Hi, really need help on my 86 f150 5.0 efi. "Emissions" light flicked off/on for a day, then stays on. pulled koeo code and said O2 sensor bad. Replaced it (really old one probably needed it anyway) cleared codes, light still on and engine idles high. (approx 145K on truck runs great otherwise)Pulled bat cable for an hour or so, then drove 15 miles to reset all...light still on... still high idle (like when cold), can hear idle slowly try to drop, but gets rougher when doing so, then pops back to high idle and smoothes out fine. Engine revs fine, and doesn't seem to have much effect on power. Pulled koer codes and rec'd 42, 13, 77 first time, restarted and "goosed engine" and rec'd 23, 44, 13, 77. From what the Ranger station codes say, sounds like the TP sensor. However earlier I unplugged the tp sensor, and engine would stumble badly if I opened throttle too quickly, but rev up if I did it slowly. Plugged it back in and pulled a koeo code for TP sensor below min vlt. Cleared codes, ran again, checked koeo code ..no codes, jut 11 pass code. KOER codes pulled after this and are listed above.
Can the TP sensor be defective, but still work sort of?? I hate to spend $47.99 for something I don't need. I only have a chiltons manual for this, and it doen't go into much detail...am going nuts! Need truck for the weekend...HELP!!!!!

 

You can check the voltage from the tps sensor. It should be .2 to .39 when the throttle is closed. This would be on the green wire. You should have 5 volts on the orange wire. You can also disconnect the tps connector and, using a meter with a needle on ohms, move the throttle and watch the needle for smooth transition from .39 to 4.84 volts.

 

**UPDATE** Replaced the TPS. Now, KOER show codes 44 (air system inop) and 13 (Air bypass). Seems like I'm chasing my own tail!! Still has high idle (which the AB would do) and emission light stays on. I'm starting to lose my balance and feel I may go over the edge at any minute!!! I guess the only way to deal with this is with an Edelbrock manifold and a good ol' 4 barrel carb!!!!!

 

The book tells you to deal with each problem, starting with the first error code it gives you. So you got the first one fixed. The 44 has something to do with the air pump system. Contrary to what I just said in the previous sentence, lets skip that one and work on the idle air bypass. I would take it out and clean up the bypass valve and the throttle butterfly. Then I would check the throttle stop adjustment screw. My book says when the engine is at idle, you should be able to unplug the idle motor, and the engine should slowly die. If it abruptly stops, or keeps running, adjust the idle stop screw for the throttle butterfly till it slowly dies when you unplug it.

 

[updated:LAST EDITED ON 29-Aug-02 AT 09:27 PM (EST)]Yeah, I went straight to the IAC also. pulled it off, cleaned it, checked the solenoid. Seems to be OK. one question, should it completely close off? Mine stays open a very slight amount, so air bypasses thru it. I looked at a new one tonight and it seems to do the same thing. I'm at a total loss. I ran it with it unplugged and it seemed to get a little slower (not much) but definitely didn't die. Backed off the idle screw while unplugged as well...nothing. Should it (the valve) be completely closed?? $80 for a new valve, but would be worth it if it'll work. So far, $35 for a O2 sensor, and $40 for the TP sensor. All this on a truck that ran great with no probs whatsoever. Emission light stays on, and high idle.

 

The emission light on your model has nothing at all to do with EEC system (which not the same as a check engine light on later models). No changes you make will turn it off. It is there for an EGR check that was supposed to be checked at about 60k miles (which a timer in the emission light circuit triggers the light on near 60K). Some of the curcuits have a reset but you can just unplug it or forget about it, it hurts nothing. The unit is a brown box about 1"x2" on the steering column under the dash on your model I think. As for your high idle, one likely cause is a vacumn leak. The gasket between the upper and lower intake is famous for leaking near the front bolts.

 

Engine has approx 150K on it, so well past the scheduled check.Been looking for vac leaks, so far no luck, but will keep plugging away at it. Light would flick on/off for a while one day. Would light up, but depending on load/speed/accell/decel, it would go back off. Finally just stayed lit.

 

Looked for reset box...can't find it! Anyone know where it is on an 86 F150? Would really like to get rid of that light...pulling the bulb is coming soon!

 

I am not doubting what the other guy said, but I would make sure the light was not from the computer. Looks to me if it was hooked to a timer, it would stay on all the time. I think you are zeroing in on your high idle problem. If you back the idle screw all the way off, and it still runs with the IAC unplugged, the engine is definitely getting air from somewhere.

 

TheIAC is unplugged. I've also made a block-off plate to mt under it just to be sure. Truck has a better idle now (even though it shouldn't even be running!) and I'm about fed up with it. Think I'll just drive it as is for a while and work on it some other time. Strange though, never had ANY troubles before replacing the O2 sensor. I did find the PCV inlet cracked which I probably did during the replacement, but it has been replaced with a new PCV. Checked hoses/tubes and sprayed everywhere looking for RPM increase. No luck. I know it has to be somewhere, but this rig just has too many possibilities. Was contemplating selling it for a 3/4 ton with a 351 to pull a trailer with, but after reading other posts, may just go ahead with the Windsor JR heads and other mods TorqueKing suggests. I really like the truck and it's in beautiful condition. Would be hard to replace for under $5K so maybe I'll just tweak the motor some for some good ol' torque and go camping!
Thanks for the help everyone, and if you think of something we haven't already tried, please let me know.

 

OK, haven't been able to find any vacuum leaks, doesn't mean it's not there though. The IAC is still blocked off and unplugged, idle still higher than should be, but not as fast as normal cold idle, idle screw backed all the way off. The only way I can get this truck to stumble and die, is to disconnect the MAP sensor either by unplugging the electrical, or the vacuum line. WHAT GIVES!!! there is a 5 line vacuum connector on the right fender, and I've traced the lines to the EGR, MAP, and a few others I don't know what they are, but all lines seem fine. Where can this thing be getting enough air to run/idle like it is?? Is there a valve connected somewhere that could be feeding the manifold enough air to be running? By the way, the truck drives just fine down the road. smooth running as always. Boy this is confusing. Remeber, all this started when I replaced the O2 sensor. i did crack the PCV but it's been replaced. What is the valve mounted behind the engine for. It has a vacuum line running to the top, and then is connected to what looks like 1/2" rubber hose, that seems to be part of the air pump circuit.

 

>Looked for reset box...can't find it! Anyone know where it
>is on an 86 F150? Would really like to get rid of that
>light...pulling the bulb is coming soon!

The reset box should be under the dash, between the wiper switch and the steering column. It's in a tough place to get at to reset. If you need help resetting it, let me know.

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[font color="green"]1995 F-150 Supercab XLT 4x4 302 M5OD 8' bed [/font]31x10.50's
[font color="green"]1986 F-150 4x4 300 I6 NP435 [/font]Warn hubs, DeeZee bed caps, '96 XLT bench seat, new paint job

 

[updated:LAST EDITED ON 03-Sep-02 AT 10:19 AM (EST)]did you possibly lean on the manifold when you replaced the O2 sensor?? you may have disurbed the manifold gaskets... may want to spray some water around the manifold mating surfaces while its running, or around other areas for that matter, this would cause it to stumble if it sucked in the water if it is a vacuum leak.. use this trick at the shop, always had good luck finding leaks like this...

 

All right everybody. I've received a lot of good advise from you folks and I appreciate every bit of it. I finally took the time to crawl back under the hood of my truck to once again, try and fix that darn idle problem. I decided I'd track down the leak using water, and figured if I shorted something out..so be it! Grabbed a gallon jug of water and started dousing my running engine any where I figured it would/could leak, and lo and behold....major stumbling right at the plenum gasket!! I've been living with this problem for several months now, figuring it just had to be a bad gasket, but didn't want to deal with pulling all the crap off to get to it. I know, lazy on my part! Anyway, Ford was nice enough to make 5 of the 6 bolts holding the plenum off accessible with a socket. The 6th one however is tucked waaaaay up underneath the plenum and behind the fuel rail. Ford decided to make this even more fun to get off by installing a TorX bolt instead of the standard hex head. Now I'm sure they have a nifty wrench in their kit just for this stupid bolt, but my 90dgree Torx set wouldn't fit, so I used a Torx bit out of an old cheap screw gun tip kit, and my trusty 1/4" wrench to get that bugger off! There might have been an easier way to achieve the removal of the plenum, but all in all it only took about an hour including the run to the parts store. $8.91 for an upper gasket kit and the truck now idles and runs beautifully!
Thanks again to all those who helped me out along the way on this.