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That valve will be open more for cold starts where the air flow has to be larger to allow the engine to develop enough internal torque to overcome the viscosity of cold oil and to maintain engine RPMs. So its opening is dynamic. The opening can change very rapidly to compensate for the sudden load changes of electric cooling fans and A/C compressor. To maintain idle speed, the air flow will be reduced as the engine warms up. The manifold vacuum will increase as this happens. (With air cooled aircraft engines where there is not a water jacked and temp sensor for that, engine warmup before takeoff is determined by the manifold pressure at a specific RPM. At least in my very old experience.)
With the T5 and T7 systems, the idle air flow is increase a moment before the engine fans are turned on or the A/C clutch is activated. This load anticipation works very well from an idle speed control point of view. However these suggen changes to engine load do create torque surges that many one can feel and hear. Many new owners find this a problem and think that it is an idle speed problem when it is quite not. Vehicles with automatic transmissions do conduct these vibrations into the chassis to a high degree, and that does not happed significantly with the manual gear boxes.
The amount that the valve has to open is also influenced by the amount of air flowing around the closed throttle plate. And that is influenced by deposits in the throttle bore. But those deposits are usually soft and oily and easy to clean with a cloth an mild spirits. The T5 IAC deposits are very hard and require an agressive solvent to clean them up. Carb cleaner works and it takes a good working over to get it all clean. Not just a quick spray. It sounds like your T5 IAC requires some work to be salvaged.
A fouled TB bore might take the IAC to an operating range where it cannot function well. A few folks have had to adjust their TB throttle stops to get more air flow to allow the IAC to maintain idle speeds. This should not be done except as a last resort, as it is very rarely needed and could make things much worse. Other problems need to be investigated and fixed first!
The T7 engines eliminate the IAC and the cruise control actuator by using a motorized throttle plate. As the T5 IAC's fouled up so bad over years of use, there has long been a suspicion that the T7 throttles would foul at their idle air flow throttle positions. At 50K my 9-5 TB showed no signs of such hard deposits, although there was some minor stuff to clean up. However there have been some reports where deposits interfered with T7 throttle movement and control which led to idle speed irregularities, stalling, and other faults. Cleaning did fix up many of those problems. Some T7 TB's failed, perhaps from early quality issues. Some revised code in the ECUs eliminated some of this. If the EUC sees that the T7 TB is not behaving correctly, it will trigger a solenoid that mechanically locks the throttle plate to the mechanical input from the gas petal. In this limp home mod, you get greatly reduced power. The T7 TB can have this solenoid manually reset. This has been done in a some cases after T7 fouling has been cleaned up and normal operation has been restored. In some post warranty situations, some folks, no doubt, have needlessly paid for new T7 TBs. However, as stated earlier, some T7 TBs do fail.
Some folks have fried their T5 IACs by trying to open them by applying 12VDC. You can test the function with a small 9V radio battery. But no point until cleaned. But once cleaned, drained, dried and oiled, it if works, it works.
Note also that with the T5 system, as there is no air mass air flow meter (MAF), you can run the engine with the throttle body transistion casting (TBTC) removed. You can also run with the inlet side of the IAC removed. So one could idle the engine and look into the IAC with a flashlight and watch the swing gate move. If the IAC inlet hose to the TB were corked, one could put there hand over the TB and modulate the air flow through the TB and watch the IAC respond. One always needs to take care to not dislodge and dust or dirt around these things that could then get inhaled onto the engine. If there is any intake opening in the T7 system between the MAF and the TB, the system will malfunction and likey CEL as well. So you can't do such with the T7. For this same reason you cannot use a dump valve with MAF equipped engines without creating significant problems.
More than anyone wanted to know.
posted by 68.91.15...
, Mon, 10 Jan 2005 09:15:09, Tue, 21 Jun 2005 07:26:56
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