One thing I do not like about the TPI system is the Idle Air Control Valve. I have replaced the valve 5 times in 10 years, driving about 2000 miles / year. When it dies, it always dies closed, causing the engine to stall when I press in the clutch. I know this is not “right”, and it should not cause the car to stall, but there is a reason .

I purposefully set the throttle body closed further than the procedure calls for. The car is an LB9 with a T5. If the throttle body is per the normal procedure, it will idle fine at a stop, at a reasonable 6-700 RPM. But, if the car is rolling, coasting, etc, the computer always opens the idle air valve too much , and it idles at 1300-1500 RPM when the car is coasting, which really annoys me. Lacking a better solution, I adjust the throttle body down so it idles at 1000 when coasting. When the car comes to a stop, such as at a light, then the idle will drop down to 6-700. I wish the idle did not bump up when the car is rolling and just stay at 6-700rpm all the time when the engine is warm. The car has acted this way since I bought it 10+ years ago. I wonder if the PROM guys know of a way to stop the car from idling high when it is rolling. Is there something in the code for a stick car that can be tweaked? Is there a reason the computer bumps up the idle when the car is rolling?

Also, I wish someone made an IAC valve that would last more than two years. The things are not even very dirty when I pull them out. They are just frozen. I did try to clean the one I pulled out last week, but I was not gentle enough and I stripped the internal threads for the pintle.

Sorry for the rant, I love these cars and the TPI puts a great bottom end on the little 305, but the IAC system has been a pain since day 3 with this car.

 

The IAC used on these cars is virtually the same used on all GM fuel injection from mid 80's on to mid 90's, whether it be car, truck, van, what not. While not fool proof, it is pretty reliable, and shouldn't be burning out every couple years. Maybe the way you're setting minimum air is causing this, but I haven't heard of it doing this before. Anyway, nearly every stick shift car bumps the idle up until a stop, to prevent stalling during odd clutch engagements, and in order to provide vacuum for power brakes without stalling. I'm sure it can be tuned out if you get on the PROM board.

 

The one on my '89 Formula is original. Perhaps find an original one in a scrap yard?

 

U could have tuned that out easily and more cheaply than throwing IAC's at it.

I would have to imagine u have something else going on other than multiple IAC failures, or its just getting gunked up since it doesnt extend/retract as intended.

I have yet to replace a IAC on any of my cars because it was bad, I did replace then on my Formula at 110k cause I felt guilty at that point and a new engine was going in.

later
Jeremy

 

Any thoughts on why the car may be eating IAC valves? I am not sure how setting the throttle body opening slightly on the low side would cause the IAC valve to suffer from such a short life. By setting the throttle body closed more than the spec calls for, the IAC valve would tend to be retraced more to allow more air in than it would otherwise be. The thing is, the IAC’s on the car are dying in the extended position, not retracted. Maybe by spending more time retracted, it caused premature failure? I do not know what the guts of the IAC look like, perhaps someone here understands why this may he happening.

The plenum and the port in the throttle body are not unusually dirty. Over time, there is a small amount of carbon buildup, but from what I understand, that is typical.

I would like to "tune" out the excess idle speed during coast. Has anyone been sucessful in doing this? I am just starting to read the DIY PROM stuff, but I have a long way to go.