Can anyone help with an idle problem I have started to experience. After startup and once driving my 1990 Bronco XLT 302, 5 speed when I push in the clutch to change gears the engine revs up to 2000-2300 rpms. It does it until the engine is warm and has been driven awhile. It probably goes away after 20 minutes or so but the idle stays at around 1000 when it used to be 700-800. The idle always has been slow to come down when I push in the clutch. Could this be the TPS??????????

 

I had the same problem on my 90 F250 302 and 5 speed. I tried replaceing every sensor I could think of and none of them made any difference. Finally figured out that it was a blown intake manifold gasket, the one between the upper and lower pieces of the intake, not the ones between the intake and the heads. Since then I have heard of other people with 302's that have had the same problem, so my guess is that is the reason your truck wont idle right.:-X23

 

Would it make sense then that the idle goes back almost to normal
after it's driven?

 

Has the air bypass valve & throttle plates been cleaned lately ? Thats the first thing you should look at if you are experiencing idle problems .

 

[updated:LAST EDITED ON 25-Mar-02 AT 10:57 AM (EST)]My 5.8L acts similar. I don't know how long at a time it would do it because I kill it and crank it back up and it stops. Makes no sense. I have changed Idle Air Control valve. The other day it did something totally different. The idle was surging up and down from 500 to 15oo or so. I'm considering getting another truck w/ a carburator if I can't get it figured out. Help!!!

 

Have seen several causes for this, first... oxygen sensor gone belly up, erratic function, two... battery terminals corroded, while the terminals are being cleaned the computer is also clearing itself, Ford uses a non-volitile computer system, it is not to retain memory, but for some reason it will on a cold startup, three... the ground wire at the battery that goes to the computer, hooked on at the battery post terminal going into a connector, the connector get corrosion in it also, this wire is a 12 guage black wire. My first guess is the oxygen sensor though. you asked can a TPS do this, only if the voltage on the green wire with the key on is above one volt, I set the ones that come in the shop at .95vdc, key on engine off. High idle: with engine running, unplug the IAC and see where the idle rpm goes, if nothing has been messed with, the engine should shut down, or should run very slow, under 400 rpm, if the engine still runs you have a vacuum leak, an it is notorious for the upper intake plenum gasket to suck in to the inside of the intake manifold. Spray some carbon cleaner around the intake and listen for an engine rpm change, when it changes you found a vacuum leak. Just some ideas, Electric1

 

I have plans to check the tps this weekend, last night I took apart the air bypass valve and it seemed to work ok and wasn't very dirty, I only cleaned it and put it back on.I have already replaced the battery cables, O2 sensor, cleaned EGR and cleaned the throttle plates. I think the problem is a little different than I explained earlier, in the 18 months I've owned it the rpm's always drop slow when the clutch is pushed, sometimes they jump a little. Now say if I am driving along at 2000rpm's and I push the clutch, sometimes the rpm's hang at 2000 or a little more before dropping. The idle sometimes stays at 1000-1200 rpm's until I rev it a little then it goes back to 700 or so. I will check the intake gasket this weekend when I try the TPS. Thanks for the help......

 

The exact same happened to my 87' F-150 automatic. It had a high idle and when it started up there was a cloud of smoke. They changed the intake gasket and it starts up and idles fine, but he said that there is something wrong with a vacuum or something (i dont know ALOT about engines but i know a little).

 

Blueman, Oxygen sensor is running rich at the time you push the clutch in, the computer is seeing the exhaust rich with hydrocarbons and what it does is rev the engine by opening the IAC valve to let in oxygen and burn the hydrocarbons down stream to get the oxygen sensor to run its normal pattern of switching over a .450 mvdc voltage line. Rich is above 450 mvdc, lean is below 450 mvdc, now this can also be oil or fuel related, if you try carbon cleaner spray on the outside of the manifold and it has a vaccum leak, the engine will make a change high or low in rpm, if you don't hear a change, pull and plug the PCV with engine running and spray carbon cleaner into oil fill hole, you are now checking the interior intake manifold gaskets for internal vaccum leaks, listen for rpm change in engine,
Just more thoughts, Electric1

 

electric1 ,if my 1993 5.8 still idles at around 4-500 rpm and will not die when i unplug my air idle control does this mean its ok.i have a rough idle im trying to get rid off .i have taken it off and it is spotless .

 

TPS(Throttle Position Sensor).......change that and see if it helps. I had a problem like that on my taurus a while ago, and the TPS fixed it. It may be sending to much fuel to the engine.