I have a 4.5L with 60,000 miles. Recently, I have experienced a loss of RPM when coming to a stop. Car idles normal otherwise. In park RPM reads 700-750, in gear 550-575 give or take headlights on or off there... When coming to a stop, now and again - its unpredictable - the idle will drop to 450 rpm quickly, then return to normal. For a split second, it almost sounds like it's going to stall.

Today, it did it when the car was stopped and I turned the lights on. Not sure if I turned the lights on right when the motor went to idle down and the alternator put a load on the engine. I also experienced a brief hesitation when accelerating from a stop.

I'll tell you what the problem isn't it. Plugs are new, wires are good, new cap and rotor. I heard of weak throttle return springs on these motors, it's not that. Cables are good. Brand new fuel injectors. I have a strong feeling it's not the throttle close switch, TPS, or ISC motor. Fuel filters have about 5,000 miles of them. EGR was replaced about 3,000 miles ago, EGR tubes cleaned 1,000 miles ago.

Can this be the beginnings of a fuel pump? I only suspect the fuel pump because weeks ago before this started happening, I could have swore I heard it whining when I was at idle.

And now that I think about it, I replaced the alternator recently. The brand new alternator does whine. I thought the whining would go away after the brushes wear in, but no. Come to think of it, I've only ran into this idle dropping problem when the headlights were on.

 

so.....lol ???? did you answer your own question?.... sometimes i also have to refresh my memory. course im from the 60s keke... anyway, what i think you are witnessing could be alternator....??? dought it though..... maybe have it checked.
i just had the same problem with a 4.6 N* and cleaned the tps, and all was fine.
keep the forum posted on your progress.... i'm real curious with this one... thanks and good luck.
rickyd

 

I keep my TB meticulously clean. They sure do carbon up. Come to think of it, I should clean my Northstar.

Can the alternator actually cause this idle issue? Would it be from some electric type thing, ie., voltage regular, or mechanical like a sticking bearing? Alt. charges fine, runs at 14.1 volts.

 

i dough the alt can effect idle... i think maybe you have other issues... let me see if i can get another tech into this one.....
rickyd

 

Well I finally scored some time I could check out the issue in the light. I think I figured it out. It is in fact the throttle return spring.

I blipped the throttle and it didn't return to idle. It remained at like 1050rpm. Reminded me the old days when carburetors had fast idle cams. You know...they could never really perfect perfection and simplicity.

So I popped the hood. I could push the throttle arm down to idle. When I throttle it up by hand slowly, it doesn't return to idle unless I push it all the way down by hand due to lack of tension from the spring.

So I think the reason it dropped idle (and the reason why I didn't figure this before) was that after I came off throttle, even though the throttle was stuck open a little bit, and ISC motor backed out to lower the idle to normal, only, it wouldn't, so when the throttle arm finally closed due to the car stopping, it clicked closed, dropping RPM, and then quickly the ISC motor brought the idle to normal.

Thanks a lot guys. I will change the spring and let you know it I have solved it.