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Hi,
If the engine does not rev up beyond the 750 computer set idle speed upon starting everytime and dropping back down to 750 there is a problem.
The engine is suppose to be on that 750 mark no matter if it runs a little rough for 20 to 30 seconds.
The computer knows no other program and the IAC is suppose to control that speed if it's not sticking. If it's in good working order and not being over run by an outside influence it will.
The split hose on the air trap can be unmetered air behind the AMM.
A split accordion hose behind the AMM is the same thing.
All hoses need to be air tight.
Any hose attached to the intake manifold has an orifice and that amount of air is limited just like the Crankcase Flame Trap system. Even though these air amounts are not metered they are fixed. They are within an allowed amount of error of the AMM but it all is corrected by the oxygen sensor on the outflow. The ECU only controls gasoline mixture between the AMM and the Oxygen sensors.
If the car is running you are best to forget there are any problems with the ECU itself!
You would be best to go back through the throttle body instructions and not follow them to the letter.
Especially the part about setting the throttle plate open with the stop screw X amount of a turn.
You only want the plate stopped in the bore not held open any amount.
This space opening is to be very close to NIL!
The TPS has to click, if and when, the plate barely moves and that's it! The IAC has a limited opening range that can be easily exceeded.
The TPS gives the ECU a heads up that the air flow is about to change and the ECU is suppose to drop the idle program and goes to a NASA program called "Throttle Up."
It is an X amount or a percent in mixture.
This compensates for transition "lag time" or otherwise the ECU/ICU would not need that switch except to shift the spark timing periods.
The TPS is also used under Wide Open Throttle conditions when the FPR adds more fuel outside of injector pulse width.
Now to be honest, the '91 I have, seems to be better at cold idling than my either of my '92 or '93 for the first few seconds after starting.
The '92 recently slowly got worse over the two years I have owned it.
I tried to blame it first on ECT or a FPR. It turn out to be more the FPR. I have used ones off of junk cars and it seems after putting on another used one it smoothed out. In fact it now runs just like the newly acquired '93 that is not quite dialed in either.
My 1991 is holding the bench mark so far for a smooth cold idle.
I'm suspicious that the two later models have issues with the ECT. It's resistances do not quite jive with the coldness of the engine. Then again, the ECU programs may be tweaked at the factory to be more stringent on emissions during warmup? I have seen several tweaks on the 1993 that set it apart from even the 1992. My '92 is a black trimmed entry level 240. Where the '93 has cruise and electric mirrors but lost the seat heaters of which I care less about!
Unfortunately the time it takes for the O2 sensor to warm up, either by its heater or exhaust, things start flying right!
I don't know if it's something to be lived with but I'm still looking for the problem. One thing is I'm not going to buy O2 sensors costing up to $100+ for the difference, especially, when they test cars hot!
So, if your car is similar, with the first start of the mornings, it could be a normal age thing?
We both will have to keep thinking and looking for a cause.
Phil
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