Originally Posted by amc49
If high mileage may be from that alone. Engine drops down fast enough to then drag it down to die as an older engine does not make enough torque to stop it from dropping to zero. It can stabilize with a second to adjust like you show (standing idle all day long) but not having that and engine in freefall the inertia takes it right past that point.
Check all normal idle air to engine like PCV positively not blocked in any way and need clean TB butterfly and bore right where they touch and IAC. Past that on one of mine I had to once all that was guaranteed open the TB butterfly up a very small amount to let more air in than normal. Older engines need slightly more air due to dragging down in normal idle, again, they have lost a lot of idle rollover torque. What ring and valve seal do when they flee due to engine wear. A compression test may show something there too.
That adjustment is touchy, you can easily get out of IAC range and then PCM sends you the typical rpm too high thing or the idle begins to hang high. There IS a small amount of adjustment room there though, I went between 1/8-1/4 turn more open on the butterfly setscrew and worlds of difference doing it. Mine mainly died when a/c went on at idle. Freshly charged so it had a lot of drag. If ATX car then watch your engine braking at throttle dropped off on decel, the braking is greatly influenced by the butterfly setting and it will likely change a bit to be less. Which helps your issue too, rpm drops at a slower rate.
Mine which used to die all the time hasn't done it in forever. Of course other things like ignition need to be in line too, watch the plug wires, they go bad very easy at pulling them to work on plugs.
The issue is that the IAC tries to respond but they do not like lightning, it takes a fractional second to move, you don't have that at engine freefall. Another thing.....if you never change the butterfly to be open more on an older engine the IAC simply opens more itself but again, that takes time. If you artificially bump the butterfly open then the IAC no longer has to OPEN, rather it will look to CLOSE meaning you already have enough idle air to keep it running hopefully. You didn't have to wait for a response from IAC as you already had the air amount built in.
Thank you for those tips!
I'm going to tinker around with the car more tonight since I'm off tomorrow.
I'll give that adjustment a try all by itself, just to see what kind of difference I can get out of it.
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