forward end of the housing in the center of the plastic end cap. Wondering if this was an adjuster for a variable resistor, I turned the hex head slowly and in small increments one way and then the other with an Allen wrench. Sure enough the resistance valves changed each time. So I reset the value to 11-12 Ohms and bolted everything back together again. After another short ride still no success - the low and erratic warm idle speed was still there.

I did more searching on the internet and discovered this: "...the base idle speed does not come from the BAC valve itself, but from some "plumbing" within the intake. The base idle speed (750 rpm) can be adjusted by turning the capped screw on top of the intake. It has nothing to do with the BAC valve, and this is why you should always adjust the base idle speed by setting the initial set coupler or disconnecting the BAC connector (at the BAC valve)."

Okay fine. So I unplug the connector to the BAC, remove the black rubber cap and adjust the AAS counter clockwise hoping to increase the base warm idle speed. I was not able increase it by enough to get the idle speed up to 750-800 RPM. I could hear the increased air volume rushing through the throttle body's internal passage - a high speed whistle like sound. I even removed the set screw all together and just inserted a temporary plug, but the idle speed just would not come up high enough.

I suppose that my only and lost resort it to remove the throttle body and reset the throttle stop stud on the main throttle shaft say one turn. Then reset the TPS and then use the AAS to try and dial in a more reasonable base idle speed.

As a side note, I also double checked the primary ignitor and coil. The low resistance spiral core spark plug wires were new this spring. I pulled and cleaned the primary spark plugs. I had recently installed an new MSD #6425 digital CD ignition control box with a CD compatible wound dual tower waste spark DIS coil on the primary system. The stock primary ignitor is used to trigger the MSD box via the white "points" lead - nothing exotic here. Switching back to the stock ignition system has made no change.

Any other thoughts or ideas before I pull the throttle body?

TIA.

j9fd3s

iTrader: (3)

sounds like you're on the right track, but when adjusting the base idle speed there is a 1 pin diagnostic plug you need to ground. its close to the battery, green, and should be next to the green 6 pin diagnostic plug.

so you ground the 1 pin connector, and set the base idle speed with the aas screw.

if its still low after that, i would look for a vacuum leak

DeaconBlue

iTrader: (31)

Yes I had the test lead grounded for both the TPS and AAS adjustments.

I replaced most of the hardened heat aged vacuum / pressure lines when I had the intake apart to repair the carbon locked 6PI.

Thanks for the advice.

satch

iTrader: (1)

And your BAC is really not adjustable like those found on a turbo.

j9fd3s

iTrader: (3)

most vacuum leaks are from the lower intake gasket

DeaconBlue

iTrader: (31)

I do know there is no official procedure for adjusting the BAC, but it does use a variable resistor and turning the hex does adjust the resistance, so that is what I did. And it seems to be working fine. Hopefully I didn't shorten its life.

DeaconBlue

iTrader: (31)

I replaced all the intake gaskets with new ones and replaced the seals in the 6PI actuators. I may re-torque all the bolts and then try the vacuum leak test with the Bernzomatic cylinder just encase.

Thanks.

DeaconBlue

iTrader: (31)

I pulled the throttle body and turned the throttle stop set screw in one full turn. I then reset the TPS once she warmed up. The base warm idle speed did not come up as much as I would have liked even after re-adjusting the AAS - still has the noticeable inlet whistle. But the hunting / erratic idle is all gone - the idle speed is now rock solid and smooth - just a bit too low, maybe. The initial throttle tip-in is also much improved. It is easier to creep in the driveway and in/out of the garage. It feels smoother off the line too - less fuss with clutch engagement. When I turn on the A/C the idle hunts a bit at first but the BAC settles down to 850 RPM in a couple tries.

All in all it was well worth the effort - anything that improves the finicky nature of the rotary's drive-ability is a good thing.

just startn

iTrader: (33)

If you need any s5 n/a sensors ect. lmk. I have tons of stuff you can have.

DeaconBlue

iTrader: (31)

Thanks for the offer - much appreciated.